Colloid chemical aspects of corrosion of metals

Abstract

The corrosion of metals and alloys results in the formation of soluble hydroxylated complexes and precipitated metal hydrous oxides, with the end-products almost invariably in colloidal state as films, particles, or particle aggregates. It is, therefore, obvious that in the course of the corrosion process the materials undergo chemical and morphological changes. Even if precipitation does not occur, the solution adjacent to the metal surface will have an entirely different composition than the bulk liquid phase. Little proven information is available or the chemical mechanism of the processes involved in the precipitation of metal (hydrous) oxides. This is, in part, due to the complexity of reactions involved in the solid phase formation and to the sensitivity of precipitation processes to various parameters, making it difficult to reproduce a given material. Recently, it was possible to develop well defined colloidal metal hydrous oxides which can serve as models for corrosion products. These systems consist of particles uniform in chemical composition, size, and shape, that can be repeatedly generated under reproducible conditions. The so prepared monodispersed sols are used to elucidate the chemical mechanism of metal corrosion processes, to study the properties of the model corrosion products (surface, electric, magnetic, optical, etc.), to investigate their interactions with various additives (chelating agents, etc.), and, finally, to establish factors governing their adhesion or the detachment from different substrates. Examples of these model dispersions are shown and various studies, relevant to corrosion of metals in general and of iron in particular, carried out with such materials are discussed.