Effect of intermetallics of 2219-T87 aluminium alloy on exposure to aggressive media

The effect of precipitating agent Al2Cu intermetallic particles of high strength 2219-T87 aluminum alloy plate (AA2219-T87) used in cryogenic propellant tank for space applications has been investigated on exposure to aggressive media like unsterile natural seawater and 0.6 M NaCl solution. The corrosion resistance of AA2219-T87 plate has been evaluated by long term weight loss method, Tafel polarization and electrochemical impedance spectroscopy (EIS) techniques while the surface morphology of bare and exposed samples have been characterized using scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX). The Tafel plots and EIS studies reveal the lower corrosion current and higher charge transfer resistance of AA2219-T87 plate in unsterile natural seawater than in 0.6 M NaCl solution. The SEM with EDX studies confirm that the improved corrosion resistance of AA2219-T87 plate for long term exposure in unsterile natural seawater is mainly attributed to the blocking of cathodic nature of Al2Cu intermetallic particles by the formation of compact passive film of oxides, carbonates and silicates of calcium and magnesium. The surface studies further confirm that a complete damage of Al2Cu intermetallic particles in AA2219-T87 plate led to pitting type corrosion on exposure to 0.6 M NaCl solution.