Alloy 825 (UNS N08825, 2.4858) is the entry point for nickel alloys in terms of price and performance. Its pitting corrosion resistance is lower than super duplex stainless steels, despite the higher cost, as reflected by a PREN of 31 versus above 40. However, PREN is only an approximation of general corrosion performance and does not tell you how a material will behave in a specific environment. In practice, Alloy 825's resistance across a wide range of oxidising and non-oxidising acids is far better than PREN alone might suggest. It has a relatively modest yield strength but good toughness at temperatures up to 500degC.
Why it resists corrosion
The high nickel content ensures resistance to stress corrosion cracking. Molybdenum and copper strongly resist reducing environments such as sulphuric and phosphoric acids; chromium resists oxidising environments such as nitric acid; titanium inhibits intergranular corrosion; and overall the alloy resists pitting and crevice corrosion.
Phosphoric acid
Phosphoric acid is used predominantly in producing phosphate fertilisers, so any equipment in that process is a candidate application: piping, tanks, mixers, pumps and valves, along with heat exchangers, evaporators, washers and immersion pipes. Ferralium® 255 and 904L are commonly specified for such applications too, at a lower price and higher strength than Alloy 825, which somewhat limits its use. Phosphoric acid is also used in the food and beverage industry and in domestic products, so Alloy 825 can find application in general engineering equipment as an alternative to Alloy 316L.
Sulphuric acid
Sulphuric acid is also consumed in fertiliser production. It is widely used in paper making, both as part of the bleaching process and to control pH during various process stages. Alternative grades in pulp processing and paper making include duplex stainless steels such as Sanmac® 2205 (1.4462, UNS S32205) and a number of lean duplex grades, mostly as piping. Sulphuric acid is also used as a pickling agent for metals, where Alloy 825's good resistance suits components such as heating coils, vessels, boilers, baskets and chains.
Nitric acid
Alloy 825 is also strongly resistant to nitric acid, a strongly oxidising acid. Nitric acid is used in fertiliser production, in the manufacture of chemicals, and in the reprocessing of spent nuclear fuel. For this reason, Alloy 825 has been used to fabricate a wide range of equipment for handling radioactive waste streams and nuclear fuel reprocessing. For instance, it has been used in nuclear fuel element dissolvers, where a variety of corrosive media, such as sulphuric and nitric acids and sodium hydroxide, are handled in the same equipment.
Seawater and marine
Seawater is widely used as a coolant across many industries, and Alloy 825 suits applications where additional contamination increases the corrosive nature of the seawater, such as marine exhaust systems. Super duplex stainless steels would be a more cost-effective solution for land-based scrubbers that run continuously. Marine scrubbers, however, see intermittent service, and when the scrubber is not in operation the temperature at any metal components can greatly exceed the permissible maximum for those alloys.
Oil and gas
The high nickel content makes Alloy 825 virtually immune to stress corrosion cracking. In the oil and gas market, it is used for pipes, tubes and fittings for extraction, in heat exchangers, evaporators, washers, immersion pipes and offshore piping.