Pit to crack transition in X-52 pipeline steel in near neutral pH environment Part 1 – formation of blunt cracks from pits under cyclic loading

Abstract

Pipeline steel specimens that had been prepitted were cyclically loaded either in air at 0·7-7 Hz or in near neutral pH solutions sparged with 5%CO₂/balance N₂ gas mixture at ∼1 × 10^–4 Hz. The pit to crack transition on the radial transverse and axial transverse surfaces of typical X-52 pipeline steel was studied while varying the loading conditions and the solutions. Crack initiation in air was very difficult and took millions of cycles even under aggressive conditions. In contrast, the number of cycles required to initiate cracks in near neutral pH solutions was thousands of times smaller even under less aggressive loading. The pit geometry affected the cracking at the pits. Smaller pits were less likely to crack. If the depth to width aspect ratio was <0·5, then the pits were less likely to crack. In addition, pit to crack transition was related to the severity of the loading conditions, the solution and the microstructure. The blunt crack growth observed can be described as anodic dissolution of the plastically deformed highly stressed region at the base of the pit and the pit to crack transition can be fitted to a Weibull distribution.