Alloy 825 (UNS N08825, Incoloy 825, 2.4858) is the entry point for nickel alloys in terms of price and performance. Pitting corrosion resistance is less than super duplex stainless steels, despite the greater cost, as defined by a PREN value of 31 versus >40. However, the PREN value is an approximation to general corrosion performance and does not inform actual performance in a specific environment. Alloy 825’s corrosion resistance in a wide range of oxidising and non-oxidising acids is far better than might have been anticipated. It has a relatively modest yield strength, but exhibits good toughness at temperatures up to 500degC.
The high nickel content ensures resistance to stress corrosion cracking, combined with molybdenum and copper to strongly resist reducing environments such as sulphuric and phosphoric acids, chromium to resist oxidising environments such as nitric acid, titanium to inhibit intergranular corrosion, and overall resistance to pitting and crevice corrosion.
Phosphoric acid is predominantly used in the production of phosphate fertilisers. Therefore, any equipment used in process could be a candidate application, including piping, tanks, mixers, pumps and valves. Also heat exchangers, evaporators, washers and immersion pipes could utilise Alloy 825. Ferralium 255 and 904L are commonly specified for such applications, at a lower price and higher strength than Alloy 825, somewhat limiting applications. Other uses of phosphoric acid are in the food and beverage industry and also domestic products, meaning that it can find application in general engineering equipment as an alternative to Alloy 316L.
Sulphuric acid is also consumed in the production of fertilisers. However, it is also widely used in paper making as part of the bleaching process and to control pH during various process stages. Alternative grades to Alloy 825 used in pulp processing and paper making include duplex stainless steels (Sanmac 2205, 1.4462, UNS S32205) and a number of lean duplex stainless steel grades, although mostly as piping. Sulphuric acid is also used as a pickling agent for metals. Components such as heating coils, vessels, boilers, baskets and chains can make use of the good resistance to sulphuric acid of Alloy 825.
Alloy 825 is also strongly resistant to nitric acid, which is a strongly oxidising acid. It is again used in the production of fertilisers, but also in the manufacture of chemicals, and also the reprocessing of spent nuclear fuel. For this reason it has been used to fabricate a wide range of equipment associated with the handling of radioactive waste streams and nuclear fuel reprocessing. For instance, it has been used in nuclear fuel element dissolvers where a variety of corrosive media, e. g. sulphuric and nitric acids and sodium hydroxide, are handled in the same equipment.
As seawater is widely used as a coolant in many industries. Alloy 825 will find use in applications where there may also be additional contamination of the seawater that increases the corrosive nature, such as marine exhaust systems. Super duplex stainless steels would provide a more cost-effective solution for land-based scrubbers i.e. power stations that are operated continuously. However, as marine scrubbers see intermittent service, the temperature seen by any metal components can greatly exceed the permissible maximum for these alloys when the scrubber is not in operation.
The high nickel content of Alloy 825 makes it virtually immune to stress corrosion cracking. Applications in the Oil & Gas market include pipes, tubes and fittings for oil and gas extraction in heat exchangers, evaporators, washers, immersion pipes and offshore piping.
Incoloy is a trademark belonging to Special Metal Inc and applied to a family of nickel–iron–chrome alloys.