Abstract
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.
Highlights
Surface modifications induced by pretreatments.—In order to observe the topography and chemical modification of the surface during each step of pretreatment, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and ToF-SIMS ion depth profiles were performed on AA2050-T8 sample after degreasing and after degreasing & pickling treatments
Cu and Ag enrichment is reduced after the pickling step with reference to the degreasing step and most of Cu and Ag is in the metallic state and distributes at the oxide/metal interface as shown by XPS calculation
The pretreatments led to formation of a thicker aluminium oxide layer with significant enrichment of Cu and Ag and decreased quantity of Li in the inner part of the oxide and the oxide/metal interface. This modified surface with Cu and Ag enrichments induced by pretreatments has promoting effects on the growth of Trivalent Chromium Process (TCP) layer compared to polished surface
Summary
Surface modifications induced by pretreatments.—In order to observe the topography and chemical modification of the surface during each step of pretreatment, SEM, XPS and ToF-SIMS ion depth profiles were performed on AA2050-T8 sample after degreasing and after degreasing & pickling treatments. It is clearly observed that most of IMPs in the alloy remain. When the degreased surface is treated with pickling bath, most of the IMPs dissolve, leaving holes and revealing the grain boundaries To facilitate a comparison of ion depth profiles, each depth profile was normalized assuming this sbigynailts[2]
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