Abstract
The roles of the alloying elements Mo, Cr and W in resisting crevice corrosion of commercial alloys C22 (UNS N06022), C625 (UNS N06625) and BC1 (UNS N10362) have been studied under galvanostatic conditions in 5 mol·L-1 NaCl at 150°C. Corrosion damage patterns were investigated using surface analytical techniques such as Scanning Electron Microscopy (SEM) and optical imaging, and the corrosion products characterized by Energy Dispersive X-ray Spectroscopy (EDS).
Once initiation had occurred, the crevice potentials increased in the order C625 < C22 < BC1; i.e as the Mo content increased. SEM and EDS analyses on crevice corroded areas of the surface showed an enrichment of Mo and depletion of Cr on all alloys. The depth of corrosion penetration increased, and the area of the alloy surface corroded decreased, in the order C625 < C22 < BC1; i.e., as the Mo content decreased. Limited penetration depth at higher Mo content can be attributed to the inhibition of metal dissolution by the rapid accumulation of insoluble molybdates at corroded locations.
In addition, once crevice corrosion initiates and the crevice acidifies, metal oxidation can also couple to proton reduction inside the crevice. The role of internal proton reduction in driving the crevice corrosion of these Ni alloys was found to be quite significant; greater than 50% of the corrosion damage is caused by proton reduction inside the crevice.
