Protection of Light Alloys: The Resistance to Corrosion of Aluminium and Magnesium Base Alloys with Methods of Protection Available to Purchase

PROBABLY one of the most remarkable features of our generation has been the ability of the metallurgist to produce alloys meeting the current needs of industry in providing the requisite mechanical characteristics, capabilities of being manipulated or fashioned by established methods, and in particular instances, specific properties of resistance to erosion, corrosion, wear, impact and fatigue. Perhaps this adaptability is best exemplified in the case of light alloys, whereby scientific alloying of aluminium or magnesium bases with other elements, combined with specific thermal treatments, have imparted increased strength, durability, and, under limited conditions, resistance to corrosion, to a whole range of alloys without any appreciable sacrifice of the great advantage in initial lightness of these two base metals. In view of this, these metals, which are conveniently designated “light alloys,” have become exceedingly popular, particularly since the majority of them are eminently suitable for fabrication in the form of diecastings, which permit the reproducibility of exact forms in large quantity production. The mechanical features of light alloys, together with their capacity to be machined, or otherwise finished to shape and dimensions, are fairly widely known, but their ability and relative merits to resist various conditions of corrosive influence are not so clearly elucidated. The readers of this journal arc naturally concerned primarily with finishes of protective value rather than with those of decorative importance. In general engineering considerable confusion exists as to the behaviour of aluminium and its alloys under deleterious influences, and this tends to retard their general adoption, even though their individual mechanical properties may make them highly desirable. In some cases most unnecessarily elaborate finishes are applied for ultra precautionary purposes, while in others a complete ban is exercised rather than incur any risk. In the aircraft industry, on the other hand, the demand for minimum weight consistent with requisite strength has provided the impetus for their adoption, which in turn has led to the development of appropriate finishing methods. Even in this sphere a comparative survey of finishing methods and their efficiency covering a range of popular alloys should prove to be of interest.