Abstract
The aim of the present work was to evaluate the effects of heat treatment on: the microstructure of Al-Li 8090 alloy; its electrochemical properties (polarisation resistance, pitting resistance); and the composition of its corrosion products in sea water. Free corrosion and electrochemical tests were carried out at 25°C in quiescent sea water at pH 8·2 with a dissolved oxygen content of 6·5 ppm. The microstructure was examined by metallographic microscopy and by X-ray microdiffractometry; electrochemical properties (polarisation resistance, pitting resistance) were studied using electrochemical impedance spectroscopy; and pit formation and growth were assessed with optical microscopy. The corrosion layer was characterised with X-ray photoelectron spectroscopy (XPS). Aging treatment led to a non-homogeneous structure which enhanced alloy pitting susceptibility; aging also led to lithium enrichment of the alloy surface. Only the initial general corrosion stage was affected by lithium enrichment, as its concentration on the alloy surface rapidly decreased owing to its preferential dissolution. Heat treatment did not affect the average compositions of the main passive layer components (aluminium and magnesium compounds). Conversely, in copper compounds, varying copper concentrations were observed which were higher for aged specimens. Copper enrichment of the surface layer enhanced pit formation and growth.
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