Nickel, as an alloying element in low alloy steels (LAS), has beneficial effects like a decrease in the ductile to brittle transition temperature and improvement in hardenability. Those effects are desirable in components for the oil and gas industry exposed to high stresses and low temperatures. In the presence of H2S, sulfide stress cracking (SSC) is a probable mechanism of failure. The effect of nickel on the resistance to SSC is still controversial and the content of nickel in steels is restricted to a maximum content of 1 wt.%, according to ISO 15156-2, which governs the use of LAS for sour service. While LAS with higher nickel content can be qualified, expensive tests are required which results in a de facto ban on nickel content. On the other hand, many authors suggest that the resistance to SSC, which is a form of hydrogen embrittlement, is controlled by the microstructure rather than the nickel content. The maximum in nickel content is controversial and several researchers have pointed out that low alloy steels with Ni>1 wt.% could perform well in service if they are heat treated to tempered martensite, which is the desired microstructure for maximum SSC resistance.
The objective of this work is to study the effect of nickel additions in electrochemical behavior, hydrogen diffusion and sulfide stress cracking of LAS heat-treated to a fully martensitic microstructure. LAS with a carbon content of 0.17 wt.% and 1.3 wt.% Mn with varying nickel contents between 0% up to 5 wt.% were fabricated and heat treated in three stages: normalizing, quenching and tempering, targeted to obtain a homogeneous tempered martensite microstructure with a hardness below 22 HRC. Electrochemical hydrogen permeation tests were carried out in a Devanathan-Stachurski cell type with gaseous charge (1 bar, H2 purity of 99.999%). This setup allowed detecting any effect of nickel on hydrogen diffusion transport through the steel lattice, independently of other effects like the formation of protective films in sulfide-containing environments. Electrochemical tests and slow strain rate tests were performed in deaerated NACE TM0177-A solution with and without thiosulfate additions. These tests allowed to characterize the effect of nickel on pit and trenches formation, features previously associated with steels with higher nickel contents, and to provide insight into the mechanism of crack propagation in high nickel steels.