Pitting Corrosion Resistance of CrMn Austenitic Stainless Steel in ...

CORROSION SCIENCE SECTION

Pitting Corrosion Resistance of CrMn Austenitic Stainless Steel in Simulated Drilling Conditions— Role of pH, Temperature, and Chloride Concentration Helmuth Sarmiento Klapper,‡,* John Stevens,* and Gabriela Wiese*

ABSTRACT Austenitic stainless steels have been used for many years in components for drilling equipment. These stainless steels are predestinated to meet the demanding requirements in terms of mechanical, magnetic, and chemical properties necessary for the latest drilling technologies. Drilling conditions might become severe in terms of corrosion because of the combination of temperature and high salinity of drilling fluids. In this research work the pitting corrosion susceptibility of 19%Cr21%Mn-Mo-N austenitic stainless steel has been investigated using electrochemical methods. Service conditions commonly observed during drilling operations have been simulated. Tests were performed in buffer solutions of pH 8, 10, and 12 containing chloride concentrations of 0.5 M, 2.25 M, and 4 M, at room temperature, 88°C, and 150°C. A detrimental effect of increasing temperature and chloride concentration on the pitting corrosion resistance of the material has been determined. On the other hand, an increase of the pH above 8 reduced the susceptibility of the CrMn stainless steel to pitting corrosion, especially at 150°C. KEY WORDS: buffer solutions, drilling, pitting corrosion, stainless steels

INTRODUCTION For more than 30 years, strain-hardened CrMn austenitic stainless steels having different nickel, nitrogen, and molybdenum contents have been specifically Submitted for publication: February 15, 2013. Revised and accepted: May 22, 2013. Preprint available online: June 5, 2013, doi: http://dx.doi.org/10.5006/0947. ‡ Corresponding author. E-mail: [email protected]. * Baker Hughes, Baker-Hughes-Strasse 1, D-29221 Celle, Germany.

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developed for drilling equipment.1-2 Besides nonmagnetic properties allowing measurement while drilling (MWD) and logging while drilling (LWD) technologies, these stainless steels meet the challenging requirements in terms of mechanical and chemical properties necessary in drilling applications. They have yield strengths typically above 1,000 MPa (145 ksi) maintaining ductility and appropriate toughness, good anti-galling properties, and excellent wear and corrosion resistance. Non-magnetic stainless steels replaced NiCu alloys, which have been widely used for years for heavy weight drill collars (HWDC), because of their better mechanical properties and price-performance ratio. From field experience it is well known that CrMn stainless steels used in drilling equipment are, nevertheless, susceptible to two different forms of corrosive damage in service. These materials might undergo pitting corrosion2 or environmental-assisted cracking, e.g., stress corrosion cracking (SCC)1,3 and corrosion fatigue (CF).4-5 However, the genesis of SCC and CF cracks, in most cases, is related to the occurrence of localized corrosion.2,4-6 Pitting corrosion, therefore, can also be considered as a precursor of cracking by SCC and CF. It becomes, therefore, a significant limiting factor to the service life of drilling equipment made of CrMn stainless steels. Components in the drill string made of nonmagnetic stainless steels like the HWDC and the MWD/LWD tools are during service, in contact with the drilling fluid. Drilling fluids can be water- or oilbased but sometimes gaseous drilling fluids are also used. Water-based muds (WBM) are commonly used for oil and gas wells as well as for geothermal drilling;

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TABLE 1 Chemical Composition of the Investigated Austenitic Staniless Steel (wt%) C

Cr

Mn

Ni

Mo

N

Si

P

0.04 18.6 21.2 1.8 0.5 0.7 0.2 0.02

most of those muds are a mixture of bentonite clay with fresh water and some polymer additives.7-8 WBM often include large amounts of chloride ions, whether from the base water, as a result of salt additions to adjust some properties in the drilling fluid, or pickup from formation waters. Chloride concentrations can go up to hundreds of thousands of parts per million (ppm) depending on the location and type of well to be drilled. During the operation of drilling tools, the drilling fluid flows down the drill pipe, through nozzles in the drill bit, and back up the annulus between the drill string and well bore wall. Inside the drill string the temperature approximates to its temperature in the annulus at the same depth, and both are close to the formation temperature. Operation temperatures above 80°C are not unusual. Therefore, depending on the depth and type of well, CrMn stainless steels might become in contact with a hot corrosive fluid leading to a high risk of pitting corrosion. An appropriate corrosion protection for downhole drilling equipment is ensured by maintaining the pH of the drilling fluid, usually between 9 and 11, but higher values are not uncommon. Additions of lime, sodium, or potassium hydroxides constitute the traditional method of increasing alkalinity in WBM during service. High pH is, in some cases, also necessary to control the effect of some well bore corrosive gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S).8 Many research works have been devoted in the past to the influence of environmental variables on the pitting corrosion resistance of CrNi austenitic stainless steels having different molybdenum contents. Ruijini, et al., observed from investigations on Type 904L (UNS N08904)(1) stainless steel that the pitting corrosion potential (Epit) of this material linearly increases with the increase of pH from 1 to 11.9 Furthermore, it is widely accepted that in the range from 25°C to 100°C a linear relationship exists between the Epit of CrNi stainless steels and the temperature.10 The higher the temperature, the lower the measured Epit has been. Investigations have shown that the morphology of the surface damage changes from deep and localized pits formed mainly at manganese sulfide (MnS) inclusions at temperatures below 150°C to a broad and shallow form of attack on austenitic stainless steels in chloride-containing solutions at higher temperatures.11 Manning and Duquette,12 and (1) UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

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later Wang, et al.,13 confirmed that the passive film on CrNi stainless steels in aqueous environments at high temperature becomes orders of magnitude thicker, more porous, and, as a consequence, less protective than those formed at atmospheric conditions. Regarding the effect of chloride ions on pitting corrosion susceptibility of austenitic stainless steels, Wang, et al., established a linear relationship between the pitting potential of Type 304 (UNS S30400) stainless steel and the chloride concentration in sodium chloride (NaCl) solutions at different temperatures. The corresponding slopes at 150°C and 200°C were lower than those observed in the range between 20°C and 80°C. The higher the chloride concentration, the lower the measured Epit was at all temperatures. When compared with the large amount of publications dealing with CrNi stainless steels, fewer efforts, however, have been focused on the pitting corrosion resistance of CrMn austenitic stainless steels. Collins introduced the use of the critical pitting temperature (CPT) as an alternative for the qualification of non-magnetic stainless steels in terms of pitting corrosion resistance for drilling applications.2 He observed a good correlation between the results from CPT tests and the performance of different CrMn stainless steels in the field. As a part of the studies regarding environmentalassisted cracking performance of austenitic stainless steels, Vichtyl, et al.,5 and Holzleitner, et al.,14 evaluated, by means of potentiodynamic polarization tests, the pitting corrosion resistance of some CrMn stainless steels in hot calcium chloride (CaCl2) and magnesium chloride (MgCl2) brines. Nevertheless, up to date, there is no documented systematic investigation on the pitting corrosion resistance of CrMn stainless steels at conditions relevant for the drilling industry. In addition, even if both have austenitic microstructure differences in their chemistry, the assumption that CrNi stainless steels would perform similarly to CrMn stainless steels is difficult to make. To improve this lack of understanding, electrochemical and surface examinations have been conducted to study the correlation between the pitting corrosion susceptibility of one CrMn stainless steel and environmental factors inherent to WBM such as pH, chloride concentration, and temperature.

EXPERIMENTAL PROCEDURES The pitting corrosion resistance of the austenitic stainless steel having the chemical composition included in Table 1 has been evaluated by means of electrochemical methods. For simplicity, the material will be referenced as CrMn stainless steel in the manuscript. Test environments have been typically NaCl solutions with different chloride concentrations, 0.5 (17.7 g/L Cl–), 2.25 (79.8 g/L Cl–), and 4 M (141.8 g/L Cl–). The tests were conducted at room temperature ([RT] 22±2°C), 88°C, and 150°C. The temperature of

CORROSION—NOVEMBER 2013


the solution was maintained at 88±2°C using an external thermostat. Electrochemical measurements at 150±5°C were performed in a fully controlled autoclave. The pH of the test solutions has been adjusted to 8, 10, and 12 using buffer solutions. The studied pH range is relevant to the conditions that would be experienced during drilling operations. The pH of drilling fluids is typically expected to be in the range of pH 9 to 12. Nevertheless, in the event of loss of pH control, the pH can drift to lower values. Therefore, solutions having pH 8 also have been considered in this study. Buffer solutions, in particular those with pH 8 and pH 10 at 88°C and 150°C, have been developed and tested specifically for this study. Their chemistry and corresponding stability measured simultaneously during selected electrochemical tests are presented in Table 2 and in Figure 1, respectively. When different from 0.5 M, the chloride concentration was adjusted to 2.25 M or 4 M by additions of NaCl. At RT and 88°C, pH measurements have been conducted with a glass electrode. The alkaline error from the high concentration Na+ ions in the buffer solutions containing considerable amounts of NaCl, typically above 0.5 M, at elevated temperatures, which strongly affects pH measurements with glass electrodes, has been considered and corrected.15 For pH measurements at 150°C, a zirconium dioxide-based pH probe in combination with a high-temperature, high-pressure silver/silver chloride (Ag/AgCl) reference electrode has been used. The maximum allowable deviation in pH of the buffer solutions was ±0.5 units from the desired pH value. In general, the developed buffer solutions showed outstanding pH stability over the entire measuring time. L-shaped specimens having a measuring surface of ca. 845 mm2 were cut from rod material. They were treated by grinding consecutively with silicon carbide (SiC) emery papers up to 360 grit. For the electrochemical measurements at temperatures up to 88°C, a conventional three-electrode configuration including a potassium chloride (KCl)-saturated Ag/AgCl reference electrode (200 mV vs. standard hydrogen electrode [SHE]) and a titanium dioxide (TiO2)-covered Ti/TiO2 counter electrode was used. For measurements at 150°C, a special Ag/AgCl reference electrode (RE) for high-temperature applications was used. Prior to the start of each experiment, the freshly prepared test solution was purged 30 min with N2. Then, the open-circuit potential (OCP) was monitored for 1 h, and subsequently, the cyclic potentiodynamic polarization test was conducted. The potential scan started at a potential 100 mV more negative than the OCP in the anodic direction using a scan rate of 0.2 mV/s. The Epit was defined as the potential at which the current density reaches 100 µA/cm2. At this point the direction of the polarization scan was switched in the cathodic direction to assess the repassivation ability of the stainless steel. Correspondingly, the repassivation potential (Erp) has been defined as the potential

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TABLE 2 Typical Chemical Composition of Buffer Solutions Used for Electrochemical Tests Chemical Composition (M) Cl– Na2B4O7· (M) NaCl 10H2O NaOH KCl HCl

pH

T (°C)

8

RT 0.50 0.485 0.0125 150 0.50 0.500 0.0100

10

RT 0.50 0.500 0.0125 150 0.50 0.450

0.023 0.132 0.05

12

RT

0.041

0.50

0.450

0.015

0.05

FIGURE 1. The pH stability of test solutions measured during electrochemical measurements at different temperatures.

at which the current density returned to 100 µA/cm2. Pit density and size were additionally determined using the criteria established in ASTM G46.16 Each experimental condition was evaluated three times (two replications) in separate experiments. The morphology of corrosion pits produced during the electrochemical tests was documented by optical and scanning electron microscopy.

RESULTS Table 3 includes the electrochemical potentials and corrosion current densities (icorr) of the CrMn stainless steel obtained at different experimental conditions. The electrochemical potential of the material after 1 h exposure at constant temperature was considered as the OCP. The values included in Table 3 correspond to the average and standard deviation of three points obtained in separate experiments. Two experimental conditions were tested at 88°C one time for statistical trend analysis. Experimental results from electrochemical tests showed, in general, excellent reproducibility. As shown in Figure 2, pits formed during the cyclic potentiodynamic polarization tests on the tested CrMn stainless steel were distributed at the surface and mostly surrounded with corrosion products. The

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Table 3 Electrochemical Parameters and Pit Morphology of CrMn Stainless Steel in Simulated Drilling Environments pH

T (°C)

Cl– OCP Epit Erp icorr Pit Pit (M) (mVSHE) (mVSHE) (mVSHE) (μA/cm2) Density(A) Size(A)

8 RT

0.50 4.00

–18±5 –46±17

542±16 242±20

454±28 107±32

0.1 0.1

3 3

1 1

88

0.50 2.25 4.00

–33 –81±15 –114

136 45±12 –36

68 –74±6