Formation of Zn-Ni alloys in the phosphating of Zn layers

The addition of Ni2+ to phosphating baths is a common practice in order to improve the corrosion resistance and the adhesion properties of phosphate coating layers. Although the major part of Ni2+ is incorporated in the structure of phosphate crystals, modifying its nucleation and growth rate, Ni deposition takes also place by a displacement reaction on corroding Zn in the coating pores. This, in turn, leads to a continuous increase of the Ni content by formation of rich Ni surface alloys of different compositions. Due to the good ionic and electronic insulating properties of the phosphate layer, the corrosion potential gives direct information about the changes of surface composition during the phosphating process in the bottom of pores. In this work, we analyse the time dependence of the cementation process and its interaction with the growing phosphate layer by means of simultaneous measurement of capacity and rest potential during phosphating. Additionally, the increase of incorporated Ni was qualitatively followed by X-ray fluorescence, voltammetry and SEM/EDX analysis.