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Abstract
In order to optimise the activation process, the effect of three activation processes on the direct electroless nickel–phosphorous plating on AZ91D was studied. The properties of the activation coating and Ni–P coating were characterised with electrochemical tests, SEM and EDX. The results showed that an optimum activation coating for nickel deposition was developed via twice activations of K4P2O7 and NH4HF2 respectively. The first activation in the K4P2O7 solution could clean the surface and remove Mg3(PO4)2 dust produced by pickling. The coating with proper 1·12–1·49 F/O ratio (at-%) composed of MgF2 and MgO or Mg(OH)2 was produced via second activation in NH4HF2 solution. Excess hydrogen produced by Mg with H+ in the plating bath could lead to high porosity and low adhesion of Ni–P coating. A compact Ni–P coating with better adhesion and corrosion resistance was explored in this research.