Improved Iron Phosphate Corrosion Resistance by Modification with Metal Ions

ron phosphate coatings have found a wide range of use in the metalI finishing industry as a convenient method for achieving reasonably high levels of corrosion resistance. Flexibility of processing parameters and ease of use are probably the biggest reasons that iron phosphating enjoys its continued popularity. The corrosion resistance of iron phosphate coatings can be dramatically improved by the selective addition of combinations of certain divalent metals. Excellent results have been attained by adding mixtures of zinc, nickel, and calcium at concentrations below 200 ppm to conventional iron phosphating baths. Changes in the iron phosphate coating brought on by these metal additions have promoted increases in corrosion resistance from 10 to 75%.