Magnesium Corrosion

In some environments magnesium part can be severely damaged unless galvanic couples are avoided by proper design or surface protection. Unalloyed magnesium is not extensively used for structural purposes. Consequently, the corrosion resistance of magnesium alloys is of primary concern. Two major magnesium alloy systems are available to the designer. *

The first includes alloys containing 2 to 10% Al, combined with minor additions of zinc and manganese. These alloys are widely available at moderate costs and their mechanical properties are good up to 95 to 120oC. *

The second group consists of magnesium alloyed with various elements (rare earth, zinc, thorium, silver etc) except aluminum, all containing a small but effective zirconium content that imparts a fine grain structure and thus improved mechanical properties. These alloys generally possess much better properties at elevated temperature, but their more costly alloying additions, combined with the specialized manufacturing technology required, result in significantly higher costs.

A clean, unprotected magnesium alloy surface exposed to indoor or outdoor atmospheres free from salt spray will develop a gray film that protects the metal from corrosion while causing only negligible losses in mechanical properties. Chlorides, sulfates and foreign materials that hold moisture on the surface can promote corrosion and pitting of some alloys unless the metal is protected by properly applied coatings. The surface film that ordinarily forms on magnesium alloys exposed to the atmosphere gives limited protection from further attack.

Corrosion of magnesium alloys increases with relative humidity. At 9.5% relative humidity (RH), neither pure magnesium nor any of its alloys exhibit evidence of surface corrosion after 18 months. At 30% RH, only minor corrosion may occur. At 80% RH the surface may exhibit considerable corrosion. In marine atmospheres heavily loaded with salt spray, magnesium alloys require protection for prolonged survival. Dissolved oxygen plays no major role in the corrosion of magnesium in either freshwater or saline solutions. The corrosion of magnesium alloys by pure water increases substantially with temperature.

Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron and nickel. Such corrosion occurs when the heavy metal, the heavy metal basic salts or both plate out to form active cathodes on the anodic magnesium surface. Chloride solutions are corrosive because chlorides, even in small amounts, usually break down the protective film on magnesium. Fluorides form insoluble magnesium fluoride and consequently are not appreciable corrosive. Oxidizing salts, especially those containing chlorine or sulfur atoms, are more corrosive than salts that are not oxidizing. Chromates, vanadates, phosphates and many others are film forming and thus retard corrosion, except at elevated temperatures.

Magnesium is rapidly attacked by all mineral acids except hydrofluoric acid (HF) and chromic acid. Hydrofluoric acid does not attack magnesium to an appreciable extent, since it forms an insoluble, protective magnesium fluoride film on the magnesium. However pitting corrosion can develop at low acid concentrations.

Aliphatic and aromatic hydrocarbons, ketones, ethers, glycols and higher alcohols are not corrosive to magnesium and its alloys. Ethanol causes slight attack, but anhydrous methanol causes severe attack. The rate of attack in the latter is reduced by the presence of water. Pure halogenated organic compounds do not attack magnesium at ambient temperatures. At elevated temperatures or if water is present, such compounds may cause severe corrosion, particularly those compounds having acidic final products. (adapted by Internet reference 46 from ASM Metals Handbook Vol. 13)