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.