Alkaline resistant phosphate conversion coatings

摘要

A method for increasing the resistance to alkaline dissolution of a phosphate conversion coating on a corrodible metal surface by an improved zinc phosphate conversion coating together with articles coated by this method are disclosed. The substrate is exposed to the phosphating solution by spraying or immersion to chemically effect a reaction with the substrate. First divalent metal cations are selected from magnesium and transition metals having a hydroxide with lower solubility in alkaline solution than zinc hydroxide. The other source of necessary metal cations is zinc and its cations are herein termed second divalent metal cations. In the preferred embodiment, the first divalent metal cations comprise about 84 to about 94 mole percent of the total first and second divalent metal cations in the bath. The deposited coating has first divalent metal cations (chemically combined with phosphate ions) present in an amount of at least 15 mole percent of the total first and second divalent metal cations in the coating.