Enhanced Corrosion Performances of Post-Treated Trivalent Chromium Protection (TCP) Coating Deposited on Aluminium Alloy 2024-T3

Abstract

There are still several drawbacks of Trivalent Chromium Process (TCP) coatings related to their low corrosion performances with reference to Chromium Conversion Coatings (CCC). However, the TCP is one of the most promising solutions for replacement of Chromium Conversion Coatings (CCC) containing hexavalent chromium for corrosion protection of aluminium alloys. In order to improve the corrosion performances of the TCP coatings the post-treatments can be applied. There is limited information in the literature on the post-treatment of TCP conversion coatings, which leads to increased corrosion resistance of the conversion layers [1-3]. This work shows the effect of Socosurf PACS post-treatment (containing lanthanum nitrate salt and hydrogen peroxide, 4.2<pH<5.3) on TCP coating. The first results performed by the mean salt spray resistance tests (according to ASTM B117 standard) showed the corrosion resistance increase from 96 hours for a TCP without post-treatment to around 360 hours for a TCP after PACS post-treatment [4]. To understand the effect of the post-treatment, different times of PACS post-treatment were applied on the TCP coating. The coating performances were evaluated by the in situ Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV) tests using different immersion times in a 0.01M NaCl + 0.1M Na2SO4 electrolyte. The LSV measurements show increase of the cathodic inhibition of oxygen reduction with increase of PACS time post-treatment. The EIS results confirm the improved corrosion resistance of the post-treated TCP layers. The surface and bulk chemical composition of the TCP coatings with and without post-treatment were performed by X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). It was found that the post-treatment has no influence on the thickness of the TCP coating however it leads to the presence of lanthanum on the surface and in the coating bulk and to the appearance of chromium (VI) only at the extreme surface of the coating layer. The long-time immersion in a 0.01M NaCl+0.1M Na2SO4 electrolyte leads to the coating degradation as evidenced by the in situ EIS measurements (a decrease of the coating resistance Rcoat). The coating degradation is less significant for the PACS post-treated conversion layers than for the conversion layers without post-treatment. Apart the cathodic inhibition of oxygen reduction provided by lanthanum, the better corrosion resistance can be related to the improved coating homogeneity and the reduced coating cracking when the PACS post-treatment is applied. Acknowledgements: Financial support of DGA-France, the Région Occitanie/Pyrénées-Méditerranée and the EU (FEDER/ERDF) in the frame of NEPAL FUI project.