Performance of sacrificial magnesium Mg and its alloys

Three factors influence the efficiency of magnesium as an anode the current density, the composition and the environment. 

Pure magnesium should have a driving potential of 850 mV to protect steel but in practice the metal corroded very rapidly with a very low efficiency. The metal suffers low polarization in the presence of chloride or sulfate ions and produce highly soluble chloride and sulfate salts. These ions are usually artificially introduced into the electrolyte as a backfill when a deficiency is expected, the hydroxide which is preferentially formed because of its low solubility becomes enriched with the backfill anions and itself functions as a backfill. Uniform general corrosion can then be obtained and well-designed inserts help to keep most or all of the anode metal available for sacrificial consumption. In freshwater or electrolytes which contain none of these ions, the hydroxide and carbonate may form, but these do not seriously polarize the anode (Morgan, 1993). 

The anode efficiency, that is the useful ampere hours per kg, increases at high current densities composition has some effect upon this, but generally maximum efficiency is obtained at current densities above 1.08A/m2 Fig. 2.13 shows this relationship. The extrapolation of these curves could suggest a certain corrosion rate even when it is not acting as sacrificial anode however, a relatively high efficiency can be maintained at low 

13 Efficiency vs. current density of high-purity magnesium as compared with that of currently employed cathodic protection anodes and commercial Mg alloys in saturated CaS04 solution (Morgan, 1993). 

Two series of anode alloys are generally considered. The first is the high-purity anode for use in soils with high resistivity (wt% 0.02 Cu, 0.003 

---