Effects of Electropolishing and Thermal Annealing on Corrosion/Release Behavior of NiCrFe Alloy 690

Electropolishing surfaces of Alloy 690 (UNS N06690) reduced the corrosion rate by about 97%, relative to as-machined surfaces, in deaerated primary water at 500 °F.  Testing carried out in a Ti autoclave 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.  The major phase identified in the corrosion film was non-stoichiometric CrOOH.  Minor amounts of spinel (AB₂O₄) were identified, along with Ni metal.  This is unlike previous tests of Alloy 600 and 304 stainless steel, where spinels were the major phase.  Thermal annealing of corrosion coupons also reduced corrosion rates of Alloy 690, but was not as effective as electropolishing.