Abstract
The corrosion behavior of Co-Cr-W-C weld deposit (UNS R30006) was tested in deaerated, pH adjusted water at 500 0F for up to 10,000 hours, and subsequently analyzed by X-Ray Diffraction, Auger Electron Spectroscopy, and Electron Spectroscopy for Chemical Analysis to determine oxide composition, and Scanning Electron Microscopy and Focused Ion Beam excavation to measure oxide thickness. The resultant weld deposit corrosion rate was three times higher than that of the analogous wrought material. The primary oxide phase composition of all surfaces was CoCr2O4; some corrosion films had a high-Co phase, likely CoFe2O4 or CoO, on their outermost surface. Thin corrosion films were found at the specimen surfaces, thinner over the Cr-rich carbide phase than the Co-Cr-W metal matrix, except where the carbide boundaries intersect the surface. Where this occurs, corrosion penetrates down and around the carbides, where the adjacent metal matrix regions are likely depleted in Cr and more susceptible to attack. These sub-surface corroded areas may have been excluded in other evaluations at shorter exposures, thus understating this material’s long-term corrosion rate. As a post-weld stress relief heat treatment applied to some specimens resulted in more carbides, it also resulted in higher corrosion rates.
