High Temperature Corrosion

High temperature corrosion is an attack on a metal at elevated temperatures in a gaseous environment rather than in a liquid. The most prominent high temperature corrosion reaction is oxidation, although sulfidation and carburization may also occur. Most metals exposed to a high temperature oxidative environment will produce an oxide scale layer which protects the metal from further corrosion. It uniformly covers the entire surface. Ionic transport through the scale is the rate controlling process. The corrosion rate will normally decrease after the scale is produced following a parabolic relationship with time. In severe corrosive environments where a protective scale cannot form, the corrosion rate will follow a more linear path.

Sulfidation occurs when the concentration of sulfur gas is high enough such a sulfide layer forms. Sulfides are less stable and grow much faster than oxides. As a result, sulfides react more readily with metals and penetrate deeper into the metal. They are replaced with the more stable oxides as reactions continue to occur. It is preferred in such environments to have a protective oxide scale first produced, which then protects the metal against subsequent sulfidation.

Hot corrosion is a term describing the high temperature attack of gas turbine engine components in the path of hot gases. It is a sulfidation process involving the formation of condensed salts containing sodium sulfate and/or potassium sulfate. Increasing the chromium content in the metal alloys improves the corrosion resistance but also results in decreased strength.

Carburization is a rare form of high temperature corrosion where carbon atoms are absorbed into a metals’ surface. It only occurs in environments with a very low oxygen partial pressure. Austenitic stainless steels are susceptible under such conditions due to the high solubility of carbon in austenite. Alloying studies to reduce carburization have shown that silicon, niobium, tungsten, titanium, and the rare earth metals increase resistance. Elements which increase damage include lead, molybdenum, boron, cobalt, and zirconium.

Metals’ Susceptible to High Temperature Corrosion

Although high temperature corrosion testing, especially on superalloy materials for gas turbine applications, has been conducted, no qualitative relationship has been determined. Materials are selected for corrosion resistance dependent upon their comparative rates of attack from tests and from field experience.

Managing High Temperature Corrosion

Methods to reduce high temperature corrosion include:

• Proper metal selection.
• Change in operating conditions.
• Structural designs to limit corrosion.
• High temperature barrier coatings (ceramics)