L00038 Variations in Electrolytic-Cell Conditions in Hard-Spot Regions of Line Pipe

Hydrogen-stress cracking (HSC) has occurred in natural gas transmission pipelines only in localized hard regions in the pipe wall (hard spots) or in hard weld zones. A specific factor that results in the initiation of HSCs in some hard spots is the possible variation in localized electrolytic cell conditions that could exist because of variations in microstructure, hardness, and residual stress in those regions. This paper presents the results of a study of these variations. The first part of the study evaluates whether local variations in pipe-to-soil potential existed in hard spot regions subjected to an imposed potential. The second part involves monitoring the current flow and hydrogen entry into high-strength, line-pipe steels as a function of the imposed half-cell potential.It was concluded that the variation in potential was greater for a section of pipe removed from a hard spot than it was for a section that did not contain a hard spot. No correlation was found between the hardness variations and the half-cell potential variations. Consequently, factors such as surface condition of the pipe or variations in the residual stresses may influence the value of the half-cell potential and the amount of variation in the half-cell potential in hard-spot regions. The variations in half-cell potential that can occur in hard spots and the variations in residual stress that have been measured in hard spots do aid in explaining why the cracks that occur in hard spots frequently initiate at locations other than the hardest region.