Abstract
As offshore oil production moves to ever deeper waters, there is an increasing need for sulfide stress cracking (SSC) resistant low alloy steel well casing materials with higher strength than those presently used. Over recent years one area of study has been the development of higher strength C- type low alloy steels for use in slight to moderately sour conditions. One emphasis has been on producing C-steel in the 100-125 ksi strength range, beyond the strength of the C-90 and T-95 type materials that have been traditionally used. As part of a High Pressure-High Temperature (HPHT) casing selection program, experiments comparing hydrogen permeation and corrosion rates were performed. This publication documents the experiments and results, and provides a tentative interpretation with respect to the performance of the P-110 and C-110 casing steels examined. The techniques used were electrochemical corrosion measurements using the linear polarization resistance (LPR) technique, and the hydrogen permeation rate as measured by the hydrogen flux through the steel. Hydrogen permeation measurements were performed on the rear side of the C-110 and P-110 steel test coupons. Simultaneously, corrosion measurements were performed on the front face of the coupons, exposed to the H2S containing environments.
Work performed as part of a parallel project has indicated that the C-110 steel is far more resistant with respect to SSC than the P-110 steel.
In this work it was found that for a given corrosion rate, once a longer term steady state has been reached, far less hydrogen passes through the C-110 steel compared to the P-110 steel examined in this work. This can be attributed to differences in lattice diffusivity, or differences in surface chemistry, or both. This phenomena could be one factor in explaining the difference in SSC performance, with respect to the advantages of the C-110 steel over P-110 steel.
