Protective Properties of a Sol-Gel Coating on Zinc Coated Steel

Galvanised and galvannealed steels are widely used due to their good corrosion resistance in aqueous solutions. However, when additional protection is required, organic coatings, corrosion inhibitors or conversion coatings are used to improve their corrosion protection. In this work, sol-gel coating was used to improve the corrosion behaviour of these two materials. This paper analyses the final protective properties of a sol-gel coating prepared by basic catalysis and its dependence on the sintering temperature and time of treatment. The influence of the sintering conditions on the galvanised and galvannealed substrates is a decisive factor for the coating quality and for the barrier affect against the aggressive media. While heat treatment time is the controlling factor for the galvannealed steels, the temperature is determining in the case of the galvanised. Corrosion mechanisms for sol-gel galvanised steels did not changed with respect to the uncoated steel. However for galvannealed steel, after coating the mechanism is not purely cathodic.