Abstract
Measurement of the amount of electric charge (coulombs) generated during electrochemical reactivation from a potential in the passive range to the corrosion potential has been used to detect susceptibility to intergranular attack associated with precipitation of chromium carbides at grain boundaries. The surfaces to be tested must be given a diamond paste polish to 1 μm. To simplify such measurements, a technique has recently been proposed for which a 140 μm or 100 grit (SiC) finish has been specified. For this test, called the double loop (DL) test, the surface is first polarized anodically through the active region before the reactivation scan in the opposite direction is conducted. In this test, the degree of sensitization is measured by determining the ratio of the maximum current generated by the reactivation scan to that of the anodic scan, Ir:Ia.
The characteristics of the DL method have been investigated. It is independent of the surface finish as well as the presence of random pitting; it is more reproducible than the single loop (SL) method and less sensitive to variations in the scan rate and solution composition. It provides a quantitative, nondestructive method for detecting relatively mild degrees of sensitization which match the metallographic ratings obtained in the oxalic acid etch test.
