Coatings have been used on metal substrates for many years to enhance corrosion resistance, mechanical properties, physical handling and appearance. Typical coatings include inorganic conversion coatings such as phosphates and chromates, direct-to-metal paints, and dry-in-place passivates hereafter referred to as thin organic coatings (TOCs). TOCs, as an evolving technology, have a changing definition, but at least one definition combines the following features: a significant organic resin component, an inorganic component for conductivity and/or corrosion protection, and a dry film thickness of < 5 µm.(1)
An intense research focus is the replacement of chromium, especially hexavalent chromium, given the increasing likelihood of chromium regulation. A growing trend of producing environmentally friendly products and the establishment of European RoHS Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment (2002/95/EC commonly referred to as the Restriction of Hazardous Substances Directive or RoHS, adopted in February 2003 by the European Union) have also contributed to this trend to eliminate hexavalent chromium.(2) A major obstacle to chromium replacement in TOCs is corrosion performance, as non-chromium coatings are generally less protective than chromium-containing coatings.(3)
Two types of TOCs in commercial products are fluoroacid-containing coatings and sol-gel coatings. Fluoroacid coatings use fluorotitanic, fluorozirconic and/or fluorosilicic acids as the inorganic basis for the coating, providing adhesion and barrier properties.(4) Those acids’ aggressive pH and fluoride content make them hazardous to employees and equipment, and are only compatible with a narrow range of organic components.