Dissociative mechanism of scales growth on metals and alloys

The present paper is an attempt to reconsider the theoretical background of the formation of microporosity in scales on metals and to present convincing experimental proofs of the validity of dissociative growth theory, explaining the mechanism of this phenomenon. This theory is based on the assumption that, due to limited dimensions of the oxidized samples, the metal consumption zone cannot be continuously compensated by the plastic flow of the scale growing owing to the outward diffusion of cations. As a consequence, a crack develops at the metal-scale interface, following the decomposition (dissociation) of the primary compact scale layer and the formation on the metal surface a porous inner layer. The dissociation of the outer layer proceeds mainly along the perpendicular-to-the-core grain boundaries of large columnar crystals of this layer, resulting in the formation of dissociation fissures, enabling eventually the inward diffusion of the oxidant. Direct experimental proofs of the formation of dissociation fissures and of the mechanism of double-layer scale growth on metals and binary alloys, obtained with the use of radioactive isotope of sulphur S-35 and stable isotope of oxygen O-18, have been presented. All these results may be considered as an ultimate demonstration that the specific micro-porosity in scales growing by the outward diffusion of cations has nothing in common with mechanical stresses developed in metal-scale-oxidant system during the reaction.