Abstract
In the oil and gas industry, materials qualification testing of corrosion resistant alloys typically involves the use of simple pass/fail test methods such as the four-point bend test, in which the observation of cracking is the main criterion for failure. However, for mechanisms such as sulfide stress corrosion cracking where pitting is a precursor to cracking, there is uncertainty over how to interpret the observation of pitting when no cracking has occurred during the test. In this work, results of systematic coupon exposure, four-point bend and electrochemical tests in a range of sour environments strongly support the perspective that pitting is controlled by mass transport of H2S to the growing surface of the pit. Pit morphology varies from occluded pits with significant undercutting of the surface at low chloride/high H2S to a more open hemispherical geometry at high chloride/low H2S. To ensure conservatism in testing, modification of existing standards is recommended for environments in which pit initiation is statistically improbable but pit propagation is relatively rapid, e.g. low chloride/high H2S. This may include the use of a higher number of test specimens, a larger sampled area per specimen and/or a longer test duration, depending on the constraints of the test programme.
