Nickel-copper and nickel-molybdenum alloys both have good corrosion resistance in reducing media, but poor corrosion resistance in oxidizing media. The nickel-chromium alloy is just the opposite. It has good corrosion resistance in oxidizing media and poor corrosion resistance in reducing media. In order to improve the shortcomings of the two, nickel-chromium-molybdenum alloys ( Ni-Cr-Mo alloy) have been developed. The alloy contains a large amount of elements such as Cr and Mo, and has a single-phase austenite structure, and has good corrosion resistance in both oxidizing and reducing media.
The nickel-chromium-molybdenum alloy has a Cr content of 7% to 22%, and a Mo content of 2% to 18%. Some grade alloys also contain elements such as W, Co, Ti, and Nb.
Typical nickel-chromium-molybdenum alloy ( Ni-Cr-Mo alloy) in HY-Industry products uses Hastelloy C alloy as an example, and its composition is 0Cr15Ni60Mo16W4. HastelloyC alloy has a corrosion rate of ≦ 0.025mm / a in natural seawater, and no pitting corrosion occurs. Corrosion resistance in sulfuric acid solution at any concentration below 70 ° C, the corrosion rate is about 0.1mm / a, and the presence or absence of air in the sulfuric acid solution has little effect on the corrosion resistance.
Corrosion resistance of HastelloyC alloy in hydrochloric acid is lower than that of nickel-molybdenum alloy Hastelloy B. However, at room temperature, the corrosion rate in various concentrations of hydrochloric acid is not higher than 0.10 mm / a; in 65 ° C, various concentrations of hydrochloric acid is not less than 0.5 mm / a. Oxygenation in hydrochloric acid has no significant effect on corrosion resistance.
HastelloyC alloy has a corrosion rate of no more than 0.25 mm / a in hydrofluoric acid at room temperature, and a corrosion rate of less than 0.05 mm / a in phosphoric acid below 100 ° C. Under the conditions of corrosion temperature and 55% H3PO4 + 0.8% HF, The corrosion rate is not more than 0.75mm / a. Therefore, it has excellent corrosion resistance in the production of wet phosphoric acid.
HastelloyC alloy is resistant to the corrosion of dry halogen gases other than fluorine, such as chlorine, bromine and iodine. The corrosion rate in dry chlorine gas at 60 ℃ is only 0.0050mm / a. This alloy is one of a small number of materials that can withstand dry and wet chlorine corrosion, and can be used under dry and wet chlorine corrosion conditions.
Hastelloy C alloy is resistant to high temperature HF gas corrosion. The corrosion rate is 0.04mm / a in HF gas below 550 ° C, and the corrosion rate is 0.16mm / a at 750 ° C, so it can be used in HF gas ≦ 750 ° C, and there is oxygen in HF gas It has no significant effect on the corrosion rate.
00Cr15Ni60Mo16W4 alloy, that is Hastelloy C-276, in which the C content does not exceed 0.02% and the Si content does not exceed 0.08%, the precipitation of carbides and intermetallic phases is reduced, and the ability to resist intergranular corrosion is improved. On this basis, further reducing C% and Fe%, removing W and adding carbide stabilizing element Ti, can further improve the resistance to intergranular corrosion, thereby forming 00Cr16Ni63Mo16Ti alloy, which is equivalent to Hastelloy C-4. Both alloys are high-grade nickel-based corrosion-resistant alloys with excellent comprehensive corrosion resistance.
Nickel-based corrosion-resistant alloys are mainly composed of scarce alloy elements such as Ni, Cr, Mo, Cu, etc., which are expensive and limit their scope of application. However, in high-temperature and strong-corrosion environments, especially in high-temperature non-oxidizing acid corrosion environments, nickel-based corrosion-resistant alloys still have positions that other materials cannot replace.
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