Fundamental investigation on the stability of the steel/coating interfaces contaminated by submicroscopic salt particles

It is known that the presence of hydrosoluble species, mainly chlorides and sulphates, at the metal/paint interface, promotes osmotic blistering of the coating and underfilm metallic corrosion when the concentration exceeds a critical level. However, since the interface is not easily accessible by experiment, not much is known about interfacial structure, reactions and the processes causing degradation. Scanning Kelvin Probe (SKP) is an ideal tool for the in situ monitoring of the degradation processes at the buried interface. In the present paper, a study of the fundamental aspects of the stability of the contaminated buried steel/coating interfaces, specially focused on the initial stages of blistering and underfilm corrosion, has been carried out. Results of the characterization study performed with a height-regulated SKP (HR-SKP) on coated steel contaminated at the interface by different salt concentrations with well-defined crystal size and distribution are shown. An effect of crystal size and distribution has been observed. While, small crystals or low density of salt spots do not show macroscopic blistering, bigger crystals or a local accumulation of them can cause significant de-adhesion and macroscopic blistering.