Anodizing magnesium

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

A similar situation is found for certain metals not forming stable ions of variable valency. In anodic magnesium dissolution, Mg+ ions are formed first. They do not undergo dismutation like the Cu+ ions, but as a strong reducing agent react with water according to... 

OCP of magnesium is 0.6 to 1.1 V more positive than the thermodynamic value. The negative difference effect that is seen in anodic magnesium dissolution (see Section 16.2) can, in part, be attributed to partial mechanical disintegration of the passivating layer during magnesium dissolution. 

Connect the anode (magnesium rod) to the positive pole and the cathode (nickel foam) to the negative pole of the power supply set the intensity at 200 mA. Monitor the reaction by GC analysis after treatment of an aliquot (0.2 mL) of the reaction mixture with iodine. 

Connect the anode (magnesium rod) to the positive pole and the cathode (nickel foam) to the negative pole of the power supply. 

Severe corrosion may occur in active A1 or Mg alloys in neutral solutions of heavy metal salts (salts of Cu, Fe, or Ni). This type of corrosion occurs when the heavy metal salts plate out to form active cathodes on the anodic magnesium surface. This type of galvanic corrosion can lead to localized pitting corrosion. 

Using a dissolving metal anode (magnesium) the cell chemistry then leads to magnesium carboxylate aiding product recovery however, as the anode metal is continuously consumed this type of anode places special restraints on the cell design. 

The early magnesium alloys suffered rapid attack under moist conditions, mainly because of the presence of impurities, notably iron, nickel, and copper. These impurities or their compounds act as minute cathodes in the presence of a corroding medium and create micro-cells with the anodic magnesium matrix (Polmear, 1989). High-purity alloys are a relatively recent development. In high-purity alloys the concentrations of these impurities are controlled to below critical concentrations and as a consequence high-purity alloys are markedly more resistant to salt water than are alloys of normal purity (Shreir, 1965). 

Galvanic corrosion or bimetallic corrosion is important to consider since most of the structural industrial metals and even the metallic phases in the microstructure alloys create galvanic cells between them and/or the a Mg anodic phase. However, these secondary particles which are noble to the Mg matrix, can in certain circumstances enrich the corrosion product or the passive layer, leading to a decrease or a control of the corrosion rate. Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron and nickel. The heavy metal, the heavy metal basic salts or both plate out to form active cathodes on the anodic magnesium surface. Small amounts of dissolved salts of alkali or alkaline-earth metal (chlorides, bromides, iodides and sulfates) in water will break the protective film locally and usually lead to pitting (Froats et al., 1987 Shaw and Wolfe, 2005). 

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. 

The ingress of corrosive ions into an anodized coating through its pores is not a slow process for a porous anodized coating. After corrosive ions reach a critical concentration threshold at the film/substrate interface and trigger the corrosion of the substrate Mg alloy, the corrosion of the anodized magnesium starts. Mg is dissolved into the solution in the pores, which gradually makes... 

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. 

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