Basic solutions, magnesium alloys

ALKALINE SOLUTIONS. Alkaline solutions generally have some action on aluminum alloys. The pH of these solutions alone is not a reliable indicator of the performance of aluminum alloys. Usually, weak bases such as ammonium hydroxide, hex-amine, alkanolamines and their aqueous solutions can be handled in aluminum because a protective film forms on aluminum after an initial period of reaction. Solutions made alkaline by hydrolysis of basic salts such as sodium carbonate form protective films on Al-Mg alloys containing 3.5% or more magnesium. Strong bases such as sodium hydroxide and potassium hydroxide dissolved in water are very corrosive and should not be handled in aluminum. See also Ref (4) pp. 35, 37. 

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). 

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