Alloy 825

The chemical composition of the alloy provides exceptional resistance to many corrosive environments. The nickel content is sufficient to provide resistance to chloride ion stress corrosion cracking. The nickel, combined with molybdenum and copper, also provides excellent resistance to reducing environments such as sulphuric and phosphoric acids. The molybdenum also aids resistance to pitting and crevice corrosion. The chromium content of the alloy provides resistance to a wide range of oxidising agents such as nitric acid, nitrates and oxidising salts. The addition of titanium, with appropriate heat treatment, stabilises the alloy against sensitisation to intergranular corrosion.

Alloy 825, also known as Incoloy 825, is a family of austenitic stainless steels that is part of the nickel-iron-chromium alloy series. It is renowned for its excellent resistance to stress corrosion cracking in the presence of chloride ions and has good oxidation and reduction resistance in acidic environments, as well as resistance to pitting and crevice corrosion. This alloy is commonly used in chemical processing equipment, oil and natural gas production equipment, and desalination systems.

The corrosion behavior of Incoloy 825 alloy in 3.5% NaCl solution at different temperatures is influenced by temperature. As the temperature increases, the degree of corrosion intensifies, and the form of corrosion changes from general corrosion to localized pitting corrosion. Moreover, the sulfidation corrosion characteristics of Incoloy 825 under subcritical/supercritical water conditions have also been studied, and it was found that under supercritical water conditions, the thickness of the corrosion layer of Incoloy 825 alloy is 4.26 μm, while Inconel 625 alloy forms a corrosion layer with an outer layer of metal sulfide and an inner layer of metal oxide.

Although Incoloy 825 and Inconel 825 have similar names, they are two different alloys. Incoloy 825 emphasizes corrosion resistance, especially in environments rich in chlorides, while Inconel 825 is a superalloy designed for high-temperature oxidizing environments, with excellent oxidation resistance, strength, and creep resistance, making it suitable for high-load applications under extreme conditions.

When welding Incoloy 825 with an electron beam, a good weld joint can be obtained. The weld area is composed of large equiaxed grains and a few columnar crystals, with no obvious element depletion in the weld area. The hardness of the weld and the heat-affected zone reaches the hardness value of the base metal, and the tensile strength of the weld joint reaches 600 MPa, which is close to the tensile strength of the base metal.

The chemical composition standard of Alloy 825 is strictly stipulated, mainly including elements such as nickel, chromium, iron, and copper, as well as a small amount of molybdenum, titanium, zirconium, and other elements. The content of carbon, silicon, manganese, and other elements also has certain requirements, and the content of trace elements such as sulfur, phosphorus, and boron also has an important impact on the performance of the alloy.

In supercritical water (SCW), the susceptibility of Alloy 825 to stress corrosion cracking increases with temperature. Its corrosion weight gain generally follows the power function growth rule, while the weight gain change under subcritical conditions shows a characteristic of first decreasing and then increasing.

Chemical Composition

Alloy 825 has good mechanical properties from cryogenic temperatures to moderately high temperatures. However, exposure to temperatures above 1000° F can result in microstructural changes that significantly lower ductility and impact strength. Alloy 825 should not be used at temperatures where creep-rupture properties are design factors.