Fes Corrosion Products Formation and Hydrogen Uptake in a Sour Environment for Quenched & Tempered Steel

metals

Article FeS Corrosion Products Formation and Hydrogen Uptake in a Sour Environment for Quenched & Tempered Steel

Elien Wallaert 1, Tom Depover 1, Iris De Graeve 2 and Kim Verbeken 1,*

1 Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 903, B-9052 Zwijnaarde, Belgium; [email protected] (E.W.); [email protected] (T.D.) 2 Department of Chemistry and Materials, Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +32-933-104-53

Received: 1 December 2017; Accepted: 13 January 2018; Published: 17 January 2018

Abstract: Surface corrosion product formation is one of the important factors affecting the corrosion rate and hydrogen uptake in a H2S environment. However, it is still unclear how the base material composition will affect the corrosion products that are generated, and consequently their impact on the corrosion rate. In this paper, corrosion product formation and the impact of the Mo content of the base material on the composition of the corrosion products and hydrogen absorption in a sour environment was investigated. The corrosion layer was composed of a double layered mackinawite (FeS1−x) structure, which was enriched with molybdenum and chromium. The layers were formed via two different mechanisms, i.e., the inner layer was created via a general oxide film formation corrosion mechanism, whereas the upper layer was formed by a precipitation mechanism. The presence of this double corrosion layer had a large influence on the amount of diffusible hydrogen in the materials. This amount decreased as a function of contact time with the H2S saturated solution, while the corrosion rate of the materials shows no significant reduction. Therefore, the corrosion products are assumed to act as a physical barrier against hydrogen uptake. Mo addition caused a decrease in the maximal amount of diffusible hydrogen.

Keywords: Quenched & Tempered (Q&T) steel; H2S; electron probe micro-analyzer (EPMA); X-ray diffraction (XRD); hydrogen absorption

1. Introduction

In the oil and gas industry, the presence of H2S gas might induce sulfide stress cracking (SSC). The interaction between steel and the H2S containing surrounding environment also results in the formation of a FexSy corrosion product. Moreover, hydrogen is also generated during this corrosion process and can be absorbed into the material. Consequently, the potential threat for hydrogen induced damage is increased [1,2]. Furthermore, the characteristics of this FexSy surface layer will significantly affect the hydrogen introduction in the material [3,4]. Different FexSy phases can be formed when steel is in contact with an aqueous H2S containing environment. Shoesmith et al. [5] were amongst the first researchers in this field. They showed that the most prominent phase appeared to be mackinawite (tetragonal FeS1−x), which readily cracked and spalled from the metal surface. Other phases that were reported to be formed in H2S environments were cubic FeS, greigite, pyrrhotite, troilite, and pyrite [6]. The corrosion rate and relative amount of the different phases were strongly dependent on the environmental parameters, such as pH, applied current, and the degree of convection. Smith and Pacheco [7] determined the minimum amount of H2S

Metals 2018, 8, 62; doi:10.3390/met8010062 www.mdpi.com/journal/metals Metals 2018, 8, 62 2 of 14 required to produce mackinawite. Therefore, they generated a phase diagram showing the temperature and H2S range in which the different phases occur. Nevertheless, the corrosion product layers have been determined to change over time [8]. Initially, the corrosion product consists of flake-like mackinawite, which grows larger and adheres with longer immersion time [9–11]. Consequently, an increase in the corrosion product film thickness is observed. Multiple studies demonstrated that a non-homogeneous corrosion layer was found on the H2S exposed material and, moreover, the corrosion product films clearly consisted of different layers of iron sulfides [12]. Often, a two-layered corrosion film is observed, with the inner layer being adherent and dense, whereas the outer layer is more porous [11,13]. Generally, the corrosion resistance of a material is strongly dependent on its alloying elements. Molybdenum (Mo) is known to influence the corrosion resistance considerably [14]. Studies have shown that Mo addition to the base material enhanced the formation of the corrosion film, which hence, influenced the material’s corrosion behavior [15,16]. Molybdenum not only generated an improved the corrosion resistance, but it also formed carbides, which can trap diffusible hydrogen, as discussed in [13,17]. This type of hydrogen plays an important role in hydrogen induced failure [1]. Therefore, the presence of carbides is generally considered to be beneficial since they are assumed to trap hydrogen, efficiently removing detrimental diffusible hydrogen from the material. A thorough understanding of the formation of the different corrosion products is of crucial importance since their presence could modify the kinetics of the corrosion process and hydrogen absorption inside the material [3,18]. The role of the corrosion products on the hydrogen/material interaction has been considered by hydrogen permeation in [3,18,19]. In some cases, it was demonstrated that the FeS corrosion products showed a protective nature since it can block the entry of H into the material [3,19]. However, this was not always the case, as this protective nature of the corrosion products was both temperature [3] and pH dependent [18]. These parameters have a large influence on the crystallographic composition of the FeS corrosion products. Other research demonstrated that the FeS is not protective and can even be considered as a reservoir of hydrogen nearby the metallic surface that can generate and sustain hydrogen absorption [20]. Surface corrosion product formation is one of the important factors affecting the corrosion rate and hydrogen uptake in a H2S environment. However, it is still unclear how the base material composition will affect the corrosion products that are generated, and consequently their impact on the corrosion rate. Moreover, the influence of the corrosion product on the hydrogen uptake is still under debate. In this study, two lab cast Quenched & Tempered (Q&T) steel, with different Mo content, were subjected to a sour environment for one month. The evolution of the corrosion products with time was characterized using scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), and X-ray diffraction (XRD). The amount of diffusible hydrogen was determined using hot extraction and the corrosion rate was evaluated by weight loss measurements. Consequently, the corrosion product formation and the impact of the Mo content of the base material on the composition of the corrosion products and hydrogen absorption with time in a sour environment was investigated.

2. Materials and Methods The chemical composition of the two lab cast materials is presented in Table1. Both materials have an identical composition, apart from the Mo content. Subsequent to casting, the materials were hot rolled to an overall thickness reduction of 84% in seven rolling passes. The materials were further austenitized for one hour at 890 ◦C to reach full austenitization and were quenched in water. The low Mo material was then tempered for one hour at two different temperatures, 670 and 700 ◦C, generating two different materials, i.e., low Mo (A) and low Mo (B), respectively. The high Mo material was ◦ tempered for one hour at 700 C. A 0.2 offset yield strength (Rp0.2) of 638 MPa for the low Mo (A) material, 554 MPa for low Mo (B) and 706 MPa for the high Mo material was determined by performing tensile tests. Metals 2018, 8, 62 3 of 14

Table 1. Compositions of the two lab cast materials.

Material C (wt %) Mo (wt %) Cr (wt %) Mn (wt %) Other Elements Low Mo Max. 0.35 0.43 0.89 Max 1.2 Si, P, S High Mo Max. 0.35 1.2 0.88 Max 1.2 Si, P, S

The materials were cut in samples with dimensions of 30 mm length, 10 mm height, and 4 mm thickness. All the sides of the samples were ground and polished to 1 µm. Immersion tests were conducted at 25 ◦C in solution A of the NACE test TM0284 [21]. This consists of 5.0 wt % NaCl (VWR, Leuven, Belgium) and 0.5 wt % CH3COOH (VWR, Leuven, Belgium) in deionized water, produced by a Milli-Q Integral System, saturated with H2S from Air Liquide with a global purity >99.5% (max. impurities 500 ppm CH4, 3000 ppm Carbonyl Sulﬁde (COS)). Prior to saturation, the test solution was deaerated with N2 gas from Air Liquide (purity > 99.99%) for 40 min. After saturation and before the start of every test, the pH of the testing solution was measured to ensure that the value was below 2.8. The pH was measured after every test as well, to check if the value has not increased above 4. Samples were then removed from the vessel after 1, 2, 3, 4, 5, 7, 14, 21, and 28 days. This was performed via special in-house designed shaft, which allowed for the removal of samples without interrupting the test. In order to study the morphology and to determine the chemical composition of the corrosion products, SEM images were taken of the surfaces and cross-sections of several samples, using a JSM-7600F FEG SEM of JEOL (JEOL, Akishima, Tokyo, Japan). Before the analysis of the cross-sections, the sample was ground and polished to 1 µm. To further characterize the features of the corrosion products in more detail, EPMA was performed, using a JXA-8530F set-up of JEOL (JEOL, Akishima, Tokyo, Japan). Additionally, the samples were analysed with XRD (Bruker, Karlsruhe, Germany) using a Siemens D5000 diffractometer with Mo Kα radiation to identify the formed corrosion products. Hot extraction experiments were performed using a G8 Galileo with Thermal Conductivity Detector (TCD) from Bruker (Bruker, Karlsruhe, Germany) at 300 ◦C to determine the amount of diffusible hydrogen, according to the deﬁnition for diffusible hydrogen, as proposed by Wang et al.[22], and frequently applied since [23]. After extracting the samples from the vessel, they were stored in liquid nitrogen to minimise the loss of hydrogen. Prior to the hot extraction measurements, the samples were thoroughly ground, using a rotary tool, to remove all of the corrosion products and rinsed with acetone. The materials were also electrochemically charged to determine the hydrogen content. This cathodic charging was performed for one hour, using a 0.5 M sulfuric acid aqueous solution containing 1 g/L thiourea at a current density of 0.8 mA/cm2. After charging, the materials were rinsed with water and acetone, dried, and the hydrogen content was measured using hot extraction, as described above. Weight loss measurements were performed using the ASTM G1-90 solution, also known as the Clarke solution [24]. This consists of concentrated HCl with 2 wt % Sb2O3 and 5 wt % SnCl2. The samples were scrubbed before they were put in the solution for 30 min, while vigorously stirring, and rinsed with acetone.

3. Results and Discussion

3.1. Corrosion Product Characterization

3.1.1. Surface Topology and X-ray Diffraction (XRD) Measurements The evolution of the corrosion products formed on the surface of the materials was investigated with SEM. Top view images are presented in Figure1, where only the SEM images of the low Mo (A) and high Mo materials are shown, as all of the materials gave very similar results. After three days, only very small nanocrystals were observed on the surface, whereas 14 days contact time clearly resulted in crystal growth of the corrosion products on the surface of all alloys. These crystals Metals 2018, 8, 62 4 of 14 Metals 2018, 8, 62 4 of 13 growfurther further as a function as a function of test duration, of test duration, as seen asafte seenr 28 afterdays. 28Similar days. crystal Similar structures crystal structures were observed were observedby Bai et byal. Bai[25] etin al. their [25 ]corrosion in their corrosion products products investigation investigation on a carbon on a steel carbon in steela hydrogen in a hydrogen sulfide sulﬁdeenvironment. environment.

Figure 1. Top view images of the low Mo (A) material after (a)) 3 days, (b) 14 days, and (c) 28 days and of the high Molybdenum (Mo) material afterafter ((d)) 33 days,days, ((ee)) 1414 days,days, andand ((ff)) 2828 days.days.

XRD measurements measurements allowed allowed further further identification identification of the of thecorrosion corrosion products. products. In Figure In Figure 2, XRD2, XRDpatterns patterns of the of corrosion the corrosion products products that were that formed were formed after 14 after and 14 28 and days 28 in days the test in the solution test solution on the onlow the Mo low (A)Mo and (A) high and Mo high materials Mo materials are shown, are shown,as well as the well XRD as the pattern XRD of pattern the corrosion of the corrosion product productresidue that residue was recovered that was recoveredfrom the bottom from the of the bottom cell after of the the celltest. after On the the surfaces test. On of theboth surfaces materials, of similar corrosion products were found, namely mackinawite (FeS1−x), which was in agreement with both materials, similar corrosion products were found, namely mackinawite (FeS1−x), which was in agreementthe general with assumption the general that assumption mackinawite that mackinawite was the first was formed the first corrosion formed corrosion product product in an inH an2S environment under ambient conditions [5,25,26]. The increasing intensity of the major diffraction H2S environment under ambient conditions [5,25,26]. The increasing intensity of the major diffraction peak of mackinawite, the {001} reflection,reflection, as a functionfunction of the test duration, indicated that a larger degree of mackinawitemackinawite is formedformed inin aa preferentialpreferential orientation.orientation. Additionally, peak intensities were further increasing with longer test durations, whichwhich indicated an increase of the amount of formed mackinawite. When When comparing comparing the the diffraction diffraction spectra spectra of of the the corrosion corrosion products products after after 14 14days days and and 28 28days, days, a peak a peak shift shift of the of the {001} {001} diffraction diffraction peak peak by by±0.1°±0.1 to ◦higherto higher diffraction diffraction angles angles was wasobserved, observed, see seeFigure Figure 2a–c.2a–c. This This corresponds corresponds to toa alattice lattice contra contractionction along along the the c-axis, c-axis, which generatedgenerated additionaladditional strain in the corrosion products, and as such, this will contribute to the poor adhesion of the corrosion products toto thethe substrate.substrate. This was in good agreementagreement withwith thethe observationobservation byby GenchevGenchev etet al.al. [[11].11]. The XRDXRD pattern pattern of of the the corrosion corrosion product product residue, resi whichdue, which was collected was collected after the after test andthe consistedtest and outconsisted of corrosion out of corrosion products products that spalled that spalled of the materialsof the materials during during the entire the entire test, test, was was composed composed of mackinawiteof mackinawite and and troilite troilite (hexagonal (hexagonal FeS). FeS). According According to Baito Bai et al. et [al.25 ],[25] troilite, troilite is generated is generated outside outside the iron-richthe iron-rich mackinawite, mackinawite, because because the unbrokenthe unbroken mackinawite mackinawite layer layer prevents prevents the diffusionthe diffusion of iron of ionsiron towardsions towards the solid-liquid the solid-liquid interface. interface. Since theSince troilite the troilite was generated was generated at the outer at the surface outer ofsurface the material, of the thismaterial, phase this will phase easily will be removed easily be from removed the sample from surfacethe sample during surface the test during and endthe uptest as and residue end atup the as bottomresidue ofat thethe cell.bottom of the cell.

Metals 2018, 8, 62 5 of 14 Metals 2018, 8, 62 5 of 13

FigureFigure 2. 2.X-ray X-ray diffraction diffraction (XRD) (XRD) patterns patterns of of corrosion corrosion products products formed formed ( a()a) after after 14 14 days days on on the the low low MoMo (A) (A) material, material, (b ()b after) after 28 28 days days on on the the low low Mo Mo (A) (A) material, material, (c ()c) after after 28 28 days days on on the the high high Mo Mo material, material, andand (d ()d the) the corrosion corrosion product product residue residue that that was was found found on on the the bottom bottom of of the the cell cell after after testing. testing. And And an an enlargementenlargement of of the the ﬁrst first peak peak of of patterns patterns a-c a-c (inset). (inset).

3.1.2.3.1.2. Cross-Section Cross-Section Analysis Analysis AA two-layered two-layered structure structure was was clearly clearly visible visible when investigatingwhen investigating the cross-sections the cross-sections of the corrosion of the productscorrosion in products Figure3. Thein Figure upper 3. layer The appeared upper layer to be appeared brittle and to wasbe brittle very often and missing.was very The often corrosion missing. productThe corrosion underneath product was underneath more adherent was to more the surface adherent of theto the material. surface Since of the the material. outer layer Since had the a veryouter variablelayer had thickness, a very variable only the thickness, thickness ofonly the the inner thickness layer was of the used inner to evaluate layer was the used corrosion to evaluate product the thickness.corrosion Theproduct inner thickness. layer grew The to inner reach layer a value grew after to 28reach days a value of 54 ±after15, 28 46 days± 16, of and 54 ±73 15,± 4618 µ± m16, forand the 73 low ± 18 Mo µm (A), for lowthe low Mo (B),Mo and(A), low high Mo Mo (B), material, and high respectively. Mo material, The respectively. presence of aThe double presence layer of indicateda double thatlayer two indicated mechanisms that two are mechanisms generating theare corrosiongenerating products. the corrosion Initially, products. the ﬁrst Initially, layer ofthe mackinawitefirst layer of wasmackinawite formed via was a solid-state formed via reaction a solid-state [4,27]: reaction [4,27]:

Fe(s) + (1 − x)H2S → FeS1−x(s) + (1 − x)H2, (1) Fe(s) + (1 − x)H2S → FeS1−x(s) + (1 − x)H2, (1) Therefore, the inner layer was generated by a uniform corrosion process. After this first step of corrosionTherefore, product the innerformation, layer sulfides was generated were also by generated a uniform by corrosion a precipitation process. mechanism After this [27], ﬁrst which step ofcreated corrosion the productupper layer. formation, Mackinawite sulﬁdes formation were also was generated much byfaster a precipitation than the formation mechanism of [other27], whichthermodynamically created the upper stable layer. products, Mackinawite such as formation troilite and was pyrrhotite much faster [4]. than The the precipitation formation ofoccurred other thermodynamicallyaccording to the following stable products, reaction scheme: such as troilite and pyrrhotite [4]. The precipitation occurred according to the following reaction scheme: Fe + H2S + H2O → FeSH−ads + H3O+, (2) − + Fe + H2S + H2O → FeSH ads + H3O , (2) Fe + HS− → FeSH−ads, (3) − − Fe + HS → FeSH ads, (3) FeSH−ads ⇔ FeSH+ads + 2e−, (4) − + − FeSH ads ⇔ FeSH ads + 2e , (4) The adsorbed FeSH+ complex can be directly incorporated in the mackinawite layer or it can + undergoThe adsorbed hydrolysis FeSH to generatecomplex Fe2+ can, as beshown directly in the incorporated reactions below: in the mackinawite layer or it can undergo hydrolysis to generate Fe2+, as shown in the reactions below: FeSH+ads → FeS1−x (s) + xSH− + (1 − x)H+, (5)

FeSH+ads + H3O+ → Fe2+ + H2S + H2O, (6)

Metals 2018, 8, 62 6 of 14

+ − +, FeSH ads → FeS1−x (s) + xSH + (1 − x)H (5) + + 2+ FeSH ads + H3O → Fe + H2S + H2O, (6) Metals 2018, 8, 62 6 of 13 Metals 2018, 8, 62 6 of 13

Figure 3. Scanning electron microscopy (SEM) images of the cross-sections of the low Mo (A) material FigureFigure 3.3.Scanning Scanning electronelectronmicroscopy microscopy (SEM) (SEM) images images of of the thecross-sections cross-sections of of the the low low Mo Mo(A) (A)material material (left), low Mo (B) material (middle), and high Mo material (right). (left(left),), low low Mo Mo (B) (B) material material ( middle(middle),), and and high high Mo Mo material material ( right(right).).

The elemental distribution in the corrosion product layers was visualised by performing EPMA TheThe elementalelemental distributiondistribution inin thethe corrosioncorrosion productproduct layerslayers waswas visualisedvisualised byby performingperforming EPMAEPMA measurements. Figures 4–6 show the Backscattered Electron (BSE) images and elemental mappings measurements.measurements. Figures Figures4 –4–66 show show the the Backscattered Backscattered Electron Electron (BSE) (BSE) images images and and elemental elemental mappings mappings of of the three materials. The two-layered structure of the corrosion products on all of the materials was theof the three three materials. materials. The The two-layered two-layered structure structure of of the the corrosion corrosion products products on on all all of of the the materials materials was was clearly demonstrated as both layers clearly have a different composition. The upper layer consisted clearlyclearly demonstrateddemonstrated asas bothboth layerslayers clearlyclearly havehave aa different different composition. composition. TheThe upperupper layerlayer consistedconsisted mainly of Fe and S enriched with Mo, while the inner layer consisted of Fe and S enriched with Cr mainlymainly ofof FeFe andand SS enrichedenriched withwith Mo,Mo, whilewhile thethe innerinner layerlayer consistedconsisted ofof FeFe andand SS enrichedenriched withwith CrCr and Mo. andand Mo.Mo.

Figure 4. Cross-sectional Backscattered Electron (BSE) image (a) and elemental mappings of (b) Fe, FigureFigure 4.4.Cross-sectional Cross-sectional BackscatteredBackscattered ElectronElectron (BSE)(BSE) imageimage ((aa)) andand elementalelemental mappingsmappings ofof ((bb)) Fe,Fe, (c) Cr, (d) O, (e) S and (f) Mo of the low Mo (A) material after 28 days of immersion in the H2S (c(c)) Cr, Cr, (d ()d O,) O, (e )( Se) and S and (f) Mo (f) ofMo the of low the Mo low (A) Mo material (A) material after 28 after days of28 immersiondays of immersion in the H2S in saturated the H2S saturated test solution. testsaturated solution. test solution.

Metals 2018, 8, 62 7 of 14 Metals 2018, 8, 62 7 of 13

FigureFigure 5. 5.Cross-sectional Cross-sectional BSE BSE image image (a )(a and) and elemental elemental mappings mappings of (ofb) ( Fe,b) (Fe,c) Cr,(c) (Cr,d) O,(d) (e O,) S ( ande) S (andf) Mo (f) of the low Mo (B) material after 28 days of immersion in the H S saturated test solution. Mo of the low Mo (B) material after 28 days of immersion in the2 H2S saturated test solution.

FigureFigure 6. 6.Cross-sectional Cross-sectional BSE BSE image image (a )(a and) and elemental elemental mappings mappings of (ofb) ( Fe,b) (Fe,c) Cr,(c) (Cr,d) O,(d) (e O,) S ( ande) S (andf) Mo (f)

ofMo the of high the high Mo material Mo material after after 28 days 28 days of immersion of immersion in the in H the2S saturatedH2S saturated test solution. test solution.

The quantitative elemental compositions that were obtained from the EPMA measurements are TheThe quantitative quantitative elemental elemental compositions compositions that that were were obtained obtained from from the the EPMA EPMA measurements measurements are are shown in Table 2. The high amount of both Fe and S for the upper layer confirmed the XRD results, shownshown in in Table Table2 .2. The The high high amount amount of of both both Fe Fe and and S S for for the the upper upper layer layer confirmed confirmed the the XRD XRD results, results, which showed that the upper layer was composed of mackinawite. Additionally, a similar amount whichwhich showed showed that that the the upper upper layer layer was was composed composed of of mackinawite. mackinawite. Additionally, Additionally, a similar a similar amount amount of of Mo was present in the upper layer of all materials, which corresponded with the assumption that Moof Mo was was present present in the in upperthe upper layer layer of all of materials, all materi whichals, which corresponded corresponded with wi theth assumption the assumption that thethat the upper layer was created via a precipitation mechanism. Since Mo has a high affinity for S, MoS2 upper layer was created via a precipitation mechanism. Since Mo has a high affinity for S, MoS2 species species will be generated and precipitate together with the FeS1−x. Cr, on the other hand, was not will be generated and precipitate together with the FeS1−x. Cr, on the other hand, was not present in present in the upper layer. These findings were in correspondence with the equilibrium phase thepresent upper in layer. the Theseupper findingslayer. These were infindings correspondence were in correspondence with the equilibrium with phase the equilibrium diagrams of phase both diagrams of both Mo and Cr in Figure 7. These equilibrium phase diagrams were made with HSC Modiagrams and Cr of in both Figure Mo7. Theseand Cr equilibrium in Figure 7. phase Thes diagramse equilibrium were phase made diagrams with HSC were Chemistry, made usingwith HSC an Chemistry, using an estimation of the concentrations of the different elements that were present at a estimationChemistry, of using the concentrations an estimation of the concentrations different elements of the that different were present elements at a that specific were place present in theat a specific place in the solution. Two different situations were simulated, Figure 7A,C show the situation solution.specific place Two differentin the solution. situations Two were different simulated, situations Figure were7A,C simulated, show the Figure situation 7A,C near show the the surface situation of near the surface of the metal, where the S concentration is lower (1 × 10−3 M) and the Fe concentration thenear metal, the surface where of the the S concentrationmetal, where the is lower S concentration (1 × 10−3 M)is lower and the (1 × Fe 10 concentration M) and the Fe is higherconcentration (1 M), is higher (1 M), while Figure 7B,D show the situation of the corrosion product in contact with the whileis higher Figure (17 M),B,D while show theFigure situation 7B,D ofshow the corrosionthe situation product of the in corrosion contact with product the electrolyte in contact Figure with7 Bthe electrolyte Figure 7B confirms the formation of MoS2 as corrosion product that was above mentioned confirms the formation of MoS2 as corrosion product that was above mentioned to be generated by a to be generated by a precipitation mechanism. Figure 7D shows that this is not the case for Cr, which

Metals 2018, 8, 62 8 of 14 precipitation mechanism. Figure7D shows that this is not the case for Cr, which will dissolve into the Metals 2018, 8, 62 8 of 13 solution as Cr3+ and will not precipitate. This conﬁrms the EPMA results, showing no Cr in the upper layerwill dissolve of the corrosion into the products. solution as Cr3+ and will not precipitate. This confirms the EPMA results, showing no Cr in the upper layer of the corrosion products. Table 2. Mean compositions of the upper and inner corrosion layer of the low Mo (A), low Mo (B), and high Mo materials, measured with electron probe micro-analyzer (EPMA). Table 2. Mean compositions of the upper and inner corrosion layer of the low Mo (A), low Mo (B), and high Mo materials, measured with electron probe micro-analyzer (EPMA). Location Fe (wt %) S (wt %) Mo (wt %) Cr (wt %) O (wt %) Mn (wt %) Upper layer LowLocation Mo (A) 60 Fe± (wt2 %) 35S (wt± 1 %) Mo 0.56 (wt± %)0.02 Cr (wt %) - O (wt 2.7 %)± 1.6 Mn (wt %) - Inner layerUpper Low layer Mo Low (A) Mo (A) 58 ± 601 ± 2 35 35± ± 11 0.561.93 ±± 0.020.54 0.92 - ± 0.10 2.7 ± 2.41.6 ± 0.4 - 0.11 ± 0.01 Upper layerInner layer Low Low Mo (B)Mo (A) 65 ± 581 ± 1 38 35± ± 11 1.930.60 ±± 0.540.03 0.92 ± 0.10 - 2.4 ± 2.50.4 ± 0.7 0.11 ± 0.01 - Inner layerUpper Low layer Mo Low (B) Mo (B) 63 ± 651 ± 1 34 38± ± 11 0.601.43 ±± 0.030.22 1.05 - ± 0.11 2.5 ± 4.70.7 ± 0.5 - 0.08 ± 0.06 UpperInner layer layer High Low Mo Mo (B) 64 ± 631 ± 1 36 34± ± 11 1.430.55 ±± 0.220.01 1.05 ± 0.11 - 4.7 ± 1.10.5 ± 0.2 0.08 ± 0.06 - Inner layerUpper High layer MoHigh Mo 57 ± 642 ± 1 31 36± ± 11 0.554.98 ±± 0.011.20 0.99 - ± 0.15 1.1 ± 7.10.2 ± 2.1 - 0.24 ± 0.03 Inner layer High Mo 57 ± 2 31 ± 1 4.98 ± 1.20 0.99 ± 0.15 7.1 ± 2.1 0.24 ± 0.03

Figure 7. TheThe equilibrium equilibrium phase diagram obtained usin usingg HSC Chemistry for the Mo and Cr specimens near the surface (A,C, respectively) and further away from the surface (B,D, respectively). During the test a pH value between 2.8 andand 4.04.0 waswas maintained.maintained.

The inner layer was mainly composed of Fe, S, Mo,Mo, and Cr. For both of the materials, the inner layers of the corrosion products werewere enrichedenriched withwith Mo in comparison to the base materials. A similar observation was made for stainless steels during activeactive dissolution, the corrosion products had a higher Mo content than the bulk material [[28].28]. The Mo amount in the inner layer was remarkably different. TheThe amountamount ofof Mo Mo in in the the inner inner layer layer of of the the high high Mo Mo material material was was more more than than twice twice that that of the of Mothe Mo in the in innerthe inner layer layer of both of both low low Mo Mo materials. materials. Chromium was only present in the inner layer of the the corrosion corrosion products products and in in the the base base material. material. Cr will react selectively with O [[14,29],14,29], the increased amount of O (cf. Figures4 4–6)–6) inin thethe innerinner layerlayer conﬁrmedconfirmed this assumption. Figure 77 againagain supportedsupported thesethese results.results. FigureFigure7 A7A shows shows the the formation formation of MoS2 near the surface. Figure 7C shows that closer to the surface, in the presence of a higher concentration of Fe, Cr2FeO4 will be formed. On the EPMA images (cf. Figures 4–6) a highly elevated concentration of O is visible in a thin layer above the metal surface. This could be a Cr2FeO4 layer that was formed. The formation of an oxide layer in between the mackinawite and the base metal was also

Metals 2018, 8, 62 9 of 14

of MoS2 near the surface. Figure7C shows that closer to the surface, in the presence of a higher concentration of Fe, Cr2FeO4 will be formed. On the EPMA images (cf. Figures4–6) a highly elevated concentration of O is visible in a thin layer above the metal surface. This could be a Cr FeO layer Metals 2018, 8, 62 2 4 9 of 13 that was formed. The formation of an oxide layer in between the mackinawite and the base metal was observedalso observed by Genchev by Genchev et al. et[11]. al. [This11]. Thisoxide oxide layer layer could could cause cause a deterioration a deterioration of the of adhesion the adhesion of the of corrosionthe corrosion products products to the to thesurface, surface, and and as assuch, such, ha haveve an an influence inﬂuence on on the the hydrogen hydrogen uptake uptake and and the corrosion rate [30]. [30].

3.2. Hydrogen Hydrogen Uptake Uptake and Corrosion Rate Evaluation InIn order to evaluate the effect of the corrosion layer on the hydrogen uptake, the amount of diffusible hydrogen present present in in the samples was was meas measuredured as as a a function of the test duration and is summarized in in Figure Figure 88.. AA peakpeak inin thethe amountamount ofof diffusiblediffusible hydrogenhydrogen waswas clearlyclearly observedobserved forfor allall materials. However, However, for the the low low Mo Mo (A) (A) material material th thee amount amount of of diffusible diffusible hydrogen hydrogen was was much much higher, higher, especially after submersion times times of two weeks an andd less. This This could could be be related related to to the amount of dislocations thatthat areare presentpresent in in the the material, material, which which have have been been demonstrated demonstrated to trapto trap hydrogen hydrogen with with low lowactivation activation energy energy [31]. [31]. Since Since the lowthe low Mo Mo (A) material(A) material was was tempered tempered at a at lower a lower temperature, temperature, cf. thecf. theexperimental experimental procedure, procedure, the amountthe amount of dislocations of dislocations will be will higher, be higher, causing causing the amount the ofamount maximal of maximaldiffusible diffusible hydrogen hydrogen to be higher. to be To higher. conﬁrm To thisconfirm statement, this statement, the base the materials base materials were also were charged also electrochemically, without immersion in H S, and the amount of diffusible hydrogen was determined, charged electrochemically, without immersion2 in H2S, and the amount of diffusible hydrogen was determined,as can be seen as in can the be inset seen of Figurein the8 inset. Since of a Figure similar 8. trend Since was a foundsimilar in trend amount was of found diffusible in amount hydrogen of diffusibleafter electrochemical hydrogen after charging, electrochemical it could be concludedcharging, it that could the be difference concluded inpeak that the values difference was intrinsic in peak to valuesthe material was intrinsic and related to the to material their dislocation and related density. to their dislocation density.

Figure 8. AmountAmount of of diffusible diffusible hydrogen, hydrogen, determined determined usin usingg a hot hot extraction extraction technique, technique, present present in the lowlow Mo Mo (A), (A), the the low low Mo Mo (B), (B), and and the the high Mo Mo materi materialal as a function of the test duration and and after after electrochemical ch chargingarging (inset).

After the initial peak, the amount of diffusible hydrogen was decreasing rapidly rapidly for for all all of the materials. This This decline decline could have several origins. On On the the one one hand, hand, cracks cracks could could be be generated generated in the materials through Hydrogen Induced Cracki Crackingng (HIC), rendering the formation of H 2 inin the cracks and a reduction in the amount of diffusible hydrogen. Therefore, all materials were investigated by by cross-sectional analysis in order to detect the presence of cracks. Only in the low low Mo Mo (A) (A) material material that that was in thethe testtest solutionsolution of of 28 28 days, days, a a crack crack was was observed, observed, as as can can be be seen seen in Figurein Figure9. However, 9. However, since since this thisHIC HIC crack crack only only occurred occurred at theat the end end of of the the testing testing period, period, it it not not likely likely to to be bethe the causecause ofof the decline of the amount of diffusible hydrogen, since this al alreadyready started after 1 to 3 days of testing. On the other hand, the decline of diffusible hydrogen could also be caused by a diminishing generation of hydrogen. Hence, the corrosion rate as a function of test duration was investigated.

Metals 2018, 8, 62 10 of 14 other hand, the decline of diffusible hydrogen could also be caused by a diminishing generation of hydrogen.Metals 2018, 8 Hence,, 62 the corrosion rate as a function of test duration was investigated. 10 of 13 Metals 2018, 8, 62 10 of 13

FigureFigure 9. Optical microscope image of a hydrogen inducedinduced crack present in the low Mo material after Figure 9. Optical microscope image of a hydrogen induced crack present in the low Mo material after 2828 daysdays ofof testing.testing. 28 days of testing. The corrosion rate (CR) evolution during the test was followed by performing weight loss The corrosion rate (CR) evolution during the test was followed by performing weight loss measurements, the corrosion rate as a function of the test duration can be seen in Figure 10. The CR measurements, the the corrosion corrosion rate rate as a function of th thee test duration can be seen in Figure 10.10. The CR does not decline as a function of time. This implies that the corrosion products that are generated does not decline as a function of time. This This imp implieslies that that the corrosion products that are generated during the test are not protective against further corrosion, in spite of the constant decrease of the during the test are not protective against further co corrosion,rrosion, in in spite of the constant decrease of the amount of diffusible hydrogen in the material. amount of diffusible hydrogen in the material.

Figure 10. The corrosion rate of the low Mo (A), the low Mo (B), and the high Mo materials as a Figure 10. TheThe corrosion corrosion rate rate of of the the low low Mo Mo (A), (A), the the low low Mo Mo (B), (B), and the high Mo materials as a function of the test duration. functionfunction of of the test test duration. A general schematic overview of the corrosion product generation and its influence on the A general schematic overview of the corrosion product generation and its influence on the diffusibleA general hydrogen schematic content overview is shown of in the Figure corrosion 11. The product first generation generation of and FeS its corrosion influence product on the diffusible hydrogen content is shown in Figure 11. The first generation of FeS corrosion product diffusiblecaused a hydrogenfirst steep contentincrease is of shown the amount in Figure of 11 di. Theffusible first hydrogen generation in of the FeS steel. corrosion After product a few days, caused a caused a first steep increase of the amount of diffusible hydrogen in the steel. After a few days, a adouble first steep layer increase of FeS corrosion of the amount products of diffusible was form hydrogened. The inouter the steel.layer Afterwas generated a few days, via a doublea deposition layer double layer of FeS corrosion products was formed. The outer layer was generated via a deposition ofmechanism FeS corrosion and was products very often was formed.missing, Theindicating outer layerits poor was adhesion. generated The via inner a deposition layer could mechanism be formed mechanism and was very often missing, indicating its poor adhesion. The inner layer could be formed andby the was diffusion very often of Fe missing, from the indicating metal through its poor the adhesion. FeS layer The or by inner the layer diffusion could of be the formed S containing by the by the diffusion of Fe from the metal through the FeS layer or by the diffusion of the S containing diffusionspecies through of Fe from the FeS the layer metal to through the metal. the The FeS latter layer is orassumed by the to diffusion occur, since of the the S original containing flat speciessurface species through the FeS layer to the metal. The latter is assumed to occur, since the original flat surface of the sample was still clearly visible in the corrosion products, as can be observed in Figure 3. In of the sample was still clearly visible in the corrosion products, as can be observed in Figure 3. In addition, the FeS layer growth could be induced by H2S solid state diffusion through the crystalline addition, the FeS layer growth could be induced by H2S solid state diffusion through the crystalline layer or by ionic conduction of S2− and HS−. Both of the situations would imply a decrease in the layer or by ionic conduction of S2− and HS−. Both of the situations would imply a decrease in the amount of diffusible hydrogen, as was experimentally observed. On the one hand, the H2S solid state amount of diffusible hydrogen, as was experimentally observed. On the one hand, the H2S solid state diffusion could create a decrease in the diffusible hydrogen content that is caused by a decrease in diffusion could create a decrease in the diffusible hydrogen content that is caused by a decrease in

Metals 2018, 8, 62 11 of 14 through the FeS layer to the metal. The latter is assumed to occur, since the original ﬂat surface of the sample was still clearly visible in the corrosion products, as can be observed in Figure3. In addition, the FeS layer growth could be induced by H2S solid state diffusion through the crystalline layer or by ionic conduction of S2− and HS−. Both of the situations would imply a decrease in the amount of diffusibleMetals 2018, hydrogen,8, 62 as was experimentally observed. On the one hand, the H2S solid state diffusion11 of 13 could create a decrease in the diffusible hydrogen content that is caused by a decrease in the corrosion rate.the corrosion On the other rate. hand,On the during other hand the ionic, during conduction the ionic of conduction S2− and HS of− S,2 a− and lot of HS hydrogen−, a lot of ishydrogen already releasedis already outside released the outside FeS layer the and FeS will layer be and lost will in the be environmentlost in the environment as H2. Since as theH2. corrosionSince the ratecorrosion does notrate clearly does not decrease clearly asdecrease a function as a offunction time, asof cantime be, as seen can inbe Figureseen in 10 Figure, the ionic10, the conduction ionic conduction of S2− andof S2 HS− and− is HS believed− is believed to be to the be mechanism the mechanism of FeS of formationFeS formation during during these these tests. tests. These These results results are are in agreementin agreement with with the resultsthe results of Huang of Huang et al. [19et ].al. Using [19]. hydrogenUsing hydrogen permeation, permeation, they observed they observed a decrease a indecrease the steady-state in the steady-state hydrogen permeationhydrogen permeation current and current the hydrogen and the diffusivity. hydrogen This diffusivity. was attributed This was to theattributed corrosion to productthe corrosion ﬁlm beingproduct an n-typefilm being semiconductor an n-type semiconductor with an anion selectivity.with an anion As such,selectivity. S2− and As HSsuch,− ions S2− and are attractedHS− ions are to and attracted H+ ions to areand repealed H+ ions are from repealed surface. from surface.

FigureFigure 11.11. SchematicSchematic overviewoverview ofof thethe corrosioncorrosion productproduct formationformation andand thethe inﬂuenceinfluence ofof thisthis corrosioncorrosion productproduct onon thethe hydrogenhydrogen contentcontent insideinside the the metal. metal.

4.4. ConclusionsConclusions InIn thisthis paper,paper, corrosioncorrosion productproduct formationformation andand thethe impactimpact ofof thethe MoMo contentcontent ofof thethe basebase materialmaterial onon thethe composition composition of theof corrosionthe corrosion products products and hydrogen and hydrogen absorption absorption with time inwith a sour time environment in a sour wasenvironment investigated. was investigated. TheThe corrosioncorrosion productsproducts thatthat werewere generatedgenerated inin anan aqueousaqueous solutionsolution saturatedsaturated withwith HH22SS hadhad aa two-layeredtwo-layered structure. The The main main composition composition of of both both layers layers was was mackinawite mackinawite (FeS (FeS1−x),1 however,−x), however, they contained different amounts of alloying elements, molybdenum, and chromium. The upper layer, which was created via a precipitation mechanism, was enriched with Mo. While the inner layer was formed via a uniform corrosion process and was enriched with Mo and Cr. Investigation of the equilibrium phase diagrams for these environments showed that indeed Mo can undergo a deposition reaction, creating MoS2, while Cr will not undergo any deposition, but remain in solution as Cr2+.

Metals 2018, 8, 62 12 of 14 they contained different amounts of alloying elements, molybdenum, and chromium. The upper layer, which was created via a precipitation mechanism, was enriched with Mo. While the inner layer was formed via a uniform corrosion process and was enriched with Mo and Cr. Investigation of the equilibrium phase diagrams for these environments showed that indeed Mo can undergo a deposition 2+ reaction, creating MoS2, while Cr will not undergo any deposition, but remain in solution as Cr . For all three of the materials that were tested, a similar trend in diffusible hydrogen content over time was observed. The amount of diffusible hydrogen reached a peak in the beginning of the test and declined after this ﬁrst peak. The maximum amount of diffusible hydrogen was much higher for the low Mo (A). This was attributed to the higher amount of dislocations present in the material. The decline of the amount of diffusible hydrogen is believed to be due to the presence of the corrosion products. Since the ionic conduction of S2− and HS− is most likely the formation mechanism of these corrosion products, a lot of hydrogen is already released outside the FeS layer and will be lost in the environment as H2. As such, the corrosion product formation can help to prevent hydrogen uptake in the material. The difference in Mo of the two different materials did not seem to have any effect on the corrosion rate or the hydrogen uptake in H2S environment. The biggest advantage of Mo addition to the base material in this research is the higher temper softening resistance that is caused by the higher amount of Mo. This allows for the material to be tempered for a longer amount of time without major loss of strength, causing the dislocation density to be lower and the maximal amount of diffusible hydrogen to decline.

Acknowledgments: The authors wish to thank the Special Research Fund (BOF) (BOF15/BAS/062 and 01P03516) and Ghent University. The authors also acknowledge the technicians and staff working at the sour laboratory at OCAS (ArcelorMittal R&D Gent) and the technical staff from the Department of Materials, Textiles and Chemical Engineering, Ghent University, for their help with the experiments and sample preparation. Author Contributions: Elien Wallaert and Kim Verbeken designed the experiments, Elien Wallaert conceived and performed the experiments; Elien Wallaert, Tom Depover and Kim Verbeken analyzed the data; Kim Verbeken and Iris De Graeve contributed analysis tools; Elien Wallaert wrote the paper. The other authors provided feedback on the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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