The influence of weld heat-affected zone on stress corrosion cracking of pipeline steel in near-neutral pH environment

Colonies of stress corrosion cracks are often seen around weld heat-affected zones (HAZs) in pipelines that are operating in the field in contact with near-neutral pH (NNPH) environment. Research on stress corrosion cracking (SCC) of welded line pipe steel can contribute knowledge that is important in establishing pipeline integrity. In the project reported in this paper, samples that had been machined from a large-diameter welded gas transmission pipe were subjected to slow strain rate testing (SSRT) at different strain rates in a soil solution and in NS4 solution, both purged with 5% CO2/balance N2 gas mixture. There were many more quasi-cleavage areas on the fracture surfaces of the weld HAZ specimens than on the fracture surfaces of the base metal. The increased SCC susceptibility of the weld HAZ samples compared to the base steel was attributed to the coarse-grained, non-uniform microstructure of the HAZ samples as well as the higher residual stress. SCC propensity increased with decreasing loading rate and with more negative potentials. SCC initiation was associated with pitting corrosion. © 2008 by NACE International.