Surface films, magnesium alloys

The effect of plasma electrolytic oxidation (PEO) treatment on the SCC of surface-modified magnesium alloys was studied [167]. PEO coating offered improved corrosion resistance. However, the barrier film did not improve the SCC resistance in ASTM D 1384 test solution. The SCC of PEO-coated specimens was attributed to the development of micro cracks in the coating, leading to substrate cracking under SSRT test conditions [167]. 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

A. N. Khramov, V. N. Balbyshev, L. S. Kasten and R. A. Mantz, Sol-gel coatings with phosphonate functionalities for surface modification of magnesium alloys, Thin Solid Films, 2006, 514, 174. 

It should be mentioned, furthermore, that MacFarlane et al. reported recently on the breakdown of films of the TFSA on magnesium and magnesium alloy surfaces... 

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]. 

Transgranular stress corrosion cracks are known [7.49] from i) austenitic steels in acidic chloride solutions, ii) low-strength ferritic steels in acidic media, iii) ferritic steels in phosphate solutions, iv) carbon steel in water saturated with CO2 and CO, v) a-brass in ammonia solutions that do not cause surface films, vi) aluminium alloys in NaCl/K2Cr04 solutions and vii) magnesium alloys in diluted fluoride solutions. For further study of fracture surface appearance, see, e.g. Lees [7.49] and Scully [7.53]. 

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial films containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

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... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

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. ... 

NDE is usually associated with accelerated corrosion of the metal. These conditions should be avoided in practice. In this, magnesium has the same shortcomings as aluminum, whereas steels do not suffer from such extra corrosion. Because aluminum alloys can form a stable surface film, NDE can often be prevented. This also indicates that if the surface Elm on magnesium alloys could be improved, then the NDE phenomenon could be prevented, and the corrosion rate would be reduced. 

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. 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

At room temperature, an insoluble passive Mgp2 film was generated on the surface of Mg alloy activated in HF solution and the mass of magnesium alloy treated in different concentrations of HF acid (10-70%) increased as function of time. When the mass ratio of Mg/F in the film was 11.3 1, the mass of the deposited MgF2 film reached a constant value. The passive film... 

H.Y. Hsiao and W.T. Tsai, Characterization of anodic films formed on AZ91D magnesium alloy , Surface Coatings and Technology, 190/2-3, (2005), 299-308. 

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