The pitting corrosion and crevice corrosion of oilfield production alloys (e.g., 13Cr/UNS S41000, 17-4PH/UNS S17400, 25Cr/UNS S32750, A286/UNS S66286, 718/UNS N07718) and proprietary austenitic stainless steels for directional drilling (PREN between ~20 to ~45) has been investigated. Specifically, series of electrochemical tests have been conducted to rank the alloys, establish simple correlations between electrochemical parameters, PRENmod, and 3-to-60-day immersion tests in 3.5% NaCl at ambient temperature. For all but one alloy, pitting was absent in stark contrast to crevices. Upon tracking populations and dimensional characteristics of crevices over time, trendlines comparing the susceptibility of the alloys towards crevice corrosion were established. Practical conclusions were reached, including the following: (1) 13Cr consistently developed crevices within days, (2) 17-4PH as well as all traditional directional drilling stainless steels developed crevices within one to five weeks, and, (3) neither 718, 25Cr, nor newer directional drilling alloys with both high nickel and high PRENmod showed any sign of crevices upon being tested up to 60 days. Through a variety of comparisons, this investigation also reveals useful technical directions for the development of new, economical, and fit-for-service Oil & Gas alloys for both production and drilling.
The stress corrosion cracking (SCC) behavior of Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys in HTHP CO2 environments was investigated through slow strain rate tests (SSRT) and electrochemical methods. The results show that a remarkable decrease in tensile strength and elongation to failure was observed when testing in CO2 environment as compared with air. Fe17Cr5.5Ni-based alloys possessed better SCC resistance than Fe13Cr5Ni-based alloys. The increase of Cr and Ni content tended to enhance the resistance to SCC and pitting corrosion. The SCC behaviors of Fe13Cr5Ni- and Fe17Cr5.5Ni-based alloys were closely associated with the repassivation capacity and the resistance to pitting corrosion.