Conversion coatings are generally required to enhance organic coating adhesion and corrosion resistance on galvanized steel. Until a few decades ago, chromate conversion coatings were the most common conversion coatings in the industry owing to their exceptional performance in this regard. However, the adverse effects associated with hexavalent chromium as found in chromate conversion coatings and certain corrosion inhibitor pigments are now widely known. As a result, various initiatives have been deployed around the globe to restrict and regulate the use of hexavalent chromium. Finding suitable alternatives for chromate conversion coatings has therefore become one of the most pertinent research topics of the moment in the field of corrosion protection.
A number of chromium-free conversion coatings have been found to show comparable corrosion resistance relative to chromate conversion coatings. For galvanized steel, conversion coatings based on zirconium and titanium have been found to be suitable alternatives to chromium. Organic additives may be incorporated in these conversion treatment solutions for galvanized steel to enhance the adhesion of organic coatings to the substrate. The effect of a given organic additive is highly dependent on the surface composition of the substrate as has been observed with the polymers polyacrylic acid (PAA), polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) on hot-dip galvanized steel and a Zn-Mg-Al alloy coated steel studied in this project.
The durability of corrosion protection may also be enhanced by embedding corrosion inhibitors in the organic coating. An increased concern for sustainability and environmental-friendliness, has resulted in a global effort toward developing and using corrosion inhibitors which are safe for the environment and for human health. The electrochemical behaviour of galvanized steel in electrolytes with green inhibitors based on silicates, phosphates, zinc oxide and calcium at various concentrations are investigated.
This project considers a multi-layer corrosion protection coating system comprising a conversion layer based on Zr/Ti cations and adhesion-enhancing polymer additives, and an organic coating embedded with 'green' corrosion inhibitors. The overarching aim is to investigate and establish new knowledge regarding the effect of polymer additives in conversion coatings on the adhesion of organic coatings as well as the identification of suitable green corrosion inhibitors. The outcome is an indication of which types of polymer additives and corrosion inhibitors work best for the two substrates tested – MagiZinc® and hot-dip galvanized steel, both supplied by Tata Steel BV.