Influence of trivalent chromium process (TCP) concentration on the anti-corrosion protection of 2024-T3 aluminium alloy

In the present work, the surface of 2024-T3 aluminium alloys was coated with different concentrations (25, 35, and 50 wt.%) of the trivalent chromium process (TCP), and they were evaluated by comparing them to hexavalent chromium coatings (HCC). The findings show that TCP coating had a porous surface regardless of the concentration of TCP coating while the HCC exhibited a cracked pattern. The TCP coating increased the surface roughness as well as the wettability compared to HCC. The immersion test in 3.5 wt. % NaCl solution revealed that the open circuit potential (Eocp) of coated samples was more positive than that of the uncoated sample. However, the HCC coating was slightly more anodic after half of the immersion. The potentiodynamic test confirmed the formation of a passive region for all samples. Furthermore, the corrosion potential (Ecorr) and the critical pitting potential (Epit) were more positive for the lowest concentration coating. The polarisation resistance of the samples suggested that the HCC coating was superior to TCP coatings. In general, the low concentration of TCP coating improved the corrosion protection of 2024-T3 but not better than HCC coating due, probably, to the porous surface morphology. Dans le present travail, on a revetu la surface des alliages d'aluminium 2024-T3 avec differentes concentrations (25% en poids, 35% en poids et 50% en poids) du procede au chromium trivalent (TCP), et on les a evalues en les comparant aux revetements de chromium hexavalent (HCC). Les resultats montrent que le revetement TCP avait une surface poreuse independamment de la concentration du revetement TCP alors que le HCC presentait un motif fissure. Le revetement TCP a augmente la rugosite de la surface ainsi que la mouillabilite par rapport au HCC. Le test d'immersion dans une solution de 3.5% en poids de NaCl a revele que le potentiel a circuit ouvert (Eocp) des echantillons revetus etait plus positif que celui de l'echantillon sans revetement. Cependant, le revetement HCC etait legerement plus anodique apres la moitie de l'immersion. Le test potentiodynamique a confirme la formation d'une region passive pour tous les echantillons. De plus, le potentiel de corrosion (Ecorr) et le potentiel critique de piquration (Epit) etaient plus positifs pour le revetement a plus faible concentration. La resistance a la polarisation des echantillons suggerait que le revetement HCC etait superieur aux revetements TCP. En general, la faible concentration de revetement TCP a ameliore la protection contre la corrosion de 2024-T3 mais pas mieux que le revetement HCC en raison, probablement de la morphologie poreuse de la surface.