Effect of δ-ferrite on sulfide stress cracking in a low carbon 13 mass% chromium steel

It is important for the 13 mass% chromium steels to keep the passivation in order to improve SSC (Sulfide Stress Cracking) resistance in sour environments. For example, the addition of molybdenum and chromium is effective to keep the passivation. However, when these elements are excessively added, delta -ferrite is formed. Therefore, the effect of delta -ferrite on SSC in the low carbon 13 mass% chromium steels was investigated. SSC occurred in the case that delta -ferrite is formed to some extent because the corrosion potential was lowered to that of the active state. The occurrence of SSC did not depend on the difference of shape of delta -ferrite. The reason for the difference in corrosion potential with and without delta -ferrite is attributed to the existence of a chromium depleted zone due to the precipitation of chromium carbides or nitrides near the delta -ferrite.