Corrosion Prevention by Adsorbed Organic Monolayers and Ultrathin Plasma Polymer Films

Abstract

Traditionally, corrosion protection of many reactive materials is associated with organic coatings, which are applied as thick multilayer films with thicknesses ranging from some 10 μm to some 100 μm. However, increasing interest exists in thin and ultrathin corrosion protecting films. This is particular true for modern areas of materials research such as microelectronic devices or micromechanics. In both cases, any corrosion protecting film must be much less than 0.1 μm in thickness and frequently even monolayer films are wanted. However, even for conventional materials such as steel, galvanized steel, or Al-based alloys, thin film coatings are of considerable interest as frequently such films are generated with unusual properties by modern surface technology. For such applications the time scale for film preparation must be in the seconds range and many techniques allow only the preparation of thin films under those restrictions. Then the thin layer coating may serve as an inner film only with top coats prepared by conventional techniques. The possible role for the thin film then lies in corrosion protection as well as in adhesion promotion of the additional polymeric layers. This chapter will focus on two surface modification techniques: molecular self-assembly and plasma polymerization. The corrosion rate of reactive metals can be reduced significantly by a modification of the metal surface by organic molecules or polymers. A well-known example is the corrosion protection by lacquers and other organic coatings [1–3], which is used in most practical applications to protect, e.g., cars against atmospheric corrosion, pipelines against corrosion in humid soil, and ships against corrosion in seawater. It has long been believed that the corrosion protection is due to the barrier properties of the coating, which impedes the penetration of water and oxygen [4] to the metal/polymer interface. However, many coatings are highly permeable for water and oxygen, and therefore it is not the barrier effect on the diffusion process that gives rise to the corrosion stability but the specific electrochemical properties of the metal/polymer interface, in particular the formation of an extended diffuse double