Conversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions

Conversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Conversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions Erdem Şahin, Meltem Alp, Ahmed Şeref This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6156428/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The surface evolution of AZ31 immersed in saturated aqueous solutions of ammonium dihydrogen phosphate (ADP) and various functional adjuvants was investigated by compositional, morphological and gravimetric analyses. The immersion process was monitored by pH and weight measurements at various intervals for a period of 21 days. Saturated aqueous solutions of ADP were initially acidic with a pH around 4 which caused a rapid degradation of the alloy surface. Apparently the dissolved cations reacted with infusing ions within the bulk of the alloy to induce a strong volumetric expansion that increased the thickness of the plates more than one order of magnitude. Close examination of the cross section by SEM revealed that monolithic crystals of struvite and other magnesium phosphate phases formed perpendicular to the rolling direction of the plates, thus intercalating and simultaneously densifying the microstructure. However such long term growth could not be sustained in any of the studied multicomponent solutions of ADP, which highlights the unique pH evolution of saturated ADP solution that is suitable for bulk conversion. Materials Chemistry Metallurgy Surface chemistry AZ31 alloy Ammonium dihydrogen phosphate Corrosion Volumetric expansion Chemical conversion Struvite Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Magnesium and its alloys have been shown to be suitable materials for the development of implants due to their superb biocompatibility (Gu and Zheng, 2010 , Saris et al., 2000 ; Zhang et al., 2015 ), low density (ranging from 1.74 to 2.0 g/cm 3 ) (Staiger et al., 2006 ) and biodegradability in human body fluids (Zhang et al., 2015 ). Although magnesium alloys are widely used in numerous industries, their advantages are countered by fast biodegradation so that its applications are limited without protective coatings (Chen et al., 2014 ; Song, 2005 ). It was aimed in this study to determine the rate and extent of phase evolution on the surfaces of AZ31 plates immersed in saturated ADP solutions as a means to understand and control degradation of the magnesium alloy. ADP is an ammonium salt of orthophosphoric acid that commonly finds use as a cement precursor to produce struvite phase. It is also produced as an intermediate compound in electrochemical waste treatment processes utilizing magnesium electrodes for phosphate recovery (Kékedy-Nagy et al., 2022 , 2021 ). Our preliminary studies yielded an interesting outcome that is worth a deeper investigation such that the immersed AZ31 plate transformed to struvite and swelled in a few days by more than an order of magnitude. Apparently the transformation did not only occur in the surface but within the bulk as well, as the sample seemed to intercalate to a porous matrix which provided a template for cement reaction products. Controlled tests with replicates were conducted upon this observation to understand the underlying mechanisms as reported in the results. In addition, effects of various adjuvants on the solution chemistry and magnesium conversion were studied. Specifically nitrate groups are known to consume the hydrogen gas produced on metal surfaces (Taub et al., 2002 ). In addition to Ca nitrate, Mg nitrate is added for its facility to keep the solution saturated with respect to Mg 2+ ions and thus limit the degradation of the alloy. MgCl 2 is added to disrupt the passivating hydroxide layer on the surface and help the transformation of the surface especially in alkaline solutions (Witte et al., 2008 ). Trisodium citrate is added to disperse the particles and ions in the suspensions more effectively due to its electrostatic stabilizing effect (Hidber et al., 1996 ) and slow down dissolution and growth of magnesium phosphates. Hydroxyethyl cellulose (HEC), a water-soluble and biocompatible cellulose derivative, is added as a viscosifier to reduce mass transport around the alloy surface (El-Haddad, 2014 ; Liu et al., 2006 ). A few studies on struvite generation with magnesium substrates have been reported in the literature. Jayaraj et al. have found out that the uniformity and defect-free thickness of the coating increased with the electroplating bath pH which in turn improved the corrosion performance of the coatings (Jayaraj et al., 2016 ). Kumari et al. have revealed that the presence of calcium in wastewater causes disruptions to the struvite crystallization process (Kumari et al., 2019 ). Huang et al. have indicated that phosphates in swine effluent can be recovered effectively by chemical conversion of magnesium alloy to high purity struvite (Huang et al., 2017 ). Although these studies provide valuable insights into reactions of magnesium with ammonium phosphate solutions, bulk conversion and the resulting volumetric expansion of the alloy is a novel finding that is investigated in this study. 2. Materials and Methods AZ31 alloy plates were manufactured in Chongqing University by hot rolling at 340°C followed by air cooling. The hot rolled alloy plates were cleaned in ultrapure water and dried before cutting to smaller dimensions. Preliminary studies were done by immersing a large plate with dimensions of 30x80x1 mm in a 200 mL saturated aqueous solution of NH 4 H 2 PO 4 with excess particles sedimented at the bottom of the beaker. Immersed sample was taken out after 21 days and dried at room temperature under ventilation. Fractured pieces were pulverized manually with mortar and pestle and subjected to phase analysis. Elemental composition analysis was done by subjecting 0.6 g of the pulverized sample to X-ray fluorescence spectroscopy in a Spectro X-Lab Pro device after diluting with 7.4 g of molten lithium tetraborate. Thermal gravimetric analysis was applied on the same sample in a Perkin Elmer Diamond TG/DTA device under oxidizing atmosphere. The sample was heated from room temperature to 1210°C at a rate of 10°C/min. For the subsequent studies hot rolled AZ31 plates were cut into samples of around 2 cm 2 area and immersed in saturated aqueous solution of ammonium dihydrogen phosphate and various multicomponent solutions containing NH 4 H 2 PO 4 above its saturation limit and the following adjuvants: 3.5 wt% magnesium chloride hexahydrate (MgCl 2 .6(H 2 O), Merck, CAS: 7791-18-6), 1.3 g (0.1 M) trisodium citrate (Na 3 C 6 H 5 O 7 , Isolab chemicals, CAS: 6132-04-3), 3 wt% Ca(NO 3 ) 2 , 3 wt% magnesium nitrate hexahydrate (Mg(NO 3 ) 2 .6(H 2 O), Sigma Aldrich CAS: 13446-18-9), 2 wt% hydroxyethyl cellulose ((C 2 H 6 O 2 ) n , Sigma Aldrich, CAS: 9004-62-0). The concentrations of the additives were set at values comparable to the sea water salt concentration as this provided a practically relevant and aggressive test environment. All chemicals were mixed into ultra pure water for 24 hours by magnetic stirrer and homogenized by ultrasound probe to ensure well mixing. Alloy samples were immersed statically in 50 mL of solutions for 21 days and analyzed at specific intervals (1, 3, 7, 14, 21 days) by gravimetry to account for the change in their mass. Also the solution pH was monitored at the same intervals. Morphological analyses of the dried samples were done by using a Philips XL-30S FEG scanning electron microscope. A secondary electron detector was used to capture micrographs at an accelerating voltage of 3.00 kV and a wedge distance of 10 mm. An electron dispersive X-ray detector was used for elemental analysis of the sample surfaces. XRD analyses were conducted by using a Philips X'Pert Pro powder diffractometer with Cu K a radiation at a generator voltage of 45 kV and a tube current of 40 mA. All XRD patterns were obtained at a scan step size of 0.05 and 4 seconds per step. Rietveld refinement method was employed for the quantitative XRD analysis of the diffraction patterns obtained from the deposited sample surfaces. Profex software from Doebelin.org and XRD references from Crystallography Open Database were utilised for phase identification and quantification. 3. Results and Discussions 3.1. Bulk conversion in aqueous saturated ADP solution It was observed in our preliminary studies that saturated ADP solution induced an extraordinary transformation of AZ31 plates as shown in Fig. 1 . The wide plate intercalated within a few days and formed new crystals all over the bulk of the alloy presumably by the infiltrating phosphate and ammonium ions. The side view given in Fig. 1 a shows the top side standing on the separated bottom side that is surrounded by excess ADP particles at the bottom of the beaker. After 21 days the thickness of the top side increased by more than an order of magnitude from 0.5 mm to about 10 mm. Manual examination of the sample shown in Fig. 1 c reveals that it is dense and mechanically stable like a cement product. The variations of sample weight and solution pH from the replicate immersion test are given in Figs. 2 a-b along with the variations for samples immersed in different solutions. A steady rise in weight indicates that the supersaturated solution acts as the precursor of a cement system, feeding the growing crystals as long as it remains supersaturated. At the same time the alloy plate acts as the cation source for the cement reactions, simultaneously dissolving like the inorganic MgO particles in a magnesium phosphate cement system. Its dissolution is expected to be fast at the initial solution pH around 4 as magnesium hydroxide cannot form as a passive layer at this acidic condition. Continuous mechanisms of dissolution of the excess ADP and integration of magnesium and phosphate ions into the bulk of the alloy resulted in a balanced pH value around 4.7 until 21 days. Such a controlled integration of phosphates into the bulk of the AZ31 plate is significant from the perspective of phosphate recovery since the initial growth rate of about 0.3 g per day for a sample with 2 cm 2 exposed area is comparable to the recovery rates achieved in electrochemical processes using large sacrificial Mg anodes (Hug and Udert, 2013 ; Kruk et al., 2014 ). Detailed characterization of the large sample reveals that ADP crystals have covered the top surface and created a solution saturated with high concentrations of hydrogen, ammonia and phosphate ions that seem to infuse through the cracks inside the sample, that are mostly aligned parallel to the top surface (Figs. 3 a-d). One or more of these species is thought to induce intercalation of the substrate and create these voids. Subsequently they formed space filling crystals with Mg ions that were leached out of the sample. Growth of monolithic crystals that are oriented perpendicular to the rolling direction of the plate may have also opened up parallel planes of the alloy matrix, contributing to volumetric expansion. In the literature it is seen that there is limited alternative ammonium phosphate phase formations in the NH 4 -Mg-PO 4 system, struvite (NH 4 MgPO 4 ·6H 2 O) being the most common one, its decomposition product dittmarite (NH 4 MgPO 4 ·H 2 O) and hannayite (Mg 3 (NH 4 ) 2 (HPO 4 ) 4 ·8H 2 O) being the others. The elemental ratio of O/N is used to distinguish between these phases (10–5 – 12 respectively) and the ADP residue (4) in the EDX analysis. On the other hand, magnesium and phosphate ions can form a long list of compounds in the three fundamental groups Mg(H 2 PO 4 ) 2 , MgHPO 4 and Mg 3 (PO 4 ) 2 . Incorporation of H 2 O, changes the atomic ratio of O/Mg/P in a wide range from 8/3/2 for trimagnesium phosphate (Mg 3 (PO 4 ) 2 ) to 30/3/2 for cattiite (Mg 3 (PO 4 ) 2 ·22H 2 O). Therefore elemental composition information has to be coupled with phase identification using XRD analysis to correctly find the composition in terms of crystalline compound fractions. The elemental composition obtained by EDX analyses points to the ADP (NH 4 H 2 PO 4 ) stoichiometry at the surface (spectra 1, 2) and a mixture of ADP and a Mg oxide phase that is not clearly identified (spectrum 3). Pure struvite stoichiometry could not be detected in the bulk (spectrum 4) where N is assumed to be present only in struvite, dittmarite or hannayite phases. Quantitative XRD analysis of the pulverized sample revealed the presence of hannayite at a weight fraction of around 10% in addition to struvite. According to the literature the monolithic struvite and hannayite crystals at the bulk may be surrounded by an amorphous Mg phosphate, MgHPO 4 that is the decomposition product of struvite (Ramlogan and Rouff, 2016 ). According to XRD, the powder also contains about 12% ammonium phosphate and barely detectable (0.3%) MgH 2 (Fig. 3 e). The former was most likely the residue on the surface of the transformed plate which was pulverized with the rest of the sample. Carbon in EDX spectra were unaccounted for in the XRD data, which may indicate miscalibration of the detector and a possible overlap of the K alpha peak of C (λ = 4.47 nm) with K alpha of N (3.16 nm) and O (2.36 nm) or with the K beta peak of P (10.38 nm) that are relatively close. XRF analysis of the powder given in Table 1 confirms the phase identification with a nearly equimolar ratio of Mg and P atoms. Slightly extra P is attributed to the hannayite phase and the ADP residue. Expected impurities of Al, Mn and Zn were detected, Zn being found less than its amount in the as cast alloy. Mn and Al are known to form stable intermetallic inclusions in AZ31 which seem to have stayed in that state. Zn is apparently leached out to the solution without reacting with phosphates. Table 1 XRF analysis results of the sample transformed in ADP solution. Elements Na Mg Al P Mn Fe Ni Zn Others Atom % 0.10 46.57 2.98 49.21 0.34 0.14 0.05 0.19 0.42 Thermogravimetric analysis of the sample shows that struvite crystals lose their bound water and ammonia upon heating up to 250°C according to the literature (Fig. 4 ) (Bhuiyan et al., 2008 ). The loss on ignition of about 48% is less than the literature value of 51% for pure struvite, as the powder also contained about 10% hannayite, 12% ammonium phosphate and some MgH 2 , which have much lower LOI values. Weight loss stopped around 500°C and accumulation of oxygen is thought to take place up to 1200°C. There was no indication of Mg evaporation at the boiling point of 1091°C. Mass balance on these compounds shows that nearly all Mg in the bulk of the alloy plate transformed into a magnesium phosphate during the 21 day static immersion test according to Fig. 2 a. It is worth noting that ADP solutions only induced this transformation in pure water without any other additive. Slight changes in the solution chemistry by various additives disrupted the continuous transformation as discussed next. 3.2. Surface evolution in aqueous saturated ADP solutions with adjuvants 2wt% HEC addition to the highly reactive ADP solution provided unstable surface transformation presumably due to slower mass transport induced by the high viscosity of the polymer. Its pH was higher than ADP solution by about 0.5 throughout immersion, indicating the slower dissolution of ADP in this medium. Still, initial high concentration of phosphates induced faster growth of the sample in the first three days. The sample structure lost its integrity afterwards and broke down to pieces, possibly due to reduced phosphate ions around the surface and continued dissolution of the alloy matrix. A mixed microstructure of the fractured surface is seen in Fig. 5 , basically consisting of monolithic crystals surrounded by much finer crystals. These were probed by EDX to yield the expected struvite stoichiometry (Mg/P/N/O = 1/1/1/10 at regions 6, 7) in Fig. 5 b, c. In addition, a mixture of phases that may contain Mg carbonate, struvite and some dehydrated MgHPO 4 was detected in spectrum 5. Analysis of the XRD pattern yielded a small fraction of bobierrite phase (Mg 3 (PO 4 ) 2 •8(H 2 O)) in addition to struvite, with a similar stoichiometry observed at region 9. Characteristic struvite monoliths were common and the small crystals around them seem to contain bobierrite, nesquehonite (MgCO 3 •3H 2 O) and struvite as a transition zone above the degrading struvite layer. It is known in the magnesium phosphate literature that struvite and bobierrite can convert to each other as a response to the solution pH, such that latter is typically stable in slightly acidic and struvite in slightly alkaline solutions (Musvoto et al., 2000 ). In fact, newberyite (MgHPO4•3H 2 O) phase can also coexist with these at a specific pH range. Apparently the slight decrease in pH in the HEC-ADP solution shifted the driving force for crystallization in favor of bobierrite but it was not strong enough to degrade all struvite completely. Magnesium carbonate phase detected by both EDX and XRD seems to be an artifact of the drying process as dissolved carbon dioxide can precipitate by reacting with magnesium ions. Carbon was detected only in spectrum 5 at a much higher fraction than the XRD average quantity that is thought to originate from a mixture of Mg carbonate and ADP residue. ADP solution with MgCl 2 seems to infiltrate the AZ31 plate less compared to the reference. Very low pH due to inhibition of Mg dissolution kept the surface without a passive hydroxide layer which transformed into newberyite in time. Two different structures are observed in Fig. 6 a which are regular prismatic blocks of newberyite and irregular lumps that look to be deposited above the newberyite layer. High definition, stoichiometric newberyite crystals seen in Figs. 6 a and c have grown monolithically without etch-pits in the constantly acidic solution. Figures 6 b and d show regions of the surface that contain different phases than newberyite like region 10 which contains high concentrations of N, Mg, and Cl, indicating presence of ammonium chloride phases. XRD analysis given in Fig. 6 e reveals that novograblenovite (NH 4 MgCl 3 ·6H 2 O) is the crystalline phase that is present by about 1.6 wt%. It seems to accumulate on the substrate like the ADP depositions on the top surface of the sample swollen in ADP solution. Presence of chlorine ions seems to have transformed ADP with the diffusing Mg ions to novograblenovite. The nitrate-added ADP solutions transformed the alloy in a completely different way such that both nitrate sources Ca(NO 3 ) 2 and Mg(NO 3 ) 2 caused complete dissolution of the samples at day 14 and 7 respectively. Nitrates are known to consume hydrogen molecules in aqueous solutions according to reaction 1 and consequently drive the magnesium corrosion reactions 2, 3, 4 forward according to the Le Chatelier principle (Wa et al., 1994 ). This self-feeding loop seems to increase the rate of Mg corrosion by reducing the activity of the gaseous H 2 corrosion product. Paralel pH evolution was recorded for both samples at a slightly lower level than the ADP solution (Fig. 2 b). The variation of pH after day 3 follows a concave-up pattern which is in contrast to the reverse pattern for the reference solution. The same upward trend in solution pH is observed after day 7 for all solutions with adjuvants which indicates the divergence in reaction mechanism from the balanced bulk conversion provided by the reference ADP solution. Apparently the adjuvants facilitate continuous corrosion of the alloy which in turn shifts the balance of water hydrolysis, resulting in a gradual increase in pH, while the reference solution appears to slow down reaction 2 presumably by inducing continuous formation of Mg compounds over the substrate. The disturbance of this balance by nitrates may be attributed to further reduction of nitrites into ammonia groups that are the products of ADP dissolution (Wa et al., 1994 ). Its solubility may be hindered as a result, reducing the concentration of other products, phosphates and hydrogen ions. $$\:{{\text{N}\text{O}}_{3}}^{-}+{\text{H}}_{2}\:\to\:{{\:\text{N}\text{O}}_{2}}^{-}+{\text{H}}_{2}\text{O}$$ 1 $$\:\text{M}\text{g}\:\:\to\:\:\:{\text{M}\text{g}}^{++}+{2\text{e}}^{-}$$ 2 $$\:{\text{H}}_{2}\text{O}\:\to\:\:{\text{O}\text{H}}^{-}+{\text{H}}^{+}$$ 3 $$\:2{\text{H}}^{+}+2{\text{e}}^{-}\:\to\:\:{\text{H}}_{2}$$ 4 Depositions on the refreshed substrates in calcium nitrate solutions occurred within a week and contained bobierrite in addition to the struvite phase. The rod-like prismatic macrocrystals seen in Figs. 7 a-c have struvite morphology. Struvite is known to be stable at relatively higher pH levels compared to its derivatives bobierrite and newberyite. Their gradual degradation due to the slightly acidic condition is indicated by the etch pits covering all crystal surfaces. Finer bobierrite groups are identified together with hexagonal rosette-like formations that are characteristic of calcium aluminate (Moranville-Regourd and Kamali-Bernard, 2019 ) and magnesium carbonate (Power et al., 2007 ) cement products. There are also groups of rod-like crystals that are associated with aluminum phosphate cements (Fig. 7 d). The high phosphorus and aluminum ratios found in the EDX results suggest that these crystals are aluminum phosphate phases and the rosette-like crystals resemble dypingite (Mg 5 (CO 3 ) 4 (OH) 2 .5H 2 O) that form by CO 2 uptake from air. However wavellite (Al 3 (PO 4 ) 2 (OH) 3 ·5H 2 O), montgomeryite (Ca 4 MgAl 4 (PO 4 ) 6 (OH) 4 ·12H 2 O) and hexaaquamagnesium(II) hypophosphite (Mg(H 2 O) 6 )(H 2 PO 2 ) 2 ) peaks were detected in the diffraction pattern which also contained peaks for the magnesium substrate and MgH 2 , as seen in Fig. 7 e. AZ31 plates completely dissolved in magnesium nitrate containing ADP solution at day 7 and the new sample continued dissolving after slight deposition at day 14. The surface of this plate immersed after a week in the solution and aged until day 21 also displays crystals with dissolution pits. Rod-like crystals seen in Figs. 8 a, b, d have struvite morphology. Irregular lumps of fine prisms are also seen together with these prismatic crystals (Fig. 8 c). The surface analysis results in Fig. 8 e show that they contain the same elements Mg, P, N in similar amounts. XRD analysis indicates that a mixed structure of struvite and newberyite have formed, on top of which struvite monoliths have grown at some regions. Detection of a significant amount of metallic Mg for both nitrate solutions indicates that the phosphate layers are thin and loose. Trace amounts of MgH 2 and Mg(OH) 2 were also detected. Comparing the results with the calcium nitrate solution, it is seen that when magnesium nitrate is added, phosphates form newberyite instead of wavellite, bobierrite and other magnesium phosphates (Fig. 8 b). The reason for this is thought to be the slight decrease in solution pH caused by the extra magnesium ions in the initial solution that prevented excessive Mg, Al dissolution and hydroxide release. ADP provided continuous deposition on AZ31 plates in 0.1M trisodium citrate solution which was arrested at the end of day 7. This produced the morphology given in Fig. 9 where the crystals around flat dense surfaces are typical monoliths of struvite. An interphase region with concentration gradients of the product phases is observed in the SEM micrographs of the sample immersed in citrate solution as shown in Figs. 9 a, c. At one end the deposited crystals have rod-like struvite morphology while the base is flat at the other end with high Mg content. C atoms are most likely present as citrate groups adsorbed on magnesium surface. The middle region 19 gives out less N emission but similar Mg, P, O. The lower O/Mg ratio in this region indicates it is a mixture of struvite with newberyite. Apparently struvite crystals formed on top of the newberyite layer that is also reflected by the gradual decrease in pH until day 14, when struvite crystals started to dissolve subsequently as evident from the etch-pits. The flat region at the other end gives the lowest O/Mg and P/Mg ratio which should result from a Mg(OH) 2 layer surrounded by magnesium phosphate phases. Mg hydroxide phase in the flat region could not be detected with XRD probably due to its small surface area and nanoscale size. The crystalline phases on the surface were found by XRD analysis as struvite, newberyite, ADP, and a sodium aluminum phosphate phase with an irregular stoichiometry of Na (3−3x) Al x PO 4 . Apparently continuous dissolution during the first week of immersion provided enough Al 3+ concentration to form this phase with Na + from trisodium citrate solution. Formation of significant amounts of both newberyite and struvite was facilitated by the slight fluctuations in the pH level around pH = 5. The pH variation is the reverse of ADP solution and its pH is constantly higher by about 0.5 which indicates lower solubility of ADP in the presence of citrate groups. As a result the balance between ADP/Mg dissolution and struvite formation has shifted towards the latter and the deposition rate diminished in a week as a sigmoidal function of time that is characteristic of inorganic cement setting kinetics (Şahin and Kalyon, 2017 ). Similarly calcium phosphate cement setting is induced by initial dissolution of an acidic phosphate salt which activates the calcium rich particles to supply cations that react with phosphates and precipitate on the surface of the particles. This creates a thick layer on the surface of exposed particles that can sustain the crystallization by dissolution. Thus the balance between dissolution and crystal growth is disrupted, leading to the end of cement setting (Şahin and Kalyon, 2017 ). As also observed for the citrate-containing solution, a continuous deposition could not be sustained in any ADP solutions with adjuvants, partially due to the physical and chemical constraints introduced by each but also because the reactions were confined to the surface where Mg dissolution and struvite formation reactions compete until one dominates the other. One extreme case is seen in nitrate solutions where the dissolution rate was kept high relative to crystallization and the sample disintegrated through progressive pitting. The other extreme is observed in Mg chloride solution where Mg dissolution was suppressed initially and the surface was quickly passivated by a small amount of newberyite deposition. The special case of the reference ADP solution seems to occur due to some additional mechanism that is responsible for significantly increasing the surface area so that both Mg dissolution and struvite formation reactions are maintained for a longer time as in cement systems. The micrographs seen in Fig. 3 hints at an intercalation step prior to the densification of the bulk by struvite crystallization which seems to be necessary for infusion of ions in the solution. The magnesium phosphate phases detected in these transformed samples have all been described in mineralogy as sheets of alternating layers of Mg octahedra and phosphate tetrahedra that are binded by hydrogen bonds into stacks making up the macroscopic crystals (Huminicki and Hawthorne, 2002 ), so it is natural to consider hydrogen as the main agent responsible for the bulk conversion of metallic Mg to phosphate compounds. Literature on H 2 embrittlement of steel describes a similar phenomenon where H + infiltration through the metal surface causes H 2 gas nucleation in the bulk and internal stresses that are known to create voids, cracks and expansion (El-Haddad, 2014 ; Huang et al., 2017 ; Kumari et al., 2019 ; Liu et al., 2006 ). Since magnesium dissolution in water is accompanied by proton and hydrogen gas evolution, the same effect may occur autogenously. To the best of our knowledge there is no report of gas formation and related stresses within the bulk of the metal in the literature. Schober’s study of microstructural changes occuring in pure Mg upon application of high pressure H 2 is the only available source on hydrogen-induced lattice expansion through formation of MgH 2 phase which was also detected in our samples in small quantities (Schober, 1981 ). In a related study by Zeng et al. it is seen that low concentrations of hydrogen and dihydrogen phosphates in saline solution only affect the surface by creating a passive Mg phosphate coating (Zeng et al., 2014 ). The high concentrations used in our study at the saturation limit of ammonium dihydrogen phosphate may be necessary for the anticipated hydrogen attack. Further studies on the role of H 2 are underway to elucidate the observed volumetric expansion in ADP solutions. Whether it is also related to the thermomechanical processing history of the hot rolled AZ31 alloy, such as inherent texturing and residual stresses is also investigated for a sound understanding of the observed phenomenon. 4. Conclusions AZ31 alloy undergoes an extraordinary transformation when immersed in aqueous saturated ammonium dihydrogen phosphate solutions that may provide an alternative route in wastewater phosphate recovery processes and a strengthening mechanism in Mg phosphate cement matrices. A steady volumetric expansion at a rate of 1 fold increase in weight per day has been observed for the first week of the immersion period of 21 days. The product is akin to a Mg phosphate cement product in terms of compactness and mechanical stability. The immersion medium acted as a dilute cement solution and the cement reaction transformed the metal plate to a magnesium phosphate ceramic. Phase analysis of the sample revealed that about 88% of the alloy transformed to struvite, 11% to hannayite and less than 1% to MgH 2 . Both the extent of transformation and composition of the products changed with adjuvants. All additives inhibited the volumetric expansion and restricted the reactions to the surface of the alloy. HEC increased struvite fraction to about 98%, while Mg containing additives MgCl 2 and Mg(NO 3 ) 2 either reduced struvite fraction or completely shifted the reaction in favor of an alternative phase, newberyite. Slight differences in their solution pH resulted in major changes in the rate, extent and composition of the formations. Aqueous saturated ADP solution provided a relatively low and constant pH around 4.7 compared to the multicomponent solutions, which seems to provide a continuous driving force for the dissolution of Mg and ADP as well as struvite formation. Further studies on the effect of hydrogen gas nucleation within the bulk of the alloy that may be responsible for the observed intercalation by creating voids and stresses in the microstructure are needed to elucidate the mechanisms leading to the observed bulk microstructural evolution. Declarations CRediT authorship contribution statement Erdem Sahin : Experimental planning, data analysis and reporting. Meltem Alp : contributed to the experiments and gravimetric data collection. Ahmed Seref : conducted the corrosion tests. Declaration of competing interest The authors declare that they do not have any conflicts of interest. Acknowledgments The authors appreciate the financial support from Scientific and Technological Research Council of Turkiye (TUBITAK) (Project No: 2020-119N759). We thank Prof. Jian Peng for his cooperation on the manufacturing and delivery of AZ31 alloy sheets. İzmir Institute of Technology Materials Research Center staff is acknowledged for their assistance in characterizations. Doebelin.org and Crystallography Open Database (Crystallography.net) are acknowledged for sharing their XRD analysis software and database. Data Availability Data will be made available on request. References Bhuiyan MIH, Mavinic DS, Koch FA (2008) Thermal decomposition of struvite and its phase transition. Chemosphere 70:1347–1356 Chen Y, Xu Z, Smith C, Sankar J (2014) Recent advances on the development of magnesium alloys for biodegradable implants. Acta Biomater 10:4561–4573 El-Haddad MN (2014) Hydroxyethylcellulose used as an eco-friendly inhibitor for 1018 c-steel corrosion in 3.5% NaCl solution. 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J Mech Behav Biomed Mater 72:252–260 Saris N-EL, Mervaala E, Karppanen H, Khawaja JA, Lewenstam A (2000) Magnesium: an update on physiological, clinical and analytical aspects. Clin Chim acta 294:1–26 Schober T (1981) The magnesium-hydrogen system: Transmission electron microscopy. Metall Trans A 12:951–957 Song G (2005) Recent progress in corrosion and protection of magnesium alloys. Adv Eng Mater 7:563–586 Staiger MP, Pietak AM, Huadmai J, Dias G (2006) Magnesium and its alloys as orthopedic biomaterials: a review. Biomaterials 27:1728–1734 Taub IA, Roberts W, LaGambina S, Kustin K (2002) Mechanism of dihydrogen formation in the magnesium – water reaction. J Phys Chem A 106:8070–8078 Wa J, Turunen I, Salmi T, Maunula T (1994) Kinetics of nitrate reduction in monolith reactor. Chem Eng Sci 49:5763–5773 Witte F, Hort N, Vogt C, Cohen S, Kainer KU, Willumeit R, Feyerabend F (2008) Degradable biomaterials based on magnesium corrosion. Curr Opin solid state Mater Sci 12:63–72 Zeng R-C, Hu Y, Guan S-K, Cui H-Z, Han E-H (2014) Corrosion of magnesium alloy AZ31: The influence of bicarbonate, sulphate, hydrogen phosphate and dihydrogen phosphate ions in saline solution. Corros Sci 86:171–182 Zhang L, Zhang J, Chen C, Gu Y (2015) Advances in microarc oxidation coated AZ31 Mg alloys for biomedical applications. Corros Sci 91:7–28 Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6156428","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":424119314,"identity":"9c7e722a-8fbb-4c58-ae38-5f3661278807","order_by":0,"name":"Erdem Şahin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2ElEQVRIiWNgGAWjYHACAxCRwMbAfADEYGwgQQtbAlQLM5FaGBh4DIjTwi99eOOjG3/s8vike75u5mGwkd1wgP/YB3xaJPvSio1z25KL2WTObrvNw5BmvOEAM/MMvK46w2MmndvAnNgmkQvScjgRpAWvw+zP8Jj/zvlTD9SS8wyo5T9hLQY8PGbMOWyHQVrYgFoOENYicYatWDq37ThQS5rZzTkGycYzDzMb49XC38O88XPOn+rE+TOSn914U2En23e88TFeLejuBGLCMTkKRsEoGAWjgBAAAIidRkM83Q0KAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-4756-3957","institution":"Muğla Sıtkı Koçman University","correspondingAuthor":true,"prefix":"","firstName":"Erdem","middleName":"","lastName":"Şahin","suffix":""},{"id":424119315,"identity":"bca06299-a919-4330-8096-e16c6e46d9a4","order_by":1,"name":"Meltem Alp","email":"","orcid":"","institution":"Muğla Sıtkı Koçman University","correspondingAuthor":false,"prefix":"","firstName":"Meltem","middleName":"","lastName":"Alp","suffix":""},{"id":424119316,"identity":"288ae524-0782-40ed-85cb-08f225696645","order_by":2,"name":"Ahmed Şeref","email":"","orcid":"","institution":"Muğla Sıtkı Koçman University","correspondingAuthor":false,"prefix":"","firstName":"Ahmed","middleName":"","lastName":"Şeref","suffix":""}],"badges":[],"createdAt":"2025-03-04 18:03:57","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-6156428/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6156428/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":77839020,"identity":"a6a08fde-f363-40dc-8408-06085c724f2c","added_by":"auto","created_at":"2025-03-06 04:27:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1132127,"visible":true,"origin":"","legend":"\u003cp\u003eMacroscopic views of AZ31 plate immersed in saturated ADP solution (A) side, (B) top, (C) section view.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/7e68e36e85d1c72b963fbb65.png"},{"id":77839179,"identity":"9e53c7b8-858d-472a-b8d2-1462b594df8e","added_by":"auto","created_at":"2025-03-06 04:35:15","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":169639,"visible":true,"origin":"","legend":"\u003cp\u003eVariations in a) sample weight and b) solution pH with time upon immersion of AZ31 plate into saturated ADP and various other solutions.\u003c/p\u003e","description":"","filename":"image2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/e6b035d98b327078555547f1.jpg"},{"id":77838269,"identity":"b2a6bbc7-c0ef-4e14-b739-5723b67ccff4","added_by":"auto","created_at":"2025-03-06 04:11:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2166418,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 sample immersed in saturated ADP solution for 21 days: (A) 100X micrograph (Inlet macrograph depicts the focus of analysis), (B) 200X micrograph and EDX results, (C) 500X micrograph and EDX results, (D) 2500X micrograph, (E) XRD pattern of the pulverized sample and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/3b3e740c8e6520e7044f20ef.png"},{"id":77838364,"identity":"2cc4b37c-817c-4fe5-bd8e-f8beccbe96ba","added_by":"auto","created_at":"2025-03-06 04:19:15","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":47491,"visible":true,"origin":"","legend":"\u003cp\u003eTGA analysis of the sample transformed in ADP solution.\u003c/p\u003e","description":"","filename":"image4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/3fe545ca1e4390b8422f4ed5.jpg"},{"id":77838273,"identity":"c9e89bb4-db80-42f5-b1fb-5b906840002e","added_by":"auto","created_at":"2025-03-06 04:11:15","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2127836,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 sample immersed in saturated ADP solution containing %2 HEC for 21 days: (A) 200X micrograph and EDX results, (B) 500X micrograph and EDX results, (C) 1000X micrograph and EDX results, (D) 5000X micrograph and EDX results, (E) XRD pattern of the fractured sample surface and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/0b5067725e03bc561019dcd7.png"},{"id":77838366,"identity":"d5959c4f-34a0-4e15-99ac-2326d398a462","added_by":"auto","created_at":"2025-03-06 04:19:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2038639,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 plate immersed in saturated ADP solution with 3% MgCl\u003csub\u003e2\u003c/sub\u003e: (A) 500X micrograph, (B) 1000X micrograph, (C) 2000X micrograph and EDX results, (D) 3000X micrograph and EDX results, (E) XRD pattern of the sample surface and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/41bb8c4c78598717b405e7cc.png"},{"id":77838370,"identity":"00ff4475-57eb-4276-b90b-28fb81aec7d6","added_by":"auto","created_at":"2025-03-06 04:19:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1625438,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 plate immersed in saturated ADP solution containing 3% calcium nitrate: (A) 500X micrograph and EDX results, (B) 1000X micrograph and EDX results, (C) 2000X micrograph and EDX results, (D) 4000X micrograph and EDX results, (E) XRD pattern of the sample surface and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/d9bba0e3060fbda523f0d942.png"},{"id":77838277,"identity":"1781b945-0e9d-454e-9377-24592a47aa71","added_by":"auto","created_at":"2025-03-06 04:11:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":2154610,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 plate immersed in saturated ADP solution containing 3% magnesium nitrate: (A) 200X micrograph and EDX results, (B) 1000X micrograph, (C) 2000X micrograph and EDX results, (D) 5000X micrograph, (E) XRD pattern of the sample surface and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/a8b5bdc97b175f06297efcae.png"},{"id":77838289,"identity":"527d73e0-6521-4404-91de-7289baaaa87d","added_by":"auto","created_at":"2025-03-06 04:11:16","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1466097,"visible":true,"origin":"","legend":"\u003cp\u003eSurface analyses of AZ31 plate immersed in saturated ADP solution containing 0.1M trisodium citrate for 21 days: (A) 100X micrograph and EDX results, (B) 500X micrograph and EDX results, (C) 1000X micrograph and EDX results, (D) 2500X micrograph, (E) XRD pattern of the sample surface and quantitative analysis results. EDX data is presented as atom percentages of the elements and XRD data is presented as weight percentages of the compounds and elements.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/019334a0174a05feaf120683.png"},{"id":77840090,"identity":"cd14fad3-75a9-4442-9a19-d5da04c9d63a","added_by":"auto","created_at":"2025-03-06 04:43:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":14841428,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6156428/v1/50150d99-e735-43d5-a4d8-525576ed7140.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eConversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMagnesium and its alloys have been shown to be suitable materials for the development of implants due to their superb biocompatibility (Gu and Zheng, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2010\u003c/span\u003e, Saris et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), low density (ranging from 1.74 to 2.0 g/cm\u003csup\u003e3\u003c/sup\u003e) (Staiger et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and biodegradability in human body fluids (Zhang et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Although magnesium alloys are widely used in numerous industries, their advantages are countered by fast biodegradation so that its applications are limited without protective coatings (Chen et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Song, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2005\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIt was aimed in this study to determine the rate and extent of phase evolution on the surfaces of AZ31 plates immersed in saturated ADP solutions as a means to understand and control degradation of the magnesium alloy. ADP is an ammonium salt of orthophosphoric acid that commonly finds use as a cement precursor to produce struvite phase. It is also produced as an intermediate compound in electrochemical waste treatment processes utilizing magnesium electrodes for phosphate recovery (K\u0026eacute;kedy-Nagy et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Our preliminary studies yielded an interesting outcome that is worth a deeper investigation such that the immersed AZ31 plate transformed to struvite and swelled in a few days by more than an order of magnitude. Apparently the transformation did not only occur in the surface but within the bulk as well, as the sample seemed to intercalate to a porous matrix which provided a template for cement reaction products. Controlled tests with replicates were conducted upon this observation to understand the underlying mechanisms as reported in the results. In addition, effects of various adjuvants on the solution chemistry and magnesium conversion were studied. Specifically nitrate groups are known to consume the hydrogen gas produced on metal surfaces (Taub et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). In addition to Ca nitrate, Mg nitrate is added for its facility to keep the solution saturated with respect to Mg\u003csup\u003e2+\u003c/sup\u003e ions and thus limit the degradation of the alloy. MgCl\u003csub\u003e2\u003c/sub\u003e is added to disrupt the passivating hydroxide layer on the surface and help the transformation of the surface especially in alkaline solutions (Witte et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Trisodium citrate is added to disperse the particles and ions in the suspensions more effectively due to its electrostatic stabilizing effect (Hidber et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) and slow down dissolution and growth of magnesium phosphates. Hydroxyethyl cellulose (HEC), a water-soluble and biocompatible cellulose derivative, is added as a viscosifier to reduce mass transport around the alloy surface (El-Haddad, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eA few studies on struvite generation with magnesium substrates have been reported in the literature. Jayaraj et al. have found out that the uniformity and defect-free thickness of the coating increased with the electroplating bath pH which in turn improved the corrosion performance of the coatings (Jayaraj et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Kumari et al. have revealed that the presence of calcium in wastewater causes disruptions to the struvite crystallization process (Kumari et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Huang et al. have indicated that phosphates in swine effluent can be recovered effectively by chemical conversion of magnesium alloy to high purity struvite (Huang et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Although these studies provide valuable insights into reactions of magnesium with ammonium phosphate solutions, bulk conversion and the resulting volumetric expansion of the alloy is a novel finding that is investigated in this study.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eAZ31 alloy plates were manufactured in Chongqing University by hot rolling at 340\u0026deg;C followed by air cooling. The hot rolled alloy plates were cleaned in ultrapure water and dried before cutting to smaller dimensions. Preliminary studies were done by immersing a large plate with dimensions of 30x80x1 mm in a 200 mL saturated aqueous solution of NH\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e with excess particles sedimented at the bottom of the beaker. Immersed sample was taken out after 21 days and dried at room temperature under ventilation. Fractured pieces were pulverized manually with mortar and pestle and subjected to phase analysis. Elemental composition analysis was done by subjecting 0.6 g of the pulverized sample to X-ray fluorescence spectroscopy in a Spectro X-Lab Pro device after diluting with 7.4 g of molten lithium tetraborate. Thermal gravimetric analysis was applied on the same sample in a Perkin Elmer Diamond TG/DTA device under oxidizing atmosphere. The sample was heated from room temperature to 1210\u0026deg;C at a rate of 10\u0026deg;C/min.\u003c/p\u003e \u003cp\u003eFor the subsequent studies hot rolled AZ31 plates were cut into samples of around 2 cm\u003csup\u003e2\u003c/sup\u003e area and immersed in saturated aqueous solution of ammonium dihydrogen phosphate and various multicomponent solutions containing NH\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e above its saturation limit and the following adjuvants: 3.5 wt% magnesium chloride hexahydrate (MgCl\u003csub\u003e2\u003c/sub\u003e.6(H\u003csub\u003e2\u003c/sub\u003eO), Merck, CAS: 7791-18-6), 1.3 g (0.1 M) trisodium citrate (Na\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, Isolab chemicals, CAS: 6132-04-3), 3 wt% Ca(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, 3 wt% magnesium nitrate hexahydrate (Mg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6(H\u003csub\u003e2\u003c/sub\u003eO), Sigma Aldrich CAS: 13446-18-9), 2 wt% hydroxyethyl cellulose ((C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003en\u003c/sub\u003e, Sigma Aldrich, CAS: 9004-62-0). The concentrations of the additives were set at values comparable to the sea water salt concentration as this provided a practically relevant and aggressive test environment. All chemicals were mixed into ultra pure water for 24 hours by magnetic stirrer and homogenized by ultrasound probe to ensure well mixing. Alloy samples were immersed statically in 50 mL of solutions for 21 days and analyzed at specific intervals (1, 3, 7, 14, 21 days) by gravimetry to account for the change in their mass. Also the solution pH was monitored at the same intervals.\u003c/p\u003e \u003cp\u003eMorphological analyses of the dried samples were done by using a Philips XL-30S FEG scanning electron microscope. A secondary electron detector was used to capture micrographs at an accelerating voltage of 3.00 kV and a wedge distance of 10 mm. An electron dispersive X-ray detector was used for elemental analysis of the sample surfaces. XRD analyses were conducted by using a Philips X'Pert Pro powder diffractometer with Cu K\u003csub\u003ea\u003c/sub\u003e radiation at a generator voltage of 45 kV and a tube current of 40 mA. All XRD patterns were obtained at a scan step size of 0.05 and 4 seconds per step. Rietveld refinement method was employed for the quantitative XRD analysis of the diffraction patterns obtained from the deposited sample surfaces. Profex software from Doebelin.org and XRD references from Crystallography Open Database were utilised for phase identification and quantification.\u003c/p\u003e"},{"header":"3. Results and Discussions","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Bulk conversion in aqueous saturated ADP solution\u003c/h2\u003e \u003cp\u003eIt was observed in our preliminary studies that saturated ADP solution induced an extraordinary transformation of AZ31 plates as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The wide plate intercalated within a few days and formed new crystals all over the bulk of the alloy presumably by the infiltrating phosphate and ammonium ions. The side view given in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the top side standing on the separated bottom side that is surrounded by excess ADP particles at the bottom of the beaker. After 21 days the thickness of the top side increased by more than an order of magnitude from 0.5 mm to about 10 mm. Manual examination of the sample shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec reveals that it is dense and mechanically stable like a cement product.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe variations of sample weight and solution pH from the replicate immersion test are given in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-b along with the variations for samples immersed in different solutions. A steady rise in weight indicates that the supersaturated solution acts as the precursor of a cement system, feeding the growing crystals as long as it remains supersaturated. At the same time the alloy plate acts as the cation source for the cement reactions, simultaneously dissolving like the inorganic MgO particles in a magnesium phosphate cement system. Its dissolution is expected to be fast at the initial solution pH around 4 as magnesium hydroxide cannot form as a passive layer at this acidic condition. Continuous mechanisms of dissolution of the excess ADP and integration of magnesium and phosphate ions into the bulk of the alloy resulted in a balanced pH value around 4.7 until 21 days. Such a controlled integration of phosphates into the bulk of the AZ31 plate is significant from the perspective of phosphate recovery since the initial growth rate of about 0.3 g per day for a sample with 2 cm\u003csup\u003e2\u003c/sup\u003e exposed area is comparable to the recovery rates achieved in electrochemical processes using large sacrificial Mg anodes (Hug and Udert, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Kruk et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDetailed characterization of the large sample reveals that ADP crystals have covered the top surface and created a solution saturated with high concentrations of hydrogen, ammonia and phosphate ions that seem to infuse through the cracks inside the sample, that are mostly aligned parallel to the top surface (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-d). One or more of these species is thought to induce intercalation of the substrate and create these voids. Subsequently they formed space filling crystals with Mg ions that were leached out of the sample. Growth of monolithic crystals that are oriented perpendicular to the rolling direction of the plate may have also opened up parallel planes of the alloy matrix, contributing to volumetric expansion.\u003c/p\u003e \u003cp\u003eIn the literature it is seen that there is limited alternative ammonium phosphate phase formations in the NH\u003csub\u003e4\u003c/sub\u003e-Mg-PO\u003csub\u003e4\u003c/sub\u003e system, struvite (NH\u003csub\u003e4\u003c/sub\u003eMgPO\u003csub\u003e4\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) being the most common one, its decomposition product dittmarite (NH\u003csub\u003e4\u003c/sub\u003eMgPO\u003csub\u003e4\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO) and hannayite (Mg\u003csub\u003e3\u003c/sub\u003e(NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e(HPO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e\u0026middot;8H\u003csub\u003e2\u003c/sub\u003eO) being the others. The elemental ratio of O/N is used to distinguish between these phases (10\u0026ndash;5 \u0026ndash; 12 respectively) and the ADP residue (4) in the EDX analysis. On the other hand, magnesium and phosphate ions can form a long list of compounds in the three fundamental groups Mg(H\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, MgHPO\u003csub\u003e4\u003c/sub\u003e and Mg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e. Incorporation of H\u003csub\u003e2\u003c/sub\u003eO, changes the atomic ratio of O/Mg/P in a wide range from 8/3/2 for trimagnesium phosphate (Mg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) to 30/3/2 for cattiite (Mg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;22H\u003csub\u003e2\u003c/sub\u003eO). Therefore elemental composition information has to be coupled with phase identification using XRD analysis to correctly find the composition in terms of crystalline compound fractions. The elemental composition obtained by EDX analyses points to the ADP (NH\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e) stoichiometry at the surface (spectra 1, 2) and a mixture of ADP and a Mg oxide phase that is not clearly identified (spectrum 3). Pure struvite stoichiometry could not be detected in the bulk (spectrum 4) where N is assumed to be present only in struvite, dittmarite or hannayite phases. Quantitative XRD analysis of the pulverized sample revealed the presence of hannayite at a weight fraction of around 10% in addition to struvite. According to the literature the monolithic struvite and hannayite crystals at the bulk may be surrounded by an amorphous Mg phosphate, MgHPO\u003csub\u003e4\u003c/sub\u003e that is the decomposition product of struvite (Ramlogan and Rouff, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). According to XRD, the powder also contains about 12% ammonium phosphate and barely detectable (0.3%) MgH\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee). The former was most likely the residue on the surface of the transformed plate which was pulverized with the rest of the sample. Carbon in EDX spectra were unaccounted for in the XRD data, which may indicate miscalibration of the detector and a possible overlap of the K\u003csub\u003ealpha\u003c/sub\u003e peak of C (λ\u0026thinsp;=\u0026thinsp;4.47 nm) with K\u003csub\u003ealpha\u003c/sub\u003e of N (3.16 nm) and O (2.36 nm) or with the K\u003csub\u003ebeta\u003c/sub\u003e peak of P (10.38 nm) that are relatively close. XRF analysis of the powder given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e confirms the phase identification with a nearly equimolar ratio of Mg and P atoms. Slightly extra P is attributed to the hannayite phase and the ADP residue. Expected impurities of Al, Mn and Zn were detected, Zn being found less than its amount in the as cast alloy. Mn and Al are known to form stable intermetallic inclusions in AZ31 which seem to have stayed in that state. Zn is apparently leached out to the solution without reacting with phosphates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eXRF analysis results of the sample transformed in ADP solution.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eElements\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eZn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eOthers\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAtom %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e46.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e49.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.42\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThermogravimetric analysis of the sample shows that struvite crystals lose their bound water and ammonia upon heating up to 250\u0026deg;C according to the literature (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) (Bhuiyan et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The loss on ignition of about 48% is less than the literature value of 51% for pure struvite, as the powder also contained about 10% hannayite, 12% ammonium phosphate and some MgH\u003csub\u003e2\u003c/sub\u003e, which have much lower LOI values. Weight loss stopped around 500\u0026deg;C and accumulation of oxygen is thought to take place up to 1200\u0026deg;C. There was no indication of Mg evaporation at the boiling point of 1091\u0026deg;C. Mass balance on these compounds shows that nearly all Mg in the bulk of the alloy plate transformed into a magnesium phosphate during the 21 day static immersion test according to Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. It is worth noting that ADP solutions only induced this transformation in pure water without any other additive. Slight changes in the solution chemistry by various additives disrupted the continuous transformation as discussed next.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Surface evolution in aqueous saturated ADP solutions with adjuvants\u003c/h2\u003e \u003cp\u003e2wt% HEC addition to the highly reactive ADP solution provided unstable surface transformation presumably due to slower mass transport induced by the high viscosity of the polymer. Its pH was higher than ADP solution by about 0.5 throughout immersion, indicating the slower dissolution of ADP in this medium. Still, initial high concentration of phosphates induced faster growth of the sample in the first three days. The sample structure lost its integrity afterwards and broke down to pieces, possibly due to reduced phosphate ions around the surface and continued dissolution of the alloy matrix. A mixed microstructure of the fractured surface is seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, basically consisting of monolithic crystals surrounded by much finer crystals. These were probed by EDX to yield the expected struvite stoichiometry (Mg/P/N/O\u0026thinsp;=\u0026thinsp;1/1/1/10 at regions 6, 7) in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, c. In addition, a mixture of phases that may contain Mg carbonate, struvite and some dehydrated MgHPO\u003csub\u003e4\u003c/sub\u003e was detected in spectrum 5.\u003c/p\u003e \u003cp\u003eAnalysis of the XRD pattern yielded a small fraction of bobierrite phase (Mg\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026bull;8(H\u003csub\u003e2\u003c/sub\u003eO)) in addition to struvite, with a similar stoichiometry observed at region 9. Characteristic struvite monoliths were common and the small crystals around them seem to contain bobierrite, nesquehonite (MgCO\u003csub\u003e3\u003c/sub\u003e\u0026bull;3H\u003csub\u003e2\u003c/sub\u003eO) and struvite as a transition zone above the degrading struvite layer. It is known in the magnesium phosphate literature that struvite and bobierrite can convert to each other as a response to the solution pH, such that latter is typically stable in slightly acidic and struvite in slightly alkaline solutions (Musvoto et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). In fact, newberyite (MgHPO4\u0026bull;3H\u003csub\u003e2\u003c/sub\u003eO) phase can also coexist with these at a specific pH range. Apparently the slight decrease in pH in the HEC-ADP solution shifted the driving force for crystallization in favor of bobierrite but it was not strong enough to degrade all struvite completely. Magnesium carbonate phase detected by both EDX and XRD seems to be an artifact of the drying process as dissolved carbon dioxide can precipitate by reacting with magnesium ions. Carbon was detected only in spectrum 5 at a much higher fraction than the XRD average quantity that is thought to originate from a mixture of Mg carbonate and ADP residue.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eADP solution with MgCl\u003csub\u003e2\u003c/sub\u003e seems to infiltrate the AZ31 plate less compared to the reference. Very low pH due to inhibition of Mg dissolution kept the surface without a passive hydroxide layer which transformed into newberyite in time. Two different structures are observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea which are regular prismatic blocks of newberyite and irregular lumps that look to be deposited above the newberyite layer. High definition, stoichiometric newberyite crystals seen in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and c have grown monolithically without etch-pits in the constantly acidic solution. Figures\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb and d show regions of the surface that contain different phases than newberyite like region 10 which contains high concentrations of N, Mg, and Cl, indicating presence of ammonium chloride phases. XRD analysis given in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee reveals that novograblenovite (NH\u003csub\u003e4\u003c/sub\u003eMgCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) is the crystalline phase that is present by about 1.6 wt%. It seems to accumulate on the substrate like the ADP depositions on the top surface of the sample swollen in ADP solution. Presence of chlorine ions seems to have transformed ADP with the diffusing Mg ions to novograblenovite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe nitrate-added ADP solutions transformed the alloy in a completely different way such that both nitrate sources Ca(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and Mg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e caused complete dissolution of the samples at day 14 and 7 respectively. Nitrates are known to consume hydrogen molecules in aqueous solutions according to reaction 1 and consequently drive the magnesium corrosion reactions 2, 3, 4 forward according to the Le Chatelier principle (Wa et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1994\u003c/span\u003e). This self-feeding loop seems to increase the rate of Mg corrosion by reducing the activity of the gaseous H\u003csub\u003e2\u003c/sub\u003e corrosion product. Paralel pH evolution was recorded for both samples at a slightly lower level than the ADP solution (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The variation of pH after day 3 follows a concave-up pattern which is in contrast to the reverse pattern for the reference solution. The same upward trend in solution pH is observed after day 7 for all solutions with adjuvants which indicates the divergence in reaction mechanism from the balanced bulk conversion provided by the reference ADP solution. Apparently the adjuvants facilitate continuous corrosion of the alloy which in turn shifts the balance of water hydrolysis, resulting in a gradual increase in pH, while the reference solution appears to slow down reaction 2 presumably by inducing continuous formation of Mg compounds over the substrate. The disturbance of this balance by nitrates may be attributed to further reduction of nitrites into ammonia groups that are the products of ADP dissolution (Wa et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1994\u003c/span\u003e). Its solubility may be hindered as a result, reducing the concentration of other products, phosphates and hydrogen ions.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{{\\text{N}\\text{O}}_{3}}^{-}+{\\text{H}}_{2}\\:\\to\\:{{\\:\\text{N}\\text{O}}_{2}}^{-}+{\\text{H}}_{2}\\text{O}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\text{M}\\text{g}\\:\\:\\to\\:\\:\\:{\\text{M}\\text{g}}^{++}+{2\\text{e}}^{-}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{\\text{H}}_{2}\\text{O}\\:\\to\\:\\:{\\text{O}\\text{H}}^{-}+{\\text{H}}^{+}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:2{\\text{H}}^{+}+2{\\text{e}}^{-}\\:\\to\\:\\:{\\text{H}}_{2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eDepositions on the refreshed substrates in calcium nitrate solutions occurred within a week and contained bobierrite in addition to the struvite phase. The rod-like prismatic macrocrystals seen in Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea-c have struvite morphology. Struvite is known to be stable at relatively higher pH levels compared to its derivatives bobierrite and newberyite. Their gradual degradation due to the slightly acidic condition is indicated by the etch pits covering all crystal surfaces. Finer bobierrite groups are identified together with hexagonal rosette-like formations that are characteristic of calcium aluminate (Moranville-Regourd and Kamali-Bernard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) and magnesium carbonate (Power et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) cement products. There are also groups of rod-like crystals that are associated with aluminum phosphate cements (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). The high phosphorus and aluminum ratios found in the EDX results suggest that these crystals are aluminum phosphate phases and the rosette-like crystals resemble dypingite (Mg\u003csub\u003e5\u003c/sub\u003e(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e.5H\u003csub\u003e2\u003c/sub\u003eO) that form by CO\u003csub\u003e2\u003c/sub\u003e uptake from air. However wavellite (Al\u003csub\u003e3\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e(OH)\u003csub\u003e3\u003c/sub\u003e\u0026middot;5H\u003csub\u003e2\u003c/sub\u003eO), montgomeryite (Ca\u003csub\u003e4\u003c/sub\u003eMgAl\u003csub\u003e4\u003c/sub\u003e(PO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003e(OH)\u003csub\u003e4\u003c/sub\u003e\u0026middot;12H\u003csub\u003e2\u003c/sub\u003eO) and hexaaquamagnesium(II) hypophosphite (Mg(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e6\u003c/sub\u003e)(H\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) peaks were detected in the diffraction pattern which also contained peaks for the magnesium substrate and MgH\u003csub\u003e2\u003c/sub\u003e, as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAZ31 plates completely dissolved in magnesium nitrate containing ADP solution at day 7 and the new sample continued dissolving after slight deposition at day 14. The surface of this plate immersed after a week in the solution and aged until day 21 also displays crystals with dissolution pits. Rod-like crystals seen in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea, b, d have struvite morphology. Irregular lumps of fine prisms are also seen together with these prismatic crystals (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec). The surface analysis results in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee show that they contain the same elements Mg, P, N in similar amounts. XRD analysis indicates that a mixed structure of struvite and newberyite have formed, on top of which struvite monoliths have grown at some regions. Detection of a significant amount of metallic Mg for both nitrate solutions indicates that the phosphate layers are thin and loose. Trace amounts of MgH\u003csub\u003e2\u003c/sub\u003e and Mg(OH)\u003csub\u003e2\u003c/sub\u003e were also detected. Comparing the results with the calcium nitrate solution, it is seen that when magnesium nitrate is added, phosphates form newberyite instead of wavellite, bobierrite and other magnesium phosphates (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). The reason for this is thought to be the slight decrease in solution pH caused by the extra magnesium ions in the initial solution that prevented excessive Mg, Al dissolution and hydroxide release.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eADP provided continuous deposition on AZ31 plates in 0.1M trisodium citrate solution which was arrested at the end of day 7. This produced the morphology given in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e where the crystals around flat dense surfaces are typical monoliths of struvite. An interphase region with concentration gradients of the product phases is observed in the SEM micrographs of the sample immersed in citrate solution as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea, c. At one end the deposited crystals have rod-like struvite morphology while the base is flat at the other end with high Mg content. C atoms are most likely present as citrate groups adsorbed on magnesium surface. The middle region 19 gives out less N emission but similar Mg, P, O. The lower O/Mg ratio in this region indicates it is a mixture of struvite with newberyite. Apparently struvite crystals formed on top of the newberyite layer that is also reflected by the gradual decrease in pH until day 14, when struvite crystals started to dissolve subsequently as evident from the etch-pits. The flat region at the other end gives the lowest O/Mg and P/Mg ratio which should result from a Mg(OH)\u003csub\u003e2\u003c/sub\u003e layer surrounded by magnesium phosphate phases. Mg hydroxide phase in the flat region could not be detected with XRD probably due to its small surface area and nanoscale size. The crystalline phases on the surface were found by XRD analysis as struvite, newberyite, ADP, and a sodium aluminum phosphate phase with an irregular stoichiometry of Na\u003csub\u003e(3\u0026minus;3x)\u003c/sub\u003eAl\u003csub\u003ex\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e. Apparently continuous dissolution during the first week of immersion provided enough Al\u003csup\u003e3+\u003c/sup\u003e concentration to form this phase with Na\u003csup\u003e+\u003c/sup\u003e from trisodium citrate solution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFormation of significant amounts of both newberyite and struvite was facilitated by the slight fluctuations in the pH level around pH\u0026thinsp;=\u0026thinsp;5. The pH variation is the reverse of ADP solution and its pH is constantly higher by about 0.5 which indicates lower solubility of ADP in the presence of citrate groups. As a result the balance between ADP/Mg dissolution and struvite formation has shifted towards the latter and the deposition rate diminished in a week as a sigmoidal function of time that is characteristic of inorganic cement setting kinetics (Şahin and Kalyon, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Similarly calcium phosphate cement setting is induced by initial dissolution of an acidic phosphate salt which activates the calcium rich particles to supply cations that react with phosphates and precipitate on the surface of the particles. This creates a thick layer on the surface of exposed particles that can sustain the crystallization by dissolution. Thus the balance between dissolution and crystal growth is disrupted, leading to the end of cement setting (Şahin and Kalyon, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAs also observed for the citrate-containing solution, a continuous deposition could not be sustained in any ADP solutions with adjuvants, partially due to the physical and chemical constraints introduced by each but also because the reactions were confined to the surface where Mg dissolution and struvite formation reactions compete until one dominates the other. One extreme case is seen in nitrate solutions where the dissolution rate was kept high relative to crystallization and the sample disintegrated through progressive pitting. The other extreme is observed in Mg chloride solution where Mg dissolution was suppressed initially and the surface was quickly passivated by a small amount of newberyite deposition. The special case of the reference ADP solution seems to occur due to some additional mechanism that is responsible for significantly increasing the surface area so that both Mg dissolution and struvite formation reactions are maintained for a longer time as in cement systems. The micrographs seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e hints at an intercalation step prior to the densification of the bulk by struvite crystallization which seems to be necessary for infusion of ions in the solution. The magnesium phosphate phases detected in these transformed samples have all been described in mineralogy as sheets of alternating layers of Mg octahedra and phosphate tetrahedra that are binded by hydrogen bonds into stacks making up the macroscopic crystals (Huminicki and Hawthorne, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), so it is natural to consider hydrogen as the main agent responsible for the bulk conversion of metallic Mg to phosphate compounds.\u003c/p\u003e \u003cp\u003eLiterature on H\u003csub\u003e2\u003c/sub\u003e embrittlement of steel describes a similar phenomenon where H\u003csup\u003e+\u003c/sup\u003e infiltration through the metal surface causes H\u003csub\u003e2\u003c/sub\u003e gas nucleation in the bulk and internal stresses that are known to create voids, cracks and expansion (El-Haddad, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Huang et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Kumari et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Since magnesium dissolution in water is accompanied by proton and hydrogen gas evolution, the same effect may occur autogenously. To the best of our knowledge there is no report of gas formation and related stresses within the bulk of the metal in the literature. Schober\u0026rsquo;s study of microstructural changes occuring in pure Mg upon application of high pressure H\u003csub\u003e2\u003c/sub\u003e is the only available source on hydrogen-induced lattice expansion through formation of MgH\u003csub\u003e2\u003c/sub\u003e phase which was also detected in our samples in small quantities (Schober, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1981\u003c/span\u003e). In a related study by Zeng et al. it is seen that low concentrations of hydrogen and dihydrogen phosphates in saline solution only affect the surface by creating a passive Mg phosphate coating (Zeng et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The high concentrations used in our study at the saturation limit of ammonium dihydrogen phosphate may be necessary for the anticipated hydrogen attack. Further studies on the role of H\u003csub\u003e2\u003c/sub\u003e are underway to elucidate the observed volumetric expansion in ADP solutions. Whether it is also related to the thermomechanical processing history of the hot rolled AZ31 alloy, such as inherent texturing and residual stresses is also investigated for a sound understanding of the observed phenomenon.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eAZ31 alloy undergoes an extraordinary transformation when immersed in aqueous saturated ammonium dihydrogen phosphate solutions that may provide an alternative route in wastewater phosphate recovery processes and a strengthening mechanism in Mg phosphate cement matrices. A steady volumetric expansion at a rate of 1 fold increase in weight per day has been observed for the first week of the immersion period of 21 days. The product is akin to a Mg phosphate cement product in terms of compactness and mechanical stability. The immersion medium acted as a dilute cement solution and the cement reaction transformed the metal plate to a magnesium phosphate ceramic. Phase analysis of the sample revealed that about 88% of the alloy transformed to struvite, 11% to hannayite and less than 1% to MgH\u003csub\u003e2\u003c/sub\u003e. Both the extent of transformation and composition of the products changed with adjuvants. All additives inhibited the volumetric expansion and restricted the reactions to the surface of the alloy. HEC increased struvite fraction to about 98%, while Mg containing additives MgCl\u003csub\u003e2\u003c/sub\u003e and Mg(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e either reduced struvite fraction or completely shifted the reaction in favor of an alternative phase, newberyite. Slight differences in their solution pH resulted in major changes in the rate, extent and composition of the formations. Aqueous saturated ADP solution provided a relatively low and constant pH around 4.7 compared to the multicomponent solutions, which seems to provide a continuous driving force for the dissolution of Mg and ADP as well as struvite formation. Further studies on the effect of hydrogen gas nucleation within the bulk of the alloy that may be responsible for the observed intercalation by creating voids and stresses in the microstructure are needed to elucidate the mechanisms leading to the observed bulk microstructural evolution.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e \u003cb\u003eCRediT authorship contribution statement\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eErdem Sahin\u003c/b\u003e: Experimental planning, data analysis and reporting. \u003cb\u003eMeltem Alp\u003c/b\u003e: contributed to the experiments and gravimetric data collection. \u003cb\u003eAhmed Seref\u003c/b\u003e: conducted the corrosion tests.\u003c/p\u003e\u003cp\u003e \u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e \u003cp\u003eThe authors declare that they do not have any conflicts of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors appreciate the financial support from Scientific and Technological Research Council of Turkiye (TUBITAK) (Project No: 2020-119N759). We thank Prof. Jian Peng for his cooperation on the manufacturing and delivery of AZ31 alloy sheets. İzmir Institute of Technology Materials Research Center staff is acknowledged for their assistance in characterizations. Doebelin.org and Crystallography Open Database (Crystallography.net) are acknowledged for sharing their XRD analysis software and database.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBhuiyan MIH, Mavinic DS, Koch FA (2008) Thermal decomposition of struvite and its phase transition. Chemosphere 70:1347\u0026ndash;1356\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen Y, Xu Z, Smith C, Sankar J (2014) Recent advances on the development of magnesium alloys for biodegradable implants. 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Corros Sci 91:7\u0026ndash;28\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Scientific and Technological Research Council of Turkey","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"AZ31 alloy, Ammonium dihydrogen phosphate, Corrosion, Volumetric expansion, Chemical conversion, Struvite","lastPublishedDoi":"10.21203/rs.3.rs-6156428/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6156428/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe surface evolution of AZ31 immersed in saturated aqueous solutions of ammonium dihydrogen phosphate (ADP) and various functional adjuvants was investigated by compositional, morphological and gravimetric analyses. The immersion process was monitored by pH and weight measurements at various intervals for a period of 21 days. Saturated aqueous solutions of ADP were initially acidic with a pH around 4 which caused a rapid degradation of the alloy surface. Apparently the dissolved cations reacted with infusing ions within the bulk of the alloy to induce a strong volumetric expansion that increased the thickness of the plates more than one order of magnitude. Close examination of the cross section by SEM revealed that monolithic crystals of struvite and other magnesium phosphate phases formed perpendicular to the rolling direction of the plates, thus intercalating and simultaneously densifying the microstructure. However such long term growth could not be sustained in any of the studied multicomponent solutions of ADP, which highlights the unique pH evolution of saturated ADP solution that is suitable for bulk conversion.\u003c/p\u003e","manuscriptTitle":"Conversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-06 04:11:11","doi":"10.21203/rs.3.rs-6156428/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b0c4f433-ddf4-4d4e-a6c9-932be67085f0","owner":[],"postedDate":"March 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":45197380,"name":"Materials Chemistry"},{"id":45197381,"name":"Metallurgy"},{"id":45197382,"name":"Surface chemistry"}],"tags":[],"updatedAt":"2025-03-06T04:11:11+00:00","versionOfRecord":[],"versionCreatedAt":"2025-03-06 04:11:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6156428","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6156428","identity":"rs-6156428","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}