Abstract
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% H2SO4 test readily detects the presence of both of these precipitates which cause rapid intergranular attack in this solution. The molybdenum-rich M6C carbide precipitate impairs resistance to intergranular, crevice, and stress corrosion. In contrast, the molybdenum-rich intermetallic compound, Mu-phase, impairs resistance only to intergranular attack, primarily in oxidizing acids.
When the carbon content is progressively increased above 0.004%, formation of the molybdenum-rich carbide precipitate at grain boundaries by one hour heat treatments results first in loss of resistance to stress corrosion cracking (SCC) in the boiling 45% MgCl2 test. Further increases in carbon content (0.05%) result in loss of resistance to intergranular attack in acids, to crevice corrosion in 10% ferric chloride at 50 C and also to SCC in this solution. All SCC is of the intergranular type. Formation of the molybdenum-rich carbide precipitate is prevented in alloy C-4 by keeping the carbon content low and by addition of titanium. Formation of the intermetallic compound is minimized by adjustment of the alloy content. In the heat investigated, titanium stabilization was effective in preventing intergranular attack in reducing acids and SCC in the MgCl2 test. However, metallographic examination and intergranular attack in the ferric sulfate test on material heated 1 hour at 1600 F (871 C) indicated that a compound was still being formed at grain boundaries.
Adjustment of the alloy content of the C-4 composition, which does not have the 4% tungsten present in alloys C and C-276, has impaired its resistance to crevice corrosion in 10% ferric chloride at 50 C in the annealed condition. Solution-annealed alloys C and C-276 are resistant, even at 65 C in this test. In 1 pound laboratory heats, it was shown that the resistance to crevice corrosion can be restored in the C-4 composition by addition of 4% tungsten or by increasing the molybdenum content from 16 to 18%. These changes in composition do not impair its resistance to SCC, even after heating 1 hour at 1600 F (871 C). Nor do they increase susceptibility to intergranular attack. They increase the resistance to general corrosion in reducing acids, but decrease the resistance in oxidizing acids.
Susceptibility of alloys C and C-276 to crevice corrosion and to SCC in chloride solutions appears to involve galvanic action between the anodic matrix and the cathodic, molybdenum-rich carbide precipitates at the grain boundaries. It is not dependent on a molybdenum-depleted zone surrounding the precipitate. Preferential corrosion by acids at grain boundaries is a function of the redox potential of the solution. Reducing acids preferentially attack molybdenum-depleted zones around molybdenum-rich precipitates. However, when the precipitates form at elevated temperature [1900 F (1038 C)], molybdenum from the matrix diffuses into the depleted zones and decreases or eliminates preferential attack by reducing acids. Oxidizing acids rapidly attack the molybdenum-rich precipitates directly and are not dependent on the presence of molybdenum-depleted zones around the precipitates.
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.
