Mechanical and environmental influences on stress corrosion cracking of an X-70 pipeline steel in dilute near-neutral pH solutions

Slow strain rate tests (SSRT) were used to investigate stress corrosion cracking (SCC behavior of an X-70 pipeline steel in a soil solution and NS-4 solution, both saturated with 5% carbon dioxide (CO2)/balance N-2 gas mixture at different potentials and strain rates. It was found that transgranular SCC occurred in this steel in the two solutions. The influence of strain rate was correlated with electrochemical potential. At a high strain rate, there were fewer cracks visible on the specimen side surface, but SCC occurred at a high cathodic protection potential. At a low strain rate, SCC susceptibility increased with decreasing an electrochemical potential. The SCC susceptibility was influenced greater in the soil solution than in the NS-4 solution. The susceptible local strain rate for near-neutral pH SCC was determined, which was lower than 5 x 10(-5) /s. There were several potential crack initiation sites including corrosion pits, inclusions, and flow lines formed during the rolling process of the steel.