Fatigue-Crack Growth in an X-65 Line-Pipe Steel at Low Cyclic Frequencies in Aqueous Environments

Abstract

The effects of salt and distilled water environments on fatigue-crack growth rates have been evaluated for an X-65 line-pipe steel. Tests were conducted over a frequency range between 10 and 0.01 Hz, under conditions of cathodic potential and free corrosion. A distinct pattern in the functional dependence of growth rates on ΔK and frequency has been found. Maximum environmental enhancement of growth rates was 50 times that of air at a cathodic potential of −1.04V, and 10 times that of air at a free-corrosion potential of −0.68V. In each case it occurred at the lowest frequency and at relatively high values of ΔK. Hydrogen embrittlement and the two-stage cracking process are examined as the main mechanisms of growth acceleration.