# Inhibition of Mg Corrosion by Sulfur Blocking of the Hydrogen Evolution Reaction on Iron Impurities

Dimitri Mercier, Jolanta Światowska, Elie Protopopoff, Sandrine Zanna, Antoine Seyeux, Philippe Marcus

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Dimitri Mercier, Jolanta Światowska, Elie Protopopoff, Sandrine Zanna, Antoine Seyeux, et al.. Inhibition of Mg Corrosion by Sulfur Blocking of the Hydrogen Evolution Reaction on Iron Impurities. Journal of The Electrochemical Society, 2020, 167 (12), pp.121504. 10.1149/1945-7111/abaf79. hal-03091388

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# Inhibition of Mg Corrosion by Sulfur Blocking of the Hydrogen Evolution Reaction on Iron Impurities

To cite this article: Dimitri Mercier et al 2020 *J. Electrochem. Soc.* **167** 121504

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# Inhibition of Mg Corrosion by Sulfur Blocking of the Hydrogen Evolution Reaction on Iron Impurities

Dimitri Mercier,<fnref n="2" /> ID Jolanta Światowska, Elie Protopopoff, Sandrine Zanna, Antoine Seyeux, and Philippe Marcus*,z ID

Chimie ParisTech-CNRS, PSL Research University, Institut de Recherche de Chimie Paris (IRCP), Physical Chemistry of Surfaces Group, 75005 Paris, France

A combination of electrochemical measurements, H₂ volume measurements and surface analysis using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) was used to investigate the role of the metallic Fe impurities (60 ppm) in high purity (99.9 wt%) magnesium and the effect of sulfur on the hydrogen evolution. The Mg corrosion was studied in NaCl solutions containing dissolved hydrogen sulfide. At OCP and under anodic polarization the hydrogen evolution reaction and the growth of dark corroded areas were significantly inhibited in the presence of H₂Saq. ToF-SIMS and XPS analyses showed the presence of sulfur adsorbed on Mg and on Fe segregated at grain boundaries. The observed inhibition of hydrogen evolution at OCP and above is assigned to a poisoning effect by adsorbed S of the H adsorption sites on segregated iron which, in the absence of sulfur, provides catalytic sites for excess HER. The equilibrium Pourbaix diagram for the Mg-Fe-S system including the adsorbed species predicts that S is adsorbed on Mg and Fe in the E-pH conditions of this work, which confirms the role assigned to adsorbed sulfur in the inhibition of excess hydrogen evolution at OCP and under anodic polarization in the presence of H₂Saq.

© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/abaf79]

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Manuscript submitted May 1, 2020; revised manuscript received August 11, 2020. Published August 25, 2020.

Despite very interesting physicochemical properties of magnesium and magnesium alloys¹ (e.g. low density, good mechanical properties, biocompatibility and recyclability), their applications in transportation,<fnref n="2" /> batteries³ and biomedical⁴ sectors are limited due to poor corrosion resistance.¹,⁵

It is well established that Mg corrosion in aqueous media occurs by Mg dissolution coupled to the Hydrogen Evolution Reaction (HER). Under anodic polarization, high rates of Mg dissolution, associated to a high rate of the HER are commonly observed. This effect, called “anomalous” Hydrogen Evolution or negative difference effect (NDE), is in apparent contradiction with the Butler-Volmer kinetic equation of charge transfer predicting a decrease of the HER rate under anodic polarization.⁶,⁷

Several mechanisms of Mg corrosion and anomalous HER have been proposed in the past¹,⁵,⁸ such as the stabilization of Mg(I) monovalent species,⁹⁻¹¹ the formation and breakdown of a partial protective film¹²⁻¹⁵ or the formation of hydride intermediates.¹⁶⁻¹⁸ One more recent explanation is that the anomalous hydrogen evolution is induced by the catalytic effect of metallic impurities.¹⁹⁻²⁴ Different papers demonstrated the presence of iron in the oxide layer (by TEM¹⁹), at the oxide(or/and hydroxide)/metal interface (by RBS²²) or redeposited at the surface of the Mg substrate (ToF-SIMS²⁴). Other works suggested that Fe film from Fe redeposition during Mg corrosion is the active cathode for the hydrogen evolution.²³ However, this conclusion is discussed in the literature and in another recent study it was reported that there is no evidence that transition metal electrodeposition is responsible for the cathodic activity on the corrosion front following the filiform corrosion propagation.²⁵ The authors suggest that Mg or metallic impurities contained in the substrate initially covered by an oxide layer become active when this oxide is removed during anodic magnesium dissolution. In this sense, several studies attempted to determine the influence of the amount of metallic impurities on the Mg corrosion.²⁶⁻²⁸ It was shown that there is a critical concentration (180 ppm for Fe) above which the Mg corrosion rate is drastically accelerated (by a factor of 10–100).²⁷

However, there is no general agreement on the metallic impurities involvement in the HER and this hypothesis remains discussed in the literature. An alternative explanation to anomalous HER is that the catalytic activity is the result of high anodic activity at the dissolving Mg interface.²⁹,³⁰ Thus, under anodic polarization, SVET²⁹ or side-view imaging coupled to gravimetric hydrogen measurements³⁰ show that the cathodic activity associated to the dark corroded propagation does not contribute to the overall increase of the HER rate. This explanation is also supported by the fact that no Fe is observed (as for example by EDX³¹). A kinetic model has been proposed for this hypothesis.³²

Our recent study,²⁴ using a combination of fine chemical surface characterization of Mg substrate, demonstrated that segregated Fe particles cause the anomalous HER and formation of dark corroded areas. Before anodic polarization, 3D ToF-SIMS imaging revealed the segregation of iron at the grain boundaries of so-called “pure” magnesium. Under anodic polarization, the H₂ evolution and the initiation and propagation of dark corroded areas occur mainly at the grain boundary terminations. Fe is also observed on the corrosion front near the dark corroded areas but there is no evidence of redeposited Fe effect on the HER. A mechanism of Mg corrosion including the role of the anodic segregation of iron impurities at the termination of grain boundaries was proposed. From this study, the so-called anomalous HER usually observed under anodic polarization can be attributed to the contribution of segregated Fe or other metallic impurities contained in the Mg substrate. At OCP, the HER is the addition of the “normal” HER occurring on the Mg substrate (governed by the cathodic Butler-Volmer relation), and of what can be defined as excess HER occurring on the segregated metallic impurities.

Although the mechanisms of anomalous hydrogen evolution associated to Mg corrosion are not completely established, many studies are now focused on Mg corrosion inhibition. Assuming the major influence of metal impurities more noble than Mg (mostly Fe) on HER and Mg corrosion, it can be expected that the addition of species lowering the reactivity of these impurities can be a way to decrease the corrosion rate of Mg.

Different inhibitors have been already proposed to hinder the HER and the Mg corrosion. For example, different studies demonstrated a significant influence of arsenate anions on the decrease of corrosion of Mg containing 280 ppm Fe.³³ The inhibition strongly depends on pH. For a neutral solution, it was assumed that the Mg

<footnote>*Electrochemical Society Fellow.</footnote>

<footnote n="2">²E-mail: dimitri.mercier@chimieparistech.psl.eu; philippe.marcus@chimieparistech.psl.eu</footnote>
corrosion was delayed by the precipitation of an insoluble Mg₃(AsO₄)₂ layer which covered the active cathodic sites. In acidic media, the inhibition of Mg corrosion over duration >36 h was evidenced and explained by the electrodeposition of elemental arsenic on the active cathodic sites, that inhibited the HER.³³

Assuming that anomalous hydrogen evolution is induced by the re-deposition of Fe on the Mg surface,²³ a research group developed a new inhibition method using complexing agents (Fe²⁺ and Fe³⁺ ions) that suppressed the iron electrodeposition.²⁸,³⁴ Using specific organic molecules, these researchers proposed three distinct mechanisms depending on the molecules:²⁸ firstly, a cathodic inhibition based on the preferential adsorption of molecules on metallic Fe-rich particles, secondly an anodic inhibition based on the adsorption of molecules on the Mg surface and thirdly a precipitation of released free Fe²⁺ and/or Fe³⁺ ions with complexing inhibitors limiting the electrodeposition of metallic iron.

Along this line, a study reported a screening of more than 150 chemical compounds and identified the most effective for decreasing Mg dissolution and anomalous HER.³⁵ However the inhibition effect remains unclear and depends on the nature of the inhibitor. No spectroscopic characterization allows concluding on the effect of inhibitors on the blocking of cathodic sites or on the improvement of the protective layer (for example by precipitation of insoluble species). Only a more detailed knowledge of the Mg corrosion mechanism would allow to determine the most effective action mode and thus find the inhibitor providing the best protection for Mg substrates.

The inhibition of the HER on pure Fe depends on the experimental conditions (e.g. pH and concentrations of inhibitors). A possible inhibitor is dissolved hydrogen sulfide (H₂Saq), although it has been shown that it can either accelerate or inhibit corrosion of Fe.³⁶ The inhibition of corrosion of iron was demonstrated in the pH range 3–5 and for H₂S concentration lower than 0.04 mmol dm⁻³,³⁷ and for alkaline medium containing H₂Saq, our group showed evidence of the adsorption of sulfur at the oxide/metal interface inhibiting the HER and promoting subsequent anodic oxidation.³⁸

The effect of Sₐ₉ₐ₅ on a non oxidized metal surface was widely analyzed in the past. On Pt, H electrod adsorption occurs alone on the bare metal surface at underpotential, i.e. positive to the H⁺/H₂ equilibrium potential, by a pseudocapacitive reaction which gives rise to a current wave in voltammetry. It was shown that a monolayer of adsorbed sulfur blocks the H underpotential adsorption sites³⁹,⁴⁰ and hinders the HER.⁴¹,⁴²

The purpose of this work is to demonstrate the role of Fe present in high purity (HP) magnesium (99.9 wt%, 60 ppm of Fe) on the excess hydrogen evolution at open circuit potential or under anodic polarization, and the poisoning by sulfur of the catalytic activity of the Fe particles for the HER. The surface chemical composition of Mg electrodes after OCP and chronoamperometric measurements in chloride solution without and with H₂Saq was characterized by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). The S surface concentration was measured and the nature of the S surface species discussed. The Pourbaix diagram for the Mg–Fe–S system including adsorbed species was drawn. This allows us to relate the decrease of the HER rate and Mg dissolution in the presence of H₂Saq to sulfur adsorbed on Mg and Fe.

## Experimental

**Sample preparation.**—A Mg foil (rolled, purity 99.9%wt, 60 ppm Fe, Goodfellow, England) with composition reported in Table I was used in this work. The microstructure obtained after chemical etching is presented in Table I. Samples (disks of 15 mm diameter and 1 mm thickness) were mechanically ground on SiC paper from 2400 grit (about 10 µm) to 4000 grit (about 5 µm) then polished on a rotating pad impregnated with alumina suspension (2–3, 1 and 0.3 µm). Between each step, the samples were rinsed with pure ethanol. Then samples were sonicated for 10 min in pure ethanol and finally dried with filtered compressed air.

All the glassware used during this study was cleaned with a Piranha solution (H₂SO₄/H₂O₂).

**Electrochemical measurements.**—Electrochemical measurements were performed using a conventional three-electrode cell with a magnesium sample as a working electrode, a platinum wire as a counter electrode and a saturated calomel reference electrode (SCE). Measurements were carried out at room temperature in 0.1 mol l⁻¹ NaCl (Analab Normapur, analytical reagent, VWR) without or with 0.01 mol l⁻¹ Na₂S·9H₂O (Sigma Aldrich). The pH was adjusted to 6.0 (±0.1) (corresponding to the measured pH of a NaCl solution without H₂Saq) by addition of concentrated HCl (1 mol l⁻¹) to make sure that dissolved hydrogen sulfide (H₂Saq) is the predominant species (pKa (H₂S/HS⁻) = 7).⁴³ Potentiodynamic polarization curves were recorded at a scan rate of 1 mV s⁻¹ using an EC-Lab SP200 potentiostat (Bio-Logic Science Instruments SAS, France). Anodic and cathodic domains have been scanned separately, starting from the OCP. In parallel, dihydrogen has been collected using a classical setup described in the literature.⁴⁴

**Surface chemical analysis and imaging.**—The entire surface of Mg samples, before and after electrochemical measurements, was observed by optical microscopy using a digital VHX-5000 Keyence microscope with a magnification from 100 to 1000 (Lens ZR1000). XPS analysis was performed using a Thermo ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic Al-Kα X-ray source (hν = 1486.6 eV) operating at a pressure of 2.10⁻⁹ mbar. Survey and high-resolution spectra were recorded with a pass energy of 50 and 20 eV, respectively, using Thermo Avantage software version 5.966. The spectrometer was calibrated using Au4f₇/₂ at 84.1 eV. During analysis, charging effects were not compensated.

ToF-SIMS analysis was performed using a ToF-SIMS V spectrometer (ION TOF GmbH, Munster, Germany) operating at 10⁻⁹ mbar pressure. Negative ions depth profiles were performed by sequentially analyzing (Bi⁺, 1.2 pA, 100 × 100 µm²) and sputtering (Cs⁺, 500 eV, 20 nA, 500 × 500 µm²) the sample surface. Data acquisition and post-processing analyses were performed using Ion-Spec software (version 4.1). MgO⁻ (39.980 amu), MgOH⁻ (40.988 amu), MgS⁻ (55.957 amu) FeO⁻ (71.930 amu) and FeS⁻ (87.907 amu) negative fragments, corresponding to Mg oxide, Mg hydroxide, Mg sulfide, Fe oxide and/or metal and Fe sulfide, respectively, have been analyzed during the depth profiling.

The combination of these two techniques allows us to obtain a quantitative surface chemical composition with a good sensitivity (<0.50 at%) by XPS (500 scans were used to reduce the signal-to-noise ratio) and obtain qualitative chemical information with a very high sensitivity (a few ppm) and a very good in-depth resolution by ToF-SIMS. XPS and ToF-SIMS are highly complementary techniques.

## Results

**Influence of H₂S at OCP.**—Figure 1a shows the evolution of the open circuit potential measured for a magnesium electrode immersed in 0.1 mol l⁻¹ NaCl electrolyte (a, black line) and 0.1 mol l⁻¹ NaCl electrolyte containing 0.01 mol l⁻¹ H₂S (b, red line) for short immersion time (30 min). First, a rapid (in the first 5 s) drop of the potential, to -1.75 V<sub>SCE</sub> is observed in both electrolytes. Without H₂S, a plateau is reached after 10 min followed by a weak decrease. After the plateau, electrochemical noise is observed and can be correlated to the initiation of the dark corroded area and the formation of H₂. With H₂S, the potential increases slowly and no plateau is observed. After 30 min no H₂ bubble or dark corroded areas are observed. Optical micrographs obtained after 30 min in NaCl electrolyte (Fig. 1C a) show the presence of the well-known dark filiform corrosion areas characteristic of magnesium corrosion in chloride media. These dark corroded areas are accompanied by strong H₂ evolution. In NaCl electrolyte containing H₂S, no dark corroded area (Fig. 1C b) and no hydrogen evolution were observed
<table><caption>**Table I.** Chemical composition in parts per million (weight ppm) of the Mg sample (99.99 wt%, purchased from Goodfellow) and microstructure obtained after chemical etching.</caption><thead><tr><th>Element</th><th>Al</th><th>Cu</th><th>Fe</th><th>Mn</th><th>Ni</th><th>Si</th><th>Zn</th><th>Mg</th></tr></thead><tbody><tr><td>ppm</td><td>50</td><td>10</td><td>60</td><td>180</td><td>20</td><td>180</td><td>30</td><td>balance</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 1.** (A): Open circuit potential (OCP) measured for short immersion time (30 min) of a Mg sample in: (a) 0.1 mol.l⁻¹ NaCl (black curve), (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S (red curve) and (c) 0.1 mol.l⁻¹ NaCl (mauve curve) after immersion for 30 min in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S. (B): Variation of open circuit potential (OCP) and evolved dihydrogen volume measured for long immersion time (>3 h) of a Mg sample in (a) 0.1 mol.l⁻¹ NaCl (black curve), (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S (red curve). (C): Micrographs of Mg samples obtained after short immersion time (<30 min) in (a) 0.1 mol.l⁻¹ NaCl, (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S and (c) 0.1 mol.l⁻¹ NaCl after 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S (mauve curve). Micrographs of Mg samples obtained after long immersion time (>3 h) in (a) 0.1 mol.l⁻¹ NaCl, (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S. The optical micrographs (size of the active area: 1.1 cm²) were obtained at the end of each OCP measurement.</figcaption></figure>

at the surface of the magnesium electrode. The magnesium sample previously exposed to H₂S containing solution during 30 min was subsequently immersed (without rinsing) in 0.1 mol.l⁻¹ NaCl solution without H₂S (Fig. 1A c). The evolution of the potential presented in Fig. 1A c shows a similar behavior to that of the OCP curve observed for sample immediately immersed in NaCl electrolyte (Fig. 1A a). Optical microscopy images show a strong limitation of the growth of the dark corroded areas and a limited evolution of hydrogen was observed during the immersion (Fig. 1C c). These results show that hydrogen sulfide inhibits the HER and the growth of the dark corroded areas both for Mg sample immersed in chloride solution with H₂S and for Mg sample “conditioned” in chloride containing H₂S and further exposure to chloride solution without H₂S. H₂ has been collected (every 5 min) during immersion at OCP for long time (>3 h) in NaCl electrolyte (Fig. 1B a)) and in

NaCl electrolyte containing H₂S (Fig. 1B b)) and H₂ volume measurements are presented in Fig. 1b.

After 30 min immersion in NaCl, OCP continues to increase slowly. This increase is not linear and can be split into two domains (30–120 min and 120 min–240 min). This evolution can be correlated to the increase of the collected H₂ evolution which presents also two distinct steps. In the presence of H₂S, OCP goes through a maximum (20 min, −1.53 V SCE⁻¹) before reaching a steady state (−1.56 V SCE⁻¹). In each electrolyte, a final pH of about 11 is measured indicating an alkalization of electrolytes. For each curve, the signal presents a strong noise which appears when H₂ evolution begins as already observed.⁴⁵ Evolution of collected H₂ volume is similar for both electrolytes and it could be split into two domains. In the presence of H₂S, H₂ evolution is always lower (red curve in Fig. 1b). In the first domain (0–100 min), hydrogen evolution is not
detectable. A first volume of 0.1 ml (which is the first quantitative value that could be measured with our experimental setup) is collected after an immersion of 40 min in NaCl and 90 min in NaCl + H₂S, showing the mitigating action of H₂S on the H₂ evolution. In the second domain (>220 min), the H₂ volume follows an almost linear rise which is smaller in the presence of H₂S (0.114 ± 0.004 ml.cm⁻².min⁻¹ in NaCl solution and 0.034 ± 0.002 ml.cm⁻².min⁻¹ in NaCl with H₂S). In the presence of H₂S, H₂ evolution is reduced by about 70%, showing a significant effect of H₂S on the hydrogen evolution at OCP.

It is to be noted that, for our Mg sample with 60 ppm Fe, in NaCl solution before addition of H₂S the amount of H₂ is high compared to values of the litterature. For example, some papers have reported a small volume of H₂ (1.5 ml.cm⁻² for UHP Mg (Fe: 0.1 ppm)⁴⁶ and 3 ml.cm⁻² for a HP Mg (Fe: 40 ppm)⁴⁴) after 80 h immersion in 0.1 mol.l⁻¹ NaCl solution while other papers reported a volume of 30 ml.cm⁻² in 3.5% NaCl after 10 h immersion (Fe: 45 ppm).²⁷ These different measured H₂ flows are attributed to the difference of purity of samples. Other impurities could modify the rate of H₂ evolution.

After the immersion tests at OCP for short time (30 min), the Mg samples were analyzed by XPS and compared to a freshly polished Mg substrate (Mg 2p in Fig. 2 and O 1s in Fig. 3). The freshly polished Mg sample spectrum (Fig. 2a, blue line) shows two components at 49.8 and 51.6 eV in the Mg 2p core level region, which can be attributed to metallic and oxide/hydroxide layer, respectively, while the third component at the highest energy (60.3 eV) corresponds to the plasmon loss characteristic of metallic magnesium. After immersion in a 0.1 mol.l⁻¹ NaCl solution with or without H₂S, the Mg 2p photopeak (Fig. 2b, black line and 2c, red line) shows only one component at a high binding energy of 52.9 eV. It is assigned to magnesium oxide/hydroxide. The binding energy shift, +1.3 eV with respect to the native oxide, is assigned to a charging effect and indicates a thickening of the insulating Mg oxide/hydroxide layer induced by exposure to the solutions. The thickening of the oxide is also confirmed by the absence of the metallic component at 49.8 eV for samples exposed to NaCl or NaCl + H₂S and indicates an oxide thickness higher than 11 nm (using a dense and continuous magnesium hydroxide layer model; mean free path: λ<sub>Mg(OH)₂</sub>² = 4.2 nm). The spectra corresponding to the second step in the OCP measurements in 0.1 mol.l⁻¹ NaCl for sample precondi- tioned 30 min in NaCl + H₂S solution are shown in Fig. 2d (dark green line) and 2e (light green line) for “uncorroded” and dark corroded areas, respectively. The absence of the metallic component at 49.8 eV and the larger energy shift (+3 eV and +5.2 eV for uncorroded and dark corroded areas, respectively) also indicates that the Mg oxide layer thickens both in the “uncorroded” and dark corroded regions. However, the most significant energy shift on dark corroded areas indicates the formation of an oxide or hydroxide thicker than on the so called “uncorroded” areas. These observations have already been reported in our previous paper.²⁴ In addition, the Mg2p spectrum recorded for the dark corroded areas presents a main component characteristic of the Mg oxide/hydroxide and a weak component at low energy (54.5 eV, Fig. 2e, marked by a green arrow). This binding energy already observed for the “uncorroded” areas (Fig. 2d) also corresponds to the binding energy of the MgO/Mg(OH)₂ components. These results, combined with the optical microscopy observations, clearly show that the dark corroded areas are rough and not homogeneous and comprise some “uncorroded” spots.

The O 1s core level spectra were also analyzed (Fig. 3a) and similarly to the Mg 2p spectra, no charge correction was done. Typically, the O 1s spectrum for the freshly polished Mg sample shows three components⁴⁷ (Fig. 3b) corresponding to the oxide Mg–O (531.4 eV), hydroxide Mg–OH (533.3 eV) and oxygen-containing contamination species CₓOᵧ (535.0 eV) such as carbonate species. It was previously proposed by Marcus and coworkers,⁴⁷ that the native layer observed on the polished Mg surface had a duplex structure composed of an inner MgO and an outer Mg(OH)₂ layer. After

<figure><img src="image_6.png" /><figcaption>**Figure 2.** XPS spectra of Mg 2p core level obtained for Mg sample: (a) after polishing (blue line, triangle), and after immersion at OCP (b) in 0.1 mol.l⁻¹ NaCl (black line, square), (c) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S (red line, circle). XPS spectra of Mg2p core level of the Mg sample immersed in 0.1 mol.l⁻¹ NaCl after immersion for 30 min in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S for the (d) uncorroded area (dark green line, diamond) and (e) dark corroded area (light green line, star). The green arrow indicates a shoulder.</figcaption></figure>

immersion in 0.1 mol.l⁻¹ NaCl with (Fig. 3A b) or without H₂S (Fig. 3A c), only one component assigned to hydroxylated species (Mg–OH) and shifted towards higher binding energies is observed. The energy shift is similar to the one previously described⁴⁸ on Mg exposed to pure water and assigned to the thickening of the outer Mg(OH)₂ layer. The thickness of the Mg(OH)₂ layer does not allow us to observe the MgO and Mg peaks by XPS and it is not possible to confirm the constant MgO thickness already observed during growing of hydroxide/oxide layer.⁴⁸ Based on inelastic mean free path values classically used⁴⁹,⁵⁰ and corrected with specific references of our group (λ<sub>Mg(OH)₂</sub>² = 2.4 nm and λ<sub>Mg2p</sub>² = 4.2 nm), a minimum Mg(OH)₂ thickness of 12.6 nm was estimated. As MgO is never observed after immersion in NaCl and NaCl + H₂S (component at 531.4 eV after charge effect correction), considering a homogeneous hydroxide layer, it is possible to determine the stoichiometry of the hydroxide layer using the O 1s/Mg 2p area ratio. From measured intensities, the at% of each species can be determined using the Eq. 1, where I is the intensity measured for each species, λ the inelastic mean free path (nm), σ the relative sensitivity factor and T the transmission factor of the spectrometer.

$$X(at\%) = \frac{100 \frac{I(X)}{\lambda_{x} \sigma_{x} T_{x}}}{\sum_{\lambda_{i} \sigma_{i} T_{i}} I(i)} \quad [1]$$

The obtained ratios are close to 2 (1.99 in NaCl solution and NaCl and 1.96 in NaCl + H₂S solution). This confirms the presence of Mg(OH)₂ in the outer part of the oxide film. Thus, it can be concluded that H₂S has no influence on the composition of the outer layer. After “conditioning” in NaCl + H₂S solution and subsequent immersion in NaCl solution, the values of the O1s/Mg2p ratio are similar (1.87 for the uncorroded area and 1.82 for the dark corroded areas) and indicate that H₂S<sub>aq</sub> does not modify the outer Mg(OH)₂ layer. The analysis of the Cl 2p core level (Fig. 4a)) reveals that after restarting the OCP measurements in 0.1 mol.l⁻¹ NaCl, a slight chloride enrichment can be observed in the dark corroded areas (~0.4 at% compared to ~0.2 at% in the uncorroded areas or on the surface after immersion in NaCl solution containing H₂S). This enrichment of chloride species on dark corroded areas was already
<figure><img src="image_7.png" /><figcaption>**Figure 3.** (A): XPS spectra of O1s core level obtained for Mg sample: (a) after polishing (blue line, triangle), after immersion at OCP (b) in 0.1 mol.l⁻¹ NaCl (black line, square), (c) in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S (red line, circle). XPS spectra of O1s core level obtained for Mg sample after subsequent immersion in 0.1 mol.l⁻¹ NaCl for the(d) uncorroded area (dark green line, diamond) and (e) dark corroded area (light green line, star). The green arrow indicates the shoulder. (B): O1s peak decomposition for a freshly polished Mg sample.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** (A): XPS spectra of Cl 2p core level obtained for Mg sample after immersion at OCP in (a) 0.1 mol.l⁻¹ NaCl (black line, square) and (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S (red line, circle). XPS spectra of Cl2p core level obtained for Mg sample immersed in 0.1 mol.l⁻¹ NaCl after immersion for 30 min in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S for the (c) uncorroded area (dark green line, diamond) and (d) dark corroded area (light green line, star). (B): XPS spectra of S 2p core level obtained for Mg sample: (a) after polishing (blue line), (b) after immersion at OCP in 0.1 mol.l⁻¹ NaCl (black line), (c) in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S (red line) and associated decomposition.</figcaption></figure>

observed during anodic polarization in NaCl in our previous work²⁴ but it was stronger (about 5 at%) and it could be explained by a higher surface fraction covered by dark corroded areas and a higher surface area due to the surface roughness. While the exact influence of chlorides on the Mg corrosion mechanisms has not been investigated in detail in the literature so far, the presence of chlorides mainly on the dark corroded areas shows an influence of chlorides on the (dendritic) growth of dark corroded areas.

Figure 4b shows the evolution of the S2p spectra obtained for polished Mg (a), and after immersion in 0.1 mol.l⁻¹ NaCl electrolyte for 3 min (b) and in 0.1 mol.l⁻¹ NaCl electrolyte containing 0.01 mol.l⁻¹ H₂S for 30 min (c). On the polished Mg surface (Fig. 4B-a), a main spin-orbit doublet at 168.6 eV (S2p₃/₂) and 169.8 eV (S2p₁/₂) (after binding energy correction from charging effect) indicates the presence of sulfate species. A very small amount

of sulfites with components at 167.4 eV (S2p₃/₂) and 168.5 eV (S2p₁/₂) is also observed. The small amount of sulfates, in the order of 0.20%, results probably from the contamination during glassware cleaning in Piranha solution (H₂SO₄/H₂O₂ mixture). After immersion in NaCl for 3 min (Fig. 4B-b) a strong decrease of the intensity of the peaks corresponding to sulfate and sulfite species is observed indicating that these species are either removed during the sample immersion (and thus do not influence the magnesium corrosion mechanism) or covered by the corrosion products layer. After immersion in NaCl containing H₂S (Fig. 4B c), similar peaks, assigned to sulfates (168.7 and 169.9 eV) and sulfites (167.4 and 168.5 eV) are also observed. The presence of sulfate and sulfite anions may originate from the chemicals used for electrolyte preparation (Na₂S·9H₂O, SO₃²⁻ contamination <0.5%). However, the presence of Mg sulfites in the corrosion products cannot be
excluded. The formation of MgSO₃ species has been already observed as corrosion products in presence of SO₂ traces.⁵¹

In Fig. 4B-c, another peak is found at 162.6 eV, characteristic of a sulfur-metal bond. This small S-Met peak observed only when H₂S is added to the electrolyte can be assigned to Sads on metal surfaces. At these low levels, and due to the detection limit of XPS, it is difficult to discriminate between a Sads-Mg or a Sads-Fe bond. Indeed the binding energies for S⁰-Met bonds (Fe,⁵² Ni,⁵³ Ni based alloys⁵⁴,⁵⁵) vary between 162 and 163 eV. The binding energy measured for the S-Au⁵⁶ in the case of SAM formation from alkanethiol is similar. At the same energies are also observed FeSx⁵⁷ and MgSx⁵⁸ species. The amount of S adsorbed is about 0.08 at% (calculated following Eq. 1). The number of scans of the S2p peak has been drastically increased (500 scans) to reduce the signal-to-noise ratio. This value corresponds to the percentage of S (at%) with respect to all species detected by XPS (except carbonaceous species) averaged over the thickness from which XPS signals are measured. This low value can be compared with the expected theoretical value for a complete monolayer on segregated Fe (see calculation in the discussion).

In order to get additional information on the nature of the S species bonded to the metal surface, ToF-SIMS ion depth profiles were performed for a polished Mg sample (Fig. 5a) and for the sample immersed in NaCl electrolyte containing H₂S (Fig. 5b). The ions characteristics of the Mg oxide (MgO⁻ at 39.980 amu), Mg hydroxide (MgOH⁻ at 40.988 amu), Mg sulfide (MgS⁻ at 55.957 amu), Fe (metal or oxide) (FeO⁻ at 71.930 amu) and Fe sulfide (FeS⁻ at 87.907 amu) were used. ToF-SIMS ion depth profiles obtained for the freshly polished Mg sample show high intensity MgO⁻ and MgOH⁻ signals in the whole range of sputtering time (600 s) indicating that the Mg metallic substrate was not reached and only the Mg oxide/hydroxide film covering the Mg substrate was analyzed (in agreement with XPS data).

Looking closely at the MgO⁻ and MgOH⁻ signals (Fig. 5a), it can be observed that MgOH⁻ intensity is higher in the outer part of the profile and MgO⁻ is higher in the inner part of the profile suggesting a bilayer structure with an inner Mg rich oxide and an outer Mg rich hydroxide as already observed and discussed above. However, there is no sharp interface. The increase of the MgO⁻ signal and the stability of the MgOH⁻ signal when probing deeper suggest a weak enrichment of MgO oxide in the inner part.

The weak MgS⁻ signal, which does not vary much during sputtering, may originate partly from a low concentration of sulfur initially present in the Mg bulk, although it was not mentioned in the impurities content by the manufacturer. ToF-SIMS has a very high sensitivity and low levels of impurities can be detected. The affinity between sulfur and Mg is well known and Mg is often used in the

desulfuration processes. A closer look at the other fragments shows that the intensity of FeO⁻ is very low through the whole depth profile and FeS⁻ is not detected. According to our previous work describing the presence of metallic Fe particles at the grain boundaries,²⁴ it is possible to consider the FeO⁻ signal as a marker of metallic Fe. Indeed, for these very low amounts (not detectable by XPS), the Fe metal signal is too low in the negative polarity and FeO₂⁻ at 85.929 amu, which is the marker of oxidized Fe, is not observed. Thus using the FeO⁻ signal as a marker of metallic Fe in contact with oxidized magnesium is consistent with the fact that the Fe particles segregated at the grain boundaries terminations are electrically connected to the Mg matrix hence at the OCP of magnesium (~-1.5 V SCE⁻¹), Fe is not oxidized.²⁴

After immersion in H₂S containing NaCl solution, the ToF-SIMS ion depth profiles (Fig. 5b) show again clearly the signals characteristic of MgO and Mg(OH)₂. The MgOH⁻ signal remains high (higher than MgO⁻) through the entire depth profile. This confirms the formation of a thicker hydroxide layer compared to the freshly polished Mg sample, as already observed by XPS.

The FeS⁻ and MgS⁻ signals shown in Fig. 5b reach a maximum intensity after about 20 s of sputtering. The MgS⁻ signal intensity decreases after 20 s of sputtering and is similar to that measured on the freshly polished sample (Fig. 5a). The FeS⁻ signal also decreases and disappears while the FeO⁻ signal, characteristic of the Fe particles, increases and stabilizes at a level similar to the one observed on the freshly polished sample (Fig. 5a). The FeS⁻ and MgS⁻ signals, observed at the beginning of the sputtering time, can be assigned to surface Mg-S and Fe-S corresponding to Sads resulting from a surface reaction between H₂S in the electrolyte and both the Mg matrix and the Fe particles. The low levels measured for these ions by ToF-SIMS and the small S 2p XPS signal at low binding energy both indicate a small surface coverage by adsorbed sulfur.

The MgS⁻ signal could also be attributed to Mg sulfite in the corrosion products, however this would imply that characteristic fragments of MgSO₃ (MgSO⁻ at 71.952 amu, MgSO₂⁻ at 87.947 amu and MgSO₃⁻ at 103.942 amu) were observed, which is not the case.

**Influence of H₂S on the polarization behaviour of Mg.**—
Figure 6 presents the potentiodynamic polarization curves obtained for a magnesium electrode in 0.1 mol.l⁻¹ NaCl without and with 0.01 mol.l⁻¹ H₂S. These experiments were performed in a non-deaerated electrolyte and in a single upward scan at a rate of 1 mV s⁻¹ after immersion at OCP for 2 min in NaCl electrolyte and 10 min in NaCl/H₂S electrolyte. Anodic and cathodic domains have been scanned separately and starting near the OCP (OCP + 0.05 V

<figure><img src="image_9.png" /><figcaption>**Figure 5.** (a): ToF-SIMS ion depth profiles (FeS⁻, MgS⁻, FeO₂⁻, MgO₂⁻ and MnO₂⁻) for a polished Mg sample. (b): ToF-SIMS ion depth profiles (FeS⁻, MgS⁻, FeO₂⁻, MgO₂⁻ and MnO₂⁻) for a Mg sample immersed at OCP in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S electrolyte.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Potentiodynamic polarization curves (A cathodic; B anodic) obtained at 1 mV s⁻¹ for Mg in (a) 0.1 mol.l⁻¹ NaCl (black line, squares) and (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S (red line, circles). The arrows indicate the direction of the scans.</figcaption></figure>

for the cathodic domain and OCP – 0.05 V for the anodic domain. The value of OCP (~-1.55 V SCE⁻¹) is similar for both immersion times. As described previously,²⁴ after this time of immersion in NaCl, no dark corroded areas and no bubble formation are observed. The shape of the curves is similar for both solutions and consistent with the literature for high purity Mg samples.²⁰,⁴⁶ For both media, no passive domain is observed. H₂S has no strong effect on the anodic current and on the corrosion potential. The cathodic current is slightly higher in the presence of H₂S, which may be explained by a blocking effect of sulfur on the formation of Mg surface oxide/hydroxide limiting the cathodic reaction.

To understand the behavior in the anodic domain, potentiostatic polarizations have been performed in NaCl and NaCl containing H₂S at low anodic overpotential (OCP + 0.25 V, corresponding to –1.32 V SCE⁻¹) and H₂ gas has been collected simultaneously (Fig. 7). Polarizations have been stopped after a flowed charge of at least 6 C.cm⁻². Micrographs obtained after a charge of

0.25 C.cm⁻² and at the end of the polarization (>6 C.cm⁻²) are also presented in Fig. 7. As at OCP, H₂ gas evolution follows a same trend for both electrolytes but is always lower in the presence of H₂S. In contrast to the OCP, the first domain attributed to the initiation of the H₂ evolution is very short. Then, the collected H₂ volume increases almost linearly. The decrease of the H₂ evolution rate in the presence of H₂S is similar to that observed at OCP (~75% decrease under anodic polarization and ~70% decrease at OCP). While the evolution rate is similar, the required time to collect 5 ml of H₂ is longer under anodic polarization (90 min) compared to OCP (300 min). Micrographs of surfaces in the presence of H₂S show a limitation of the dark corroded areas.

XPS analyses performed on the Mg samples after the anodic polarizations at OCP + 0.25 V and OCP + 0.5 V for a charge of up to 250 mC.cm⁻² allowed us to determine the O 1s/Mg 2p ratio. The dark corroded areas covering the surface at high charges (6 C.cm⁻²) led to inhomogeneous surfaces and interpretation of data obtained by

<figure><img src="image_11.png" /><figcaption>**Figure 7.** Volume of evolved H₂ as a function of charge obtained during anodic polarization (OCP + 0.25 V) in (a) 0.1 mol.l⁻¹ NaCl and (b) 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ Na₂S and micrographs recorded at the end of the polarization and for a charge of up to 250 mC.cm⁻².</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 8.** XPS spectra of S 2p core level obtained for Mg samples after anodic polarization in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S at OCP + 0.25 V and OCP + 0.5 V.</figcaption></figure>

XPS on these surfaces is quite complex. This O1s/Mp2p ratio (1.9 close to expected 2) does not depend on the nature of the electrolyte or the overpotential, which indicates that the anodic polarization has no effect on the global stoichiometry of the layer which remains mainly composed of hydroxide in the outer part. A significant charging effect is also observed for these two samples indicating an insulating character of the surface explained by a thickening of the hydroxide layer. The corresponding binding energy shift in NaCl solution containing H₂S is estimated to 4.1 eV and 4.2 eV for the samples polarized at OCP + 0.25 V and OCP + 0.5 V, respectively. For comparison, in NaCl solution, this shift is estimated to 1.9 and 2.2 eV for the samples polarized at OCP + 0.25 V and OCP + 0.5 V, respectively. The amount of chlorine is also similar (about 0.5 at%) without or with H₂S and after the two polarizations.

After anodic polarization in NaCl + H₂S solution a sulfur signal is also observed by XPS (Fig. 8) as after OCP measurements (Fig. 4). Again it shows the doublet peaks corresponding to sulfate (168.6 and 169.8 eV), sulfite (167.4 and 168.5 eV) and the peak indicative of a surface S-Met bond (at 162.6 eV). By selecting the best XPS experimental conditions for the analysis of the latter peak,

particularly by increasing the number of scans to 500, we could reduce the signal-to-noise ratio and thus enhance the quantitative analysis. The amount of adsorbed sulfur measured by XPS is low and varies from 0.08 at% for the sample immersed at OCP, to 0.05 at% for the sample polarized at OCP + 0.25 V and to 0.03 at% for the sample polarized at OCP + 0.5 V. These at% correspond to the S amount measured in the analyzed volume. These atomic percents are determined considering all species quantified by XPS (using Mg2p, O1s, Cl2p, S2p) except carbonaceous species (C1s peak).

As already mentioned in the analysis of Fig. 4, the low binding energy peak is assigned to S<sub>ads</sub> on segregated Fe particles and on Mg.

The ToF-SIMS ion depth profiles obtained after anodic polarization in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S at OCP + 0.25 V and OCP + 0.5 V are presented in Figs. 9a and 9b, respectively. As after immersion at OCP in NaCl + H₂S solution (Fig. 5), the profiles show a strong modification compared to the ToF-SIMS depth profiles obtained on the freshly polished Mg sample (see discussion about Fig. 5). The two ToF-SIMS ion depth profiles in Fig. 9 show intense MgOH⁻ signals, that remain higher than the MgO⁻ signal over the whole analyzed thickness. This indicates an outer Mg-rich hydroxide, as already observed by XPS (see discussion about Fig. 3). Compared to the ToF-SIMS analysis performed after immersion at OCP in NaCl + H₂S electrolyte (Fig. 5), the analyses carried out after anodic polarization show a slightly different behaviour for the FeS⁻ and MgS⁻ fragments. In particular, the maximum of the MgS⁻ signal is slightly shifted (~20 s for E<sub>ocp</sub> + 0.25 V and ~70 s for E<sub>ocp</sub> + 0.5 V) suggesting that the species with Mg–S bond is slightly buried into the hydroxide layer when the potential increases. The FeS⁻ fragment follows the same trend as previously observed (Fig. 5) with a complete and fast decrease of the signal indicating that the S–Fe species is located at the surface. Compared to the results obtained after immersion without anodic polarization (see Fig. 5), the FeO⁻ fragment (characteristic of metallic iron as explained previously) appears simultaneously with the FeS⁻ for the low anodic polarization and in a large domain similar to that observed for the FeS⁻ fragment for the high anodic polarization suggesting the presence, at the same location, of Fe–S and metallic Fe species in the outer layer. The presence of the FeO⁻ signal in the whole profile confirms the existence of metallic Fe segregating at the grain boundary terminations. Under anodic polarization, a porous Mg(OH)₂ layer covers the segregated Fe. However the connectivity of the segregated iron particles and the porosity of the Mg(OH)₂ layer can maintain the catalytic activity of Fe on the HER. The evolution of the MgS⁻ signal is similar to that observed previously (Fig. 5a) with a final intensity slightly higher than for the freshly

<figure><img src="image_13.png" /><figcaption>**Figure 9.** (a): ToF-SIMS ion depth profiles (FeS⁻, MgS⁻, FeO₂⁻, MgO₂⁻ and MnO₂⁻) of a Mg sample obtained after anodic polarization (OCP + 0.25 V, 500 mC.cm⁻²) in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S electrolyte. (b): ToF-SIMS ion depth profiles (FeS⁻, MgS⁻, FeO₂⁻, MgO₂⁻ and MnO₂⁻) of a Mg sample obtained after anodic polarization (OCP + 0.5 V, 500 mC.cm⁻²) in 0.1 mol.l⁻¹ NaCl + 0.01 mol.l⁻¹ H₂S electrolyte.</figcaption></figure>
polished sample. Moreover, the shape of the MgS⁻ signal profile suggests the presence of Mg–S species in the Mg hydroxy layer.

Discussion

As reported in the introduction, it has been suggested that metallic impurities in the Mg substrate, particularly Fe play a main role in the hydrogen evolution reaction and the enhancement of Mg corrosion.<sup>19,22–24,27</sup> Recently our group has shown the role of residual metallic Fe segregated at grain boundaries, on the Mg corrosion and HER.<sup>24</sup> From the experiments reported in the present work it is observed that the growth of dark corroded areas associated with a significant rate of HER is strongly limited by the addition of sulfides into the NaCl electrolyte. This effect is explained by the XPS results showing a S2p photopeak at 162.6 eV characteristic of a metal-adsorbed sulfur bond, and the detection, by ToF-SIMS, of FeS⁻ and MgS⁻ ions, which are assigned to the adsorption of S on Fe and Mg.

The adsorption of S and its effect on hydrogen evolution has been previously studied on different metallic surfaces. It was shown on platinum single crystal surfaces that the adsorption of H at underpotential strongly decreases when the coverage of adsorbed sulfur increases, and is prevented by a complete sulfur monolayer, which indicates a strong blocking of the H sites by adsorbed sulfur.<sup>39,40</sup> The HER rate at a given potential is also strongly reduced (the HER overpotential increases) when the sulfur coverage increases, which was explained by a partial blocking of the HER sites.<sup>41,42</sup> The effects of adsorbed sulfur on different metal substrates have been reviewed, and the mechanisms of the blocking of the reactive sites by sulfur rationalized.<sup>59,60</sup> It was also observed that adsorbed sulfur drastically impedes the HER on Ni<sup>61</sup> in acidic solution and on Fe<sup>38</sup> in alkaline solution. Moreover, it was previously shown that a stable sulfur monolayer accelerates the metal dissolution on Ni, NiFe and Fe by blocking the O adsorption sites required for the growth of a passive film.<sup>52,62–64</sup>

The electrochemical measurements combined with H₂ volume collection and optical observations show that sulfur has a significant effect both on the Mg corrosion and the HER. A decrease of ~70%–75% of the evolved H₂ volume is observed in the presence of H₂S at OCP and under anodic polarization.

In order to highlight the relationship between sulfur and Mg corrosion in H₂S-containing solution, we examine the possibility that sulfur adsorbing on a Mg electrode surface from H₂Saq could block the HER sites on metallic Fe particles segregated at the Mg grain boundaries.<sup>24</sup>

The amount of S adsorbed on Fe segregated during immersion at OCP can be assessed as follows. Using the polarization curve (Fig. 6), the corrosion current density has been only estimated using the Tafel slopes extrapolation (not shown here because this is only to get an approximate value, as the potential range of Tafel behaviour is limited). A value of about 90 µA.cm⁻² is obtained in the presence of H₂S. With this current density for a time of 1800 s, using the Faraday law, the amount of dissolved Mg is calculated to be 2.0 10⁻⁵ g.cm⁻² which corresponds to a 5.0 10¹⁸ at.cm⁻². The initial content of Fe impurity in Mg is 60 weight ppm hence the atomic ratio Fe/Mg in the bulk is equal to 2.6 10⁻⁵. Whereas Mg is dissolved, Fe accumulates at the surface by anodic segregation, and the amount of Fe at the surface is calculated to be 1.3 10¹³ at.cm⁻². Considering that on Fe a complete monolayer of S<sub>ads</sub> corresponds to a ratio S/Fe = 0.5, as determined on Fe surfaces,<sup>65,66</sup> a complete monolayer of S on Fe segregated at the Mg surface would give a surface density of 6.5 10¹² at.cm⁻². From the XPS spectra, the global S% (average value for the depth analyzed by XPS) was estimated at 0.08 at%. Assuming that the surface layer is mainly composed of Mg(OH)₂ (molar density: 0.04 mol.cm⁻³) for the analyzed volume (depth of 12 nm), the sum of surface densities for all measured species is estimated to 8.7 10¹⁶ at.cm⁻². The S surface density corresponding to the percentage of 0.08 measured by XPS is about 7.0 10¹³ at.cm⁻², i.e. much higher than the value

estimated above (for a dissolution of 1800 s) for a full monolayer of S adsorbed on segregated Fe, showing that S is also adsorbed on Mg. Similar calculations have been carried out after anodic polarization. The same charge (250mC.cm⁻²) has been flowed at the two anodic potentials (OCP +0.25 V and OCP + 0.5 V) (see Fig. 8). The S<sub>ads</sub> surface density obtained from XPS data is 4.3 10¹³ at.cm⁻² at OCP +0.25 V and 2.6 10¹³ at.cm⁻² at OCP + 0.5 V. These values are lower than the value at OCP but still higher than the density corresponding to a full monolayer of S<sub>ads</sub> on segregated Fe.

In addition to S bonded to Fe (evidenced by the FeS⁻ signal in ToF-SIMS, and consistent with the S2p signal at low binding energy in XPS), the excess of the full monolayer on Fe is assigned to Mg–S species, which is consistent with the MgS⁻ signal in ToF-SIMS. The Mg–S species, resulting from the reaction of H₂S with Mg, could remain trapped in the hydroxide layer.

Summarizing our results, it appears that sulfur is adsorbed on Mg and on segregated Fe. The amount of adsorbed sulfur is sufficient to form a complete monolayer on segregated Fe and this explains the inhibiting effect observed on HER. When the applied potential is increased anodically from OCP, the S<sub>ads</sub> surface density decreases, which can be related to the observed limited inhibition of HER and of associated dark corroded areas growth (Fig. 7). This suggests that the coverage of S<sub>ads</sub> on segregated Fe is lower when the potential increases.

In order to better understand the influence of S on Mg and Fe electrochemical behaviour, the equilibrium potential-pH diagram for the Mg–Fe–S system including the adsorbed species was calculated (Fig. 10) (the principle of calculation of the E-pH lines for adsorbed species is described in Ref. 67). This diagram shows the stability domains of atomic S adsorbed on Mg, denoted S<sub>ads</sub>(Mg) and S adsorbed on Fe, denoted S<sub>ads</sub>(Fe).

The electrochemical equilibrium of S adsorption on a metal (Mg or Fe) surface from H₂S<sub>aq</sub> can be written as<sup>67</sup>:

<figure><img src="image_14.png" /><figcaption>**Figure 10.** E-pH equilibrium diagram for the Mg–Fe–S system, showing the thermodynamically stable forms of dissolved S, dissolved species and solid compounds of Mg and Fe, and also S adsorbed on the Mg and Fe surfaces from dissolved sulfides (H₂Saq or HS⁻). The activities (on the molal scale) are: a<sub>S</sub> =10⁻², a<sub>Mg</sub> =1 to 10⁻⁶, and a<sub>Fe</sub> = 10⁻² to 10⁻⁶. The bold lines indicate the E-pH lines for the S adsorption equilibria at half-saturation of the surface. The method for estimating thermochemical data used in calculation of the lines for adsorption is given in Appendix. (blue circle: OCP, green circle: OCP + 0.25 V and brown circle OCP + 0.50 V), the potentials investigated are indicated as color segments: blue line: OCP, green line: OCP + 0.25 V and brown line: OCP + 0.50 V.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure 11.** Mechanisms of Mg corrosion at OCP and above (anodic overpotential) and inhibition of excess HER by S adsorption on Fe segregated at the surface.</figcaption></figure>

$$H_2S_{aq} + (M) = S_{ads}(M) + H^+ + e^- \quad [2]$$

where (M) represents a free M surface adsorption site.

The diagram indicates that at pH 6, corresponding to the experimental conditions used in this work, sulfur should adsorb on the Mg surface when increasing the potential from the cathodic region.

At pH 6, above ~-1 V/SHE, S can adsorb on the Fe surface at lower (more negative) potentials than the potentials of formation of the Fe sulfides, indicating the excess of stability of the 2D phase as compared to the 3D compounds. The potentials at which the experiments reported here were performed (from -1.3 V/SHE to -0.8 V/SHE), are located only partially in the E-pH stability domain of sulfur adsorbed on Fe. The lower potentials corresponding to OCP are located ~0.2 V below the bold full line representing the S adsorption equilibrium at half-saturation coverage on the surface. In our previous work on S adsorption on silver,⁶⁸ it was shown that the half-saturation adsorption potential measured from experiments using voltammetry combined with electrochemical in situ STM⁶⁹ was lower by ca. 0.1 V than the value calculated from gas phase experiments.⁶⁸ Moreover it should be noted that the S adsorption line drawn in Fig. 10 is calculated at half-saturation coverage from estimated data for S adsorption (cf. Appendix), using the Langmuir model of adsorption where all the adsorption sites are similar, whereas real S adsorption begins on the more stable sites of the surface (such as defects) at potentials more negative (some hundreds

of mV) than the line at half-saturation coverage. Drawing the lines for low S coverage would give a larger thermodynamic stability domain for S<sub>ads</sub>(Fe).

On a Mg electrode with metallic Fe segregated at the surface, the potential is imposed by the Mg matrix to the surface Fe. Our thermodynamic calculations indicate that for the potentials used in this work (from OCP to OCP+0.5 V), S is adsorbed from H₂S<sub>aq</sub> both on Mg and segregated Fe.

In contrast to our findings, other authors argue that the anomalous hydrogen evolution is not associated with the presence of metallic impurities in the Mg matrix but is the result of very high corrosion current densities at the actively corroding front.²⁹,³⁰ This hypothesis is mostly supported by the fact that no Fe is observed during characterization (for example by EDX³¹). These observations are in contrast with other works showing the presence of Fe, as for example using TEM,¹⁹ RBS²² and ToF-SIMS.²⁴ Using ToF-SIMS imaging, a very sensitive technique allowing the detection of a few ppm, the presence of Fe at the emergence of grain boundaries was evidenced.²⁴ In the present work, the presence of FeS⁻ and FeO⁻ fragments (in very low amount, too low to be detected by XPS) confirms the presence of surface Fe. Thus, this work, associated to our previous work²⁴ confirms the role of segregated Fe on the HER at OCP and above.

Finally, a schematic representation of the mechanism of Mg corrosion showing the role of metallic Fe impurities segregated at grain boundaries surface terminations in the excess hydrogen evolution was proposed in a previous paper.²⁴ An extended
model incorporating the blocking by S<sub>ads</sub>(Fe) of the excess
hydrogen evolution sites on the surface Fe particles is shown in
Fig. 11.

**At OCP in chloride solution.**—During Mg dissolution, Fe enrichment at the Mg surface occurs by anodic segregation. The Fe particles (shown in red on Fig. 11a) segregated at the grain boundaries surface terminations supply catalytic sites for the HER, explaining the excess H₂ evolution on Mg compared to the HER deduced from extrapolation of the cathodic branch.

**At OCP in the presence of H₂S in the chloride solution.**—In the presence of H₂S<sub>aq</sub>, (orange on Fig. 11b) S is adsorbed on the Fe particles segregated at the grain boundaries surface terminations during the Mg corrosion. This S<sub>ads</sub> monolayer inhibits the HER and the development of the dark corroded areas. Simultaneously, a porous Mg(OH)₂ forms from Mg²⁺ released during Mg dissolution. Some Mg released by the corrosion process reacts with S and the Mg–S species remain trapped in the Mg(OH)₂ layer.

**Under polarization in H₂S containing chloride solution.**—
Under anodic polarization (Fig. 11c), a similar mechanism is
proposed. Anodic segregation of metallic Fe at the grain boundaries
surface terminations takes place, and the adsorption of S blocks the
iron surface sites for HER.

Conclusions

The influence of iron impurities corrosion of magnesium (containing 60 ppm of Fe) and on the Hydrogen Evolution Reaction at OCP and above (OCP + 0.25 V; OCP + 0.5 V) in 0.1 mol·l⁻¹ chloride solution (pH6) without and with Na₂S (0.01 mol·l⁻¹) was investigated by XPS, ToF-SIMS, electrochemical measurements and H₂ volume measurements.

The addition of H₂S to the electrolyte leads to a significant
corrosion mitigation associated with a inhibition of the Hydrogen
Evolution Reaction, of ~70% at OCP and of ~75% under anodic
polarization.

Combining XPS and ToF-SIMS analyses, the presence of adsorbed sulfur, corresponding to a low surface coverage on Mg and a high coverage on Fe, was evidenced.

Sulfur adsorbed from H₂S<sub>aq</sub> on the Fe particles segregated at the grain boundaries terminations inhibits the excess HER (also called “anomalous” HER) by poisoning the Fe catalytic sites.

The surface analysis also indicates the presence of Mg–S species trapped in the Mg(OH)₂ layer.

The potential-pH diagram for the Mg–Fe–S system including the adsorbed sulfur species (S<sub>ads</sub>) was calculated. It shows the E-pH domains of stability of S<sub>ads</sub> on Fe and Mg, which are consistent with the experimental results.

A model was proposed for the mechanisms of Mg corrosion including the role of the segregated Fe impurities and the inhibition of the excess HER at OCP and above by adsorption of S on Fe particles.

**Appendix A**

The equilibrium potential-pH diagram for the Mg–Fe–S system was calculated using thermochemical data from NBS.⁷⁰ The Gibbs free energies of formation for sulfur adsorbed on iron were calculated from gas phase data and presented in a previous paper.⁶⁷ Since there was no available data for sulfur adsorbed on Mg (2D phases), we estimated the thermodynamic values from the ones available for the most stable 3D sulfide, i.e. MgS,⁷⁰ as follows: for the adsorption enthalpies, we used an empirical correlation established between the enthalpies of formation of the most stable metallic sulfides and the enthalpies of adsorption of sulfur.⁶⁵ The entropy of adsorbed sulfur was estimated from the entropy of MgS and Mg using a relation presented previously.⁶⁷

ORCID

Dimitri Mercier https://orcid.org/0000-0002-9637-1484

Philippe Marcus https://orcid.org/0000-0002-9140-0047

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