Ni-Cr-Mo alloys

Ni-Cr-Mo and Ni-Cr-Mo-W corrosion-resistant alloys are the nickel-based corrosion-resistant alloys with the largest amount and the widest range of use. Their early grades were the Hastelloy C alloy introduced in the early 1930s, and this alloy was widely used because of its resistance to corrosion by both oxidizing and reducing acidic media. During its long-term application, the problem of intergranular corrosion of this alloy caused widespread concern and was the focus of research on corrosion-resistant alloys in the 1950s and 1960s.
It was finally confirmed that the carbide and intermetallic phase precipitation of the high Mo Ni-Cr-Mo alloy was the source of its sensitivity to intergranular corrosion. Based on this research result, Hastelloy C-276 with reduced carbon (C ≤ 0.005%) and silicon (Si ≤ 0.004%) was introduced in 1965, which effectively alleviated the intergranular corrosion problem caused by M6C precipitation, but the intergranular corrosion caused by μ-phase precipitation was not effectively alleviated. In addition to the reduction of carbon and silicon, a new alloy Hastelloy C-4 was created by removing tungsten from the alloy and reducing iron to less than 2.0% (Fe) = 2.0%. Its thermal stability was greatly improved and the susceptibility to intergranular corrosion caused by M6C and μ-phase precipitation was suppressed.
However, C-4 alloy, due to the removal of tungsten, its resistance to all-round corrosion and resistance to pitting and crevice corrosion has been reduced. Subsequently, the Hastelloy C-22 alloy developed in 1982 and Alloy 59 (Nicrofer5923hMo) developed in 1990 made up for the shortcomings of the C-4 alloy. In order to further improve the corrosion resistance of Ni-Cr-Mo alloy, Inconel 686, which is based on C-276 alloy, was put into industrial use in 1993 by increasing the chromium in the alloy to (Cr) = 21%, controlling the iron to (Fe) <1% and adding Ti. Now 686 alloy is the best corrosion resistance, intergranular corrosion resistance and C-4 and alloy 59 equivalent high molybdenum Ni-Cr-Mo corrosion resistant alloy