Temporary corrosion protection of a new generation of magnesium alloys

Abstract

Magnesium (Mg) and its alloys have shown a great potential as a structural material because of their properties and because of the abundance of Mg in the earth crust. Consequently, they have been gaining wider acceptance in industry, especially because of ever tighter limits imposed on the carbon emissions by the governments around the world. Nowadays the Mg alloys are successfully used in many industries including aerospace, automotive and electronics. One factor that limits their more widespread use is the high reactivity and high corrosion rates of Mg and its alloys.Many critical components in military aircraft are manufactured from the Mg alloys EV31A, WE43B and ZE41A, which were designed for high temperature use. Review of the available literature identified significant gaps in understanding the corrosion behaviour of these alloys, especially in the case of EV31A and WE43B as they are new alloys and have not yet received much research attention. One of the gaps identified was the influence of the immersion period on the corrosion of the alloys. Thus, one of the aims of the current research was to understand and characterise corrosion of these Mg alloys in comparison to that of pure Mg. In particular, this research explores the effect of immersion period, the role of second phases, impurities and various solutions.WE43B, immersed in 3.5 wt % NaCl solution had the lowest corrosion rate (0.23 mm y–1), lower than the intrinsic rate of pure Mg (0.4 mm y–1). The corrosion rate of EV31A was somewhat higher than that of WE43B. The low corrosion rate of WE43B was attributed to no corrosion acceleration by the second phases and a more protective film. Furthermore (a) the extent of corrosion correlated with the immersion period, (b) corrosion rates were higher in NaCl solution compared with Na2SO4 solution, attributed to the presence of aggressive Cl– and (c) corrosion rates measured by electrochemical techniques were typically lower than those evaluated from the evolved hydrogen, consistent with the corrosion mechanism involving the Mg+ ion.In military aircraft, the components manufactured from EV31A, WE43B and ZE41A are typically supplied with protective paint system, which can be damaged in service, thus exposing the Mg metal. In such cases, substantial corrosion rates of the Mg alloys could compromise the structural integrity of the naval aircraft, which operate in a harsh marine environment. This in turn, could significantly increase maintenance costs and jeopardise the mission readiness. Consequently, there is a need to apply surface coatings to provide temporary protection against corrosion to the Mg alloy components.Among the many corrosion inhibiting compounds (CICs) are Ardrox 3961, LPS 2, LPS 3 and AMLGuard. These products are petroleum based, containing inexpensive corrosion inhibitors and have been used on mainly copper, aluminium and steels. Thus, another aim of the project was to examine the potential of these corrosion inhibiting compounds (CICs), applied by dip coating, to provide temporary corrosion protection to the Mg alloys in different environments. As such, the second part of the thesis concerns (i) the effectiveness and efficiency of the CICs to inhibit aqueous and atmospheric corrosion and (ii) the mechanism of inhibition of each CIC.The results revealed that all four CICs reduced corrosion of the Mg alloys and pure Mg immersed in 3.5 wt% NaCl solution with various efficiencies. Ardrox 3961 was least effective whilst LPS 3 was most effective, completely suppressing corrosion with an efficiency of 100% in most cases. This was attributed to a thicker and more durable barrier film formed by chemical adsorption by the aromatic long chain hydrocarbons. The atmospheric exposure revealed that both Ardrox 3961 and LPS 2 were ineffective as corrosion inhibitors with corrosion rates comparable to those of the bare alloys and pure Mg. Whilst LPS 3 was effective in inhibiting the atmospheric corrosion, the AMLGuard coating resulted in further reduction of the corrosion rates, with rates an order of magnitude less than of the bare alloys and pure Mg. The mechanism of inhibition of atmospheric corrosion of LPS 3 and AMLGuard was similar to that in corrosion by immersion, whereby the CICs formed a thicker, more-durable, more-protective barrier film by chemical adsorption. It was found that (a) the second phases in alloys had a lesser role in atmospheric corrosion compared with corrosion by immersion, and (b) there was good correlation between the corrosion rates of the bare alloys and pure Mg.