Surface Modifications Induced by Pretreatments and Effects on The Chemical Structure of TCP Conversion Coating on Al-Cu-Li Alloy (AA2050)

Trivalent Chromium Process (TCP) was developed as a promising conversion process aiming at enhancement of corrosion properties and adhesion of aluminium alloys. The alkaline etching and acid pickling are usually applied as surface pretreatments on aluminium alloys before TCP deposition in order to optimize their properties. In this work, the surface modifications of an Al-Cu-Li alloy (AA2050-T8) at different stages of pretreatment (alkaline degreasing and acid pickling) and subsequent TCP are investigated by X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and scanning electron microscopy. The pretreatments leading to significant enrichments in Cu and less marked in Ag, show also a strong influence on the subsequent formation of TCP in comparison to the non-pretreated, polished AA2050-T8 alloy. A slight decrease in Li content is observed in the oxide layer after pretreatment and no Li is present in the outer TCP layer. A thicker TCP conversion coating (outer oxide) with less cracks is formed on the pretreated alloy surface than on the polished one. Moreover, a Cu enrichment is evidenced in the TCP layer of the pretreated sample, which results in enhanced cathodic activity of pretreated sample with reference to polished sample as demonstrated by linear sweep voltammetry.