Lack of adhesion of paint in galvanised steel strip

Specimens of galvanised and painted steel sheets were analysed by means of confocal and scanning electron microscopy to determine the reasons for paint detachment from the surface during manufacturing operations of home and automotive appliances. It was found that samples showing paint adhesion failure presented a thicker galvanised layer, compared to painted samples without failure and galvanised only samples. In addition, enrichment of carbon and oxygen was localised at the top of the galvanised coating, which points to the formation of a layer made of oxides, hydroxides and carbonates of zinc at this location. The formation of this layer can be responsible for reducing the adhesive strength established between the galvanised coating and paint, therefore favouring its removal when subjected to external stresses. The mechanisms responsible for paint detachment and subsequent removal are discussed.