Experimental pH Potential Diagram of Aluminum for Sea Water

The theoretical Pourbaix diagram for the aluminum-water system does not take into account pitting corrosion, the usual form of corrosion for aluminum in chloride containing environments. To obtain a practical representation of aluminum corrosion usable in sea water, pitting potentials, protection potentials, and uniform attack potentials have been measured with 5086 AA specimens in 3% NaCl solutions buffered within the 4 to 9 pH range. These potentials limit passivity, pitting corrosion, and uniform corrosion areas in the potential-pH diagram. The passivity area of 5086 AA in 3% NaCl solution is bordered by pitting areas both in the most noble direction (anodic polarization) where crystallographic pits are formed, and in the most active direction (cathodic polarization) where semi-spherical pits are formed. The results are interpreted on the basis of local pH evolutions during polarization of the specimens. This experimental diagram has been extended both to more acidic and more alkaline environments.