The Role of Microstructure in the Atmospheric Corrosion of Selected Light Alloys and Composites

Doctoral thesis, 2016

Magnesium-aluminum (Mg-Al) alloys, Mg-based metal matrix composites (MMCs) and Al alloys are among the best material candidates for future lightweight solutions. However, the use of these materials is limited by a number of issues, including the ability to resist corrosion. For example, Mg alloys and Mg-based MMCs exhibit relatively poor corrosion resistance in many environments and there are issues in the corrosion behavior of Al alloy weldments. This thesis aims to provide new knowledge on the role played by microstructure for the corrosion properties of light materials. The corrosion studies were done using well-controlled laboratory exposures and lasted 10-3200 h. The rate of corrosion was determined and the composition of the corrosion products was investigated using several techniques. The alloy microstructure and the microstructure of corrosion was investigated using a wide range of techniques, including TEM/EDX/EELS and statistical approaches. Cross sections and thin foils were produced using BIB and FIB milling methods. The atmospheric corrosion of several Mg-Al alloys was investigated and the influence of various environmental parameters, such as exposure temperature, chloride ions, CO2, and SO2 was studied. Alloys produced by the newly developed rheocasting (RC) technique were found to be more corrosion resistant than their high pressure die-cast (HPDC) counterparts. The superior corrosion resistance of the RC materials was explained in terms of differences in the alloy microstructure, based on a systematic characterization of the micro-constituents at macro, micro and nanoscales. The rate of corrosion decreased with increasing Al content and the atmospheric corrosion of Mg-Al alloys exhibited a strong positive correlation with temperature. It was proposed that the temperature dependence of corrosion was linked to the chloride-assisted breakdown of an Al3+- rich layer, which was detected in the bottom part of the surface film formed on Mg-Al alloys. Mg-Al alloys-based MMCs were produced by the RC method. The microstructure of the composites was investigated and the products the interfacial reactions were identified. The reaction products included a hitherto unknown Al carbide (AlC2) and MgH2. The MMCs corroded faster than the monolithic alloy. However, it was shown that the corrosion resistance of Mg alloy-based MMCs can be improved by selecting appropriate process parameters. The corrosion behavior of an Al alloy (6xxx series), which was welded/processed using friction stir welding (FSW), and its variants bobbin friction stir welding (BFSW) and friction stir processing (FSP) was investigated. The thermal history of the specimens was investigated in detail. As expected, the heat affected zones (HAZs) were most susceptible to the atmospheric corrosion. The extent of corrosion in the HAZ was proportional to the size of iron- and silicon-rich intermetallic particles in the stir zone (SZ). The formation of intermetallic compounds (IMCs) in Al/Mg dissimilar FSW joints was investigated.