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Equipment for use in sour gas wells is exposed to high-chloride brines saturated with hydrogen sulfide (H2S) and carbon dioxide (CO2) at temperatures up to 260°C (500°F). Both hydrogen-induced cracking and stress corrosion cracking are possible. However, traditional methods for testing environmental cracking resistance, such as the National Association of Corrosion Engineers (NACE) tension test or the magnesium chloride (MgCl2) test, have limited applicability to this service. The slow strain rate method overcomes many of the disadvantages of more traditional test methods, while posing some unique questions of its own. Apparatus and procedure are described. The theoretical interaction between slowly rising dynamic strain and hydrogen-related cracking is discussed. Selection of appropriate strain rate and sample size will be described. The possible evaluation criteria will be compared, including: time to failure, percent reduction in area, loss of ductile fracture surface area, and the appearance of secondary cracking. Certain ambiguous features in the transition zones between cracking and noncracking environments will be described. The strengths of the method (rapidity, positive results) will be balanced and compared with its limits of use.
hydrogen embrittlement, sulfide stress cracking, test methods, strain rate effects, low-alloy steels, corrosion-resistant alloys
Mclntyre, Dale R.
Cortest Laboratories, Inc., Cypress, TX