DFO: Too much of a good thing

Time and again, coating problems such as adhesion problems or insufficient corrosion resistance are caused by inadequate pre-treatment. In this case, excessive concentrations of substances in a silane-based pre-treatment process led to adhesion problems.

Excessive concentrations of zirconium-containing compounds in the bath lead to thick and brittle passivation layers that tend to break cohesively. To ensure optimum passivation quality with zirconium-containing passivations, bath hygiene and the relatively narrow tolerances for the material concentrations are the most important parameters that must be observed during the process.

The condensation constant climate test is suitable for detecting pre-treatment defects. In this case, coated components exhibited severe blistering under the coating after storage in a constant condensation climate. The steel component had been treated with a silane-based, zirconium-containing passivation. At the beginning of the investigations, the chemical composition of the coating material and the substrate was determined using energy dispersive X-ray spectroscopy (EDX). The underside of the coating in the area of the bubble showed high amounts of iron oxide and zirconium. This was an indication of a cohesive fracture within the passivation layer.