Corrosion without mechanical loading is understood to be corrosion forms in which mechanical loading is not relevant to the damage caused by the corrosion process itself. This does not mean that no mechanical overloading can occur during or after the corrosion process; for example, overloading resulting from sectional weakening or the notch-effect of corrosion pittings. Corrosion forms without mechanical loading often occur in connection with cell action due to contact with another engine part and/or at contaminants (e.g. wear products) or a material inhomogeneity near the surface. The most common corrosive medium is moisture with deposits from an ocean atmosphere. However, maintenance and manufacturing processes can also result in attacks corrosive media. During manufacture or repair, there is an increased danger for unprotected parts which have not yet have had coatings applied or have had coatings removed.
Through the use of new materials such as carbon fiber-reinforced plastics or synthetic based, metal powder-filled rub coatings, corrosion can occur even in modern engines with blading and housings made from titanium and nickel alloys.
The use of chemically noble tip armor (to minimize rubbing wear; WC/Co, etc.) and erosion protection coatings (such as TiN) on steels and Al alloys can result in cell action and pitting corrosion.
Figure "Marine atmosphere": Sea atmospheres contain most of the corrosively acting substances that can cause corrosion in engines. Aside from chlorine, the most corrosive substance, sea air also includes compounds such as NaCl, MgCl2, and MgSO4,, both in watery media as well as in dry deposits. After chlorine, sulfur and sodium are important causes of high temperature corrosion forms (such as sulfidation, Chapter 5.4.5).