Hastelloy B is commonly used in nickel-molybdenum alloys. It has good corrosion resistance to all concentrations of boiling hydrochloric acid. In addition, it has non-oxidizing acids and non-oxidizing acids such as phosphoric acid, hydrofluoric acid, bromic acid, sulfuric acid, acetic acid and other organic acids. Oxidative salt solution and wet hydrogen chloride also have good corrosion resistance.
The nickel-molybdenum alloy is not resistant to corrosion in oxidizing ion-containing solutions, oxidizing acids such as nitric acid, and oxidizing hydrochloric acid and sulfuric acid.
Reducing the carbon content of Hastelloy B to ≦ 0.02% is Hastelloy B-2, which can reduce and delay the precipitation of carbide phases at high temperatures and improve the resistance to intergranular corrosion in the sensitized state. The nickel-molybdenum alloy has two intergranular corrosion sensitization zones, 600-900ºC and 1300ºC. In these regions, precipitated phases such as MOC, α phase, and Ni7Mo6 will be precipitated, thereby reducing the Mo content of the grain boundary attachments and causing intercrystalline Sensitivity to corrosion.
The addition of chromium to the nickel-molybdenum alloy is the nickel-molybdenum alloy, and the amount of chromium is similar to that of stainless steel. Commonly used are Hastelloy C, Hastelloy C-276 and Hastelloy C-4. Because it contains chromium, it is resistant to oxidizing acids such as nitric acid. Corrosion of mixed acid of nitric acid and sulfuric acid, mixed acid of chromic acid and sulfuric acid, etc., can also resist corrosion of oxidizing salts or media containing other oxidants such as hypochlorite at a higher temperature than normal temperature. It has good corrosion resistance in seawater, formic acid, and wet chlorine, and can also be used in chlorine or chloride-containing media. However, the corrosion resistance of nickel-molybdenum alloy in hydrochloric acid is not as good as that of nickel-molybdenum alloy.
Hastelloy C is susceptible to intergranular corrosion in oxidizing media and chloride-containing media when used in a sensitized state. This is because the chromium contained in Hastelloy C precipitates chromium during sensitization and depletes chromium at the grain boundaries, so intergranular corrosion occurs. Hastelloy C sensitizes in the range of 704-1038ºC in less than 10 seconds and will cause intergranular corrosion.
Hastelloy C-276 is made by reducing the carbon and Si content of Hastelloy C. Due to the reduction of carbon content, Hastelloy C-276 has enhanced resistance to intergranular corrosion. The sensitized and post-welded Hastelloy C-276 has much better pitting resistance than Hastelloy C and better crevice corrosion resistance. Hastelloy C-276 has good corrosion resistance in the case of oxidizing, reducing and containing halogen ions.
Hastelloy C-4 has a lower carbon content (C ≦ 0.015) and titanium, and has fewer carbides and intermetallic compounds in the sensitized state. Hastelloy C-4 can withstand a long time at a sensitization temperature of 530ºC-1090ºC without causing severe intergranular corrosion sensitivity. Hastelloy C-4 has good corrosion resistance to various media such as organic acids and chloride-containing acid solutions.
Hastelloy C-4 has full corrosion resistance similar to Hastelloy C. Hastelloy C-4 has a higher corrosion rate than Hastelloy C-276 in a strong reducing acid such as hydrochloric acid, but Hastelloy C-4 has a lower corrosion rate than Hastelloy C-276 in hydrofluoric acid.