A NEW APPROACH TO THE EVALUATION OF PITTING CORROSION OF SUPERMARTENSITIC WELDMENTS

摘要

Supermartensitic stainless steel (SMSS) weldments are found to be prone to pitting corrosion at ambient temperature in simulated formation waters containing traces of H_2S. The present work has been undertaken to clarify how the occurrence of low temperature pitting corrosion depends on the presence of H_2S and the surface condition of the weldment. It appears that the aswelded SMSS root surfaces are not fully passivated when they are tested at ambient temperatures and that the main effect of H_2S is to hinder repassivation and to accelerate corrosion occurring in the oxidized region close to the fusion line. The increased corrosion susceptibility seems to be a result of high temperature oxidation during welding modifying the surface condition. Corrosion tests have been used to simulate repassivation of "as-welded" SMSS root surfaces during preexposure to various oxidizing electrolytes. Some practical implications with respect to qualification of SMSS weldments are discussed. Based on these findings, it is recommended that a common pre-exposure in an oxidizing electrolyte is carried out before the samples are subjected to corrosion testing. The test procedure should reflect the modification of the SMSS weldments taking place prior to exposure to H_2S.