Combination of chemical conversion with water-based nanoparticle coatings for the corrosion protection of magnesium alloys

摘要

Magnesium and its alloys have an excellent strength to weight ratio and are attractive materials in cars, aircrafts or high end portable devices. Nevertheless magnesium is very reactive and does not form a naturally passivating oxide layer, like titanium or aluminium. Therefore its alloys need additional corrosion protection. One widely used technique is chemical conversion. These conversion layers are excellent primers for organic coatings. Established chromate conversion coatings, providing excellent corrosion protection with self-healing abilities, cannot be used anymore due to the toxicity of Cr(VI). Since most non-chromate conversion coatings have inferior self-healing abilities, new developments in this field are demanded by industry. A new approach to develop conversion coatings with improved properties is the combination of chemical conversion with nanoparticulate coatings. The nanoparticles are added to established conversion solutions and are deposited simultaneously during the formation of the conversion layer. Scheme 1 shows this process. Here the main focus is the increase of the barrier properties of the coating by an increase of the coating thickness and the abrasion resistance. First experiments showed a good miscibility of commercially available aqueous nanoparticle dispersions with commercial conversion solutions. The performance of pure conversion coatings and pure nanoparticle coatings are compared to coatings prepared from combined nanoparticle-conversion solutions. The influence of dipping time, pH and post-treatment on the coating performance is discussed.