CORROSION AND PASSIVATION BEHAVIOR OF AL, AL-CU, AL-6061 AND AL-7075 IN NITRIC-ACID AND NITRIC ACID-CONTAINING CHLORIDE SOLUTIONS

Due to the technological importance of Aluminium (Al) and its alloys a nd the wide industrial applications of these materials, the electroche mical behaviour of naturally passivated aluminium and aluminium alloys (Al-Cu, Al-6061 and Al-7075) was investigated in nitric acid and nitr ic acid-containing chloride solutions. The effect of concentration of the ambient electrolyte on the corrosion behaviour of these materials was traced. Electrochemical impedance spectroscopy (EIS) and polarizat ion techniques were used. The 6061 alloy was found to be the most corr osion resistant even in the presence of chloride ions up to 0.35 M. Th e native oxide film present on Al or its alloys is too thin to impart complete passivity. An equilibrium occurs between barrier film dissolu tion and surface passivation especially in dilute solutions (less than or equal to 0.1 M HNO3). The electrode/electrolyte interface was fitt ed to a parallel resistor/capacitor model using a fitting program to f it the experimental impedance data to computer-generated data accordin g to the proposed model. The fitting results revealed that the barrier layer on Al and Al-6061 behaves like a perfect dielectric. The barrie r layer on Al-Cu and Al-7075 alloys deviate from ideal capacitor behav iour.