Influence of Ca on Corrosion Resistance of AZ91D

Abstract

By adding 3% Ca in AZ91D, the reticular Al 2 Ca phase on grain boundaries formed and the corrosion rate decreased to 14.1% of the AZ91D corrosion rate in 5 wt % NaCl solution. The corrosion morphologies showed that the reticular Al 2 Ca phase effectively confined the development of corrosion from grain to grain and hence significantly reduced the corrosion rate. The Auger depth profiles indicated the corrosion films on AZ91D + 3% Ca had a three-layered structure: a thick Al enrichment outer layer, an intermediate layer, and a thin protective inner layer. The thin inner layer should consist of an Al 2 Ca phase, which acted as an effective corrosion barrier.Magnesium alloys with Al and Zn have found widespread applications in the automobile sector. Several studies 1-4 have shown that the corrosion behavior of Mg-Al alloys is significantly dependent on the microstructure, particularly on the amount and distribution of the intermetallic phases, and the grain size. Song et al. 5 suggested that the Mg 17 Al 12 phase in an AZ alloy could act as either a corrosion barrier or a galvanic cathode accelerating corrosion. However, finely and continuously distributed Mg 17 Al 12 phase could effectively stop the development of corrosion in an AZ alloy.The corrosion resistance of magnesium-aluminum alloys has been partly explained in terms of increased passivity by incorporation of aluminum, which stabilizes the magnesium oxide layer even though the layer is thin when the amount of Al is increased. [6][7][8] Investigation of the corrosion films formed in water by transmission electron microscopy ͑TEM͒ 9 revealed a three-layered structure, consisting of a hydrated inner layer, an apparently dense intermediate layer, and an outer layer with platelet like morphology. On magnesium-aluminum alloys, it was found that a significant enrichment of the aluminum oxide occurs in the innermost layer. This results in increased stability of the oxide and, consequently, improved corrosion resistance.The purpose of the present paper is to investigate the effect of Ca on corrosion resistance of AZ91D magnesium alloy. Alloying AZ91D with Ca formed reticular Al 2 Ca phase on grain boundary. The corrosion surface morphologies and corrosion cross section morphologies were observed by scanning electron microscopy ͑SEM͒. Changes of composition across the corrosion films of the alloy were investigated in a qualitative manner by Auger in-depth profiling. Open circuit potential tests and potentiodynamic polarization measurements were performed in order to investigate the electrochemical property. ExperimentalIn this work, five new AZ91D alloys with different Ca content were studied and were named AZC alloys. These alloys were prepared with AZ91D and pure Ca. The composition of AZ91D determined by inductively coupled plasma-Auger electron spectroscopy ͑ICP-AES͒ is listed in Table I. AZ91D ingots were melted in an aluminum oxide crucible at 730°C. Appropriate amount of Ca ͑99.9% purity͒ was added into the molten AZ91D alloy. The above procedures were condu...

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