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The corrosion of Mg alloy Mg-10Gd-3Y-0.4Zr, in the as-cast (F), solution treated (T4) and aged (T6) conditions, was investigated in 5% NaCl solution by immersion tests and potentiodynamic polarization measurements. The as-cast (F) condition had the highest corrosion rate due to micro-galvanic corrosion of the a-Mg matrix by the eutectic. Solution treatment led to the lowest corrosion rate, attributed to the absence of any second phase and a relatively compact protective surface film. Ageing at 250°C increased the corrosion rate with increasing ageing time to 193 h attributed to increasing micro-galvanic corrosion acceleration of the Mg matrix by increasing amounts of the precipitates. Ageing for longer periods caused a decrease in the corrosion rate attributed to some barrier effect by a nearly continuous second-phase network. Electrochemical measurements did not give accurate evaluation of the corrosion rate in agreement with the immersion tests.

The corrosion behaviour of AZ91D and NZ30K (Mg-3Nd-0.2Zn-0.4Zr) alloys was investigated in 5% NaCl solution by immersion tests and electrochemical measurements. The immersion tests showed that the corrosion rate of NZ30K was about half that of AZ91D. The localized corrosion of AZ91D was more severe than that of NZ30K due to the higher cathode-to-anode area ratio and higher difference in potential of the cathode phase and the anode matrix. The corrosion of AZ91D concentrated on certain areas and resulted in much deeper corrosion pits, whilst that of NZ30K spread across the surface and led to more uniform and shallow corroded areas. The corrosion products of NZ30K were more compact than those of AZ91D as indicated by SEM. The cyclic polarization curves showed that NZ30K had higher pitting corrosion resistance than AZ91D.

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