Electrochemical impedance measurements on Ni–P coated magnesium alloy,chromated magnesium alloy,and anodised aluminium alloys in aqueous salt solutions

Abstract

Both electroless nickel coated and chromated surfaces of magnesium alloy AZ 31 and hydrothermally sealed anodised surfaces of unalloyed aluminium Al-2S and aluminium alloy 6061 have been prepared and the corrosion behaviour of these surfaces in aqueous K Cl and K₂SO₄ solutions has been studied using electrochemical impedance spectroscopy (EIS). The effect of the concentration of hypophosphite in the plating bath used for electroless plating on the Ni-P layers formed and their corrosion behaviour has also been studied. The EIS studies have shown that the diffusion of ionic species is less for yellow chromated surfaces of magnesium alloy than for black chromated ones. In the case of anodised aluminium surfaces, impedance values are much higher in KCl than in K₂SO₄ solutions. The effect of anodising time on pore resistance R _po and barrier capacitance C_b has also been studied. Data from EIS indicate that the pore resistance of aluminium alloys A1-2S and AA 6061 attains a saturation value after anodisation for ~45 min.