Initiation of Stress Corrosion Cracking on X-65 Linepipe Steels in Near-Neutral pH Environment

Abstract

Initiation and early growth of stress-corrosion cracking of X-65 linepipe steel in near-neutral pH environment were studied in NS-4 solution saturated with N2/5%CO2. The occurrence of SCC is discussed in terms of: 1. Generation of an environment that causes cracks to initiate; 2. Initiation and early growth of cracks; and 3. Effect of the mechanical loading spectrum. SEM observations of cracks at specimen surfaces showed that stress-corrosion cracks were initiated at bottoms of corrosion pits. Pits that lead to initiation of SCC can develop from surface discontinuities, which can be geometrical, such as surface roughness, or metallurgical, such as inclusions, second phase particles, etc. The formation of large cracks resulting from coalescence of micro-cracks was studied.