Effect of Pretreatment on Performance of Zinc and Zinc Alloy Electroplated Sheet in a Cyclic Corrosion Test

Effect of pretreatments on the performance of zinc and zinc alloy electroplated sheet in a cyclic corrosion environment has been evaluated. It is shown that Zn-Ni and Zn-Fe coatings are less sensitive to variations in the phosphating pretreatment than zinc and steel. These latter two materials benefit from modifications to the phosphate and show better scribe creep resistance when Ni⁺⁺ or Ni⁺⁺ and Mn⁺⁺ containing zinc phosphates are used instead of the unmodified zinc phosphate. A nonchromate post-phosphate rinse improves the scribe creep resistance of steel, but it has little impact on the scribe creep resistance of zinc and zinc alloy coatings. Electrolytic Cr+CrOx pretreatment significantly enhances the scribe creep resistance of zinc. The likely reasons for the improvements are discussed in terms of the physical and chemical characteristics of the pretreatment layers and their stability in the underfilm corrosion environment.