Thin metallic films for corrosion control

Metallic alloys and composites, often engineered for a desired combination of mechanical properties, may not have sufficient resistance to corrosion, and a coating is usually proposed as protection. Many coatings have good corrosion resistance themselves, but are not adherent or do not offer protection when penetrated. Indeed, corrosion resulting from penetrations may be especially disastrous. However, it is possible to achieve metallic coatings with excellent adherence which protect substrates even when relatively large areas are breached. A suitable choice of deposition processing, film composition and film structure is necessary. The current situation with respect to aqueous, especially saline, corrosion is reviewed. The roles of microcrystalline and amorphous structures and of open circuit potentials (Eco) are discussed. In general, it is best to have an amorphous or microcrystalline film with an Eco value more negative than that of the substrate. Excellent adhesion and structure may be achieved by processes such as sputtering or ion plating which employ energetic particles. (orig.)