Anodizing, magnesium alloys

The corrosion performance of an anodized magnesium alloy depends on the corrosion performance of the substrate alloy. For example, the corroded areas of the anodized commercial alloys are measured and plotted versus the corrosion rates of these alloys in Fig. 16.12. There is a good correlationship in corrosion damage degree between the anodized specimens and the corresponding im-anodized alloy. 

Bonilla, F, Berkani, A, Skeldon, P, Thompson, GE, Habazaki, H, Shimizu, K, John, C Stevens, K (2002a), Enrichment of alloying elements in anodized magnesium alloys . Corrosion Science, 44, 1941-1948. 

Shi, Z (2005), The corrosion performance of anodized magnesium alloys. PhD Thesis, University of Queensland. 

Song, G, Shi, Z, Hinton, B, McAdam, G, Talevski, J Gerrard, D. (2006), Electrochemical evaluation of the corrosion performance of anodized magnesium alloys. In 14th Asian-Pacific Corrosion Control Conference, Shanghai, China. Keynote-11. 

Magnesium Alloys, Anodic Treatment Of, MUitary Specification MIL-M-45202, Dept, of the Army, Army Materials and Mechanics Research Center, Watertown, Mass., Oct. 3, 1968. 

Magnesium. This molten salt electrolysis process is the current principal method of magnesium production. The graphite anodes can be either round or rectangular in nature (see Magnesiumand magnesium alloys). 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

Table 6-4 Composition (wt.%) and properties of magnesium alloys for anodes...

The special forms consist of the many types of anode which are used for protecting smaller containers. Boilers, heat exchangers and condensers belong to this group. Besides the rod anodes already mentioned with tube screw joints which can be screwed into the container from outside, there are also short and round anode supports as well as more or less flat ball segments which are bolted onto the protected surface with cast-on supports. These shapes are mostly manufactured from magnesium alloys. In addition, there are star-shaped or circular anodes for installation in condensers and pipes. The weight of these anodes lies between 0.1 and 1 kg. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

King, J. F., Adamson, K. G. and Unsworth, W., Impregnation of Anodic Films for the Protection of Magnesium Alloys, Ministry of Defence D.Mat. Report No. 193, February (1973) Adamson, K. G., King, J. F. and Unsworth, W., Evaluation of High Temperature Resistant Coatings for the Protection of Magnesium Alloys, Ministry of Detence D.Mat. Report No. 196, July (1973)... 

Anode efficiency is of little practical significance and can be misleading. For example, magnesium alloy anodes often have an efficiency ca. 50% whilst for zinc alloys the value exceeds 90% it does not follow that zinc alloy anodes are superior to those based on magnesium. Efficiency will be encountered in many texts on sacrificial anode cathodic protection. 

Cathodic protection equipment has been used very successfully in water tanks and HW and steam boilers as anticorrosion devices for 100 years or more. Such equipment comes in many shapes and sizes, and comprises a sacrificial anode of either zinc or magnesium alloy, either bolted directly to a suitable internal water-wetted (cathodic) metal surface, or self-contained by enclosing the anode with a suitable cathode (such as a silver plated base metal). Usually several devices are required for any boiler, more for larger units and less for smaller ones, and these require replacement every one to two years. 

Sheets of magnesium or magnesium alloys were used as anodes in the developed magnesium-air cells. 

The comparison of the data from Figure 4 shows that the pure magnesium (99.99%) is the most suitable material to be used as anode in magnesium-air cells with NaCl-electrolyte. The magnesium alloy type... 

Magnesium-air air cells with NaCl-electrolyte were developed and investigated. The current-voltage and the discharge characteristics of the cells with were studied. Air gas-diffusion electrodes suitable for operation in NaCl-electrolytes were designed. Various carbon-based catalysts for the electrochemical reduction were tested in these air electrodes. Magnesium alloys suitable for use as anodes in Mg-air cells were found. 

The neutral electrolyte, often sea water, can be replaced by aqueous NaOH providing that the anode is alloyed with magnesium and tin. Alkaline aluminium-air cells have significantly greater power densities. 

Military Specification, MIL-M-45202C, Magnesium Alloy, Anodic Treatment of April 1981. 

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