Chromium valence change in trivalent chromium conversion coatings on aluminium deposited under applied potentials

Jiantao Qi (1, 2) , Jolanta Swiatowska (3) , Peter Skeldon (2) , Philippe Marcus (3)

1 China University of Petroleum
2 University of Manchester [Manchester]
3 IRCP - Institut de Recherche de Chimie Paris

The electroassisted (EA) deposition of trivalent chromium conversion coatings on aluminium is investigated with a focus on the influence of applied potential on the chromium valence state, as determined by Raman spectroscopy. The morphology and compositions of the coatings were investigated by scanning electron microscopy and energy-dispersive spectroscopy. The EA coatings were formed in naturally-oxygenated SurTec 650 chromitAl solution at constant potentials of −1.5 and −0.5 V SCE. The coatings contained chromium and zirconium constituents. The potentials resulted in net cathodic and anodic current densities, respectively, during the coating growth. Comparisons were made with coating formation at the open-circuit potential (OCP). The coating thickness increased in order −0.5 V SCE < OCP < −1.5 V SCE , a result of increasing alkalinity from the cathodic reaction that facilitates deposition of the coating constituents. Fresh coatings formed at −1.5 V SCE revealed the presence of Cr 6+ species. By contrast, Cr 6+ species were not resolved in the coating formed at −0.5 V SCE. It is proposed that less H 2 O 2 is generated at −0.5 V SCE to oxidize Cr 3+ coating species.

Keywords: Trivalent chromium; Hexavalent chromium; Conversion coating; Hydrogen peroxide; Oxygen reduction

Domains: Material chemistry; Inorganic chemistry

Jiantao Qi, Jolanta Swiatowska, Peter Skeldon, Philippe Marcus. Chromium valence change in trivalent chromium conversion coatings on aluminium deposited under applied potentials. Corrosion Science, 2020, 167, pp.108482. ⟨10.1016/j.corsci.2020.108482⟩. ⟨hal-03049594⟩