Effect of Electropolishing and Thermal Annealing on the Corrosion/Release Behavior of UNS N06690

摘要

Electropolishing surfaces of UNS N06690 reduced the corrosion rate by about 90%, relative to as-machined surfaces, in deaerated, pH-adjusted water at 500 °F. Testing was carried out in a Ti autoclave, in an effort to minimize pick-up of ferrous corrosion products from the autoclave. It was demonstrated that base metal was released to solution more rapidly at the beginning of the test, before a stable oxide film was able to passivate the coupon surface. After the film was established, metal release and oxide film growth proceeded more slowly. The film was Cr-rich, and consisted of fine crystals with a diameter of about 20 nm on electropolished surfaces, but consisted of fine and coarse crystals on the as-machined surfaces. Unlike previous corrosion/release tests, the major phase identified in the corrosion film was non-stoichiometric CrOOH rather than spinels of the form AB2O4. Minor amounts of spinel were identified, along with Ni metal. Thermal annealing of corrosion coupons also reduced corrosion rates of UNS N06690, but was not as effective as electropolishing.