Nickel is the primary alloying element for forming austenite, but its effects are fully realized only when combined with chromium. If nickel is used alone, to achieve a pure austenitic single-phase structure in low-carbon nickel steel, the nickel content needs to be above 24%. In fact, the nickel content needs to reach 27% to significantly enhance the corrosion resistance of stainless steel. Therefore, nickel is not used as a sole alloying element in stainless steel. When nickel is combined with chromium, its ability to enhance the corrosion resistance of steel becomes prominently evident.
For instance, by adding a small amount of nickel to ferritic stainless steel, the microstructure transitions from single-phase ferrite to a duplex structure of austenite and ferrite. This transformation allows for strength improvement through heat treatment. With further increases in nickel content, a single-phase austenitic structure can be achieved. For example, adding 8% nickel to a steel with 18% chromium yields a fully austenitic structure. This is the reason behind the widespread application of the 18-8 type chromium-nickel austenitic stainless steel, as it possesses high corrosion resistance, good formability, weldability, and is non-magnetic.
Nickel expands the passivation range, enhancing corrosion resistance, particularly in non-oxidizing media (such as dilute sulfuric acid).
Nickel has a weaker tendency to form carbides compared to iron, promotes graphitization, and slightly increases the hardenability of steel. In medium-carbon and high-carbon steels, when the nickel content reaches an effective level, retained austenite tends to be preserved after quenching.
Furthermore, adding nickel to chromium stainless steel enhances its corrosion resistance in sulfuric acid, acetic acid, oxalic acid, and neutral salts (especially sulfate salts).