Sulfidation: High temperature turbine blade corrosion
Mechanical erosion like pitting, FOD, cracking, and burning is the main form of damage to engine compressor and turbine sections, with corrosion usually reserved for the parts of the airplane in closest contact to the atmosphere.
But there is one form of corrosion that exists in the blistering environment of a turbine engine hot section, and it is called sulfidation (or sulphidation in some texts). The galvanic corrosion that eats away aircraft structures exists at normal temperatures and is an electro-chemical process. Sulfidation is a chemical process that occurs in high temperature environments, and typically has two types: Type I sulfidation occurs between about 1,500 F to 1,750 F (825 C to 950 C), and Type II sulfidation occurs in the 1,300 F to 1,500 F range (700 C to 800 C).
One definition of sulfidation is a reaction of a metal or alloy with some form of sulfur to produce a sulfur compound that forms on or under the surface of a metal or alloy. Sulfur in the fuel and airborne salts like sodium and chlorine reacts with the oxide layer on the blades in the high temperature environment of the turbine to attack the base metal of the blades. As a normal by-product of combustion, sulfur oxides are formed that combine with the salts and other elements ingested into the engine. This reaction forms sodium sulfates that expose the blade's protective oxide layer to decay. Water is also produced as a by-product of hydrocarbon fuel combustion, and this water can combine with the sodium sulfur compounds to produce sulfuric acid.