Magnesium Corrosion Protection

Elemental magnesium has a density of 1738.0 kg/m3 (108.50 lb/ft3), a melting point of 649°C (1200°F, and a boiling point of 1091°C (1995°F).

Commercially pure magnesium is the starting point for producing magnesium alloys. However, commercially pure magnesium produced in the United States is contaminated with various elements, especially iron, and these contaminants are highly detrimental to corrosion resistance.

For example, the corrosion rate of commercially pure magnesium in 3% sodium chloride solution is 100 to 500 times greater than that of chemically pure magnesium (99.95 + % Mg) containing less than 10 ppm of iron, nickel, and copper combined.

Because the corrosion rate of chemically pure magnesium in 3% sodium chloride solution is about 250 pm/yr (10 mpy), a rate 100 to 500 times greater is unacceptable.
Therefore, commercially pure magnesium has little direct application.
Alloying with various elements, however, improves the corrosion resistance and the mechanical properties of commercially pure magnesium.

The corrosion rate of chemically pure magnesium is greatly increased when it is alloyed with the following elements in excess of the concentrations listed: 2% zinc, 0.3% calcium, 0.5% silver, 0.1%copper, 0.017% iron, and 0.0005% nickel. These concentrations are the magnesium tolerance limits. The increase in corrosion rate when the tolerance limit is exceeded is gradual with zinc, more rapid with calcium and silver, and precipitous with
copper, iron, and nickel.

Magnesium is highly active in the electromotive force (EMF) series, and it has little tendency to polarize in most environments. Therefore, magnesium and magnesium alloys are also highly active in the galvanic series and will corrode when placed in contact with most metals. For this reason magnesium is used as a material for galvanic anodes in cathodic protection.

Methods for protection Magnesium from corrosion include paintings, coatings and surface treatments, including chromate and anodic coatings.

Because the corrosion of magnesium results in the formation of magnesium hydroxide, the paint should be alkali resistant, and there are many paints based on vinyl, epoxy, acrylic, alkyl, and urethane resins that have excellent resistance to alkali.

Magnesium shall never be placed in contact with more noble materials in order to avoid the possibility of galvanic corrosion.

reference: MATERIAL DETERIORATION PREVENTION AND CONTROL GUIDE OF ARMY MATERIAL, PARTONE, METALS (MlL HDBK-73S (MR)

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