Stainless steels resist corrosion through formation of an oxygen-rich passive film. Halide ions damage this film and promote localized attack. Chloride ions are the most prevalent halide ions in our environment and are involved in many stainless steel corrosion failures. Failures typically are manifested as pitting or crevice corrosion. Several laboratory tests are used to measure resistance to these modes of attack. Among the commonly-used tests are the ASTM(1) G1501 Electrochemical Critical Pitting Temperature (CPT) test, the ASTM G482 Practices A, C, and E CPT tests, the ASTM G48 Practices B, D, and F Critical Crevice Temperature (CCT) tests, and ASTM G613 Cyclic Polarization tests. While all tests measure resistance to chloride-induced local corrosion, the existence of multiple tests allows multiple, inconsistent measurements of corrosion resistance. Different alloy types – austenitic stainless steels, duplex stainless steels, and nickel alloys – may exhibit different relationships among the various CPT and CCT determinations. Fabrication, especially welding, may affect further the relationship among the various tests. The CPT and CCT temperatures of several alloys (UNS S31603, S32003, S32205, and N08367 alloys) in these tests will be compared and insights into the relationship between laboratory test results and predicted in-service corrosion performance will be developed.
Susceptibility to localized corrosion, typically caused by chlorides, is one of the major shortcomings of stainless steels and nickel alloys. This localized corrosion is typically expressed as pitting or crevice corrosion. While crevice corrosion is generally the more severe mode of attack and is the failure mechanism more often encountered in-service, pitting corrosion has fewer variables and is often considered to be of greater theoretical interest. Many tests have been developed to measure resistance to these modes of attack. The oldest are in-service or simulated service exposures to the environment of interest.
