Influence of activation on performance of nickel–phosphorous coating

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 K 4 P 2 O 7 and NH 4 HF 2 respectively. The first activation in the K 4 P 2 O 7 solution could clean the surface and remove Mg 3 (PO 4 ) 2 dust produced by pickling. The coating with proper 1?12-1?49 F/O ratio (at-%) composed of MgF 2 and MgO or Mg(OH) 2 was produced via second activation in NH 4 HF 2 solution. Excess hydrogen produced by Mg with H þ 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.

Effect of pH on the characteristics of electroless Ni-P coatings

“…One of the most important requirements for electroless coatings is to achieve proper adhesion between the substrate/coating interfaces [18][19]. In case of steel substrates, the adhesion improving effect of nitriding process has also been reported [20].…”

Influence of pickling time on electroless Ni–P coating on magnesium alloy

“…The main reason is attributed to the formed MgF 2 particles on the substrate surface, which can provide corrosion resistance to the matrix and hinder the oxidization and dissolution of the substrate. [ 15 ] So the solution B with NH 4 HF 2 is more suitable as a pickling solution for electroless Ni–P plating of Mg alloy.…”

“…It is well‐known that fluoride could prevent the Mg alloy substrate from further corrosion, and the oxides acted as active sites in the acidic plating bath. [ 15 ] The substrate surface with low F/O ratio can provide more active sites for nickel deposition, but the substrate dissolution and rapid replacement reaction between Mg and Ni 2+ lead to the loose Ni–P coating. [ 29 ] Table 3 shows that the F/O ratio of the substrate surface is always very low as the pickling time increases.…”

“…The conventional processing of pretreatment is acid pickling by chromic oxide and nitric acid, followed by activation in hydrogen fluoride. [15,16] However, chromic oxide is forbidden due to its carcinogenic effect and environmental hazards. [17] Therefore, exploiting an environment-friendly pretreatment process has been a research hotspot in recent years for electroless plating of magnesium alloy.…”

Activation of zinc phosphate coating by silver nitrate pretreatment

“…As presented in Figure 4, the activated phosphate coating has more positive corrosion potential and lower corrosion current density, which corresponds to a lower corrosion rate [31] (1.49 × 10 −2 mpy), compared to bare steel surface and inactivated phosphate coating (11.1 × 10 −2 mpy and 3.85 × 10 −2 mpy, respectively). The corrosion current density of the activated coating was reduced by 86.5 and 61.3%, with respect to the bare steel surface and the inactivated coating, respectively.…”