The Various Forms of Aluminium Corrosion in Natural Waters

THE VARIOUS FORMS OF ALUMINIUM CORROSION IN NATURAL WATERS 

Natural waters, both surface waters (river water, spring water) and seawater, generally have a pH close to neutral. Municipal waters mostly have a pH between 6.5 and 7.5. Distilled water is slightly more acidic, with a pH ranging from 6 to 6.5, and seawater is shghtly alkaline, ranging from 8 to 8.2. 

Pitting corrosion of aluminium in waters develops at preferential sites where the natural oxide film is less resistant because of heterogeneous features such as A Fe intermetallics or defects related to very localised thinning or rupture of the natural oxide film. These sites are anodic with respect to their vicinity, and corrosion pits can develop according to the electrochemical mechanism described above (see Section B.1.2). Pitting is initiated by anions that penetrate into the defects of the natural oxide layer. 

As in the case of atmospheric corrosion (see Section C.5.2), experience shows that in most cases, the penetration rate of pitting in contact with waters (freshwater, seawater) decreases with time. Trials conducted in the early 1950s with 25 different Canadian waters [4] showed that the deepening rate of pitting on alloy 1100 follows the equation  

This law has been checked by measuring the pitting depth at regular intervals over 13 years on an installation including over 100 km of pipes in alloy 5052 (unit length 15 m) for 

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The Various Forms of Aluminium Corrosion in Natural Waters... 

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