Accelerated test techniques for measuring SCC resistance of alloys

Testing of alloys in specific environments for susceptibility to stress corrosion cracking is, at best, difficult in some cases, and virtually impossible in others. This is particularly true for alloys where susceptibility is slight, and where service failures may occur over periods of years, decades or even centuries (e.g., for nuclear waste disposal cases). However, a number of test techniques have been developed which attempt to at least rank order alloys for susceptibility, even if they cannot always provide actual expected failure times or accurate stress corrosion crack growth data. Among the most common accelerated test techniques is the slow strain rate (SSR) or constant extensive rate test (CERT). This test has the advantage of allowing relatively rapid, reasonably reproducible tests in a known specific environment. Unfortunately, the test may prove to be either too severe, leading to rejection of alloys which might otherwise be acceptable, or conversely, may prove to be too conservative if crack initiation events are slow to develop relative to the extension rates which are utilized. This presentation will address the parameters which are relevant to the CERT technique and also indicate some of the possible shortcomings of the technique. Other test techniques which may be used to determine susceptibility of specific alloys to IGSCC will be described, including differential current-potential scans, current transients associated with film rupture, and the electrochemical potentiokinetic reactivation (EPR) technique. It will be pointed out that no single test technique may be adequate to qualify alloys for specific service conditions, but that a combination of mechanical, electrochemical and microscopic techniques are required to differentiate between different alloys or thermo-mechanical treatments of families of alloys