Polarization magnesium alloys

Figure 2.9 Polarization curves for polyoxadiazole coated and uncoated magnesium alloy.

Calculate the expected current output of a single 48 lb Galvamog magnesium alloy anode. The size of the backfill package is 10" x 40". The steel has been polarized to a potential of -0.85 V. The resistivity of soil is 2000 ohm-cm. The solution potential of Galvamog is —1.75 V. Calculate the life of 48 lb magnesium anode. 

The open circuit potential of a single 48 lb Galvomag magnesium alloy anode is —1.75 V. The surrounding backfill has dimensions of 8" x 30". The anode has polarized the steel in a soil of resistivity 3000 ohms-cm to —0.85 V. Estimate the current output of the anode. 

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). 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

Figures 8.4 and 8.5 indicate the critical potentials noble to which S.C.C. of 18-8 stainless steel initiates when exposed to magnesium chloride solution boiling at 130°C with and without inhibiting anion additions [27]. Anodic polarization induces shorter cracking times the more noble the controlled potential cathodic polarization, on the other hand, extends the observed cracking times. Below the critical value of -0.145 V (S.H.E.), the alloy becomes essentially immune (Fig. 8.4). Addition of various salts, such as sodium acetate, to the magnesium chloride solution shifts the critical potential to more noble values. When the amount of...

Anodic protection is apphcable only to metals and alloys (mostly transition metals) which are readily passivated when anodically polarized and for which /passive is very low. It is not apphcable, for example, to zinc, magnesium, cadmium, silver, copper, or copper-base aUoys. Anodic protection of aluminum exposed to high-temperature water has been shown to be feasible (see Section 21.1.2). 

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