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

摘要

Both Electroless nickel coated ad chromated surfaces of magnesium alloy AZ31 and hydrothermally sealed anodized surfaces of unalloyed aluminium Al-2S and aluminium alloy 6061 have been prepared and the corrosion behaviour of these surfaces in aqueous KCl and K_2SO_4 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 K_2SO_4 solutions. The effect of anodizing 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 Al-2S and AA 6061 attains a saturation value after anodisation for approx 45 min.