L51683 Environment Sensitive Cracking of High-Pressure Pipelines with Carbon Dioxide-Containing Solutions

The great majority of instances of stress corrosion cracking in high pressure gas pipelines are associated with the propagation of intergranular cracks, but several instances of transgranular cracking have now been experienced. The latter have involved service as well as hydro test failures, and transgranular cracks frequently have been found in the region of dents in those lines that display this mode of cracking. In all of those respects the transgranular and intergranular forms are identical, but the difference in crack morphology suggests that the mechanism of growth, and therefore the conditions under which the two forms occur, are different. Since the steels involved and the operating pressures are not essentially different for pipelines displaying the two different modes, the implication would appear to be that the different mechanisms result from variations in the environmental conditions. Field data relating to the transgranular stress corrosion cracking of high pressure gas pipelines indicates that the solutions found between coating and pipe in regions where cracks have occurred have low concentrations of dissolved salts and pH's of about 6.5, distinguishing them from those that are associated with the more common intergranular form of cracking. The lower pH solutions involved with transgranular cracking are the result of the presence of free carbon dioxide and the lack of significant cathodic current arriving at the pipe surface in those locations. Slow strain rate and cyclic loading tests employing solutions simulating those found in the field have shown that transgranular stress corrosion cracks can be generated under laboratory conditions and that they have the same characteristics as are observed in samples from lines that have developed cracks in service. Those similarities extend to the data from both field and laboratory sources showing considerable scatter, indicating the need for appropriate statistical treatment. It is suggested that the mechanism of transgranular cracking involves dissolution and the ingress of hydrogen to the steel and that implies that some of the concepts that have been employed in modeling the intergranular form of cracking will need to be modified for the transgranular case.