Effect of aging on anodic behavior of Ni--Cr--Mo alloys

The precipitation response of corrosion resistant alloys based on the Ni-Cr-Mo phase diagram is complex with both intermetallic and carbide phases being formed during aging in the 650 to 1090 C (1202 to 1994 F) range. Although these phases markedly affect the corrosion resistance, their composition and structure have not been fully defined. Data are presented to show that the precipitating intermetallic phase in Hastelloy alloy C-276 is a (Ni, Fe, Co)3 (W, Mo Cr)2 Mu phase and the carbide is an M6C. Both phases have been shown to significantly increase the anodic dissolution rate of the passive region in boiling 10% sulfuric acid. Intergranular attack on aged alloy C-276 samples occurred due to attack of depleted regions in reducing acids and direct dissolution of precipitates under oxidizing conditions.