Evaluation of the “nickel effect” in sulfide stress cracking of low alloy steels

Abstract

The “nickel effect” refers to the cause of the 1 wt %Ni maximum limit in low alloy steels for the oil and gas production industry to avoid sulfide stress cracking (SSC). The Ni restriction dates to the first edition of the NACE MR 0175/ISO 15156 standard in 1975 and the specific reasons for that limitation remain disputed. This work evaluates the nickel effect with electrochemical tests in the presence and absence of tensile loads at anodic and cathodic potentials and at the open circuit potential (OCP). Thiosulfate was used as a satisfactory alternative to H2S bubbling. The effect of nickel on hydrogen transport and trapping was analyzed with gaseous hydrogen permeation tests. A strong influence of Ni, even at Ni contents below 1 wt%, on SSC initiation at OCP due to trench formation was observed. Compared to other common alloying elements, Ni had a modest role in hydrogen trapping.