Why does the G61 test often indicate that the alloy will not pit, yet it is known that the alloy is susceptible to pitting?
A5. The standard G61 test is optimized for austenitic stainless steels. Nitrogen purging makes the test more reproducible and eliminates the chemical tug of war between passivating and depassivating the surface that would occur in an aerated electrolyte. Based on my experience, what this test actually detects is segregation, inclusions, and coarse grain structure. These all contribute to the susceptibility to pitting corrosion but there are other factors, which this test does not account for, concentration cells and MIC. Further, the standard deaerated NaCl electrolyte used in this test is not realistic of what the material will be exposed to in service. Other alloys, such as copper and brasses, do not give reproducible results because it can take hundreds of hours of exposure before the surface has equilibrated with the electrolyte, and the surfaces are extremely altered by the polarization.
