Carbon dioxide – an unknown factor in the corrosion of light metals

The need for fuel efficiency and increased performance in transportation systems continually places new demands on materials. The design criteria are primarily concerned with density, strength, stiffness, and corrosion resistance. In recent years, the automotive industry has directed much effort towards decreasing the weight of vehicles. A weight reduction of 100 kg leads to a reduction in fuel consumption of 3-5 per cent, depending on whether the weight reduction is used to downsize the driveline or not. A decrease in fuel consumption also decreases emissions of greenhouse gases, e.g. CO2.

One way to reduce the weight of vehicles is by replacing steel by lighter materials such as aluminium or magnesium alloys. The use of aluminium and magnesium alloys by vehicle manufacturers critically depends on the ability to withstand atmospheric corrosion. This goes for uses where the load-bearing capacity is the main concern as well as for the cases where appearance is important.

However, there is a marked lack of information on the environmental degradation in the atmosphere, especially for magnesium alloys. This is partly why the use of magnesium alloys in vehicles is mostly restricted to interior applications. The corrosion of these alloys is strongly dependent on alloy microstructure. Magnesium and aluminum alloys mainly suffer from localized forms of corrosion in the atmosphere. This includes pitting, crevice corrosion and galvanic corrosion. In engineering designs, different types of attachments and fasteners in nobler materials cannot be avoided. This creates problems with galvanic corrosion. Magnesium acts anodically in contact with all other construction materials. Choosing the right type of surface treatment, type of alloy and design is therefore of utmost importance if magnesium is to become a viable alternative in exterior vehicle applications.

“Recently, we showed that the corrosion of aluminium in humid air is strongly inhibited by ambient concentrations of CO2. This discovery throws new light on the known tendency of aluminium to corrode in crevices and in other environments where the supply of CO2 may be poor. Our results show that CO2 is also important in the corrosion of magnesium, “ says Daniel Bengtsson Blücher.

The different phenomena in atmospheric corrosion have vastly different characteristic length scales. The chemical and electrochemical reactions that occur in the aqueous film on the surface involve individual ions and molecules. On the nanometer scale, the materials exhibit noble precipitates and dislocations in the alloy as well as flaws in the passive film that are of decisive importance for corrosion. On the micrometer scale one must consider alloy grain boundaries and, e.g., the distribution of a-phase in magnesium-aluminum alloys. The extent of the electrochemical cells involved in localized corrosion often reaches millimeters. At the other end of the scale, corrosion rate is often measured in mm/year, averaging over the whole surface. Typically, laboratory investigations of the atmospheric corrosion of metals use one or two selected techniques that can only provide information on certain aspects of corrosion. Especially, the connection between surface chemistry, alloy microstructure and the rate of corrosion is seldom made. This project has usd recent advances in methods and instrumentation (In-situ SKPFM, FIB, SEM and TEM) in order to change this and take a qualitative step forward in understanding the mechanisms of atmospheric corrosion.

The thesis “Carbon Dioxide: The Unknown Factor in the Atmospheric Corrosion of Light Metals” was defended at a public defence in My 2005 at Chalmers University of Technology, Gothenburg, Sweden.