Experimental research on stress corrosion cracking of X70 pipeline steel in nitrate solutions

API X-70 pipeline steel was widely adopted in the West-East Natural Gas Transportation Project in China. Pipelines normally carry natural gas or oil, but may suffer from H2S, liquid water, nitrate solutions or other electrolyte. The above complex mediums probably induce stress corrosion cracking (SCC) and then seriously impact the pipeline integrity. Therefore, many works were performed to evaluate cracking susceptibility of X70 pipeline steel. This paper researched the SCC behavior of X70 spiral welded pipe by carrying out experimental tests hi nitrate solutions. The mechanical performance of X70 pipeline steel was introduced. Slow strain rate test (SSRT) and modified pre-cracked wedge-open loading constant displacement (WOL) specimens test were employed respectively. The results showed that cracks of the base metal under static loading with constant displacement in saturated sodium nitrate aqueous solutions with near-neutral P-II didn't propagate within a long duration of loading. Either did the samples in saturated sodium nitrate solutions composed of CI (mass fraction: 1%, 3%, 5%) and H2S (50, 500, 1000ppm). Meanwhile, pitting corrosion was observed on the surface of X70 WoL specimens. It's analyzed that aggressive chlorine ions enhanced pitting corrosion and NO3- inhibited the cracking. The results indicated that the susceptibility to nitrate stress corrosion cracking of X70 material was little by using SSRT tests. While, the SCC susceptibility to 11,S (500, 1000ppm) of the tested material was rather high.