Stress Corrosion Cracking Index Research Articles

Modeling 3D finite element analysis of a semi-elliptical crack on stress corrosion cracking of API X52 pipeline

In this study, an external semi-elliptical crack was modeled in a 3D API 5 L X52 pipeline with stress corrosion cracking (SCC). To make the crack, the finite element method (FEM) was used to obtain the failure pressure (Pf) and its mechanical behavior. The longitudinal crack had a constant length (2c) and varied with depth (a). The properties of X52 steel subjected to SCC using the slow strain rate technique (SSRT) were considered. True stress-strain curves were obtained in air and in an NS4 solution with pHs of 3.5 and 9.5 at temperatures of 25 and 50 °C. According to the SCC index calculated from the mechanical properties of the SSRT, the X52 steel is susceptible to SCC at pH 3.5 and 50 °C. The mechanical properties of the tensile test using the stress-strain curves decreased as the pH and temperature changed, compared to those carried out at room temperature. This was due to the corrosion and hydrogen embrittlement produced by the solution on the X52 steel. The failure pressure is sensitive to the stress-strain curve; if the stress-strain curve decreases, the failure pressure also decreases. Corrosion defects, such as longitudinal cracks in X52 steel, which could be susceptible to SCC in an NS4 solution, decrease the failure pressure (Pf) and its capacity to withstand pressures of up to 75 % in pipe-grade steel that transports hydrocarbons.

Role of Mexican Clay Soils on Corrosiveness and Stress Corrosion Cracking of Low-Carbon Pipeline Steels: A Case Study

This work analyzed the physicochemical effect of different types of Mexican clay soils on corrosion and stress corrosion cracking (SCC) behavior in contact with X60 and X65 steels. Four soils were obtained from the right of way land in southern Mexico at 1.5 m depth close to pipelines. Two soils were from the state of Oaxaca (SO1 and SO2), and two others from the state of Veracruz (SV1 and SV2). Physicochemical and textural analysis of soils was performed and correlated to SCC susceptibility and corrosion mechanism. It was observed that soil texture might be related to corrosivity. A texture index (ratio between sand and silt + clay), which was seen to have a relationship with the corrosive tendency of soils, was estimated. It showed that soil with a higher index (SV1) has a higher corrosion rate. Electrochemical impedance spectroscopy and polarization curves were performed and correlated to the corrosion rate and the SCC susceptibility of steels. Steels exposed to SV1 soil exhibited a higher corrosion rate related to a higher content of chlorides and acid pH than those seen in other soils, which resulted in the pitting of such steels. Two types of corrosion were observed. Soils from SV1 and SV2 generated pitting, and soils from SO1 and SO2 produced general corrosion. Inclusions caused pitting in the gauge section of X60 and X65 steels exposed to SV1 soil by anodic dissolution. Galvanic coupling between inclusions and the base metal and dissolution of the inclusions might have enhanced the nucleation of pits at these sites. SCC susceptibility was evaluated using slow strain rate tests (SSRT). After SSRT, the fracture surfaces were analyzed through scanning electron microscopy. The SCC index obtained from SSRT indicates that X60 and X65 steels exhibited good resistance to SCC. A highly corrosive soil, such as SV1, causes the formation of pits instead of cracks, which is attributed to the dissolution process; however, lower SCC indexes were obtained for this system. The higher corrosion resistance of X60 steel is related to a more homogenous microstructure and a higher content of elements, such as Ni and Cr, than those of X65 steel that decrease the corrosion rate.

Stress Corrosion Cracking Process of X60 Steel Immersed in a Brine Solution with Corrosion Inhibitor through the Electrochemical Noise Technique

Stress corrosion cracking (SCC) process of an API X60 steel exposed to a brine solution with and without addition of corrosion inhibitor was analyzed through electrochemical noise (EN). A NACE brine solution without and adding 100 ppm of inhibitor was used like electrolyte at atmospheric pressure and 60°C. The SCC assessment was carried out using slow strain rate tests (SSRT) at strain rate of 1x10-6s-1in accordance with NACE TM-198. During SSRT the SCC process was monitored using EN using a set up with three electrodes. A decrease in mechanical properties for samples without corrosion inhibitor was observed. The SCC index obtained from the mechanical properties of SSRT indicate that X60 steel could be susceptible to SCC in brine solution without adding corrosion inhibitor. The time series show fluctuations with high frequency, but of low amplitude (for current and potential) which indicate the presence of a type of localized corrosion, which was corroborated with the index location calculated, where a mix corrosion process type was obtained. A clear effect of corrosion inhibitor was observed, which decrease the corrosion rate considerably.

Elucidating the role of microstructural modification on stress corrosion cracking of biodegradable Mg4Zn alloy in simulated body fluid.

This paper investigates the effect of microstructure modification by heat treatment on stress corrosion cracking (SCC) behavior of Mg4Zn alloy in simulated body fluid (SBF). Mg4Zn alloy in as cast, solution heat treated and peak aged conditions was susceptible to SCC in SBF when strained at 3.6 × 10-6 s-1. SCC index based on fracture energy is least for solutionized alloy (0.84), while 0.88 for as cast and peak aged alloys. Fractographic analysis indicates predominantly intergranular SCC for solution treated alloy initiated by anodic dissolution near grain boundaries. As cast and peak aged alloy shows mainly transgranular failure due to hydrogen embrittlement adjacent to secondary phase particles.

Stress corrosion cracking behavior in 2297 Al–Cu–Li alloy at different grain orientations

The effect of annealing and aging on mechanical properties, texture and stress corrosion cracking (SCC) susceptibility of 2297 Al–Cu– Li alloy after cold rolling was studied. Alloy annealed at 380 °C/60 min (sample B), which showed the best performance of SCC resistance, had an SCC index of only 3.8%. After aging at 175 °C for 31 h, the fracture surface of sample B had the finest dimples, which corresponded to the best ductility (elongation, 12.8%). The textures of the alloys treated by different annealing processes varied significantly, and grains gradually transferred from deformed to recrystallized textures during the aging process. Cracks tended to propagate along the grain boundaries with {100}//RP orientation grains, whereas the grain boundaries related to {111}//RP and {110}//RP orientation grains hindered SCC of the alloy.

Effect of non-conventional heat treatment of API X60 pipeline steel on corrosion resistance and stress corrosion cracking susceptibility

PurposeThe purpose of this paper is to study the effect of heat treatment (HT) applied to an API X60 steel in corrosion resistance and stress corrosion cracking (SCC) susceptibility through slow strain rate tests (SSRT) in NS4 solution and congenital water (CW) to assess external and internal SCC, respectively.Design/methodology/approachAPI X60 steel was heat treated at a temperature of 1,200°C for 30 min followed by water quenching. Specimens from this steel were machined according to NACE TM 198. SSRT were performed in a constant extension rate tests (CERT) machine at room temperature at a strain rate of 1 × 10–6s–1. For this purpose, a glass cell was used. Corrosion behavior was evaluated through polarization curves (PCs).FindingsThe SCC index obtained from SSRT indicates that the steel heat treated could be susceptible to SCC in CW and NS4 solution; the mechanism of SCC was hydrogen embrittlement. Thus, CW may promote the SCC phenomenon in pipelines. HT improves the steel corrosion resistance. Higher corrosion rate (CR) was observed when the steel is exposed to CW. The corrosion process in X60 steel shows that the oxidation reaction in the anodic branch corresponds to an activation process, and the cathode branches reveal a diffusion process.Originality/valueThe purpose of the heat treatment applied to X60 steel was to generate a microstructure of acicular ferrite to improve the corrosion resistance and SCC behavior.

Retrogression on corrosion behavior of spray formed Al-7075

Abstract

Environmentally assisted cracking resistance of Al–Cu–Li alloy AA2195 using slow strain rate test in 3.5% NaCl solution

The general corrosion and environmental cracking resistances of Al–Cu–Li alloy AA2195 were investigated in 3.5% NaCl environment and compared with those of another high strength alloy AA2219. The general corrosion resistance of these alloys was examined using immersion corrosion and potentiodynamic polarization tests, while the stress corrosion cracking (SCC) resistance was evaluated by slow strain rate test (SSRT) method. The tested samples were further characterized by SEM–EDS and optical profilometry to study the change in corrosion morphology, elemental content and depth of corrosion attack. The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of the alloys. The results indicated that the corrosion resistance of AA2195 alloy was better than that of AA2219 alloy as it exhibited lower corrosion rate, along with lower pit depth and density. However, the SCC index (ɛNaCl/ɛair) measured was greater than 0.90, indicating good environmental cracking resistance of both the alloys. Detailed fractography of the failed samples under SEM–EDS, in general, revealed a typical ductile cracking morphology for both the alloys.

Evolution of Microstructure and Stress Corrosion Cracking Behavior of AA2219 Plate to Ring Weld Joints in 3.5 Wt Pct NaCl Solution

AA2219 aluminum alloy plate (T87) and ring (T851) were joined by tungsten inert gas (TIG) welding using multi-pass welding. The mechanical properties and stress corrosion cracking (SCC) resistance of the above base metals (BMs) in different directions (L, LT, and ST) were examined. Similarly, the weld metal joined by plate to plate and plate to ring (PR) joints was evaluated. The results revealed that the mechanical properties of the ring were comparatively lower than the plate. This was found to be due to the extremely coarse grain size of the ring along with severe Cu-rich segregation along the grain boundaries when compared to the plate material. The SCC resistance of the base and weldments were found to be good and not susceptible to SCC. This was shown to be due to high values of SCC index (>0.9) and the typical ductile cracking morphology of the BM and the weld joints after SCC test in the environment (3.5 wt pct NaCl) when compared to test performed in the control environment (air). However, the corrosion resistance of the weld interface between the FZ and ring was inferior to the FZ–plate interface.

Stress corrosion cracking assessment of API X65 steel non‐conventionally heat treated

Stress corrosion cracking (SCC) susceptibility of API X65 steel non‐conventionally heat‐treated was evaluated through slow strain rate tests (SSRTs) using the synthetic soil solution NS4 at 20, 40 and 60 °C. Polarization curves (PCs) and corrosion potential (Ecorr) measurements were also performed. The non‐conventional heat treatment (HT) was made by heating plates at 1050 °C for 30 min followed by water quenching. The tensile strength of the heat‐treated plates increased to values similar to API X80 steel. The SCC index obtained by ratios of mechanical properties indicated that X65 steel without HT is not susceptible to SCC whereas with HT is slightly susceptible to SCC. The SCC mechanism in the soil solution in the heat‐treated plates was influenced by hydrogen diffusion as confirmed by the internal cracks and the brittle fracture observed in the specimens. The electrochemical study suggests that the corrosion process of X65 steel with and without HT occurs via a mix mechanism where the charge transfer resistance is influenced by a mass transfer process. Crevice corrosion features were found in the surface of the steel. It is likely that this type of corrosion could have sensitized the X65 steel and induced cracks when the steel was under tensile stress.

Stress Corrosion Cracking Behaviour of Flux Bounded TIG Welded AA2219 T87 Aluminum Alloy in 3.5 Weight Percent NaCl Solution

Limitation in penetration depth is a concern in conventional TIG welding. To improve penetration capability of TIG process, Flux Bounded TIG (FBTIG) has been developed. Stress corrosion cracking (SCC) behavior of FBTIG welds of aluminum alloy AA 2219 T87 is evaluated in 3.5 weight percent NaCl solution using Slow Strain Rate Test technique (SSRT) as per ASTM G129. SCC index defined as the ratio of the elongation of tensile tested specimen in NaCl to that of air is taken as a measure of the susceptibility to cracking. Based on the SCC index, it is concluded that the SCC resistance of FBTIG joints are good and comparable to that of conventional TIG welds.

The Influence of Multiple Welding Repairs of Pipelines in Residual Stress Assessment Related to Stress Corrosion Cracking

The effect of residual stress of multiple welding repairs in seamless API 5L X52 on the stress corrosion cracking (SCC) susceptibility was evaluated. Four repairs of the girth weld were evaluated through X-ray diffraction (XRD) on the internal face of the pipe to measure residual stresses in longitudinal and circumferential direction. Residual stresses in the circumferential and longitudinal direction reach values of about 77% and 58% of the UTS (460 MPa) respectively, approaching to the YS of the steel (360 MPa). It was observed that its magnitude increases as move away from weld center line. SCC susceptibility of X52 steel welded joints with the residual Original Research Article Contreras et al.; BJAST, 6(6): 621-634, 2015; Article no.BJAST.2015.117 622 stresses generated was evaluated through slow strain rate tests (SSRT) in a soil solution at low and high pH. Relation between mechanical properties and residual stresses on the SCC susceptibility was analyzed. SCC index obtained from the mechanical properties of SSRT indicate good SCC resistance of X52 steel exposed to soil solution at low and high pH. From these results, it is suggested that the region with high residual stresses before to generate cracks in the steel surface due to the combination of soil solution and the strain applied, should favor pitting formation and not cracking. Initiation of micro-cracks from these pits will depend on the geometrical form of the pit and the stresses state can be established at given pitting. It is suggested that initiation of micro-cracks from these pits may depend on the dissolution rate and morphology of the pitting and stresses on the bottom of the pitting. This favorable condition could appear increasing the soil exposition time of the steel and reducing the strain rate applied on the SSRT.

The Role of Residual Stresses in Circumferential Welding Repairs of Pipelines in SCC Susceptibility

The main objective of the present study is to measure the residual stresses in the circumferential welding joints of X52 pipeline steel with multiple welding repairs, and then relate these residual stresses to study the influence on the high pH stress corrosion cracking (SCC) susceptibility. Four conditions of shielded metal arc welding (SMAW) repairs of the girth weld were evaluated. The residual stresses were measured through X-ray diffraction (XRD) on the internal side of the pipe in longitudinal and circumferential direction. Residual stresses in the circumferential and longitudinal direction reach values of about 98 and 74% of the yielding strength (360 MPa) respectively. The effect of residuals stresses in the high pH-SCC susceptibility of X52 pipeline steel was evaluated through slow strain rate tests (SSRT) in a simulated soil solution. Relation between SCC index and residual stresses on the SCC susceptibility was analyzed. Results of SCC index taking account the ratios obtained from the mechanical properties of the welding joints evaluate (containing different levels of residual stresses) showed good SCC resistance. It was observed that increasing the magnitude of residual stresses, the SCC susceptibility increases. For all the SSRT specimens the failure occurs in the base metal and heat affected zone (HAZ) interface.

Residual Stress Assessment and its Effect on SCC of Pipelines Steel in Acidic Soil Environment

ABSTRACTEffect of residual stresses of multiple welding repairs on API 5L X52 pipeline steel on stress corrosion cracking (SCC) in a simulated acidic soil solution was studied. Four conditions of repairs of the girth weld were evaluated. The residual stresses were measured through X-ray diffraction (XRD) on the internal side of the pipe in longitudinal and circumferential direction. The circumferential and longitudinal residual stresses values are compressive on the inner surface of the welding joints. The highest residual stresses were measured in the hoop direction reaching values of about 98% of the yielding strength (360 MPa). It was observed that its magnitude increases as move away from weld center line. The effect of residuals stresses in the SCC susceptibility of X52 pipeline steel was evaluated through slow strain rate tests (SSRT) in a simulated acidic soil solution. Relation between mechanical properties obtained from SSRT and residual stresses on the SCC susceptibility was analyzed. Results of SCC index taking account the ratios obtained from the mechanical properties of the welding joints evaluate, showed good SCC resistance in acidic soil solution at low pH. Scanning electron microscopy (SEM) observations showed that the region with high residual stresses prior to generate cracks in the steel due to the combination of soil solution and the strain exerted, should favor pitting formation and not cracking.

Effect of Repair Welding on Electrochemical Corrosion and Stress Corrosion Cracking Behavior of TIG Welded AA2219 Aluminum Alloy in 3.5 Wt Pct NaCl Solution

The stress corrosion cracking (SCC) behavior of AA2219 aluminum alloy in the as-welded (AW) and repair-welded (RW) conditions was examined and compared with that of the base metal (BM) in 3.5 wt pct NaCl solution using the slow strain rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both BM and welded joints. The results show that the ductility ratio (eNaCl/(eair)) of the BM was close to one (0.97) and reduced to 0.9 for the AW joint. This value further reduced to 0.77 after carrying out one repair welding operation. However, the RW specimen exhibited higher ductility than the single-weld specimens even in 3.5 wt pct NaCl solution. SSRT results obtained using pre-exposed samples followed by post-test metallographic observations clearly showed localized pitting corrosion along the partially melted zone (PMZ), signifying that the reduction in ductility ratio of both the AW and RW joints was more due to mechanical overload failure, caused by the localized corrosion and a consequent reduction in specimen thickness, than due to SCC. Also, the RW joint exhibited higher ductility than the AW joint both in air and the environment, although SCC index (SI) for the former is lower than that of the latter. Fractographic examination of the failed samples, in general, revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy. Microstructural examination and polarization tests further demonstrate grain boundary melting along the PMZ, and that provided the necessary electrochemical condition for the preferential cracking on that zone of the weldment.