abstract |
A bonding composition for a heat curable overlay coating for coating a substrate is provided. The bonding composition comprises phosphate ions, permanganate ions, at least one species of metal cation having a valence greater than +1, and water. The bonding composition has a pH in the range from about 1.5 to about 4.5 and is substantially free of chromate ions and molybdate ions. Preferably, the molar ratio of phosphate ions to permanganate ions in the bonding composition is in the range from about 2:1 to about 580:1 and the nonvolatile content of the bonding composition is in the range from about 10% (wt) to about 35% (wt). Aluminum ions are a preferred species of metal cation in the bonding composition. Overlay coating compositions are also provided containing metallic aluminum particles dispersed in the bonding composition of the invention. |