Methods for salt contamination of steel corrosion products: A characterization study

It is known that for painted metals the presence of hydrosoluble species within the layer of atmospheric corrosion products, mainly chlorides and sulphates, beneath the coating promotes osmotic blistering and underfilm metallic corrosion when the concentration of the soluble salts exceeds a critical level. However, a more in-depth understanding of the coating degradation mechanisms involved especially the initial stages of blistering, and underfilm corrosion is still missing. On the other hand, from the technical point of view, the determination of safe critical limits for soluble salt contamination at the interface is also necessary. Hence, new methods for artificial saline contamination of steel surfaces must be developed. The work presented here shows that the chloride distribution achieved with salt fog cabinet is much closer to the real case of natural contamination than that achieved with the commonly used homogeneous dosing method. Therefore, it should be used for field-technical studies aimed to establish critical levels of this contaminant at the steel/coating interfaces. On the other hand, it has been also proven that by means of the so-called 'impactor' and 'droplet' methods, very low and ultra-low contaminated samples with well defined crystal size and distribution of chloride can be prepared. They are necessary on research works planned by the authors to study the fundamental aspects of the stability of the contaminated buried steel/coating interfaces.