Anodization and corrosion of magnesium Mg alloys

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

At 100% efficiency, the output is limited by local corrosion cells. To avoid this, magnesium is alloyed with A1 and Zn. Practically, an efficiency of 60-70% can only be obtained. Galvomag anodes are used for seawater service. A typical composition is shown in Table 5.3a. The consumption rate is 17.52 lb/A-year. The compositions of high purity and Galvomag Mg anodes are also shown in Table 5.3a. 

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. 

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). 

Magnesium and its alloys are definitely anodic to the A1 alloys and, thus, contact with aluminum increases the corrosion rate of magnesium. For example, in sodium chloride solutions (3-6%), the potential of Mg alloys is -1.67 V/SHE while that of Al-12%Si and pure aluminum are -0.83 to -0.85, respectively. However, such contact is also likely to be harmful to aluminum, since magnesium may send sufficient current to the aluminum to cause cathodic corrosion in alkaline medium. Aluminum oxide is amphoteric and so it is soluble in acid as well as in alkaline solutions. The standard reduction potentials of these two half-reduction reactions are (-1.66 V/SHE) and (-2.35 V/SHE), respectively. Alkaline reaction of the possible existence of aluminum phase in sacrificial Mg anodes is ... 

The corrosion process of a sacrificial Mg alloy is evaluated in the laboratory in either 3% NaCl solution or a solution containing 5g CaS04. 2H2O and 0.1 g Mg (0H)2/L), as recommended after the ASTM G97 standard. Beside the ASTM procedure, there is also the Mexican test method (NMX-K-109-1977, Magnesium anodes used in cathodic protection ) that considers a test environment made of artificial seawater. Both standards consider galvanostatic tests, in which a known direct current is passed through test cells connected in series in order to determine efficiency of sacrificial anode materials (Guadarrama-Mu-oz et al., 2006). 

NMX standard. Figure 2.16 shows the obtained polarization curves in the simulated backfill solution. It is clear that the slope of each polarization curve is different for each specimen in each environment. This difference can be associated with the different corrosion rates as related to the chemical composition of the magnesium alloys. The polarization curves show that the pure Mg anode (Ml) is more active by 36 mV if compared with that of M3. The calculated corrosion rates of pure Mg Ml and M2, M3 alloys (icon- in mA/cm ) are 0.0079, 0.0072 and 0.046, showing the accelerating influence of impurities (alloy M3) and the beneficial content of manganese combined with fewer impurities (M2). 

When an Mg alloy does not fulfill the chemical composition specified for a sacrificial magnesium anode, features as inductive loops at lower frequencies appear in the Nyquist representation of the measured impedance. As the magnesium alloy is polarized further away from its E, in the anodic direction, the Nyquist representation of the impedance exhibits inductive loop behavior (Fig. 2.18). This fact leads to the consideration of an inductor component in the corresponding electrical equivalent circuit. This inductive loop can be associated with the adsorption and desorption phenomena occurring on the surface of the sample and leading to the process of formation of the corrosion product layer on the surface of the electrode (Guadarrama-Mu-oz et al., 2006). 

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