Corrosion Failure of Aluminum Heat Exchanger Tubes
This paper investigates the corrosion failure of aluminum tubes on a heat exchanger user to remove heat from the windings of an electric motor. Only the tubes on the perimeter (bottom, top, and sides) at the air inlet location were affected, which is associated with the heat distribution. This investigation concludes that the observed corrosion on the tube is caused by increased chloride concentration from dry out or alternate wetting and drying of the protective aluminum oxide which breaks down, forming a saturated solution of aluminum chloride. The initial aqueous chloride solution is formed by condensation and the high salt content of the inlet air from a marine environment which is then gradually concentrated as it dried out due to heat from the hot nitrogen side of the tube, hence the observed white powder at the tube inlet and vicinity. The concentrated aluminum chloride with an acidic pH of approximately 3.5 becomes very corrosive to the aluminum tubes resulting in the observed pitting and intergranular corrosion. The failure may be seasonal, based on ambient air conditions and the operating temperature of the tube inlet end. Recommendations for prevention are fourfold: change of process/design e.g., dehumidification of the inlet air in order to prevent condensation on the outer tubes; change of material (e.g., use of Incoloy 625 tubes); application of internal coating to prevent condensation (e.g., blast cleaning and coating with a solvent-free epoxy); and replacement of corroded tubes with annual cleaning to remove salts. Selection of the preferred option should involve evaluation of the relative advantages/disadvantages.