Environmental factors affecting aluminium corrosion

The corrosion of aluminium alloys in high purity water is complex and many of the factors responsible for this corrosion are interrelated. In high purity, deionized water, general thinning of the cladding caused by uniform corrosion is very low. The fuel enters the basin with, in some cases, several millimetres of protective oxide coating formed at high temperatures. When corrosion by water 

In general, soft water is less aggressive than hard water with respect to pitting corrosion of aluminium. The hardness of water is due to the presence of calcium carbonate (CaCOj) and other ions such as sulphates and chlorides. Water with a CaCOg content of about 1-60 ppm is considered soft, with over 60 ppm hard and with over 181 ppm very hard. 

The major factors believed to influence the pitting of aluminium alloys are conductivity, pH, and bicarbonate, chloride, sulphate and oxygen content [2.6]. Because of the interrelationship of the composition and service factors, it is difficult to predict the influence of water on aluminium corrosion from a table of water composition alone. A number of studies have been conducted of synthetic waters containing several metal and salt ions alone and in combination [2.15-2.17]. They found that the corrosion of aluminium was accelerated when salts of copper, chlorides and bicarbonates were present together, compared with cases where only a single impurity was present. In some cases where two of the three constituents were present, there was little corrosion, but with the three species present together, nodular corrosion occurred. 

As corrosion processes associated with aluminium fuels in storage basins are electrochemical, the nature of the electrolyte, or basin water, plays a key role in the flow of electric current and electrons in the process. The amount of metal removal by corrosion is directly related to the current flow. By increasing the resistance of the water, the corrosion of the aluminium can be reduced. Very pure water has a high resistance and is much less corrosive than impure or natural waters. The low corrosiveness of high purity water is primarily due to its high electrical resistance (low conductivity) [2.12]. 

The electrical resistivity of water is measured in ohm centimetres, and conductivity, given by the reciprocal of resistivity, in mho/cm. In international units, 1 (imho/cm = 1 pS/cm, where S is the unit siemens. Extremely pure water is produced by deionization. This water has a conductivity of less than 1 pS/cm. 

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