Stress Corrosion Cracking Evolution of Low Alloy Downhole Tubular Steel in CO2 Containing Environment at 175 °C
Arshad Bajvani Gavanluei Cameron International Corporation
Subsea Systems Houston TX 77041
USA
David Olson Colorado School of Mines
1500 Illinois Street Golden, CO 80401
USA
Brajendra Mishra
Colorado School of Mines 1500 Illinois Street Golden, CO 80401
USA
ABSTRACT Slow strain rate tests were performed to study the stress corrosion cracking evolution of a low alloy tempered martensite downhole tubular steel at 350 °F (175 °C) in CO2 saturated environment. Experiments were carried out in a high pressure high temperature nickel base super alloy autoclave which was connected to a constant extension rate machine. Slow strain rate tests were interrupted at specific strain values to examine the specimens and study crack initiation and growth. It was found that up to a certain strain value cracks didn’t form, but after that, formation of cracks on the gauge length of the tensile specimens was observed and increasing the strain value cracks grew both in length and width. Crack growth velocity estimated from the length of the largest (deepest) crack and the experiment time period after initiation. Corrosion products were studied and identified using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, and after cleaning the corrosion products cracking behavior and cracks’ evolution was studied using scanning electron microscopy. It was proposed that the formation of protective corrosion products on the steel surface and local break down of the scale lead to localized anodic dissolution and crack propagation. Key words: stress corrosion cracking, carbonate-bicarbonate, CO2 corrosion, tubular steels, downhole environments, crack growth rate, crack evolution
INTRODUCTION The search for new oil and gas resources has forced the industry towards deeper, deviated wells and the use of extended reach drilling technology, horizontal wellbores and multiple lateral completions. Operational activities have moved toward harsher environments in deeper high pressure/high temperature wells and deep water for the exploration of new reserves of hydrocarbons. Therefore, accurate prediction of material performance becomes important in these harsh and aggressive environments. Additionally, the economic incentive of multi-phase transportation through subsea completions and long infield flowlines causes increased risk of corrosion. Control and management of corrosion becomes essential for the effective design and safe operations as it is one of the main operational issues.1-2
Traditionally, carbon and low alloy steels are used for transportation pipelines as well as downhole tubing and casing. Even though these steels do not show appropriate performance for uniform CO2 corrosion, the industry still depends on the carbon and low alloy steels. Utilizing of these steels despite their low corrosion resistance is due to their availability, fabricability and lower cost, although many corrosion resistance alloys have been developed.1 For over five decades there has been a continuous need to develop new materials for use in the oil and gas industry applications involving exposure to sour and sweet gases. Therefore, attempts have been focused on producing higher strength and more corrosion resistance materials that can resist high temperature/high pressure aggressive downhole environments. Higher pressure and temperature and more aggressive environments will be encountered as drilling goes deeper. In addition, toughness requirements for materials increases substantially as drilling activities move toward deeper wells. Tensile loads are higher due to the hanging weight and hoop stresses increase as a result of higher pressures.3
Investigations on producing higher strength and more corrosion resistance materials lead to the production of materials that can withstand service conditions in which temperatures and pressures exceed 200 °C and 135 MPa in the presence of considerable amount of acid gases such as CO2 and H2S as well as other aggressive species, such as sulfur components and chloride ions. Due to the variety of hydrocarbon resources and higher level of impurities, downstream petroleum refining and petrochemical environments exhibit higher corrosivity. Aqueous environments, containing CO2 and/or H2S, can be aggressive to the carbon and low alloy steels, depending on the conditions such as temperature, partial pressure of the gases and pH. These conditions can lead to rigorous uniform corrosion and stress corrosion cracking.4 These issues necessitates better understanding of the corrosion and stress corrosion cracking behavior of low alloy tempered martensite steels in these environments. Uniform corrosion of mild and high strength low alloy steels such as tank and API(1) 5L pipeline steels in aqueous environments containing CO2 has been extensively studied and reported in the literature. Corrosion mechanisms are now very well defined and are already incorporated in prediction models. 1,5-
7 Extensive studies have been carried out to address the sulfide stress cracking (SSC) susceptibility of strength low alloy steels in H2S containing environments at lower temperatures since SSC is unlikely to take place at higher temperatures. However, there is less information about the CO2 corrosion of low alloy steels with microstructures such as tempered martensite as microstructure plays an important role in the corrosion behavior of steel. Very few publications exist describing the occurrence of stress corrosion cracking (SCC) of these steels in CO2 containing environment.8-9 Therefore, understanding the SCC behavior of low alloy tempered martensite tubular steels at higher temperatures and in CO2 containing environments can help to avoid failures of these materials. SCC susceptibility of these steels
(1) American Petroleum Institute (API), 1220 L Street, NW, Washington, DC 20005-4070
in environments containing CO2 as they are subjected to static and dynamic mechanical stresses along with wet CO2 gas seems to have been neglected in favor of H2S corrosion and sulfide stress cracking. SCC susceptibility of a low alloy tempered martensite steel at different temperatures from 25-175 °C in carbonate-bicarbonate containing environments was studied and the results have already been reported.10 The results showed that increasing temperature SCC susceptibility increased and the maximum loss of ductility obtained at 175 °C. Study11 on the effects of CO2 partial pressure and strain rate on the SCC susceptibility of this low alloy steel at 175 °C showed that increasing CO2 partial pressure reduced the SCC susceptibility and decreasing the strain rates increased SCC susceptibility of the low alloy tempered martensite downhole tubular steel. This investigation, which is the extension of previous works, presents stress corrosion cracking evolution and crack growth on the tubular steel at 175 °C (350 °F) in CO2 saturated environment.
EXPERIMENTAL PROCEDURE
Initially, the microstructure, chemical composition and mechanical properties of the as received tubular steel were characterized. Tempered martensite microstructure of the steel is shown in Figures 1 and 2. Prior austenite grain boundaries were delineated using saturated picric acid etchant.
Figure 1: Optical microscopy image of as received tubular steel showing tempered
martensite microstructure. Prior austenite grain boundaries delineated using saturated
picric acid etchant, 50X
Figure 2: Scanning electron microscopy image of microstructure of tempered
martensite tubular steel showing precipitate and coarsening of carbides along the grain
boundaries and within the grains
Chemical composition and mechanical properties of the as received tubular steel for this study are listed in Table 1.
Table 1
Mechanical properties and chemical composition (wt. pct.) of as received tubular steel
Rectangular and subsized smooth tension test specimens cut from longitudinal direction of the tubular steel were used to perform slow strain rate tests (SSRTs).12 Geometry and dimensions of designed SSRT specimen is shown in Figure 3.
Figure 3: Geometry and dimensions (in mm) of rectangular tension test specimen for SSRT
Tension test samples were machined and ground to 0.25 µm (10 µin.) surface roughness in accordance with the standards.13,14 Before experiments, specimens were washed with DI water, rinsed with acetone and dried with hot blowing air. SSRTs were carried out using a constant extension rate test (CERT) machine that is activated by a ¼ HP 1725 rpm 0-90 VDC permanent magnet motor. Experiments were conducted in an autoclave made from UNS N06600 nickel base superalloy and in the CO2 containing deionized and ultra-filtered
(DI.U.F.) water at 175 ⁰C. Figure 4 shows the schematic illustration of CERT machine.
To perform the experiments, the autoclave was filled with 850 ml DI.U.F water and after tightening the autoclave, CO2 gas was purged one hour to deaerate and saturate the solution and finally heated to the
desired test temperature, 175 ⁰C, using a proportional integral derivative (PID) temperature controller.
Figure 4: Schematic demonstration of constant extension rate apparatus.
Slow strain rate tests, with strain rate (extension rate) of 10-6 S-1, were interrupted in certain strain values to evaluate SCC occurrence on the specimens. After any experiment, specimens were
examined by scanning electron microscopy to study the cracking morphology as a function of applied strain. Phase analysis of the corrosion products was obtained using X-ray diffraction (XRD) method with a Philips PW 3040/60 X’Pert Pro† spectrometer using Cu Kα radiation. FEI Quanta 600i† scanning electron microscopy (SEM) was used to study the morphology of the corrosion products as well as cracks morphology after cleaning the corrosion products.
RESULTS and DISCUSSION
Corrosion Products Analysis Formation of a compact rhombohedric crystalline corrosion product, iron carbonate, on the surface of the steel specimens studied at 175 °C in CO2 containing environment analyzed and identified using SEM, EDS, and XRD and the results have already been presented.10,11 In this study, evolution of stress corrosion cracks was studied at different strain values and after cleaning of the corrosion products. Stress Corrosion Cracking Evolution during Slow Strain Rate Test SSRTs were interrupted at the certain strain values to examine the specimens and study crack initiation and growth. These experiments conducted on the tubular steel in CO2 saturated DI water at 175 °C. After any experiment, specimens were examined by SEM to study the cracking morphology and density of cracks as a function of applied strain. Figures 5 shows the SEM image of the gauge length of the tensile specimens at different applied strains.
Figure 5: Schematic illustration of strain interruption values in a stress – strain curve of the low alloy tubular steel. Corresponding SEM images of the tensile specimens’ gauge length. SSRTs
with 1×10-6 S-1 strain rate were performed in CO2 saturated DI water at 175 °C.
No sign of cracking appeared on the gauge length of the specimen stopped at strain value of 0.019. Immediately after the yield point and at a strain value of 0.025, cracks are initiated and formed throughout the gauge length of the specimen perpendicular to the load direction. Increasing the plastic strain to 0.045, prior to the ultimate tensile strength (UTS), growth of existing cracks occurs while new cracks also are formed such that increase in the density of cracks, was observed. At a strain value of 0.047, at the UTS, cracks’ growth continued, but change in cracks’ density was insignificant. At strain value of 0.055, after the UTS, growth of cracks continued specially in width and it was mostly limited to the necking area which is the highly deformed area on a tensile specimen. Decrease in the density of cracks after UTS is attributed to the crack coalescence and increase in the crack size. Figure 6 demonstrates the occurrence of cracks and their distribution on the gauge length of the tensile specimens. Initiation of cracks after the yield point indicates that plastic deformation can play an important role on crack initiation. Yielding of steel promotes crack initiation on the surface, but crack propagation is very slow and it increases with increasing the value of strain.
Figure 5: SEM image of the SSRT specimens interrupted at strain value of: a) 0.019 which shows no cracking; b) 0.025, immediately after yield stress of the specimen indicating the initiation of cracks; c) 0.045, just before the UTS of the specimen showing increase in cracks’ density and also growth of the cracks; d) 0.047 which is the UTS of the specimen indicating cracks’ growth and no change or even decrease in density. e and f) 0.055, between UTS and fracture indicating growth of cracks in highly deformed necking area of the specimen. All SSRT experiments with
strain rate of 10-6 S-1 were performed in CO2 saturated DI water at 175 °C.
To calculate the crack growth rate (CGR), the length of the deepest crack in cross section (longest crack in thickness direction) was measured using SEM. Then, it was divided by the time that the SSRT specimens were under straining. Results are shown in Table 2:
Table 2 Calculation of the crack growth rate
Strain Value Longest crack (µm) Time (h) CGR (m/s)
0.019 0 6.63 0
0.025 68 10.93 1.72817E-09
0.045 200 16.65 3.33667E-09
0.047 327 18.57 4.8914E-09
0.055 441 17.67 6.93265E-09
It needs to be reminded that the calculated CGR numbers are estimated values because only direct distance between the start and end of the crack was measured as shown in Figure 7. Studies showed that cracks followed intergranular path in this material and environment combination, therefore, the exact crack length are longer than that was measured meaning that CGR shall be higher.
Figure 7: Illustration of the crack length measurement in specimen with strain value of 0.025 (left) and strain value of 0.055 (middle). The image in the right shows growth in width of a crack
at strain value of 0.055.
CGR increased as strain rate increased. Higher CGR at UTS and after UTS can be attributed to the stress concentration at the highly deformed necking area. Stress concentration and localization due to the necking phenomena and formation of slip bands promotes the crack propagation around the highly strained necking area. Furthermore, this highly strained region causes the relief of stress in areas far from the necking area and consequently faster repassivation that retards crack growth in those areas.17 Figure 8 shows SEM images of the larger cracks formed on the necking area which impedes crack growth in locations far from the highly strained regions.
Figure 8: SEM images of larger cracks formed on the necking area which results in cracks not growing far from the highly strained areas. (a) Necking area (b) area close to necking area (c)
area far from necking area (d) higher magnification of the image shown in (c). The highly strained necking area causes the relief of stress in areas far from this region and consequently
faster repassivation that retards crack growth in those areas.
Mechanism of Stress Corrosion Cracking of the Low Alloy Tempered Martensite Tubular Steel in CO2 Containing Environment
The mechanisms of SCC in different aqueous environments have been comprehensively investigated.15-32 In most of these systems, local breakdown of the passive film and repassivation under certain stress conditions are crucial to initiate stress corrosion cracking.15-17 Propagation of cracks mainly occurs due to two kinds of mechanisms. These mechanisms are hydrogen embrittlement and anodic dissolution at the crack tip within the certain potential range. Several investigations have been conducted to study the SCC of pipeline steels in carbonate bicarbonate environments.19-32 In addition, extensive research has been carried out on the various kind of hydrogen embrittlement, especially in H2S containing environments. This work is an attempt to study the cracking behavior of the low alloy tempered martensite tubular steels in high temperature CO2 containing environments. Anodic dissolution and film breakdown are assumed to be the most dominant mechanisms in this material and environment combination. Formation of a protective iron carbonate scale on the steel surface leads to a localized dissolution and formation of corrosion pits in the weak areas of the steel or localized corrosion of high energy regions such as grain boundaries. Corrosion pits at a certain stage of
their development can activate cracks initiation. Like any opening in the material, pits cause stress concentration and it might reach twice of the applied stress.33 As SCC evolution experiments indicated, cracks initiated after yield point suggesting that the plastic deformation of the steel is an important factor for corrosion film breakdown and crack initiation during SSRTs. However, passivity of the surface of steel before the yield and during the elastic deformation remains stable. The brittle surface film which formed on the surface, fractures under the applied stress. Fracture of the film exposes bare metal which rapidly reacts with the environment to re-form the surface film. The crack propagates by alternate film growth and fracture.34 Therefore, it can be concluded that after yield the dominance of repassivation and local stress concentration controls the initiation and growth of cracks. Stress corrosion cracks being intergranular or transgranular always are perpendicular to the tensile component of stress.35 knowing the type and morphology of SCC, remedies can be suggested to decrease susceptibility of materials to SCC. For this steel and environment combination, it was determined that11 intergranular stress corrosion cracking occurred. In other words, cracks propagated along the grain boundaries of prior austenite grains suggesting that segregation of trace elements such as sulfur and phosphorous precipitates impairs film formation hence enhancing dissolution of grain boundary regions.20 SEM evaluation of the cracks indicated the phenomenon of crack coalescence which is shown in Figure 9. It has been reported22 that if two adjacent cracks are coplanar and either or both grow sufficiently at their nearest tips then they will coalesce into a single crack, but, if two adjacent cracks of a particular average length are sufficiently far apart in the transverse direction, then their respective stress field will not interact and coalescence will not occur. If the adjacent cracks are sufficiently close in the transverse direction, their stress fields interact and achieve coalescence. For coalescence to occur the distance of separation, y (in mm), in the hoop direction is linearly dependent upon the crack length as shown in Equation 1: 19,22,26
(1)
where 2a is the crack length in millimeter (mm) It has been reported20,22 that at a lower crack tip strain rate which is proportional to specimen strain rate, the crack will cease to grow as long as the tips remain filmed. Current study showed that the straining condition is sufficient to rupture the film and expose bare metal to the solution again leading to the growth of cracks. Therefore, the rate determining step becomes the frequency with which crack tip films are ruptured. However, the mechanism of crack advance is still by dissolution.20 Figure 9 shows coalescence of separate cracks to create a larger and longer crack at different strain values. As strain values increases, cracks grow in length and width.
Figure 9: SEM images at different strain values showing coalescence of separate cracks to create larger and longer crack perpendicular to the applied stress. It seems the propagation of
Stress corrosion cracking occurred when a low alloy tempered martensite downhole tubular
steel examined at 175 °C in CO2 saturated environment using slow strain rate test. Cracks propagated along the grain boundaries of prior austenite grains suggesting that segregation of trace elements such as sulfur and phosphorous precipitates impairs film formation hence enhancing dissolution of grain boundary regions.
Cracks did not appear on the steel specimen at strain value of 0.019 which is below the yield
point. Increasing the strain values to 0.025, 0.045, 0.47, and 0.055 cracking appeared. Density of cracks increased up to ultimate tensile strength and then decreased after necking.
As strain value increased, cracks’ growth continued. Growth of the cracks after ultimate tensile
strength was limited to the highly deformed necking area which caused a relief of stress at the areas far from necking area leading to not growing of the cracks at those far areas.
Some of the cracks formed on the surface may become inactive, but some may be reactivated
by coalescence with later nucleated cracks and perhaps by variation of stress condition. Coalescence of cracks to create a larger and longer crack played an important role in crack propagation and growth.
The type of SCC occurred in this steel was intergranular stress corrosion cracking. Anodic
dissolution and film breakdown are assumed to be the most dominant mechanisms in the SCC of low alloy tempered martensite tubular steels.
ACKNOWLEDGEMENTS
The authors would like to express their acknowledgments and appreciation to the support of Petroleum Institute Abu Dhabi, UAE. Also they are thankful of DEVASCO International Inc (Welding Products) for their help and support. The first author is grateful of Technology division of Subsea Systems of Cameron International Corporation for their support and encouragement.
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