Magnesium alloys generally have a fairly good corrosion resistance, but as the most electrochemically active engineering materials, they require corrosion protective measures if they are used in aggressive environments and/or in contact with other materials. Another large concern is stress corrosion cracking. The combination of mechanical stresses and corrosive attack can promote the initiation and growth of cracks which may result in abrupt failure without preceding plastic deformation.

After a basic introduction to magnesium and magnesium processing, the major corrosion problems and their mechanisms are discussed in this chapter. This is followed by a summary of corrosion prevention strategies and examples of typical applications.

Corrosion of Magnesium and its Alloys

Application and use of different aluminium alloys with respect to workability, strength and welding parameter optimization

Effects of nano-particles strengthening activating flux on the microstructures and mechanical properties of TIG welded AZ31 magnesium alloy joints

A systematic study of the effects of activating flux in the weld morphology, arc profile, and angular distortion and microstructure of two different arc welding processes, namely, Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW), was carried out. The results showed that the activating fluxes affected the penetration capability of arc welding on stainless steel. An increase in energy density resulting from the arc constriction and anode spot reduction enhanced the penetration capability. The Depth/Width (D/W) ratio of the weld played a major role in causing angular distortion of the weldment. Also, changes in the cooling rate, due to different heat source characteristics, influenced the microstructure from the fusion line to the centre of the weld.

Research on the activating flux gas tungsten arc welding and plasma arc welding for stainless steel

An innovative phosphate–permanganate surface treatment (PPT) was developed to improve the static strength of adhesive-bonded 4 mm thick magnesium AZ31 sheets. The phosphate coating having the chemical composition of 1.43% P, 1.63% F and 0.15% Mn (in mass %) was formed after the treatment with PPT solution which has the formulation of KMnO4, K2HPO4, Na2SiO3 and NaF. The combination of additives NaF and Na2SiO3 and the pH values in the range of 5–6 for a phosphate–permanganate solution was found to be the key elements for the formation of the phosphate coating. The appearance of the phosphate coating and corrosion resistance to 3.5%NaCl solution was assessed. To study the durability of the coating, the effect of an exposure in a hot-humid environment (96% R.H. at 40 °C) on the static strength of adhesive-bonded magnesium AZ31 was investigated. Test results showed that the phosphate coating improved not only the static strength of bonded magnesium AZ31 joints in an ambient condition but also the durability in a hot-humid environment. These results suggest that PPT surface pretreatment is capable of improving the static strength and thermal durability of adhesive-bonded magnesium AZ31 sheets.

D.-H. Cai

Papers

Gas tungsten arc welding of magnesium alloy using activated flux-coated wire