New conversion coatings for aluminium alloys are under development to replace hexavalent chromium and one of the most promising solutions is the Trivalent Chromium Protection (TCP) coating.This work focuses on the effect of post-treatment solutions on TCP, used to reinforce the conversion layer and to improve the corrosion resistance. To understand the effect of the post-treatment, containing hydrogen peroxide and a lanthanum salt as an inhibitor, our approach was to study the effect of these two constituents separately. Surface characterisation techniques, X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), have been used. It was found that the post-treatment has no impact on the thickness of the TCP coating. After the post-treatment, the XPS and ToF-SIMS analyses show the presence of chromium (VI) only at the extreme surface of the coating layer, while lanthanum is found also in the bulk of the TCP layer. Lanthanum plays a significant role in improving the homogeneity and reducing the cracking of the TCP conversion layer as observed by Field Emission Gun-Scanning Electron Microscopy (FEG-SEM).
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