Two types of sulfate-induced hot corrosion are generally distinguished: Type I takes place above the melting point of sodium sulfate, whereas Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO.
In Type I, the protective oxide scale is dissolved by the molten salt. Sulfur is released from the salt and diffuses into the metal substrate, forming grey- or blue-colored aluminum or chromium sulfides. With the aluminum or chromium sequestered, after the salt layer has been removed, the steel cannot rebuild a new protective oxide layer. Alkali sulfates are formed from sulfur trioxide and sodium-containing compounds. As the formation of vanadates is preferred, sulfates are formed only if the amount of alkali metals is higher than the corresponding amount of vanadium.
The same kind of attack has been observed for potassium sulfate and magnesium sulfate.