7471 Alloy 825

Alloy 825 is developed from alloy 800 with the addition of molybdenum (3%), copper (2%), and titanium (0.9%) for providing improved aqueous corrosion resistance in a wide variety of corrosive media. Its high nickel content of about 42% provides excellent resistance to chloride-ion stress corrosion cracking, although not immune to cracking. When tested in boiling magnesium chloride solutions, this alloy has been an upgrade to the 300 series stainless steels, when localized corrosion and stress corrosion cracking has been a problem. The high nickel in conjunction with the molybdenum and copper provides good resistance to reducing environments such as those containing sulfuric and phosphoric acids. Laboratory test results and service experience have confirmed the useful resistance of alloy 825 in boiling solutions of sulfuric acid up to 40% by weight and at all concentrations up to a maximum temperature of 66° C. In the presence of oxidizing species other than oxidizing chlorides, which may form HCl by hydrolysis, the corrosion resistance in sulfuric acid is usually improved. Hence the alloy is suitable for use in mixtures containing nitric acid, cupric acid, and ferric sulfates. In pure phosphoric acid the alloy is resistant at concentrations and temperatures up to and including boiling 85% acid. The alloy's high chromium content confers resistance to a variety of oxidizing media such a nitric acid, nitrates, and oxidizing salts. The titanium addition with an appropriate heat treatment serves to stabilize the alloy against sensitization to intergranular attack.

Some typical applications include various components used in sulfuric acid pickling of steel and copper, components in petroleum-refineries and petrochemicals (tanks, agitators, valves, pumps), equipment used in production of ammonium sulfate, pollution control equipment, oil and gas recovery, acid production, nuclear fuel reprocessing and handling of radioactive waste, and phosphoric acid production (evaporators, cylinders, heat exchangers, equipment for handling fluorsilicic acid solution, and many others). Alloy 825 is a versatile alloy handling a wide variety of corrosive media but has begun to be gradually replaced in the industry by other alloys due to their superior localized corrosion resistance, such as the G family alloy and the 6% moly superaustenitic stainless steels, such as alloy 1925hMo (N08926) and alloy 31 (N08031).

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