Magnesium alloys film formation

Magnesium alloys are very lightweight, and are being used in the aerospace industry. Because they are very reactive, these alloys need to be protected from corrosion. Dr. Birss holds a patent on a new approach to the electrochemical formation of protective oxide films on magnesium alloys. Dr. Birss also works on developing new catalysts for fuel cells, and studies the factors that lead to the breakdown of fuel cells. 

C. Chemical modification of the glued surfaces by the formation of passivating layers. The modification technique depends on the nature of the metal. The parts are most often subjected to acid pickling, e.g. aluminum alloys are anodized in sulfuric and chromic acids. It is preferable to anodize aluminum parts in sulfuric acid followed by treatment of the anodic film in a bichromate. There are several methods of pickling carbon and stainless steels, chemical oxidation of magnesium alloys as well as copper and titanium alloys before gluing [4]. 

Efthimiadis J, Neil WC, Bunter A, Howlett PC, Hinton BRW, MacFarlane DR, Forsyth M (2010) Potentiostatic control of ionic liquid surface film formation on ZE41 magnesium alloy. ACS Appl Mater Interfaces 2(5) 1317-1323. doi 10.1021/am900889n... 

Forsyth M, Neil WC, Howlett PC, Macfarlane DR, Hinton BRW, Rocher N, Kemp TF, Smith ME (2009) New insights into the fundamental chemical nature of ionic liquid film formation on magnesium alloy surfaces. ACS Appl Mater Interfaces 1(5) 1045-1052. doi 10.1021/ am900023j... 

To date, research efforts regarding protective thin films have primarily focused on compact textures. In fact, smdies have confirmed that mesoporous thin films could confer corrosion protection for magnesium alloys. Furthermore, a mesoporous structure is beneficial to the integrity of the outer surface. This mesoporous structure could effectively release stress in the thin film and ameliorate the mismatch between the substrate and coating thus, crack-free thin films can be successfully prepared. In addition, smdies have reported that mesoporous thin films can easily induce apatite formation and have high bone-forming ability owing to the enhancement of cell activity and protein adsorption. ... 

The subject of surface films on electrodes in non-aqueous solutions is mostly important for the field of batteries. The performance of both Li and Li-ion batteries depends strongly on passivation phenomena that relate to surface film formation on both the anodes and the cathodes. Lithium and lithiated carbon anodes reduce all the solvents and salt anions in electrolyte solutions relevant to Li batteries. The products of these surface reactions always contain insoluble Li salts that precipitate on the electrodes as surface films. All charge transfer processes of Li, Li-C, and Li alloy anodes in Li batteries involve the critical step of Li-ion migration through the surface films. Thereby, the composition, structure, morphology, and electrical properties of surface films on Li, Li-C, and Li alloy electrodes were smdied very intensively over the years. In contrast, reversible magnesium electrodes can function only in surface film-free conditions. ... 

This reaction stops in alkaline electrolytes because of the formation of an insoluble film of magnesium hydroxide on the electrode surface which prevents further reaction. Acid tends to dissolve the film. An important consequence of the film on magnesium elecfrodes (see also Chap. 9) is that there is a delayed response to an increase in the load because of the need to disrupt the film to create new bare surfaces for reaction. Pure magnesium anodes usually do not give good cell performance, and several magnesium alloys have been developed for use as anodes tailored to provide the desired characteristics. 

Xia S J, Birss Va I and Rateick Jr. R G (2003), Anodic oxide film formation at magnesium alloy WE43 , Electrochemical Society Proceedings, 25, 270-280. 

Z. Shi, P.K. Mallick, R.C. McCune, Characterization of film formation on magnesium alloys due to corrosion in engine coolants , SAE technical paper 2008-01-1155, SAE International, Detroit. 2008. 

L. Kouisni, M. Azzi, M. Zertoubi, F. Dalard, S. Maximovitch, Phosphate coatings on magnesium alloy AM60 part 1 study of the formation and the growth of zinc phosphate films. Surf. Coat. Technol. 185 (2004) 58-67. 

Bonilla, F A, Berkani, A, Liu, Y, Skeldon, P, Thompson, G E, Habazaki, H, Shimizu, K, John, C Stevens, K (2002b), Formation of anodic films on magnesium alloys in an alkaline phosphate electrolyte . Journal of the Electrochemical Society, 149, B4-B13. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

A passive film is formed on the Mg electrode surface in alkaline solutions, which creates problems when the Mg-air cell is to be discharged. However Mg and especially some Mg alloys are suitable to be used in magnesium-air cells with NaCl-electrolyte [4], where the formation of the passive film on the magnesium surface is not observed. 

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