CORROSION BEHAVIOR OF CRYOROLLED ULTRAFINE GRAINED 7075 ALUMINIUM ALLOY

Abstract

Aqueous corrosion study by salt immersion test and electrochemical test in 3.5% Nacl solution of cryo rolled 7075 Al alloy subjected to peak ageing treatment are reported in the present investigation. The results are compared with corrosion resistance of starting materials i.e bulk solution treated alloy and bulk peak aged alloy.The specimens were artificially aged at different temperatures to optimize the peak ageing conditions. Comparison among the four types ,of materials was done by measuring weight change due to corrosion through salt immersion salt immersion test and corrosion current density by electrochemical test. Among the four, cryo rolled peak aged material has been found to have best corrosion resistance followed by bulk solution treated materials followed by cryo rolled material and at last bulk peak aged material. Cyro rolled peak aged material has the advantage of thick A1203 layer precipitates pinning the grain boundaries and lower dissolution rate due to recovery which increase its corrosion resistance. The surface morphology of the corroded alloys was characterized by SEM, Fe-SEM studies and type of corrosion was predicted. It was observed that the pitting corrosion was less rigorous in both the peak aged alloys. Cross -sectional analysis was done using X-Ray mapping and line analysis method and penetration of various elements was investigated .Cro rolled peak aged material was founded to have thick A1203 layer and the order of corrosion resistivity suggested by it matched the results ofa salt immersion and electrochemical test.XRD was employed to determine the phases present on the surface of the materials before and after corrosion which revealed the presence of oxides of A1203.ZnAl2O3,MgAl2O4,CuAl2O4 etc after corrosion had taken place. 3