Stress corrosion cracking tests for low and high alloy steels in sour oilfield service: Tests performed at VTT

The purpose of the studies was to validate the usefulness of the proposed NACE slow strain rate testing method /1/ and compare it with the EFC document /2/. In the NACE document the testing takes place at a temperature of 177°C, the test solution contains 20 wt% NaCl, the partial pressure of H2S varies between 14 and 28 bar and the partial pressure of CO2 between 14 and 55 bar. In the NACE document the strain rate is determined as 4 . 10-6 1/s and in the EFC document 1 . 10-6 1/s. The results showed brittle behaviour for the test material in all of the test environments, and in each case the elongation was less than 5%. For comparison purposes the SSRT was conducted with the test material also in an inert environment (N2 gas), where the fracture was ductile and elongation 65%. The tests conducted with different strain rates gave the same result, which shows that the difference between EFC /2/ and NACE /1/ documents within the strain rate is not significant in the environments studied. However, since the alloy 654 SMO, which is considered to have a high resistance to corrosion, failed the SSRT test in the environments determined in the NACE document, the NACE document can be considered too severe for testing of austenitic stainless steels. Since contrary to the NACE document the EFC document does not determine levels for hydrogen sulphide and carbon dioxide, the EFC document can be considered more suitable than the NACE document for testing of austenitic stainless steels for sour service.