Pyrophosphate conversion coating of zinc surface - using pyrophosphate- and nitrate-contg. soln. of controlled pH

摘要

A zinc pyrophosphate-based protective coating is formed on a zinc surface using a soln. contg. 0.01-0.2 (pref. 0.05-0.1) moles/l pyrophosphate ions or precursors (expressed as pyrophosphate ions) and nitrate ions in a molar ratio nitrate ions:pyrophosphate ions of max. 2, pref. 0.4-1.2, esp. about 1, the pH of the soln. being adjusted to 3-6.5, pref. 3.2-6.4, esp. 4.2-5.3. The soln. pref. also contains Zn(2+) ions in amt. max. 0.02 (pref. 0.0001-0.01, esp. 0.001-0.002) moles/l. The process is esp. useful for treating galvanised ferrous metals. A highly corrosion resistant coating is formed using a soln. which is less acidic and less expensive than prior art solns.. Formation of pptes is reduced and the soln. is easily regenerated.