The Oxidation of Aluminum

Date:2019-08-29 07:24:30 Posted by:mrpower View:349

Writer:  Ron Knight (Chemist)

Aluminum being the most abundant metal in the earth’s crust (at 8.2% of the total metals) is never found free in nature. Aluminum is reactive and will react spontaneously with water and/or air to form aluminum oxide. Aluminum oxide, Al2O3, forms a stable passive layer that protects aluminum from corrosion or further oxidation. This layer is about 4 nm thick and will provide corrosion protection as long as this oxide layer is stable.

Aluminum is an amphoteric metal and can react with an acid as well as a base. The protective layer of aluminum oxide will deteriorate in environments with high or low pH or even in environments where aggressive ions are present. The result is that the oxide layer is only stable in a pH range of 7.0 to 9.0. Below 7 or above 9 the rate of corrosion increases at an increasing rate.
Among the most aggressive of ions is chlorine. When the layer of aluminum oxide comes in contact with the chlorine, it becomes unstable and a reaction develops, releasing hydrogen gas and producing dialuminum hexachloride. The AlCl3 hydrolyses in water and forms an acid mist that breaks down the protective layer. The most typical results are: formation of corrosion pits, crevice corrosion, inter-granular corrosion and galvanic corrosion. As this process continues the corrosion creates stress raisers and ultimately stress cracks and metal fatigue and failure.

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