Sulfide Stress Cracking (SSC) of Low Alloy Linepipe Steels in Low H₂S Content Sour Environment

Abstract

Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H 2 S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.

A Review of the Factors That Can Increase the Risk of Sulfide Stress Cracking in Thermomechanical Controlled Processed Pipeline Steels

“…This in turns leads to increased susceptibility to hydrogen embrittlement (HE), which results in crack initiation at the tip of the corrosion pit. [ 37 ] For example, pipeline steels consisting mainly of granular bainite have been observed to result in at least 1.25 times higher density of corrosion pits compared to that of lath bainite when exposed to low H 2 S partial pressure of 0.15 bar. However, the same experiment also revealed that cracks were observed at the tip of the corrosion pit in the microstructure consisting mainly of lath bainite but not that of granular bainite.…”

“…However, the same experiment also revealed that cracks were observed at the tip of the corrosion pit in the microstructure consisting mainly of lath bainite but not that of granular bainite. [ 37 ]…”

“…When comparing between microstructure consisting mainly of lath and granular bainite, the lower dislocation density in granular bainite results in lower hydrogen trap sites, which lead to higher SSC resistance. [ 36,37 ] But overall, the microstructure consisting mainly of acicular ferrite mixture has a lower efficiency as a hydrogen trap site, leading to a higher SSC resistance when compared to those of mainly bainite or pearlite. This is mainly due to the higher fraction of cementite present in bainite and pearlite as compared to acicular ferrite mixture.…”

“…Dunlop [ 89 ] observed that the applied cathodic polarization was able to inhibit the propagation of cracks in steel formed in environment containing low 0.01 atmospheric partial pressure of H 2 S. Thus, Yamane et al [ 6 ] proposed a model for SSC whereby initiation stage of SSC was dominated by localized anodic dissolution at the tip of the corrosion pit followed by HE as the dominant mechanism during the propagation stage of SSC. [ 37 ] Based on this proposed model, Samusawa et al [ 90 ] also went on to analyze the depth of corrosion pit within pipeline steels exposed to 0.15 bar H 2 S partial pressure under different applied stress and concluded that the increase in stress concentration increases the dislocation density and thus the corrosion rate, therefore increasing the depth of corrosion pit. This in turn further increases the localized stress concentration which promotes HE.…”

“…However, the same experiment also revealed that cracks were observed at the tip of the corrosion pit in the microstructure consisting mainly of lath bainite but not that of granular bainite. [37] Zhang and Kelly [38] observed that granular bainite has a similar crystallographic structure as martensite and suggested that a displacive transformation for granular bainite is involved. The lack of carbides in granular bainite also reduces the need for alloying elements such as Si or Al which acts to stabilize surrounding austenite and inhibit the precipitation of cementite which can act as hydrogen sinks that may contribute to HE.…”