Corrosion of aluminium alloys in supply waters

The corrosion characteristics of wrought aluminium and a number of wrought and cast alloys when immersed in typical public supply waters under stagnant or slowly moving water conditions have been investigated. Most of the tests have been carried out at room temperature but tests have also been made at temperatures up to 80°. Corrosion‐time curves have been drawn, using pitting depth and loss of weight as measures of corrosion. Changes in the composition of the waters caused greater differences than changes in alloys, but the order of attack was similar in all waters tested. Corrosive attack was most severe in the harder waters, the typical form of attack being the development of deep pits covered by mounds of corrosion product consisting mainly of aluminium hydroxide (‘nodular pitting’) Experiments with synthetic waters have shown that this form of pitting develops when calcium bicarbonate, chlorides, copper salts and dissolved oxygen are present together in the water. These constituents are all normally present in various proportions in public supply waters. A mechanism accounting for the development of the pits is proposed.