L51743 Stress Corrosion Cracking of Pipelines in Contact with Near-Neutral pH Solutions

While much has been learned about low pH stress corrosion cracking in the decade since it was recognized, a review of the published papers and reports since the last overview of the subject in 1992 indicates that there is still much to be understood about this matter. Most of the laboratory studies have involved dilute solutions based upon those found in the vicinity of cracks in operating lines, but the possible role of bacteria, for which there is supporting field evidence, has not received systematic study. The trans-granular service cracking has been reproduced in the laboratory, most readily when relatively high stresses and/or strains are applied to specimens, but there is still difficulty in reproducing cracking in the laboratory with stressing conditions similar to those on an operating pipeline.

If meaningful modeling of low pH cracking is to be achieved, there is need for more data on crack initiation and the early stages of growth with stressing conditions no overly excessive by comparison with service conditions. There is also a need, related to modeling, of an understanding of the mechanistic aspects of cracking, since while it is known, not least from visible evidence of corrosion on the sides of cracks, that dissolution occurs within the crack enclave, there is indirect evidence that the ingress of hydrogen into the steel may be involved also in the overall crack growth process. If hydrogen is involved then existing models based upon high pH cracking, and involving a quantifiable dissolution mechanism, will not be directly applicable to the low pH problem.