High temperature corrosion

High temperature corrosion is also known as "hot corrosion".

Hot corrosion by sulfates
Two types of sulfate-induced hot corrosion are generally distinguished : Type I takes place above the melting point of sodium sulfate and Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO3.

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 discrete grey/blue colored aluminum or chromium sulfides so that, after the salt layer has been removed, the steel cannot rebuilt a new protective oxide layer.

The same kind of attack has been observed for potassium and magnesium sulfate.

Hot corrosion by vanadium
Most fuels contain small traces of vanadium. The vanadium is oxidized to different vanadates. Molten vanadates present as deposits on metal can flux oxide scales. Furthermore, the presence of vanadium accelerates the diffusion of oxygen through the fused salt layer to the metal substrate.

Hot corrosion by lead
Lead can form a low melting slag capable of fluxing protective oxide scales.