Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) susceptibility of three different steels were determined using the methodology of the slow strain rate (SSR) testing as currently discussed within ASTM G129 and more specifically in the proposed NACE T-1F-9e test method. The materials included 13% Cr and 13% Cr-5% Ni- 2% Mo martensitic stainless steels and 22%Cr-5%Ni-3%Mo (22Cr) duplex stainless Steel. They were tested under slow strain rate experiments, conducted in simulated service environments containing H2S, CO2 and Cl-containing brine.
The parameters selected to quantify the susceptibility to environmentally assisted cracking (EAC) were ductility (as measured by elongation and reduction in area), fracture energy (as measured by the area under stress versus strain curves) and time to failure. These parameters were analyzed considering the ratio of the property in the corrosive environment to the result in an inert environment at the same temperature. The classification of fracture type defined by the proposed NACE T 1F-9 test method was also used.
It was concluded that the SSR testing was able to screen corrosion resistant alloys for stress corrosion cracking in sour oil field service. It was shown that, although 22Cr (duplex steel) had a higher corrosion resistance compared to 13Cr-5Ni-2Mo and of 13Cr martensitic stainless steels, there was a region, pH above 4, temperatures up to 150 C, H2S partial pressure below 1.5 psia and CO2 partial pressure of 130 psia, where 13 Cr failed but where 13Cr-5Ni-2Mo gave acceptable results. It was concluded that pH and temperature of the environment work together and play an important role on the mechanism of failure.
Keywords: Slow strain rate, testing, corrosion resistant alloys, stainless steels, NACE T1F9, evaluation, material selection, screening. stress corrosion cracking, sulfide stress cracking, SCC, SSC, hydrogen sulfide.