EFFECT OF COMPOSITION AND STRUCTURE ON CREVICE, INTERGRANULAR, AND STRESS-CORROSION OF SOME WROUGHT NI-CR-MO ALLOYS

The relationship of composition and microstructure to the occurrence of localized corrosion in Hastelloy alloys C, C-276, and C-4 was investigated. One hour exposures of these alloys in the range of 1200 to 2200 F (649 to 1204 C) may result in the formation of a molybdenum-rich intermetallic compound and, in alloys with more than about 0. 004%C, a molybdenum-rich carbide. The boiling ferric sulfate-50% H//2SO//4 test readily detects the presence of both of these precipitates which cause rapid intergranular attack in this solution. The molybdenum-rich M//6C carbide precipitate impairs resistance to intergranular, crevice, and stress corrosion. In constrast, the molybdenum-rich intermetallic compound, Mu-phase, impairs resistance only to intergranular attack, primarily in oxidizing acids. Susceptibility of alloys C and C-276 to crevice corrosion and to stress corrosion cracking (SCC) in chloride solutions appears to involve galvanic action between the anodic matrix and the cathodic, molybdenum-rich carbide precipitates at the grain boundaries. Proposals are included for improvements of the compositions of Ni-Cr-Mo alloys, for acceptance test procedures, and for optimum use of these alloys in acid and chloride environments.