SSC PERFORMANCE OF A SUPER 13% Cr MARTENSITIC STAINLESS STEEL FOR OCTG:

THREE-DIMENSIONAL FITNESS-FOR-PURPOSE MAPPING ACCORDING TO PH2S, pH AND CHLORIDE CONTENT

Hervé MARCHEBOIS, Jean LEYER Vallourec Research Center

Corrosion Department 59620 Aulnoye Aymeries

France

Bertine ORLANS-JOLIET Vallourec and Mannesmann Oil & Gas

OCTG Division 59620 Aulnoye Aymeries

France

ABSTRACT The growing demand of energy has resulted in a general need to produce fields with a significant CO2 content. The presence of corrosive species such as CO2, H2S, organic acids, chloride has a pronounced effect on corrosion in producing wells. For the last 25 years, the use of 13% Cr seamless steel tubing has been a cost effective solution for preventing sweet CO2 corrosion. More recently, Super 13% Cr alloys were introduced for improving the resistance to specific forms of corrosion, especially Sulfide Stress Cracking (SSC) at high yield strengths. They combine low carbon content and additions of Ni and Mo in order to achieve its superior properties over conventional 13% Cr steels. This paper summarizes the results of electrochemical and SSC tests that were undertaken to assess the role of chloride content when combined with PH2S and pH in environments where SSC is likely to occur. NACE Uniaxial Tensile tests were performed according to the Fitness-For-Purpose approach recommended in the new edition of NACE MR0175/ISO 15156. The influence of the three parameters led to the creation of a three-dimensional mapping of SSC susceptibility on the whole range of yield strength. This mapping enables the definition of usage domains for Super 13% Cr stainless steel in sour service.

KEYWORDS Super 13% Cr, Super martensitic stainless steel, modified 13% Cr, Sulfide Stress Cracking (SSC), depassivation pH, electrochemistry, fitness-for-purpose (FFP)

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FOREWORD The terms conventional, alloyed or super 13% Cr martensitic Stainless Steels (SS) are commonly used as material denominations in the literature. Conventional 13% Cr martensitic SS refers to AISI 420 like steels with carbon wt. % between 0.15 and 0.22 and chromium wt. % between 12 and 14. Modified or Super 13% Cr martensitic SS include steels with a wt. % of carbon lower than 0.03 and additions of molybdenum and nickel. A wide range of terminology requires clarification with respect to the alloying content especially such as C, Cr, Ni and Mo: “lean” (13Cr-<2.5Ni-<1Mo), “medium” (13Cr-<4.5Ni-<2Mo such as 13-4-1 martensitic SS) and “fat” (13Cr-<6.5Ni->2Mo such as 13-5-2 martensitic SS) have also been proposed for specific applications (see UNS 41425-41426 and 41427). The units adopted in the significant number of publications vary considerably as well: pressure (MPa, bar, atm, psi) but also temperature (°C, °F) or concentrations (mol eq., vol. %, wt. %, ppm or g/L) and so on. This is why, units of data presented in this paper will be expressed essentially in MPa or bar (0.1 MPa = 1 bar) for pressure, °C for temperature and g/L or ppm for concentrations.

INTRODUCTION The growing demand for energy has resulted in a general need to produce fields with a significant CO2 content. The presence of corrosive species (among them CO2, H2S and chlorides) has a pronounced effect on corrosion in producing wells. Carbon dioxide is almost always present and its effect on corrosion is primarily by decreasing the pH of the water phase. Carbon dioxide promotes general corrosion, localized corrosion or formation of hard iron carbonate scales. For the last 25 years, the use of 13% Cr seamless steel pipes has been the most cost effective solution to protect against sweet wet CO2 corrosion. The conventional 13% Cr martensitic stainless steels were developed in the 1970s and gained popularity as downhole tubulars for oil and gas production in sweet environments at elevated temperatures typically up to 130°C or so. They have high strength and excellent corrosion resistance in CO2 containing oil and gas wells. Super 13% Cr Martensitic SS with superior strength and CO2 resistance were developed in the last fifteen years. These steels present a lower carbon content than conventional 13% Cr steels while nickel is proved to be the most efficient alloying element to produce the austenitic structure at elevated temperature leading to the development of the martensitic microstructure. Molybdenum is also used to increase the critical Ac1 temperature as well as to improve the corrosion resistance. Jackman et al.1 summarized the challenges for the steelmaker and the tube maker to resist corrosion in oil and gas wells. Temperature limits for conventional 13% Cr martensitic SS are assumed to be about 150°C. The development of Super 13% Cr martensitic SS (led by the oil operators’ needs) fills the niche between conventional 13% Cr and higher CRA alloy (typically 22% Cr duplex SS). Today, there are two common applications for Super 13% Cr: tubing with superior CO2 corrosion resistance and higher strength than conventional 13% Cr, and equal SSC

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resistance to conventional 13% Cr; and linepine with weldable (low carbon) martensitic SS, replacing 22% Cr solution annealed duplex SS. This paper deals with SSC susceptibility of Super 13% Cr martensitic SS tubing for Oil Country Tubular Goods (OCTG). Experimental data are presented that illustrate some aspects learned over the past ten years, about Super 13% Cr. Uniaxial tensile test in various environments, fractographic investigation and electrochemical tests are shown in order to assess the role of pH, partial pressure of H2S and chloride content leading to the creation of a 3-Dimension mapping of the SSC susceptibility.

LITERATURE SURVEY ON SSC SUSCEPTIBILITY OF SUPER 13 % Cr While the NACE MR0175 / ISO 15156 standard restricts the use of 13% Cr alloys at a maximum H2S partial pressure of 0.1 bar and pH ≥ 3.5, Kermani et al.2 defines a wider domain of non sour service for L80 13% Cr (conventional 13% Cr – 420P type) according to the pH – PH2S diagram. Further works showed that Super 13% Cr materials presented the same / or a slightly higher cracking susceptibility than 13% Cr martensitic SS, taking into account some testing parameters as well as chemical composition and mechanical properties3. Therefore, SSC susceptibility should be considered carefully since martensitic stainless steels display good CO2 corrosion resistance but less cracking resistance than carbon steels required for sour service application. A great number of SSC tests are available in the literature. This section tries to gather the main references according to seamless tube pipes for OCTG. Nevertheless, SSC threshold can be confusing since many of the investigation use different stress levels from 80 percent of the SMYS to 90 (and more) percent of the AYS. Recent investigation mainly displays SSC results performed under 90 percent of the AYS applied stress as required in the NACE MR0175 / ISO 15156 standard according to the Fitness-For-Purpose philosophy. Cayard et al.4 investigated the serviceability of 13% Cr tubulars in oil and gas production environments in 1998. They showed some SSC behavior in the pH – PH2S diagram and presented some corrosion rates versus pH (Figure 1) for both conventional and modified 13% Cr SS. SSC characterization was first investigated in the NACE environment at pH 3.5 with 0.1 bar H2S and 50 g/L NaCl. Scoppio et al.3 investigated SSC behavior of Super 13% Cr steels with NACE TM0177 Uniaxial Tensile test or by Slow Strain Rate Test (SSRT). The authors observed that the Super 13% Cr steel is more resistant to SSC than conventional 13% Cr in slightly sour environments. The main advantage of Super 13% Cr materials is due to the ability to increase strength level up to 110 ksi grade, maintaining SSC resistance substantially equivalent to an 80 ksi conventional 13% Cr grade. Sakamoto et al.5 investigated the effects of environmental factors on SSC resistance of Super 13% Cr materials. They observed that the occurrence of pitting on the passive breakdown of the material, SSC mechanism is principally hydrogen embrittlement as observed on conventional 13% Cr in H2S environments. Taking into account the Cl- content, when the passive layer is stable in chloride free solution, the SSC resistance is far superior to that of conventional 13% Cr SS.

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According to the test condition, Kimura et al.6 showed that the pH stability of the test solution affects the test results on SSC resistance, i.e. guidelines and NACE standard must be respected carefully. The deaeration procedure with nitrogen should be made outside the test vessel in order to avoid oxygen contamination of the specimens. Moreover, the SSC susceptibility of sub-size specimens was higher compared to standard-size specimens. Vitale et al.7 observed that chloride content had a significant effect on the SSC resistance of the 13% Cr alloys. A no fail limit was observed at 7 bar PH2S, pH 3.5 and 100 ppm chloride for Super 13% Cr when heat treated at 99 ksi (YS level) as per the requirements of NACE MR0175 / ISO 15156 standard. Higher strength (YS = 123 ksi) showed significantly less SSC resistance with a threshold at 0.1 bar PH2S, pH 5.5 and 100 ppm chloride or 0.01 bar PH2S, pH 4.5 and 100 ppm chloride (see Figure 2). Nose et al.8 showed that Super 13% Cr material containing 10% or more retained austenite if dispersed very finely in the matrix, led to higher SSC resistance. Kondo et al.9 showed that retained austenite could be effective in the SSC resistance even if the passive film is broken. Indeed the SSC susceptibility could be reduced as a result of the decrease of hydrogen content inside the martensitic matrix. Further papers tried to investigate different test environments than those required by NACE / ISO standard. Two environments should be taken into consideration: formation and condensed waters. Hashizume et al.10 observed that high strength super 13% Cr SS (above 110 ksi) presented good SSC resistance in a milder environment than pH 4.0 with 0.03 bar PH2S for simulating formation water at room temperature. Recent papers or discussions showed that the SSC resistance of Super 13% Cr SS is strongly influenced by the chloride content. Nice et al.11 investigated the SSC susceptibility of a 110 ksi grade Super 13% Cr SS in both high chloride (200,000 ppm) and low chloride (1,000 ppm) solutions. pH vs. PH2S maps of the SSC resistance are displayed on Figure 3.

SSC PERFORMANCE OF SUPER 13% Cr SS IN SLIGHTLY SOUR ENVIRONMENTS: EXPERIMENTAL DATA

The usual environment for SSC testing of 95 ksi Super 13% Cr is PH2S 0.1 bar, pH 3.5 and NaCl 50 g/L (NACE MR0175 / ISO 15156 allows the use of 95 ksi SMYS Super 13% Cr – UNS S414126 – with a max AYS of 105 ksi for PH2S ≤ 0.1 bar and pH ≥ 3.5. The 50 g/L NaCl content is conventionally used in SSC testing). However, past studies have shown that Super 13% Cr could exhibit either complete failures or secondary cracks, in this environment. The second part of the paper deals with the role of both H2S partial pressure and chloride content on the SSC susceptibility of Super 13% Cr SS thanks to electrochemical investigation while exhaustive SSC mappings were drawn on both 95 and 110 ksi grade material according to NACE Tensile tests. Testing parameters Previous tests were performed on a 95 ksi grade Super 13% Cr martensitic SS heat treated at 101 ksi of Yield Strength. Four environments were used (as shown in Table 1) with NACE Uniaxial Tensile or NACE C-ring testing. Solutions were made up of either 1 or 50

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g/L NaCl at pH 3.5 or 4.0 with or without H2S containing environments. Tests were made according to the EFC 17 guidelines: pH values were achieved through 4 g/L sodium acetate (CH3COONa) and hydrochloric acid (HCl) additions. The initial pH were adjusted before H2S saturation and checked after H2S saturation. A Uniaxial Tensile test was also performed under 100% CO2 gas at atmospheric pressure. The samples were electropolished before tested. The test duration was 720 hours according to the standard at 24°C. Note: According to NACE MR0175 / ISO 15156 standard the use of 4 g/L CH3COONa could be detrimental to SSC resistance because of an accelerated corrosion in such a medium. The stress level was 80% SMYS (Specified Minimum Yield Strength) according to the Company’s specifications for 95 ksi grade and not per NACE MR 0175 / ISO 15156 standard. SSC test results are displayed in Table 2. While secondary cracks were sometimes noticed in low pH or high chloride content media in combination with 0.1 bar partial pressure H2S (pH 3.5 with 1 g/L NaCl and pH 4.0 with 50 g/L NaCl), both SSC tests in the less severe media did not exhibit any secondary crack (pH 4.0 with 1 g/L NaCl in 0.1 bar partial pressure H2S mixture and pH 3.5 with 50 g/L NaCl without H2S). Tests #1 and #2 led to secondary cracks at the surface of the specimens. Hence, it is clear that both a low pH (in the range of 3.5 to 4.0) and a high NaCl content (in the range of 1 to 50 g/L) could initiate depassivation / pitting leading to secondary cracks. Fracture analyses of secondary cracks No fractures were observed on specimens tested in pH 3.5 / 1 g/L NaCl and pH 4.0 / 50 g/L NaCl, which remained shiny after testing, but some of them exhibited secondary cracks. Firstly, very thin cracks were observed on the surface of the specimens, but in most cases, severe corrosion types were observed on the cross section after cutting off them in order to see the cracks propagation inside the material. Uniaxial Tensile specimens presented transverse secondary cracks onto both the head radius and the gauge section (see Figure 4). Even though the surface morphology was shiny after a visual examination, the surface also showed heterogeneity at larger magnification. This heterogeneity appeared to be passivated and depassivated spots on the surface. The pictures (longitudinal section transverse to secondary crack) show at low magnification a case of pitting corrosion as well as crack propagations inside the specimen (see Figure 5). On some pictures, the anomalies on the metal look like pitting corrosion and / or depassivation. The case of pitting corrosion is characterized by a small pin hole appearing at the surface of the specimen as well as a drastic degradation within the specimen. This kind of corrosion mechanism is due to the acidification inside the pin hole (local decrease of the pH) leading to anodic dissolution. Moreover, intergranular cracks / grain de-cohesion induced by intergranular corrosion (and sometimes trans-granular cracks / quasi-cleavage) were observed on etched samples – see Figure 6 and Figure 7.

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Investigation of crack initiation Complementary analysis by SEM was undertaken to determine what kind of process was initiated first: corrosion or cracking process? The observations were sometimes difficult to analyze. It is difficult to differentiate prior-to-cracking pitting corrosion or post-cracking crevice corrosion. Moreover, the SEM examination at the border line between brittle and ductile fractures show inter-granular fissures without corrosion products (even though they are less pronounced that in the vicinity of the exposed area to the solution). Figure 5 shows an initial hemispherical pitting followed by cracking within the pit combined with crevice corrosion within the crack. A large secondary crack of UT specimen tested at pH 3.5 with 50 g/L NaCl was mechanically opened in order to analyze the surface of the brittle fracture. Figure 8 shows a view of the investigated fracture before and after mechanical opening. A dark zone on the picture on the right corresponds to the brittle fracture of the specimen. As previously presented, the mechanically opened crack (see Figure 9) shows different zones with regards to the corroded area in contact with the solution, the propagation of the fissure located below the corroded area and a ductile zone due to mechanical opening. While zone “A” shows a large quantity of corrosion on the surface in contact with the testing solution, the morphology of the fracture in zone “B” shows both intergranular cracking and transgranular cleavage. Zone “C” shows a ductile fracture pattern (with presence of dimples) due to mechanical opening. Both micrographic examinations and mechanical openings show the same pattern in all media. However, the degradation seems to be slightly more pronounced on specimens tested in pH 3.5. Drastic corrosion was observed in pits directly in contact with the aggressive solution, then as the inner part of the tested specimens was investigated, crack propagation was observed leading to both trans and pseudo-intergranular fractures combined with inter-granular corrosion. The crack morphologies clearly show both intergranular fractures and transgranular cleavages. With regards to the hydrogen embrittlement phenomenon, it is generally accepted that this kind of fracture leads to both intergranular fissures and transgranular cleavages. The higher the Yield Strength of tested steels in sour media, the higher the content of intergranular fissures is. However, a more important presence of transgranular cleavage patterns than intergranular fissures was observed, while it is generally admitted that the contrary is true for hydrogen embrittlement phenomena (higher content of intergranular fissures in case of high YS material). Apart from a possibly greater pronounced degradation (corroded pattern) on specimens tested at pH 3.5 compared to those tested at pH 4.0, it seems to be very difficult to quantify the role of NaCl in fracture mechanisms. Electrochemical tests (see next chapter) could allow us to improve our knowledge of the effect of pH and chloride content on the SSC resistance of Super Martensitic Stainless Steel used for OCTG.

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SSC PERFORMANCE OF SUPER 13% Cr SS

IN SLIGHTLY SOUR ENVIRONMENTS: INFLUENCE OF pH/PH2S/Cl- Electrochemical investigation Corrosion potential recording. Uniaxial Tensile SSC tests according to the standard NACE TM0177 were carried out in order to evaluate the susceptibility of Super 13% Cr to cracks. Corrosion potentials by means of a reference electrode (Saturated Calomel Electrode: SCE) placed in the same solution as the metal specimen were recorded; Ecorr is measured relative to the potential of the reference electrode. Figure 10 shows the testing configuration of Ecorr measurements during a tensile test (NACE TM0177 Method A). Table 3 summarizes the testing parameters before and after testing. Firstly, a slight difference of final pH in the two solutions can be observed, less than 3.5 and more than 3.8 respectively for solution #1 (pH = 3.5 + 1 g/l NaCl) and solution #2 (pH = 4.0 + 50 g/l NaCl). The pH drift is more pronounced on solution #2 with higher Cl- concentration. Small hydrogen bubbles were observed onto the surface of specimens #3 and #4 (solution #2) while nothing had appeared onto the surface of specimens #1 and #2 (solution #1) after a one week duration. At the end of the test, these bubbles were correlated with the location of pits (which could lead to secondary cracks) observed onto the surface of the specimens. Figure 11 shows the presence of a hydrogen bubble on the surface of specimen #4 during NACE tensile test. The bubbles were first observed after 70 hours of exposure time for specimens #3 and #4, i.e. the specimens tested at pH 4.0 with addition of 50 g/L NaCl (solution #B). Figure 12 presents the evolution of the corrosion potentials of specimens #1 to #4 in the two different environments. Two specimens were tested in each solution in order to be sure of the reproducibility of the measurement. Specimens #1 / #2 and #3 / #4 were respectively tested in solution #1 (pH = 3.5 + 1 g/L NaCl) and solution #2 (pH = 4.0 + 50 g/L NaCl). Each solution lead to different values of corrosion potentials. For both specimens #1 and #2 the corrosion potentials were close to -240 mV/SCE while for both specimens #3 and #4 the corrosion potentials were close to -510 mV/SCE. The corrosion potential measurements gave accurate values regarding the state of the steel in the given solution. However, in order for us to separate more precisely the two media (and the influence of their parameters), the behavior of the material in the solution has to be known, i.e. the location of the different domains of activity of the steel as a function of pH, PH2S and all the species present in the solution. For this reason, polarization curve measurements were carried out. Polarization curves recording Test conditions and measurements. In order to record the potentiodynamic polarization curves, a potentiostat EG&G Instruments model VersaStat II was used with the software Softcorr m352©. Figure 13 shows the test apparatus used for plotting the potentiodynamic polarization curves (I versus E).

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In order to obtain reproducible results, the same procedure and configuration was used for testing Super 13% Cr. The specimens were embedded in a Teflon cylinder leading to a 1 cm2 area exposed to the solution. The surface of the probe was wet ground at 1200 grit polish (SiC). Polarization curves were recorded at 24°C with an electrochemical conventional assembly: glass cell, Saturated Calomel Electrode (SCE) immersed in the solution by means of Luggin and platinum foil electrode as counter electrode. The conditions of nitrogen de-aeration and H2S saturation were the same as those required by the NACE TM0177 standard. During de-aeration and saturation steps, the specimens were kept under cathodic protection to avoid any degradation onto its surface and to remove any passivation layer formed when the specimen was exposed to air. Since these electrochemical tests were carried out without apply stress, a potential effect of CP hydrogen charging was not a concern. Table 4 summarizes the setup parameters used for plotting the potentiodynamic polarization curves by using the potentiostat. Influence of H2S saturation. Since the measured current response can vary over several orders of magnitudes, the log current function versus the potential on a semi-logarithmic chart was plotted. The first tests were performed in only one medium (pH = 3.5 with addition of 1 g/L NaCl) for assessing what is the role of H2S in the degradation process of Super 13% Cr. Two tests were conducted with a 10% H2S – 90% CO2 gas mixture producing a 0.1 bar partial pressure of H2S: - For the first test, the solution was saturated at a high flow rate and a slow continuous bubbling was maintained for the rest of the test (which corresponds to the usual procedure for SSC testing as required by the NACE TM0177 standard). - For the second test, the slow continuous bubbling was applied directly without prior saturation. Figure 14 shows the influence of H2S saturation on Super 13% Cr specimens tested in the same environment. This figure shows the worse corrosion resistance (according to protectiveness and generalized corrosion) on the specimen tested in the “NACE” solution with a complete H2S saturation. As discussed previously, the current flow is related to the rate of the electrochemical reactions, i.e. to corrosion processes: the higher the current, the faster corrosion processes will occur. The red arrows drawn on Figure 14 indicate the passivation stage for which the stainless steel is naturally protected by means of a thin protective layer. As it is generally assumed that under a current density of 10 µA/cm2 the stainless steel is protected and remains in a passive state. It is clear that the H2S saturation, as required by the NACE TM0177 standard, leads to the absence of a true passivation stage in comparison with the case without saturation before testing. The current density is higher and the passivation stage is lower due to H2S content. H2S appears as a depassivation agent for this steel. Influence of pH and NaCl content (PH2S = 0.1 bar) In order to correlate the susceptibility to cracking phenomena on Super 13% Cr, electrochemical experiments in the same media as those tested according to NACE Method

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A were carried out (respectively pH = 3.5 with addition of 1 g/L NaCl and pH = 4.0 with addition of 50 g/L NaCl). Figure 15 presents the polarization curves. Two curves are presented in the environment at pH 4.0 in order to show the reproducibility of such an experiment. Again, the polarization curves show a clear difference between both environments. Even if the passivation stage is higher than 10 µA, a slight increase of the critical current density for passivation can be observed, respectively Icrit = 0.11 mA/cm2 at pH 3.5 with addition of 1 g/L NaCl (solution #1) and Icrit = 0.16 mA/cm2 at pH 4.0 with addition of 50 g/L NaCl (solution #2). In the same way, the width of the passivation stage is smaller in solution #2 than in solution #1. The values of the corrosion potential after one week of exposure are also reported on Figure 15 in order to evaluate how the steel is tested in the NACE Method A environment with regards to the polarization curves recorded without loading. The corrosion potentials (also called “open-circuit” potential, i.e. without any applied current) of the specimens lead to current densities that are high enough for inducing degradations on the surface (I > 10 µA/cm2). The corrosion potential also remain apart from the passivation stage where Super 13% Cr is able to remain spontaneously passivated in a particular medium. Solution #2 was first defined as the most severe environment (see previous section) compared to solution #1: active current density higher as well as passivation stage lower. Nevertheless, when corrosion potentials were compared as a function of the polarization curve shapes, the open-circuit potentials of the specimens when SSC testing (NACE TM0177 Method A) were higher in solution #1 than in solution #2. These values also lead to higher current densities. The presence of corroded areas (some pits and crevices) on the surface of the specimens after electrochemical tests could upset the passivation stage inducing an increase of the current densities as the values of the applied Ecorr is increased. The two media led to different electrochemical behavior as well. While the Super 13% Cr SS behaved according to the active dissolution domain in solution #2 (pH 4.0 + 50 g/L NaCl), it behaved according to a non stable passive dissolution domain in solution #1 (pH 3.5 + 1 g/L NaCl). Specimens tested at pH 4.0 + 50 g/L NaCl showed earlier pitting than specimens tested at pH 3.5 + 1 g/L NaCl. Taking into account that alloying elements of stainless steels should be chosen according to the electrochemical state in a given environment (different solubility in solution #1: Fe-Fe2+ / Fe-Fe3+ [all soluble] respectively, Ni-Ni+ [soluble] vs. Ni2+/4+ [oxide] respectively, Cr-Cr3+ vs. Cr-Cr6+ [all soluble] respectively and in solution #B: Mo-Mo3+ vs. Mo-Mo6+ [all soluble] respectively), it seems very important to investigate the threshold when Super 13% Cr remains in the active or in a non stable passivation domain leading to dissolution. In other words, it means that the SSC susceptibility could be assessed according to alloying elements taking into account its electrochemical dissolution domains (among other parameters such as hydrogen adsorption / uptake and applied stress influence), even if Super 13% Cr showed secondary cracks in both the environments. Electrochemistry according to pH – PH2S diagram. For Super 13% Cr characterization with regard to the SSC susceptibility, two environments should be taken into consideration: formation and condensed waters. While formation water originates in the geological

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formations in oil and gas reservoirs, condensed water is produced in the tubes as the pressure and the temperature decrease. As mentioned in the literature survey a typical formation water extracted with the oil and gas will contain chloride ions with concentrations up to about 200,000 ppm with Na and/or Ca carbonates up to 100 ppm leading to pH values close to 4.5 and a typical condensed water will contains about 1,000 ppm chloride without bicarbonates leading to a pH value between about 3.2 and 3.5 for the condensate phase. These typical pH values of the water phase are of course correlated with specific CO2 and H2S partial pressures. This is why, in order to observe an evolution of electrochemical parameters, tests were performed according to the environments “pH 3.5 – 0 up to 100 g/L NaCl (condensed water type)” and “pH 4.5 – 0 up to 100 g/L NaCl (formation water type)” at 0, 0.1 and 1 bar partial pressure H2S (see Table 5). Electrochemical tests were carried out according to the same procedure as the one used for previous tests. These results will be compared to the mapping of the conventional SSC NACE “A” tests in order to assess the role of corrosion processes in the cracking phenomena of Super 13% Cr. Figure 16 (displayed on next page) shows that chloride content seemed to be very influential at environments with low pH plus H2S, while tests performed at pH 4.5 did not exhibit significant evolution of the critical current densities. The influence of the chloride concentration seems very important when combined with H2S at low pH. The critical current densities recorded at pH 3.5 did not present significant variations what ever the H2S partial pressure (0.1 or 1 bar). Indeed, these electrochemical tests were carried out without applied stress on the specimens; it means that the effect of load, hydrogen adsorption on the surface of the specimen and formation/stability of the iron sulfide layer were not investigated. Only the anodic dissolution of the metal was taken into account related to the corrosion behavior. Electrochemical measurements under loading could be combined with hydrogen permeation or absorption tests according to the SSC susceptibility of Super 13% Cr SS as well. This could allow to rank the influence of H2S partial pressure thanks to electrochemical measurements.

SSC PERFORMANCE OF SUPER 13% Cr SS IN SLIGHTLY SOUR ENVIRONMENTS: SSC MAPPING

ACCORDING TO NACE UNIAXIAL TENSILE TESTS pH – PH2S – NaCl diagrams according to SSC susceptibility. NACE Uniaxial Tensile tests were performed in various environments according to Fit-For-Purpose philosophy as Super 13% Cr SS did not pass the SSC tests in the EFC 17 guidelines / NACE MR0175 – ISO 15156 environments. NACE Uniaxial Tensile tests were performed according to the EFC 17 testing conditions modified with 1 and 100 g/L NaCl instead of the 50 g/L specified in the guidelines. Specimens were stressed under 90% AYS on a 110 ksi Grade Super 13% Cr heat treated at 124 ksi and on a 95 ksi Grade Super 13% Cr heat treated at 109 ksi. Figure 17, Figure 18, Figure 19 and Figure 20 show 2-Dimension diagrams generated by NACE Uniaxial Tensile tests on both C95 and C110 grade range Super 13% Cr material, with respectively 1 g/L and 100 g/L NaCl. Solid circles refer to tests which passed the 720

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hours in the corrosive environment without any cracks or secondary cracks, while crossed circles refer to tests which did not pass the 720 hours in the corrosive environment. Figure 21 is a combination of the 2-Dimension diagrams in order to focus on the influence of chloride content when assessing the SSC susceptibility of Super 13% C stainless steels. An slight evolution of the SSC threshold is brought out between C95 (YS = 109 ksi) and C110 (YS = 124 ksi) grade range materials.

CONCLUSION Martensitic stainless steels have been widely used for tubing and casing applications because they present high strength and excellent corrosion resistance in CO2 containing oil and gas wells. Super martensitic stainless steels with superior strength and CO2 resistance were developed in the last decade: this is the family of the so-called Super 13% Cr. They combine low carbon content and additions of Ni and Mo leading to superior CO2 corrosion resistance over 13% Cr with higher strength while keeping SSC resistance. The Super 13% Cr materials exhibit significantly better corrosion resistance than the standard grades. This is attributed in the various papers of the literature to the lower carbon content (leading to lowering the carbide content) and to the beneficial effect of the alloying elements such as Mo, Ni, and so on. The corrosion rates of Super 13% Cr are generally ten to twenty times lower than those of the conventional 13% Cr. However, the pitting rates (measured as the pit depths) are reduced by a smaller margin, typically two to ten times. The limit of use proposed in the literature considering corrosion resistance at high temperature is 180°C at pH ≥ 3.5 in high chloride environments (up to 200 g/L NaCl). Nevertheless, at higher pH and/or lower chloride content, acceptable corrosion resistance might be obtained at higher temperature. At lower temperature (close to room temperature), Sulfide Stress Cracking may be the limiting factor for the Super 13% Cr. This paper mainly focuses on SSC susceptibility and especially on the influence of chloride content. Both micrographic examinations and mechanical openings show the same pattern in all media. Corrosion patterns were observed in pits/crevices directly in contact with the aggressive solution, then, as the inner part of the tested specimens were investigated, crack propagations leading to both trans and pseudo-intergranular fracture were observed. Electrochemical tests clearly showed the influence of both H2S saturation and chloride content when recording the polarization curves in different media. The two testing media (pH 3.5 + 1 g/L NaCl and pH 4.0 + 50 g/L NaCl) led to different electrochemical behavior. While the Super 13% Cr SS behaved according to the active dissolution domain in the first test solution, it behaved according to a non stable passivation domain in the second one. Electrochemical tests in formation and condensed water solutions allowed the determination of the evolution of the critical current density (Jc) according to pH / PH2S and Chloride content. Electrochemical tests performed on Super 13% Cr showed that chloride content seemed to be very influential at low pH (e.g. 3.5) H2S environments, while tests performed at higher pH

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(e.g. 4.5) did not exhibit significant evolution of the critical current densities. The influence of the chloride concentration seems very important when combined with H2S at low pH. 2-Dimension pH / PH2S SSC mappings in formation and condensed water like solutions were drawn on both 95 ksi and 110 ksi grade Super 13% Cr in the Yield Strength range of 95 to 125 ksi. These investigations with regards to the influence of chloride content among pH and PH2S led to the creation of a 3-Dimension mapping of both a 95 ksi and a 110 ksi grade Super 13% Cr SS showing that the steel can be used in Fit-For-Purpose environments according to the chloride content. Super 13% Cr martensitic stainless steels heat treated in the both 95 ksi and 110 ksi grade range led to the definition of the approximately the same domains of SSC susceptibility. Nevertheless, while the SSC threshold is strong on 110 ksi grade Super 13% Cr, secondary cracks and pits are observed after testing on 95 ksi grade Super 13% Cr.

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Table 1: Test environments used in order to assess the role of both H2S and Cl-

Tests pH NaCl (g/L) PH2S (bar) PCO2 (bar) #1 3.5 1 0.1 0.9 #2 4.0 50 0.1 0.9 #3 4.0 1 0.1 0.9 #4 3.5 50 0 1

Table 2: Cracking resistance results compiling in various environments

Test method pH NaCl

(g/l) PH2S (bar) Applied YS SSC results

C-Ring +

Uniaxial Tensile tests

3.5 1 0.1 No failures but

Secondary cracks

C-Ring +

Uniaxial Tensile tests

4.0 50 0.1 No failures but

Secondary cracks

Uniaxial Tensile tests 4.0 1 0.1

No failures

No cracks

Uniaxial Tensile tests 3.5 50 0

80% SMYS

No failures

No cracks

Table 3: NACE tensile tests + corrosion potential in two different environments

Environment #1 (pH = 3.5 + 1 g/L NaCl) #2 (pH = 4.0 + 50 g/L NaCl) Testing specimens #1 #2 #3 #4 Testing conditions: Final pH [H2S] (ppm)

3.43 340

3.45 340

3.83 357

3.90 340

H2S partial pressure 0.1 bar in balance with 0.9 bar CO2 Constant load 85% S.M.Y.S. Duration of the tests 1 week Corrosion potential at the end of the test -220 mV/SCE -260 mV/SCE -480 mV/SCE -530 mV/SCE

Observations Surface still shiny after testing No secondary cracks

Surface still shiny after testing Presence of secondary cracks Apparition of H2 bubbles onto the surface of both specimens

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Table 4: Setup of the electrochemical experiments (potentiodynamic polarization)

Table 5: Testing environment for electrochemical investigation

Partial Pressure H2S (bar) 0 0.1 1

pH 3.5

pH 4.5

0 1 g/L NaCl 100

Setup Values Observations

Conditioning time 4700 s Controls the time that the condition potential is applied to the cell to protect the working electrode

Conditioning potential -1 V/SCE Cathodic potential to keep the working electrode protected during de-aeration and H2S saturation steps

Sweep rate 0.3 mV/s Relative to the scan rate and the scan increment

Initial potential -1.2 V/SCE Potential at which the scan begins

Final potential When current densities reached 1 mA/cm2 Potential at which the scan ends

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Figure 1: Corrosion rate versus system pH

for both conventional and modified Super 13% Cr (from Corrosion 98 – Paper #112)

Figure 2: SSC susceptibility of Super 13% Cr

(from Corrosion 99 – Paper #584)

1,000 ppm chloride 200,000 ppm chloride

Figure 3: SSC domain of a Super 13% Cr grade 110 tested at various chloride contents

(from Corrosion 2005 – Paper #05091)

15

Figure 4: Visual examination of UT specimens onto the surface (M × 9.5)

(M × 50) (M × 500)

Figure 5: Micrographic examination of a corrosion pits as well as crack propagation

Figure 6: Crack details of tensile specimen #10383 after Villela etching (M × 500)

16

Figure 7: Intergranular fissures / corrosion observed onto the fracture surface by means of SEM after mechanically opening the crack (M× 800)

(M × 13) (M × 16)

Figure 8: Secondary crack transverse to the edge

before and after mechanically opening

17

General view of mechanically opened crack

(A: corroded area / B: crack propagation / C: mechanically opened ductile fracture)

(M × 120) (M × 380)

Zone A: General view of mechanically opened crack

(M × 300) (M × 800)

Zone B: Brittle fracture with trans and intergranular cracking

(M × 450)

Zone C: Ductile fracture of mechanically opened crack showing dimples

Figure 9: SEM investigation of different zones of mechanically opened secondary crack

A

B

C

18

Figure 10: Measurement of Ecorr from a metal/solution system:

application to NACE TM0177 tensile tests

Figure 11: Formation of a hydrogen bubble on the surface of specimen #4

Hydrogen bubble

19

-600

-500

-400

-300

-200

-100

00 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Time (hour)

Ecor

r (m

V)

#1 (pH 3.5 / 1 g/l NaCl)#2 (pH 3.5 / 1 g/l NaCl)#3 (pH 4.0 / 50 g/l NaCl)#4 (pH 4.0 / 50 g/l NaCl)

Figure 12: Corrosion potential versus time recorded simultaneously

during NACE TM0177 Tensile tests

Figure 13: Test apparatus for electrochemical measurements.

- 240 mV after equilibrium

- 510 mV after equilibrium

Solution #1Solution #2

20

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

-1.20

E+03

-1.10

E+03

-1.00

E+03-90

0-80

0-70

0-59

9-49

9-39

9-29

9-19

9 -99

Potential (mV)

Log

I (m

A)

pH 3.5 NaCl 1 g/l - Test 1

pH 3.5 NaCl 1 g/l - Test 2

10 µA

0.1 bar PP H2Safter H2S saturation

0.1 bar PP H2Sw ithout H2S saturation

Figure 14: Polarization curves of Super 13% Cr: Influence of the H2S saturation of the solution

-4

-3

-2

-1

0

1

2

-1.20

E+03

-1.10

E+03

-1.00

E+03-90

0-80

0-70

0-59

9-49

9-39

9-29

9-19

9 -99

Potential (mV)

Log

I (m

A)

pH 3.5 NaCl 1 g/l - Test 2

pH 4.0 NaCl 50 g/l - Test 1

pH 4.0 NaCl 50 g/l - Test 2

10 µA

0.1 bar PP H2S after H2S saturation

Figure 15: Polarization curves of Super 13% Cr in two different environments

Passivation area ~ resistance

-510 mV -240 mV

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Figure 16: 3D NACE – ISO diagram vs. Critical Current Density (Jc) on Super 13% Cr

0 g/L NaCl

1 g/L NaCl

100 g/L NaCl

1 0.1 0

4.54.5

4.53.5

3.53.5

0

200

400

600

800

1000

1200

Jc (µA/cm²)

P H2S (bar)

pH

22

Figure 17: 2-Dimension SSC susceptibility diagram of a 110 ksi Grade Super 13% Cr SS (Y.S. = 124 ksi)

with 1 g/L NaCl (specimens were stressed at 90% AYS)

Figure 18: 2-Dimension SSC susceptibility diagram of a 110 ksi Grade Super 13% Cr SS (Y.S. = 124 ksi)

with 100 g/L NaCl (specimens were stressed at 90% AYS)

23

Figure 19: 2-Dimension SSC susceptibility diagram of a 95 ksi Grade Super 13% Cr SS (Y.S. = 108 ksi)

with 1 g/L NaCl (specimens were stressed at 90% AYS)

Figure 20: 2-Dimension SSC susceptibility diagram of a 95 ksi Grade Super 13% Cr SS (Y.S. = 108 ksi)

with 100 g/L NaCl (specimens were stressed at 90% AYS)

24

Figure 21: 3-Dimension SSC susceptibility diagram of a Super 13% Cr SS

(specimens were stressed at 90% AYS)

SSSSSSCCC rrreeesssiiissstttaaannnccceee

NNNooo SSSSSSCCC rrreeesssiiissstttaaannnccceee

25

REFERENCES 1 P. Jackman, H. Everson, Development of new martensitic stainless steel for OCTG: the challenges for the steelmaker and the tubemaker, Corrosion 95, Paper #89, 1995 2 B. Kermani, D. Harrop, M. Truchon, J-L Crolet, Experimental limit of sour service for tubular steels, Corrosion 91, Paper #21, 1991 3 L. Scoppio, M. Barteri, G. Cumino, Sulfide Stress Cracking resistance of super martensitic stainless steel for OCTG, Corrosion 97, Paper #23, 1997 4 M. Cayard, R. Kane, Serviceability of 13Cr tubulars in oil and gas production environments, Corrosion 98, Paper #112, 1998 5 S. Sakamoto, K. Maruyama, H. Asahi, H. Kaneta, Effects of environmental factors on SSC property of modified 13Cr steels in oil and gas fields, Corrosion 97, Paper #21, 1997 6 M. Kimura, Y. Miyata, T. Toyooka, Y. Nakano, F. Murase, K. Takojima, Effect of test method on SSC performance of modified 13Cr steel, Corrosion 98, paper #114, 1998 7 D. Vitale, Effect of hydrogen sulfide partial pressure, pH and chloride content on the SSC resistance of martensitic stainless steels and martensitic precipitation hardening stainless steels, Corrosion 99, Paper #584, 1999 8 K. Nose, H. Asahi, Effect of microstructure on corrosion resistance of a martensitic stainless linepipe, Corrosion 2000, Paper #19, 1997 9 K. Kondo, H. Amaya, T. Ohmura, K. Moriguchi, M. Ueda, Effect of cold work on retained austenite and on corrosion performance in low carbon martensitic stainless steels, Corrosion 2003, Paper #03094, 2003 10 S. Hashizume, K. Masamura, K. Yamazaki, Performance of high strength super 13% Cr martensitic stainless steels, Corrosion 2003, Paper #03095, 2003 11 P. Nice, J. Martin, Application limits for super martensitic and precipitation hardened stainless steel bar-stock materials, Corrosion 2005, paper #05091, 2005

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