Quantifying the amount of reducing substance in the corrosion products on magnesium electrodes

Quantifying the amount of reducing substance in the corrosion products on magnesium electrodes | 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 Method Article Quantifying the amount of reducing substance in the corrosion products on magnesium electrodes Egil Gulbrandsen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8652822/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 This brief report describes an ad hoc two-stage method for quantifying the amount of reducing substance, presumed to be magnesium hydride, found in the corrosion products on pure magnesium electrodes exposed to localised corrosion. In the first stage, the corrosion product was dissolved in a small amount (approx. 20 mL) of an aqueous solution of 200 g/L chromic (VI) trioxide (CrO 3 ) with volumetric measurement of small amounts of H 2 gas (1–3 mL) produced by the oxidative hydrolysis of the hydride. In the second stage, the quantity of Cr(III) ions generated from the reduction of chromic acid by the reaction with magnesium hydride in the first stage was measured by a spectrophotometric technique utilising the absorbance from Cr(III) ions at 650 nm. Materials Chemistry magnesium localised corrosion corrosion products magnesium hydride analysis chromic acid Figures Figure 1 Figure 2 Figure 3 Figure 4 1 Introduction Magnesium hydride (MgH 2 ) has been detected in the corrosion products on magnesium by various analytical methods, and it has been documented that the corrosion product has reducing properties, see the review by Nazarov et al. [ 1 ]. The reducing substance is presumed to be MgH 2 , or a reducing reaction product of it. However, these findings related to MgH 2 have so far received little attention in the study of the corrosion mechanisms of magnesium and its alloys. MgH 2 has a strong tendency to hydrolyse in water with formation of hydrogen gas. However, previous studies have shown that this reaction is slowed down by a layer of reaction products in neutral to alkaline solutions[ 2 ], and a residual amount of MgH 2 can therefore be present in the corrosion product layer on Mg. This brief report describes a two-stage analytical technique for determining the total amount of reducing substances present in the corrosion products on pure Mg specimens. The objective is to document the performance of the method for use in future corrosion studies of Mg. The corrosion products on Mg can be dissolved by a 200 g/L CrO 3 (chromium (VI) trioxide) solution according to ASTM Standard Practice G1-03 [ 3 ]. CrO 3 dissolves in water to form chromic acid, H 2 CrO 4 . The acidic solution dissolves the alkaline corrosion products (MgO, Mg(OH) 2 ) by acid-base reactions, while the Mg metal becomes protected (chromated) by the chromic acid and suffer no measurable corrosion during the treatment [ 4 ]. The speciation of Cr(VI) compounds at such high total concentration of CrO 3 is described by the acid-base equilibrium between chromic acid and the dichromate ion, Cr 2 O 7 2− , rather than with HCrO 4 − which would be the case at lower total concentrations of Cr(VI) [ 5 , 6 ]: 2 H 2 CrO 4 = Cr 2 O 7 2- + H 2 O + 2 H + pK a = -0.18 ([5]) (1) This indicates that the pH of the chromic acid solution is slightly negative. Chromic acid and dichromate solutions have historically been used in many analytical and preparative methods [ 7 ], due to their oxidising properties [ 5 ]: Cr 2 O 7 2− + 14 H + + 6e − = 2 Cr 3+ + 7 H 2 O E 0 = 1.333 V (SHE) (2) The oxidation is known to proceed stoichiometrically in three consecutive electron transfer steps from Cr(VI) ions via the Cr(V) and Cr(IV) oxidation states to Cr(III) [ 8 ]. MgH 2 , with hydrogen in oxidation state -I, is a very reducing compound [ 9 ]: MgH 2 = Mg 2+ + 2 H + + 4 e − E 0 = -1.114 V (SHE) (3) If MgH 2 is present in the corrosion products, three overall red-ox reactions can take place during the dissolution in chromic acid solution. Firstly, a fraction of MgH 2 can be oxidatively hydrolysed by the acidic solution to form H 2 (gaseous and dissolved): MgH 2 + 2 H + = Mg 2+ + 2 H 2 (4) Secondly, the strongly oxidising dichromate ion (Eq. 2) can oxidise the hydride ions in MgH 2 to H + and itself be reduced to Cr(III) ions. This reaction can in simplified notation be written as: MgH 2 + 4/3 Cr(VI) = Mg 2+ + 2 H + + 4/3 Cr(III) (5) Finally, dissolved H 2 molecules that do not immediately enter the gas phase during the acid hydrolysis of MgH 2 can be oxidised to H + by the chromic acid as well, since the H + /H 2 reversible potential ≈ 0.0 V (SHE) is far below that for the chromic acid: H 2 (aq) + 2/3 Cr(VI) = 2 H + + 2/3 Cr(III) (6) Since the sum of Eq. 4 and Eq. 6 yields Eq. 5, it is seen that the dissolved fraction of H 2 from Eq. 4 will be formally measured by Eq. 5. The relative fractions of these competing electron transfer reactions are unknown, and further investigation is outside the scope of the present method development. However, the total number of electrons produced in the oxidation of MgH 2 can be calculated from the amount of H 2 left as gas plus the amount of Cr(III) ions formed. The first stage of the analytical method was therefore developed to measure small volumes of H 2 gas evolved from the corrosion products, and the second stage to determine the amount of chromic acid ions that was reduced to Cr(III) ions by MgH 2 in the dissolution stage. Note that the method in the present version is directly applicable only for pure Mg specimens. Possible interferences from alloying elements have to be considered before the method can be applied to Mg alloys. 2 Volumetric measurement of small amounts of H gas 2.1 Experimental method Test solutions were prepared using distilled water and analytical reagent grade chemicals. The chromic acid solution was prepared by dissolving 200.00 g CrO 3 (99.99 g/mol) in water to form 1 litre solution. This yields a concentration of 2.000 mol/L, which will be referred to as 2 M for convenience. Note that the addition of AgNO 3 prescribed by the Standard Practice [ 3 ] was not added to avoid possible formation of colloidal Ag which might interfere with the spectrophotometric analysis. A schematic drawing of the measurement device is shown in Fig. 1. This device is a simple small-scale adaptation of the classical eudiometer [ 10 ] for measuring small volumes of H 2 gas (typically 1 to 3 mL) being evolved from the corrosion products during hydrolysis of MgH 2 in small volumes of chromic acid solution. It was necessary to keep the volume of the chromic acid solution small ( approx. 20 ml) to avoid too much dilution to be able to measure the concentration of Cr(III) ions in the next step. The device included a 30 mm high cylindrical PVC sleeve (25.2 mm ID) at the lower end, which served as a holder for the corroded test electrode. The upper section of a small glass funnel was attached upside down to the sleeve with epoxy resin. A 5 ml graded syringe (13 mm OD, 60 mm height, 0.5 mm wall thickness) was glued to the top of the inverted funnel for collection of the gas bubbles. The opposite end of the syringe was cut away and replaced by a 5 mm thick slice of a rubber stopper. Two 1.3 mm diameter syringe needles were pushed through the rubber stopper, and a 7.5 cm long 3 mm OD hose was fitted to one of the needles before the stopper was fitted on top of the syringe. A short 5 mm OD glass tube led from the inside of the sleeve right above the corroded electrode. A 4 mm ID plastic hose was connected to this tube. The hose acted as a receiver for the liquid being displaced by the evolved gas and as a supplementary means for estimating the gas volume by measuring the length of the column of displaced liquid. The measurement procedure started by mounting the test electrode assembly into the sleeve from below. The electrode assembly consisted of a 20.0 mm OD disc of Mg embedded in epoxy resin inside a 21.7 mm ID, 24.7mm OD PVC sleeve. A layer of PTFE tape was applied to the outside of the electrode assembly, and silicon grease applied to the inside of the sample holder sleeve to assure tightness. The liquid displacement hose was then plugged at the end, and the entire measurement assembly tilted slightly clockwise from the vertical position. The needle equipped with the hose was used for filling chromic acid solution rapidly from a 20 mL syringe down to the funnel part, with the other needle acting as an air vent. Once the funnel and syringe were filled, the two needles were rapidly plugged, the plugged liquid displacement hose was re-opened, and the apparatus was tilted back to vertical position. Gas evolution started within a few seconds and lasted up to around five minutes. When the gas evolution was finished (stable readings), the gas volume was read from the syringe grading relative to the bottom of the rubber stopper. The volume was corrected for the volume of the inner filling hose through the gas pocket. As a check, the volume of the displaced liquid was estimated by measuring the length of the liquid column in the displacement hose. In most cases this volume was within ± 0.1 ml of the syringe reading. The vent needle was then opened, and the chromic acid solution drained through the displacement hose into a pre-weighed conical flask. The system was finally flushed with a small portion of chromic acid solution (approx. 2 ml) through the filling needle. The total amount of solution was determined by weighing the conical flask again and the liquid volume calculated using a measured density of 1.147 g/mL for the liquid. The cleaned electrode was removed from the sleeve, washed in distilled water, and dried to constant weight for metal mass loss measurement. The number of moles of MgH 2 in the fraction oxidised by H + to H 2 gas (Eq. 4) can, by means of the universal gas equation, be calculated as: n MgH2 = ½ n H2(g) = ½ ( P a - P w ) V H2 /(R T ) (7) where P a and P w are the ambient and water vapour pressures, respectively. V H2 is the volume of the gas, T is the absolute temperature, and R is the universal gas constant. 2.2 Estimated uncertainty Precision The dominating uncertainty of the volumetric method is the reading of the volume on the graded inverted syringe. The syringe has a grading line every 0.2 mL. The uncertainty of reading the volume can be estimated to about half of the grading, i.e., 0.1 mL. With measured volumes in the range from 1 mL to 3 mL, the relative uncertainty can be estimated to be within 10%. Systematic errors The accuracy of the gas collection method is hard to assess in detail. Complete dissolution of corrosion product was ensured by keeping the chromic acid solution in the system for about 5 minutes after visible gas evolution ended. The loss of H 2 by permeation through the polymer walls of the gas collecting device was estimated using published values for H 2 permeability in polymers [ 11 , 12 ]. It was found that permeation through the 0.5 mm thick polypropylene (permeability 3.1x10 − 9 mol/(s m MPa)) syringe wall constituted the major loss of H 2 . However, a conservative estimate, assuming the entire syringe section was gas filled, yielded only ≈ 20 µL of gas lost by permeation during a 10 min period, which is negligible compared to the evolved gas volumes. As argued in Section 1, the dissolved H 2 is assumed to be oxidised by the chromic acid and is therefore not a source of error. Some H 2 gas may escape from the system during the filling stage which takes about 10 seconds. Observation of the dissolution process indicated a few seconds initiation time before H 2 started to evolve from the corrosion products. Furthermore, the gas evolution could last for several minutes. This error is therefore assumed to be relatively small. 1 3 Spectrophotometric determination of Cr(III) ions The spent chromic acid solution collected in the conical flask was analysed for the concentration of Cr(III) ions using the spectrophotometric method described below. 3.1 Absorption spectra Cr(III) ions are present as hexaaquo ions [Cr(H2O) 6 ] 3+ in acidic solutions since that ion is a cation acid with pKa = 3.81 [ 5 ]. Previous work showed that the hexaaquo Cr(III) ion possess two broad absorption peaks with maxima at 575 nm and 410 nm [ 13 , 14 ], with a considerable overlap between the two and a certain pH-dependence of the peak wavelengths and absorbances. The blue-filled spectrum in Fig. 2 is the visual light absorption spectrum measured for a solution of 0.05 M Cr(NO 3 ) 3 ∙9H 2 O (chromium(III) nitrate nonahydrate) in 0.1 M H 2 SO 4 . The spectrum was in fair agreement with the cited literature data, both in terms of peak wavelengths and absorbances at low pH [ 14 ]. The spectrum of the orange pure 2 M chromic acid solution is also shown, together with the spectra of that solution diluted 10, 100, and 1000 times, respectively. The 2 mM solution spectrum shows a plateau at around 430 nm in agreement with previously published data for dichromate solutions [ 15 ], where it was also shown that this plateau extended into the UV range with a very strong peak at 350 nm. The true absorbance of the plateau at 430 nm in the undiluted solution is very high and not measurable in practice. The noisy absorbance data below 550 nm for the less diluted solutions were therefore dominated by internally scattered light (stray light) in the spectrophotometer [ 7 ] and have therefore been plotted in grey in Figs. 2 and 3. The absorbance of the 2 M solution dropped steeply between 560 nm and 600 nm, and then gradually to zero around 700 nm. Adding 50 mM Cr(III) ions to the 2 M chromic acid solution caused additional absorption above 560 nm, see Fig. 3. By subtracting the pure chromic acid solution spectrum, the contribution of 50 mM Cr(III) appeared as a wide peak around 600 nm, as shown by the upper, blue-filled spectrum in Fig. 3. Note that this peak is distorted by the stray light from the chromic acid near the sharp edge of its absorbance curve, and the exact location of the peak maximum and the peak absorbance are therefore uncertain. However, the expectation of a Cr(III) peak around 580 nm is fulfilled. Spent chromic acid solutions from the dissolution tests described above showed the same features. The difference spectrum for such a sample is shown as the lower, blue-filled curve in Fig. 3. It exhibits a similar wide absorption peak around 580 nm. Based on chemical reasoning concerning the strong thermodynamic driving force for red-ox reaction between Cr(VI) and MgH 2 [ 5 , 9 ], and the similarity with the spectra with Cr(III) added, as well as the cited literature data, it is reasonable to conclude that the additional absorption observed at wavelengths above 560 nm in the spent solution was caused by the formation of Cr(III) ions. Since the maximum of the difference peak around 580 nm appeared to be influenced by stray light and the steeply dropping chromic acid peak it was chosen to use the absorbance at a higher wavelength for the quantitative determination. The wavelength of 650 nm was chosen as a compromise, where the absorbance of the pure chromic acid solution was small (0.02–0.05), while the absorbance of Cr(III) had not dropped too low. 3.2 Experimental method The amount of Cr(III) ions in the spent 2 M chromic acid solution was determined spectrophotometrically at 650 nm using 1 cm path-length disposable plastic cuvettes. The concentration of Cr(III) ions was determined by the Beer-Lambert law [ 7 ] A = εlc (8) where A is absorbance, ε is the molar absorption coefficient, l is the optical path length, and c is the concentration of Cr(III) ions in the sample. A 0.100 M Cr(III) stock solution was made by dissolving Cr(NO 3 ) 3 ∙9H 2 O in the 2 M chromic acid solution. A series of Cr(III) standard solutions were made by mixing portions of this stock solution with 2 M chromic acid solution. The calibration curve was established in the concentration range from 0.005 M to 0.1 M Cr(III). The absorbance measured at 650 nm, subtracted the baseline for chromic acid, has been plotted against the Cr(III) ion concentration in Fig. 4. This curve shows a linear correlation in the range from 0 M to 0.05 M, while the absorbance dropped below this linear trend at concentrations above 0.05 M. The calibration curve was established by linear least squares (LLS) fitting to the data points up to and including 0.05 M. Based on 8 parallels, each curve having a coefficient of determination ≥ 0.998, the average slope of the curve (the molar absorption coefficient ( ε ) times the path length ( l )) was determined to be 17.0 (mol/L) −1 . The relative standard deviation (SD rel% ) of the slope was 3%. This variation is associated with both the spectrophotometer, the cuvettes, and preparation of the test solutions. The standard test solutions were prepared by weighing and pipetting, where the uncertainty can be shown to be less than 1%, both in terms of accuracy and precision. 2 Furthermore, the total amount of chromic acid solution from the first stage was determined by weighing, and with the flushing step during transfer of the solution to the conical flask it is assumed that the sampling error also is less than 1%. The number of moles of MgH 2 in the fraction oxidised by Cr(VI) to H + (Eq. 5) can be calculated as n MgH2 = ¾ n CrIII = ¾ cV s (9) Where n CrIII is the number of moles of Cr(III) formed, and V s is the volume of the sample of spent chromic acid solution. 3.3 Estimated uncertainty Precision The relative uncertainty of the product εl is 3% as determined above. The uncertainty of the absorbance measured for a single sample was SD = 0.007 (9 measurements, 3 measurement each for 3 cuvettes). The number of moles of Cr(III) is n CrIII = cV s (10) The estimated relative uncertainty of V s is 1%. Inserting Eq. 8 for c yields n CrIII = AV s / εl (11) For the total combined uncertainty in n CrIII we have (uncertainties denoted u), see, for example, section 8.2.6 of [ 16 ]: u(n CrIII ) = n CrIII √[( u(A) / A ) 2 + ( u(V) / V ) 2 + ( u(εl) / εl ) 2 )] (12) (√ means the square root of the expression within the square backets). Inserting the uncertainties discussed above 3 , we have for the relative uncertainty as a function of A for a single measurement: u(n CrIII ) / n CrIII = √[(0.007/A) 2 + 0.01 2 + 0.03 2 ] = √ [(0.007/A) 2 + 1x10 − 3 ] (13) Since the lowest absorbances measured for the samples could be as low as 0.1, the estimated uncertainty is accordingly reported as SD within 8% for this method, see Table 2 . Table 2 Effect of the measured absorbance on the relative uncertainty of the determination of n CrIII . (Relative standard deviation in %). Measured absorbance Relative uncertainty in calculated n CrIII SD rel% 0.1 7.7% 0.2 4.7% 0.3 3.9% 0.4 3.6% Systematic errors Systematic errors were minimised by using calibrated equipment, establishing a calibration curve with known concentrations, with a chemical matrix very close to the samples. During the development work it was found that the molar absorbance coefficient for Cr(III) at 650 nm increased about 4 times with increasing chromic acid concentration from the value in 0.1 M H 2 SO 4 . The chemical interactions behind this increase are unknown and outside the scope of this work. However, when the chromic acid is present in a large and constant excess, and the calibration curve is straight and reproducible, it should not influence the reliability of the method. It was observed that the chromic acid could etch the epoxy resin that was normally used for preparing of the test electrodes. If this was due to an oxidation process rather than just acid dissolution it could increase the amount of H 2 and/or Cr(III). Furthermore, it would also introduce an error in the electrode’s metal mass loss. This etching error was negligible for the standard size electrodes (20 mm diameter discs, where the exposed area of epoxy was small, but had to be prevented for smaller electrodes by masking the epoxy with a resistant tape or using a more resistant epoxy. The epoxy that was sensitive to etching was a commercial blend for glueing and filling, which most likely contained an inorganic filler. Further testing showed that a resin for mounting specimens for metallography 4 was resistant to the chromic acid solution, at least during the time frame of the exposure (10 min). Finally, the standard solutions should not be stored in capped plastic cuvettes for a long time (> days) due to possible oxidation of the plastic material, and, anyway, the risk of water loss by evaporation. However, for short time (1 day) no degradation could be measured. 4 Summary and conclusions A two-stage analytical method was developed to quantify reducing substances (presumed to be magnesium hydride, MgH₂) in corrosion products on pure Mg electrodes. The method is intended for corrosion studies of pure Mg. The corrosion product was dissolved in about 20 mL of chromic acid solution (200 g/L CrO 3 ). This solution dissolves the corrosion products, while the metal is protected (chromated) and suffers negligible corrosion during the cleaning. In this solution, the hydride ions in MgH 2 were oxidised by protons and by chromic acid. In Stage 1, the amount of H 2 gas formed by hydrolysis was measured by a small-scale volumetric gas collecting device. The amount of MgH 2 in this fraction can be calculated by Eq. 7. In this stage the main uncertainty was reading the height of the gas column, estimated to be around ± 0.1 mL, thus within 10% relative error at the lowest measured volume of ≈ 1 mL. The main systematic error was assumed to be a small loss of H 2 gas during filling the measurement devices with chromic acid. An estimate of gas permeation, using published values for the permeability of H 2 in polymers, showed that such loss was negligible for the duration of this measurement (approx. 10 min). In Stage 2 the amount of Cr(III) ions formed from Cr(VI) in the oxidation of MgH 2 was measured by a spectrophotometric method. The amount of MgH 2 in this fraction can be calculated by Eq. 9. The Cr(III) ion had a wide absorption peak around 580 nm in the solution. However, to avoid interference from a strong absorption from Cr(VI) at lower wavelengths, the absorption was measured at 650 nm for the analysis. A straight calibration curve was established for the concentration range 0.005 M to 0.05 M Cr(III) in the chromic acid solution by adding known amounts of Cr(NO 3 ) 3 ∙9H 2 O. The main uncertainty was connected to the spectrophotometric determination. The total uncertainty was estimated to be ± 8% relative error. The main systematic error was that the chromic acid solution could etch certain epoxy resins, which may be a redox process and increase concentrations of H 2 gas and/or Cr(III) and influence mass loss measurements. Therefore, researchers should qualify the epoxy resin used for mounting the electrode assembly before applying the method. Declarations Competing interests The author has no conflicts of interest related to the content of this method article. The author is the owner and an employee of Gulbrandsen Technology AS. The company has no commercial or financial interests in the topic of this article. No funding was received for conducting this study. 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Dissolution of Na 2 CO 3 ∙10 H 2 O in a dilute H 2 SO 4 was slower. However, with a solubility of about 34 mM for CO 2 in water at room temperature [17], it can be estimated that 20 mL of the dilute acid solution may contain about 16 times more CO 2 than a 1 mL gas pocket. A small deviation from equilibrium saturation would therefore produce large errors in the measured gas pocket volume. Due to these complications, this topic was not further pursued. Using a calibrated variable micropipette 5.00 to 0.50 mL. 1% applies to the lowest volume, with smaller errors for the higher volumes. Ref. [16] states that the individual uncertainties should be given in the same format, e.g., as SD’s, when calculating the combined uncertainty. The extreme (maximum) error limits established in the present report, stated as “within x%”, may be recalculated to SD’s by dividing the extreme value by √3 if the maximum error is likely to occur, or dividing by √6 if unlikely[16]. However, the maximum values have here been used without reduction to SD’s. This gives a conservative estimate of the SD (i.e. somewhat overestimated), since they are based on a limited amount of qualification testing and estimates. Epofix™ from Struers. 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-8652822","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Method Article","associatedPublications":[],"authors":[{"id":577698390,"identity":"07ac14ce-47df-440c-ad3c-3df7d52614fe","order_by":0,"name":"Egil Gulbrandsen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYFACxsYDHxgYZAxAbB6oGDMBLQ0HZwAVI7SwEdTCwHCYhyQt/PyLGw7b/DrMYy59+NmDN39s8hjkmw9+LsCjRXLGw4bDuX2HeSz70swN57alFTOwsSVLz8CjxeDGQaCWnsM8BmcYzKR5Gw4nNrDxmDHz4NFiD9JiCdbC/k2a589/oBb+b3i1GPA3Nhxm+AHSwmMmzcN2AGQLG14tEjeAgdzbkM5j2cNTJjm3LTmxjS3NWBqfFv7+4w8f/PhjLWfOw75N4s0fu8R+5sMPP+PTwiCRAIzNNiQBNnyqIdYcABJ/CCobBaNgFIyCkQwAdmVNLpdPdEoAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-8184-6977","institution":"Gulbrandsen Technology AS","correspondingAuthor":true,"prefix":"","firstName":"Egil","middleName":"","lastName":"Gulbrandsen","suffix":""}],"badges":[],"createdAt":"2026-01-20 20:32:24","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-8652822/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8652822/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":100773541,"identity":"69b0b993-1c7e-4163-91b4-337b584c3d6e","added_by":"auto","created_at":"2026-01-21 10:18:12","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":198714,"visible":true,"origin":"","legend":"","description":"","filename":"20260120Quantifyingtheamountofreducingsubstancefinal.docx","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/4f5fa9215d6ad25ad7c4feb5.docx"},{"id":100773486,"identity":"2d53af47-afc0-498b-bb5a-98378dbeaab7","added_by":"auto","created_at":"2026-01-21 10:17:39","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":342,"visible":true,"origin":"","legend":"","description":"","filename":"rs8652822.json","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/5a1b7d70516542c9276279c8.json"},{"id":100773571,"identity":"f102ba83-3101-40a1-a2bf-c49070286c99","added_by":"auto","created_at":"2026-01-21 10:18:53","extension":"xml","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":58574,"visible":true,"origin":"","legend":"","description":"","filename":"rs86528220enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/5cad6d961c45894038d4d61e.xml"},{"id":100773424,"identity":"87b85fde-f1eb-4dae-96b0-d8e7af1848c0","added_by":"auto","created_at":"2026-01-21 10:17:04","extension":"xml","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":57446,"visible":true,"origin":"","legend":"","description":"","filename":"rs86528220structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/3eb9fcf4a58c39030508aacb.xml"},{"id":100773652,"identity":"ffede906-eb56-4e46-a5bf-2e261fe9b149","added_by":"auto","created_at":"2026-01-21 10:21:27","extension":"html","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":63098,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/99379e0d617fc5187c1c42c7.html"},{"id":100773584,"identity":"371f5956-3d28-4ee0-9f19-baf18c2a4c7e","added_by":"auto","created_at":"2026-01-21 10:19:07","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":8214030,"visible":true,"origin":"","legend":"\u003cp\u003eDevice for measuring small volumes of H2 gas produced during dissolution of the corrosion products on magnesium electrodes in a chromic acid solution after corrosion tests. The drawing is not to scale. See the text for detailed descriptions.\u003c/p\u003e","description":"","filename":"Fig1.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/3829d4d8faf5ee271d30fab0.jpg"},{"id":100773685,"identity":"5dc35f56-a2f6-49de-a2e4-01de457b49cd","added_by":"auto","created_at":"2026-01-21 10:22:22","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":269729,"visible":true,"origin":"","legend":"\u003cp\u003eVisual light absorption spectra of chromic acid solutions with concentrations in the range from 2 mM to 2 M. The blue filled curve is the spectrum of a 0.01 M H2SO4 solution with 0.05 M Cr(NO3)3∙9H2O added. The greyed out upper parts of the spectra were dominated by internal light scattering since the real absorbances were unmeasurably high.\u003c/p\u003e","description":"","filename":"Fig2.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/eea5efea58a45dc1bd3657d6.jpg"},{"id":100773669,"identity":"cae81263-d506-4ed9-9121-4b90f00f9129","added_by":"auto","created_at":"2026-01-21 10:21:44","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":356628,"visible":true,"origin":"","legend":"\u003cp\u003eVisual light absorption spectra of 2 M chromic acid solution, pure and with addition of 0.05 M Cr(III). The graph labelled Sample 1 is the spent solution after dissolving the corrosion products on an actual test specimen that had been galvanistatically polarised for 150 minutes at 10 mA/cm2 in 0.1 M NaCl solution. The blue-filled graphs are the differences between the respective spectra with Cr(III) and pure 2 M chromic acid solution.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig3.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/479abedd8c34ed8930ec39b2.jpg"},{"id":100773650,"identity":"88101ec4-4918-4088-8c2d-cb25055a6f80","added_by":"auto","created_at":"2026-01-21 10:20:57","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":181352,"visible":true,"origin":"","legend":"\u003cp\u003eCalibration curve showing the absorbance at 650 nm vs. concentrations of Cr(III) added to the 2 M chromic acid solution.\u003c/p\u003e","description":"","filename":"Fig4.tif.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/a6213f83729cd37c040fd2fe.jpg"},{"id":100857970,"identity":"db56cb46-b620-4223-a195-6341a2d94779","added_by":"auto","created_at":"2026-01-22 07:23:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9587596,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8652822/v1/b1cb331a-ef06-40e2-aef0-40942fb1f540.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eQuantifying the amount of reducing substance in the corrosion products on magnesium electrodes\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eMagnesium hydride (MgH\u003csub\u003e2\u003c/sub\u003e) has been detected in the corrosion products on magnesium by various analytical methods, and it has been documented that the corrosion product has reducing properties, see the review by Nazarov et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The reducing substance is presumed to be MgH\u003csub\u003e2\u003c/sub\u003e, or a reducing reaction product of it. However, these findings related to MgH\u003csub\u003e2\u003c/sub\u003e have so far received little attention in the study of the corrosion mechanisms of magnesium and its alloys. MgH\u003csub\u003e2\u003c/sub\u003e has a strong tendency to hydrolyse in water with formation of hydrogen gas. However, previous studies have shown that this reaction is slowed down by a layer of reaction products in neutral to alkaline solutions[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], and a residual amount of MgH\u003csub\u003e2\u003c/sub\u003e can therefore be present in the corrosion product layer on Mg. This brief report describes a two-stage analytical technique for determining the total amount of reducing substances present in the corrosion products on pure Mg specimens. The objective is to document the performance of the method for use in future corrosion studies of Mg.\u003c/p\u003e \u003cp\u003eThe corrosion products on Mg can be dissolved by a 200 g/L CrO\u003csub\u003e3\u003c/sub\u003e (chromium (VI) trioxide) solution according to ASTM Standard Practice G1-03 [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. CrO\u003csub\u003e3\u003c/sub\u003e dissolves in water to form chromic acid, H\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e. The acidic solution dissolves the alkaline corrosion products (MgO, Mg(OH)\u003csub\u003e2\u003c/sub\u003e) by acid-base reactions, while the Mg metal becomes protected (chromated) by the chromic acid and suffer no measurable corrosion during the treatment [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The speciation of Cr(VI) compounds at such high total concentration of CrO\u003csub\u003e3\u003c/sub\u003e is described by the acid-base equilibrium between chromic acid and the dichromate ion, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, rather than with HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e which would be the case at lower total concentrations of Cr(VI) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]:\u003c/p\u003e\n\u003cp\u003e2 H\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e = Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e + H\u003csub\u003e2\u003c/sub\u003eO + 2 H\u003csup\u003e+\u003c/sup\u003e pK\u003csub\u003ea\u003c/sub\u003e = -0.18 ([5]) (1)\u003c/p\u003e\n\u003cp\u003eThis indicates that the pH of the chromic acid solution is slightly negative. Chromic acid and dichromate solutions have historically been used in many analytical and preparative methods [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], due to their oxidising properties [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e + 14 H\u003csup\u003e+\u003c/sup\u003e + 6e\u003csup\u003e\u0026minus;\u003c/sup\u003e = 2 Cr\u003csup\u003e3+\u003c/sup\u003e + 7 H\u003csub\u003e2\u003c/sub\u003eO E\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.333 V (SHE) (2)\u003c/p\u003e \u003cp\u003eThe oxidation is known to proceed stoichiometrically in three consecutive electron transfer steps from Cr(VI) ions via the Cr(V) and Cr(IV) oxidation states to Cr(III) [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMgH\u003csub\u003e2\u003c/sub\u003e, with hydrogen in oxidation state -I, is a very reducing compound [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eMgH\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Mg\u003csup\u003e2+\u003c/sup\u003e + 2 H\u003csup\u003e+\u003c/sup\u003e + 4 e\u003csup\u003e\u0026minus;\u003c/sup\u003e E\u003csub\u003e0\u003c/sub\u003e = -1.114 V (SHE) (3)\u003c/p\u003e \u003cp\u003eIf MgH\u003csub\u003e2\u003c/sub\u003e is present in the corrosion products, three overall red-ox reactions can take place during the dissolution in chromic acid solution. Firstly, a fraction of MgH\u003csub\u003e2\u003c/sub\u003e can be oxidatively hydrolysed by the acidic solution to form H\u003csub\u003e2\u003c/sub\u003e (gaseous and dissolved):\u003c/p\u003e \u003cp\u003eMgH\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2 H\u003csup\u003e+\u003c/sup\u003e = Mg\u003csup\u003e2+\u003c/sup\u003e + 2 H\u003csub\u003e2\u003c/sub\u003e (4)\u003c/p\u003e \u003cp\u003eSecondly, the strongly oxidising dichromate ion (Eq.\u0026nbsp;2) can oxidise the hydride ions in MgH\u003csub\u003e2\u003c/sub\u003e to H\u003csup\u003e+\u003c/sup\u003e and itself be reduced to Cr(III) ions. This reaction can in simplified notation be written as:\u003c/p\u003e \u003cp\u003eMgH\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4/3 Cr(VI)\u0026thinsp;=\u0026thinsp;Mg\u003csup\u003e2+\u003c/sup\u003e + 2 H\u003csup\u003e+\u003c/sup\u003e + 4/3 Cr(III) (5)\u003c/p\u003e \u003cp\u003eFinally, dissolved H\u003csub\u003e2\u003c/sub\u003e molecules that do not immediately enter the gas phase during the acid hydrolysis of MgH\u003csub\u003e2\u003c/sub\u003e can be oxidised to H\u003csup\u003e+\u003c/sup\u003e by the chromic acid as well, since the H\u003csup\u003e+\u003c/sup\u003e/H\u003csub\u003e2\u003c/sub\u003e reversible potential\u0026thinsp;\u0026asymp;\u0026thinsp;0.0 V (SHE) is far below that for the chromic acid:\u003c/p\u003e \u003cp\u003eH\u003csub\u003e2 (aq)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2/3 Cr(VI)\u0026thinsp;=\u0026thinsp;2 H\u003csup\u003e+\u003c/sup\u003e + 2/3 Cr(III) (6)\u003c/p\u003e \u003cp\u003eSince the sum of Eq.\u0026nbsp;4 and Eq.\u0026nbsp;6 yields Eq.\u0026nbsp;5, it is seen that the \u003cem\u003edissolved\u003c/em\u003e fraction of H\u003csub\u003e2\u003c/sub\u003e from Eq.\u0026nbsp;4 will be formally measured by Eq.\u0026nbsp;5.\u003c/p\u003e \u003cp\u003eThe relative fractions of these competing electron transfer reactions are unknown, and further investigation is outside the scope of the present method development. However, the total number of electrons produced in the oxidation of MgH\u003csub\u003e2\u003c/sub\u003e can be calculated from the amount of H\u003csub\u003e2\u003c/sub\u003e left as gas plus the amount of Cr(III) ions formed. The first stage of the analytical method was therefore developed to measure small volumes of H\u003csub\u003e2\u003c/sub\u003e gas evolved from the corrosion products, and the second stage to determine the amount of chromic acid ions that was reduced to Cr(III) ions by MgH\u003csub\u003e2\u003c/sub\u003e in the dissolution stage.\u003c/p\u003e \u003cp\u003eNote that the method in the present version is directly applicable only for pure Mg specimens. Possible interferences from alloying elements have to be considered before the method can be applied to Mg alloys.\u003c/p\u003e"},{"header":"2 Volumetric measurement of small amounts of H gas","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Experimental method\u003c/h2\u003e \u003cp\u003eTest solutions were prepared using distilled water and analytical reagent grade chemicals. The chromic acid solution was prepared by dissolving 200.00 g CrO\u003csub\u003e3\u003c/sub\u003e (99.99 g/mol) in water to form 1 litre solution. This yields a concentration of 2.000 mol/L, which will be referred to as 2 M for convenience. Note that the addition of AgNO\u003csub\u003e3\u003c/sub\u003e prescribed by the Standard Practice [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] was not added to avoid possible formation of colloidal Ag which might interfere with the spectrophotometric analysis.\u003c/p\u003e \u003cp\u003eA schematic drawing of the measurement device is shown in Fig.\u0026nbsp;1. This device is a simple small-scale adaptation of the classical eudiometer [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] for measuring small volumes of H\u003csub\u003e2\u003c/sub\u003e gas (typically 1 to 3 mL) being evolved from the corrosion products during hydrolysis of MgH\u003csub\u003e2\u003c/sub\u003e in small volumes of chromic acid solution.\u003c/p\u003e \u003cp\u003eIt was necessary to keep the volume of the chromic acid solution small ( approx. 20 ml) to avoid too much dilution to be able to measure the concentration of Cr(III) ions in the next step. The device included a 30 mm high cylindrical PVC sleeve (25.2 mm ID) at the lower end, which served as a holder for the corroded test electrode. The upper section of a small glass funnel was attached upside down to the sleeve with epoxy resin. A 5 ml graded syringe (13 mm OD, 60 mm height, 0.5 mm wall thickness) was glued to the top of the inverted funnel for collection of the gas bubbles. The opposite end of the syringe was cut away and replaced by a 5 mm thick slice of a rubber stopper. Two 1.3 mm diameter syringe needles were pushed through the rubber stopper, and a 7.5 cm long 3 mm OD hose was fitted to one of the needles before the stopper was fitted on top of the syringe. A short 5 mm OD glass tube led from the inside of the sleeve right above the corroded electrode. A 4 mm ID plastic hose was connected to this tube. The hose acted as a receiver for the liquid being displaced by the evolved gas and as a supplementary means for estimating the gas volume by measuring the length of the column of displaced liquid.\u003c/p\u003e \u003cp\u003eThe measurement procedure started by mounting the test electrode assembly into the sleeve from below. The electrode assembly consisted of a 20.0 mm OD disc of Mg embedded in epoxy resin inside a 21.7 mm ID, 24.7mm OD PVC sleeve. A layer of PTFE tape was applied to the outside of the electrode assembly, and silicon grease applied to the inside of the sample holder sleeve to assure tightness. The liquid displacement hose was then plugged at the end, and the entire measurement assembly tilted slightly clockwise from the vertical position. The needle equipped with the hose was used for filling chromic acid solution rapidly from a 20 mL syringe down to the funnel part, with the other needle acting as an air vent. Once the funnel and syringe were filled, the two needles were rapidly plugged, the plugged liquid displacement hose was re-opened, and the apparatus was tilted back to vertical position. Gas evolution started within a few seconds and lasted up to around five minutes. When the gas evolution was finished (stable readings), the gas volume was read from the syringe grading relative to the bottom of the rubber stopper. The volume was corrected for the volume of the inner filling hose through the gas pocket. As a check, the volume of the displaced liquid was estimated by measuring the length of the liquid column in the displacement hose. In most cases this volume was within \u0026plusmn;\u0026thinsp;0.1 ml of the syringe reading. The vent needle was then opened, and the chromic acid solution drained through the displacement hose into a pre-weighed conical flask. The system was finally flushed with a small portion of chromic acid solution (approx. 2 ml) through the filling needle. The total amount of solution was determined by weighing the conical flask again and the liquid volume calculated using a measured density of 1.147 g/mL for the liquid. The cleaned electrode was removed from the sleeve, washed in distilled water, and dried to constant weight for metal mass loss measurement.\u003c/p\u003e \u003cp\u003eThe number of moles of MgH\u003csub\u003e2\u003c/sub\u003e in the fraction oxidised by H\u003csup\u003e+\u003c/sup\u003e to H\u003csub\u003e2\u003c/sub\u003e gas (Eq.\u0026nbsp;4) can, by means of the universal gas equation, be calculated as:\u003c/p\u003e \u003cp\u003e \u003cem\u003en\u003c/em\u003e \u003csub\u003e \u003cem\u003eMgH2\u003c/em\u003e \u003c/sub\u003e = \u0026frac12; \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eH2(g)\u003c/em\u003e\u003c/sub\u003e = \u0026frac12; (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e-\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003ew\u003c/em\u003e\u003c/sub\u003e)\u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eH2\u003c/em\u003e\u003c/sub\u003e/(R\u003cem\u003eT\u003c/em\u003e) (7)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003ew\u003c/em\u003e\u003c/sub\u003e are the ambient and water vapour pressures, respectively. \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eH2\u003c/em\u003e\u003c/sub\u003e is the volume of the gas, \u003cem\u003eT\u003c/em\u003e is the absolute temperature, and R is the universal gas constant.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Estimated uncertainty\u003c/h2\u003e \u003cp\u003ePrecision\u003c/p\u003e \u003cp\u003eThe dominating uncertainty of the volumetric method is the reading of the volume on the graded inverted syringe. The syringe has a grading line every 0.2 mL. The uncertainty of reading the volume can be estimated to about half of the grading, i.e., 0.1 mL. With measured volumes in the range from 1 mL to 3 mL, the relative uncertainty can be estimated to be within 10%.\u003c/p\u003e \u003cp\u003eSystematic errors\u003c/p\u003e \u003cp\u003eThe accuracy of the gas collection method is hard to assess in detail. Complete dissolution of corrosion product was ensured by keeping the chromic acid solution in the system for about 5 minutes after visible gas evolution ended. The loss of H\u003csub\u003e2\u003c/sub\u003e by permeation through the polymer walls of the gas collecting device was estimated using published values for H\u003csub\u003e2\u003c/sub\u003e permeability in polymers [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. It was found that permeation through the 0.5 mm thick polypropylene (permeability 3.1x10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e mol/(s m MPa)) syringe wall constituted the major loss of H\u003csub\u003e2\u003c/sub\u003e. However, a conservative estimate, assuming the entire syringe section was gas filled, yielded only\u0026thinsp;\u0026asymp;\u0026thinsp;20 \u0026micro;L of gas lost by permeation during a 10 min period, which is negligible compared to the evolved gas volumes. As argued in Section 1, the \u003cem\u003edissolved\u003c/em\u003e H\u003csub\u003e2\u003c/sub\u003e is assumed to be oxidised by the chromic acid and is therefore not a source of error.\u003c/p\u003e \u003cp\u003eSome H\u003csub\u003e2\u003c/sub\u003e gas may escape from the system during the filling stage which takes about 10 seconds. Observation of the dissolution process indicated a few seconds initiation time before H\u003csub\u003e2\u003c/sub\u003e started to evolve from the corrosion products. Furthermore, the gas evolution could last for several minutes. This error is therefore assumed to be relatively small.\u003csup\u003e1\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Spectrophotometric determination of Cr(III) ions","content":"\u003cp\u003eThe spent chromic acid solution collected in the conical flask was analysed for the concentration of Cr(III) ions using the spectrophotometric method described below.\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Absorption spectra\u003c/h2\u003e \u003cp\u003eCr(III) ions are present as hexaaquo ions [Cr(H2O)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e in acidic solutions since that ion is a cation acid with pKa\u0026thinsp;=\u0026thinsp;3.81 [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Previous work showed that the hexaaquo Cr(III) ion possess two broad absorption peaks with maxima at 575 nm and 410 nm [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], with a considerable overlap between the two and a certain pH-dependence of the peak wavelengths and absorbances. The blue-filled spectrum in Fig.\u0026nbsp;2 is the visual light absorption spectrum measured for a solution of 0.05 M Cr(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙9H\u003csub\u003e2\u003c/sub\u003eO (chromium(III) nitrate nonahydrate) in 0.1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The spectrum was in fair agreement with the cited literature data, both in terms of peak wavelengths and absorbances at low pH [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The spectrum of the orange pure 2 M chromic acid solution is also shown, together with the spectra of that solution diluted 10, 100, and 1000 times, respectively. The 2 mM solution spectrum shows a plateau at around 430 nm in agreement with previously published data for dichromate solutions [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], where it was also shown that this plateau extended into the UV range with a very strong peak at 350 nm. The true absorbance of the plateau at 430 nm in the undiluted solution is very high and not measurable in practice. The noisy absorbance data below 550 nm for the less diluted solutions were therefore dominated by internally scattered light (stray light) in the spectrophotometer [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] and have therefore been plotted in grey in Figs.\u0026nbsp;2 and 3.\u003c/p\u003e \u003cp\u003eThe absorbance of the 2 M solution dropped steeply between 560 nm and 600 nm, and then gradually to zero around 700 nm. Adding 50 mM Cr(III) ions to the 2 M chromic acid solution caused additional absorption above 560 nm, see Fig.\u0026nbsp;3. By subtracting the pure chromic acid solution spectrum, the contribution of 50 mM Cr(III) appeared as a wide peak around 600 nm, as shown by the upper, blue-filled spectrum in Fig.\u0026nbsp;3. Note that this peak is distorted by the stray light from the chromic acid near the sharp edge of its absorbance curve, and the exact location of the peak maximum and the peak absorbance are therefore uncertain. However, the expectation of a Cr(III) peak around 580 nm is fulfilled. Spent chromic acid solutions from the dissolution tests described above showed the same features. The difference spectrum for such a sample is shown as the lower, blue-filled curve in Fig.\u0026nbsp;3. It exhibits a similar wide absorption peak around 580 nm. Based on chemical reasoning concerning the strong thermodynamic driving force for red-ox reaction between Cr(VI) and MgH\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], and the similarity with the spectra with Cr(III) added, as well as the cited literature data, it is reasonable to conclude that the additional absorption observed at wavelengths above 560 nm in the spent solution was caused by the formation of Cr(III) ions. Since the maximum of the difference peak around 580 nm appeared to be influenced by stray light and the steeply dropping chromic acid peak it was chosen to use the absorbance at a higher wavelength for the quantitative determination. The wavelength of 650 nm was chosen as a compromise, where the absorbance of the pure chromic acid solution was small (0.02\u0026ndash;0.05), while the absorbance of Cr(III) had not dropped too low.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Experimental method\u003c/h2\u003e \u003cp\u003eThe amount of Cr(III) ions in the spent 2 M chromic acid solution was determined spectrophotometrically at 650 nm using 1 cm path-length disposable plastic cuvettes. The concentration of Cr(III) ions was determined by the Beer-Lambert law [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e \u003cem\u003eA\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003eεlc\u003c/em\u003e (8)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e is absorbance, \u003cem\u003eε\u003c/em\u003e is the molar absorption coefficient, \u003cem\u003el\u003c/em\u003e is the optical path length, and \u003cem\u003ec\u003c/em\u003e is the concentration of Cr(III) ions in the sample. A 0.100 M Cr(III) stock solution was made by dissolving Cr(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙9H\u003csub\u003e2\u003c/sub\u003eO in the 2 M chromic acid solution. A series of Cr(III) standard solutions were made by mixing portions of this stock solution with 2 M chromic acid solution. The calibration curve was established in the concentration range from 0.005 M to 0.1 M Cr(III). The absorbance measured at 650 nm, subtracted the baseline for chromic acid, has been plotted against the Cr(III) ion concentration in Fig.\u0026nbsp;4. This curve shows a linear correlation in the range from 0 M to 0.05 M, while the absorbance dropped below this linear trend at concentrations above 0.05 M. The calibration curve was established by linear least squares (LLS) fitting to the data points up to and including 0.05 M. Based on 8 parallels, each curve having a coefficient of determination\u0026thinsp;\u0026ge;\u0026thinsp;0.998, the average slope of the curve (the molar absorption coefficient (\u003cem\u003eε\u003c/em\u003e) times the path length (\u003cem\u003el\u003c/em\u003e)) was determined to be 17.0 (mol/L)\u003csup\u003e\u0026minus;1\u003c/sup\u003e. The relative standard deviation (SD\u003csub\u003erel%\u003c/sub\u003e) of the slope was 3%. This variation is associated with both the spectrophotometer, the cuvettes, and preparation of the test solutions. The standard test solutions were prepared by weighing and pipetting, where the uncertainty can be shown to be less than 1%, both in terms of accuracy and precision.\u003csup\u003e2\u003c/sup\u003e Furthermore, the total amount of chromic acid solution from the first stage was determined by weighing, and with the flushing step during transfer of the solution to the conical flask it is assumed that the sampling error also is less than 1%.\u003c/p\u003e \u003cp\u003eThe number of moles of MgH\u003csub\u003e2\u003c/sub\u003e in the fraction oxidised by Cr(VI) to H\u003csup\u003e+\u003c/sup\u003e (Eq.\u0026nbsp;5) can be calculated as\u003c/p\u003e \u003cp\u003e \u003cem\u003en\u003c/em\u003e \u003csub\u003e \u003cem\u003eMgH2\u003c/em\u003e \u003c/sub\u003e = \u0026frac34; \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e = \u0026frac34; \u003cem\u003ecV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e (9)\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e is the number of moles of Cr(III) formed, and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e is the volume of the sample of spent chromic acid solution.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Estimated uncertainty\u003c/h2\u003e \u003cp\u003ePrecision\u003c/p\u003e \u003cp\u003eThe relative uncertainty of the product \u003cem\u003eεl\u003c/em\u003e is 3% as determined above. The uncertainty of the absorbance measured for a single sample was SD\u0026thinsp;=\u0026thinsp;0.007 (9 measurements, 3 measurement each for 3 cuvettes).\u003c/p\u003e \u003cp\u003eThe number of moles of Cr(III) is\u003c/p\u003e \u003cp\u003e \u003cem\u003en\u003c/em\u003e \u003csub\u003e \u003cem\u003eCrIII\u003c/em\u003e \u003c/sub\u003e = \u003cem\u003ecV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e (10)\u003c/p\u003e \u003cp\u003eThe estimated relative uncertainty of \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e is 1%. Inserting Eq.\u0026nbsp;8 for \u003cem\u003ec\u003c/em\u003e yields\u003c/p\u003e \u003cp\u003e \u003cem\u003en\u003c/em\u003e \u003csub\u003e \u003cem\u003eCrIII\u003c/em\u003e \u003c/sub\u003e = \u003cem\u003eAV\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eεl\u003c/em\u003e (11)\u003c/p\u003e \u003cp\u003eFor the total combined uncertainty in \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e we have (uncertainties denoted u), see, for example, section 8.2.6 of [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003e \u003cem\u003eu(n\u003c/em\u003e \u003csub\u003e \u003cem\u003eCrIII\u003c/em\u003e \u003c/sub\u003e \u003cem\u003e)\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e \u0026radic;[(\u003cem\u003eu(A)\u003c/em\u003e/\u003cem\u003eA\u003c/em\u003e)\u003csup\u003e2\u003c/sup\u003e + (\u003cem\u003eu(V)\u003c/em\u003e/\u003cem\u003eV\u003c/em\u003e)\u003csup\u003e2\u003c/sup\u003e + (\u003cem\u003eu(εl)\u003c/em\u003e/\u003cem\u003eεl\u003c/em\u003e)\u003csup\u003e2\u003c/sup\u003e)] (12)\u003c/p\u003e \u003cp\u003e(\u0026radic; means the square root of the expression within the square backets). Inserting the uncertainties discussed above\u003csup\u003e3\u003c/sup\u003e, we have for the relative uncertainty as a function of \u003cem\u003eA\u003c/em\u003e for a single measurement:\u003c/p\u003e \u003cp\u003e \u003cem\u003eu(n\u003c/em\u003e \u003csub\u003e \u003cem\u003eCrIII\u003c/em\u003e \u003c/sub\u003e \u003cem\u003e)\u003c/em\u003e/\u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e = \u0026radic;[(0.007/A)\u003csup\u003e2\u003c/sup\u003e + 0.01\u003csup\u003e2\u003c/sup\u003e + 0.03\u003csup\u003e2\u003c/sup\u003e] = \u0026radic; [(0.007/A)\u003csup\u003e2\u003c/sup\u003e + 1x10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e] (13)\u003c/p\u003e \u003cp\u003eSince the lowest absorbances measured for the samples could be as low as 0.1, the estimated uncertainty is accordingly reported as SD within 8% for this method, see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e2\u003c/span\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 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEffect of the measured absorbance on the relative uncertainty of the determination of \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e. (Relative standard deviation in %).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMeasured absorbance\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRelative uncertainty in calculated \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003eCrIII\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e \u003cp\u003eSD\u003csub\u003erel%\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.7%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.7%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.9%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.6%\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\u003eSystematic errors\u003c/p\u003e \u003cp\u003eSystematic errors were minimised by using calibrated equipment, establishing a calibration curve with known concentrations, with a chemical matrix very close to the samples. During the development work it was found that the molar absorbance coefficient for Cr(III) at 650 nm increased about 4 times with increasing chromic acid concentration from the value in 0.1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The chemical interactions behind this increase are unknown and outside the scope of this work. However, when the chromic acid is present in a large and constant excess, and the calibration curve is straight and reproducible, it should not influence the reliability of the method.\u003c/p\u003e \u003cp\u003eIt was observed that the chromic acid could etch the epoxy resin that was normally used for preparing of the test electrodes. If this was due to an oxidation process rather than just acid dissolution it could increase the amount of H\u003csub\u003e2\u003c/sub\u003e and/or Cr(III). Furthermore, it would also introduce an error in the electrode\u0026rsquo;s metal mass loss. This etching error was negligible for the standard size electrodes (20 mm diameter discs, where the exposed area of epoxy was small, but had to be prevented for smaller electrodes by masking the epoxy with a resistant tape or using a more resistant epoxy. The epoxy that was sensitive to etching was a commercial blend for glueing and filling, which most likely contained an inorganic filler. Further testing showed that a resin for mounting specimens for metallography\u003csup\u003e4\u003c/sup\u003e was resistant to the chromic acid solution, at least during the time frame of the exposure (10 min).\u003c/p\u003e \u003cp\u003eFinally, the standard solutions should not be stored in capped plastic cuvettes for a long time (\u0026gt;\u0026thinsp;days) due to possible oxidation of the plastic material, and, anyway, the risk of water loss by evaporation. However, for short time (1 day) no degradation could be measured.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 Summary and conclusions","content":"\u003cp\u003eA two-stage analytical method was developed to quantify reducing substances (presumed to be magnesium hydride, MgH₂) in corrosion products on pure Mg electrodes. The method is intended for corrosion studies of pure Mg.\u003c/p\u003e \u003cp\u003eThe corrosion product was dissolved in about 20 mL of chromic acid solution (200 g/L CrO\u003csub\u003e3\u003c/sub\u003e). This solution dissolves the corrosion products, while the metal is protected (chromated) and suffers negligible corrosion during the cleaning. In this solution, the hydride ions in MgH\u003csub\u003e2\u003c/sub\u003e were oxidised by protons and by chromic acid.\u003c/p\u003e \u003cp\u003eIn Stage 1, the amount of H\u003csub\u003e2\u003c/sub\u003e gas formed by hydrolysis was measured by a small-scale volumetric gas collecting device. The amount of MgH\u003csub\u003e2\u003c/sub\u003e in this fraction can be calculated by Eq.\u0026nbsp;7. In this stage the main uncertainty was reading the height of the gas column, estimated to be around \u0026plusmn;\u0026thinsp;0.1 mL, thus within 10% relative error at the lowest measured volume of \u0026asymp;\u0026thinsp;1 mL. The main systematic error was assumed to be a small loss of H\u003csub\u003e2\u003c/sub\u003e gas during filling the measurement devices with chromic acid. An estimate of gas permeation, using published values for the permeability of H\u003csub\u003e2\u003c/sub\u003e in polymers, showed that such loss was negligible for the duration of this measurement (approx. 10 min).\u003c/p\u003e \u003cp\u003eIn Stage 2 the amount of Cr(III) ions formed from Cr(VI) in the oxidation of MgH\u003csub\u003e2\u003c/sub\u003e was measured by a spectrophotometric method. The amount of MgH\u003csub\u003e2\u003c/sub\u003e in this fraction can be calculated by Eq.\u0026nbsp;9. The Cr(III) ion had a wide absorption peak around 580 nm in the solution. However, to avoid interference from a strong absorption from Cr(VI) at lower wavelengths, the absorption was measured at 650 nm for the analysis. A straight calibration curve was established for the concentration range 0.005 M to 0.05 M Cr(III) in the chromic acid solution by adding known amounts of Cr(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙9H\u003csub\u003e2\u003c/sub\u003eO. The main uncertainty was connected to the spectrophotometric determination. The total uncertainty was estimated to be \u0026plusmn;\u0026thinsp;8% relative error. The main systematic error was that the chromic acid solution could etch certain epoxy resins, which may be a redox process and increase concentrations of H\u003csub\u003e2\u003c/sub\u003e gas and/or Cr(III) and influence mass loss measurements. Therefore, researchers should qualify the epoxy resin used for mounting the electrode assembly before applying the method.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cstrong\u003eCompeting interests\u003c/strong\u003e \u003cp\u003eThe author has no conflicts of interest related to the content of this method article. The author is the owner and an employee of Gulbrandsen Technology AS. The company has no commercial or financial interests in the topic of this article. No funding was received for conducting this study.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe raw data for the graphs presented here are available from the author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eNazarov A, Yurasova T, Marshakov A (2024) Hydrogen Absorption and Self-Corrosion of Mg Anode: Influence of Aqueous Electrolyte Species. 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Chem Rev 83:651\u0026ndash;731. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/cr00058a004\u003c/span\u003e\u003cspan address=\"10.1021/cr00058a004\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Footnotes","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e Control tests with some gas evolving solid compounds were explored. Calcium hydride (CaH\u003csub\u003e2\u003c/sub\u003e) reacted extremely fast even in distilled water (\u0026lt;\u0026thinsp;1 sec.) and was therefore useless. Dissolution of Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e∙10 H\u003csub\u003e2\u003c/sub\u003eO in a dilute H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was slower. However, with a solubility of about 34 mM for CO\u003csub\u003e2\u003c/sub\u003e in water at room temperature [17], it can be estimated that 20 mL of the dilute acid solution may contain about 16 times more CO\u003csub\u003e2\u003c/sub\u003e than a 1 mL gas pocket. A small deviation from equilibrium saturation would therefore produce large errors in the measured gas pocket volume. Due to these complications, this topic was not further pursued.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003e Using a calibrated variable micropipette 5.00 to 0.50 mL. 1% applies to the lowest volume, with smaller errors for the higher volumes.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003e Ref. [16] states that the individual uncertainties should be given in the same format, e.g., as SD\u0026rsquo;s, when calculating the combined uncertainty. The extreme (maximum) error limits established in the present report, stated as \u0026ldquo;within x%\u0026rdquo;, may be recalculated to SD\u0026rsquo;s by dividing the extreme value by \u0026radic;3 if the maximum error is likely to occur, or dividing by \u0026radic;6 if unlikely[16]. However, the maximum values have here been used without reduction to SD\u0026rsquo;s. This gives a conservative estimate of the SD (i.e. somewhat overestimated), since they are based on a limited amount of qualification testing and estimates.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003e Epofix\u0026trade; from Struers.\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":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"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":"magnesium, localised corrosion, corrosion products, magnesium hydride, analysis, chromic acid","lastPublishedDoi":"10.21203/rs.3.rs-8652822/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8652822/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis brief report describes an ad hoc two-stage method for quantifying the amount of reducing substance, presumed to be magnesium hydride, found in the corrosion products on pure magnesium electrodes exposed to localised corrosion. In the first stage, the corrosion product was dissolved in a small amount (approx. 20 mL) of an aqueous solution of 200 g/L chromic (VI) trioxide (CrO\u003csub\u003e3\u003c/sub\u003e) with volumetric measurement of small amounts of H\u003csub\u003e2\u003c/sub\u003e gas (1\u0026ndash;3 mL) produced by the oxidative hydrolysis of the hydride. In the second stage, the quantity of Cr(III) ions generated from the reduction of chromic acid by the reaction with magnesium hydride in the first stage was measured by a spectrophotometric technique utilising the absorbance from Cr(III) ions at 650 nm.\u003c/p\u003e","manuscriptTitle":"Quantifying the amount of reducing substance in the corrosion products on magnesium electrodes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-21 09:19:51","doi":"10.21203/rs.3.rs-8652822/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":"9a806a41-5c11-4316-a306-15ab2e20e8a6","owner":[],"postedDate":"January 21st, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61463939,"name":"Materials Chemistry"}],"tags":[],"updatedAt":"2026-01-21T09:19:51+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-21 09:19:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8652822","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8652822","identity":"rs-8652822","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}