Environmentally induced cracking of API grade steel in near-neutral pH soil

Abstract

Environmentally induced cracking of pipeline steels in contact with soil have been investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using electrolytes obtained from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46, X60 and X80 steels were obtained at different electrode potentials (Ecorr, 100 mV below Ecorr and 300 mV below Ecorr) with 9 x 10-6 s-1 strain rate. The results obtained with API 5L X46 in soil solution were compared with slow strain rate test (SSRT) results obtained using a strain rate of 9 x 10-7s-1 to the same steel. All results showed the incidence of cracking and their dependence on the potential imposed, revealing the SCC (Stress corrosion cracking) susceptibility of the steels. It was also observed that the hydrogen embrittlement - HE - has an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility. © 2009 by ABCM.