Effects of surface contamination and pre-exposure on the corrosion rate of Mg alloys

Corrosion resistance is one of the key material properties that are assessed in selecting Mg alloys for automotive applications. For a given alloy, its corrosion rate data can vary considerably depending on the method used in the corrosion test and on the sample surface condition involved. In many automotive applications, high-pressure die cast parts are used in as-cast surface conditions. Surface contaminations derived from the manufacturing process can increase the superficial rate of corrosion. This result was observed in the ASTM B117 tests of AM60 plates with and without the as-cast skin removed. For practical applications it is important to know the corrosion rates of Mg alloys after they have been pre-exposed to a corrosion environment. Contrary to common belief, corrosion products on Mg surface do have some passivating effect and the presence of such products can lead to a reduced rate of corrosion attack during long-term service. In order to investigate the latter surface effect, linear polarization resistance (LPR) measurements were performed using AM60B, AE42, AE44 and AJ62LX alloys that were pre-exposed to GM9540P test solution for various durations. Between the samples pre-exposed for one day and those pre-exposed for seven days, the increase in corrosion resistance, resulting from the pre-exposure, is generally greater than five times. Microscopic analyses were also carried out to characterize the surface corrosion products that were developed during pre-exposure.