It is well known that the extensive use of magnesium alloy is limited by its poor corrosion resistance. Alloying RE elements into magnesium alloy is one of useful methods to enhance its corrosion resistance. The previous results have revealed that RE elements could act as the nucleations to promote rapid formation of the oxides at high temperature, which significantly increases the oxidation resistance. However, whether RE elements still have effect as their performance at high temperature to improve the corrosion resistance of magnesium alloys in aqueous solution is not clear at present. Therefore, the influnence of RE elements on the formation, stability and pitting corrosion resistance of the passive film formed in alkaline system on AZ91 magnesium alloy was studied in detail from theoretical aspect in this work, which could help people understand the role of RE elements in the formation of passive film. The experimental results are following.
The results of AZ91 alloy containing 1.5 % Ce in 0.01MNaOH system showed that RE element Ce participated in the formation of passive film and enriched as CeO2 in the inner layer. During the formation of passive film, CeO2 acted as a block layer, which changed the machnism of passivation. Ce4+ in CeO2 replaced Mg2+ in passive film with n-type semiconductive property. The replacement increased the carrier density and conductivity, which accelerated the electrochemical reactions. Therefore, the stability of passive film on AZ91 alloy containing 1.5 % Ce decreased.
In the testing system containing Cl-, Ce decreased the pitting corrosion resistance of AZ91 alloy. Ce increased the adsorped concentration of Cl- on the outer layer of passive film. The adsorped Cl- promoted the break down of passive film, which decreased the thickness and enhanced the rupture of passive film under lower anodic potential.
Although RE element decreases the pitting corrosion resistance of AZ91 magnesium alloys in some degree, the magnesium alloys have outstanding machnical and anti-oxidation property, which have extensive application in aviation and spaceflight in future. Therefore, the topic of looking for effective methods to increase the corrosion resistance still attracts more attention in corrosion field. Some studies suggest that nanocrystalline / microcrystalline coating by magnetron sputtering can significantly improve the pitting corrosion resistance. Therefore, GW102K （Mg-10Gd-2Y-0.5Zr）microcrystalline coating has been prepared by magnetron sputtering in this paper and the influence of microcrystallization on the stability and pitting corrosion resistance of passive film has been investigated, which will provide the experimental guidance to find effective techniques improving the corrosion resistance of magnesium alloys.
The results in 0.2MNa2SO4 and 0.6M (3.5wt %) NaCl systems revealed that microcrystallization increased the corrosion current density and decreased the corrosion resistance. Microcrystallization changed the cathodal reactional mechanism and formed hydrides on the surface. Microcrystalline coating has a lot of grain boundaries, which improved the formation of hydrides and accelerated the cathodic reactions. Therefore, the acceleration of cathodic reactions leads to the increase of natural corrosion rate and the decrease of corrosion resistance.
Meanwhile, microcrystallization improved the formation of anodic corrosion products and decreased the anodic current density, which increased the stability of corrosion products. On the other hand, the passive film with more uniform composition and more compact structure formed on microcrystalline coating increased the alloy’s pitting corrosion resistance. The further AFM observation and the analysis of carrier structure in passive film indicated that the corrosion rate of microcrystalline coating decreased with time, which means the pitting corrosion resistance gradually increased. The rich of hydroxides of RE elements Gd and Y in the corrosion products and adsorption of Cl- transformed the semiconductive type of corrosion products on microcrystalline coating, which lead to the increase of pitting corrosion resistance.
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