In accordance with Annex A of ANSI(1)/NACE(2)MR0175/ISO(3) 15156-2, low-alloy steel (LAS) components for oil and gas (O&G) are acceptable if they contain less than 1 wt.% Ni. As an alloying element in steels, nickel improves low-temperature toughness and hardenability and does not promote the formation of carbides that could retain hydrogen into the steels. In this work, a set of five steels with different nickel content was specially fabricated to study the effect of that element on the sulfide stress cracking (SSC) resistance. Quenching and tempering (Q&T) heat treatments were customized to each chemical composition to obtain a tempered martensite microstructure with a hardness level below but near the 22 HRC threshold. The effect of nickel content on the anodic and cathodic behavior of those steels was evaluated by electrochemical polarization tests in substituted NACE TM0177 A test solution, where H2S bubbling was replaced by thiosulfate additions (i.e., Tsujikawa Method). Hydrogen permeation tests were conducted in a Devanathan-Stachurski cell, to evaluate the effect of nickel on the hydrogen diffusion. Additionally, slow strain rate tests (SSRTs) were performed at the open circuit potential (OCP) to assess the role of nickel on the SSC resistance.
The first reported failures of oil and gas (O&G) steel components due to sulfide stress cracking (SSC) occurred in Canada, France, and the United States in the early 1950s.1 Since then, several combinations of chemical composition and microstructure for different grades of steels were thoroughly evaluated. Fraser, Treseder and Eldredge studied the metallurgical factors affecting SSC resistance among 104 low-alloy steels (LASs).2,3 In a following investigation by Treseder and Swanson, when comparing steels with the same hardness level, the SSC susceptibility increased if the nickel content was higher than 1 wt.%.4 Considering the metallurgical advantages of adding Ni to steels, such as the increase in hardenability and the decrease of the ductile-to-brittle transition temperature, this limit has been discussed since the late 1960s.5-7 Snape pointed out the possible presence of deleterious phases, i.e., retained austenite and untempered martensite, in the final microstructure of the steels studied by Treseder and Swanson.4,7 Nickel stabilizes the austenitic phase at lower temperatures due to its effect on the lower critical temperature (A1). If A1 is exceeded during tempering, untempered martensite can be present in the final microstructure, which greatly reduces SSC resistance.4,7 Snape proposes that steels with an adequate quenched and tempered (Q&T) microstructure can be resistant to SSC even at Ni contents beyond 1 wt.%.7 However, a maximum limit for nickel as an alloying element was incorporated in the requirements for SSC resistant LASs as detailed in Annex A of ANSI(1)/NACE(2) MR0175/ISO(3) 15156-2 standard.8 The standard sets a maximum hardness level of 248 HV (22 HRC) and a nickel content below 1 wt.%, and it excludes free-machining steels and those in the cold-worked state.8 On the other hand, LASs that do not comply with those guidelines should be extensively qualified for SSC according to the test methods required by Annex B of ANSI/NACE MR0175/ISO 15156-2 and described in the NACE TM0177 standard.8 Nonetheless, many authors suggest that microstructure, rather than the content of a given alloying element, is in charge of determining SSC resistance.7,9,10 Another important factor to be considered is the steel cleanliness to avoid undesired effects between the alloying and residual elements.9