Unravelling the Fe Effect on the Corrosion of Chromium Coatings: Chemical Composition and Semiconducting Properties

A model trivalent chromium-based electroplating bath doped with different concentrations of Fe was used to obtain different metallic coatings. The impact of the Fe was investigated on both the Cr layer and its native passive film by a detailed characterisation using X-ray Photoelectron Spectroscopy (XPS), Angle Resolved XPS and Auger Electron Spectroscopy. Moreover, the semiconducting properties of their oxide layers were explored by Mott-Schottky and the corrosion performance of the coating by linear polarisation resistance and kinetics of the oxide formation. Results revealed not only a homogeneous Fe distribution in the Cr layer but also the presence of an iron-chromium duplex oxide layer for concentrations ≥ 100 mg l −1 Fe in the bath. The Mott-Schottky analysis showed a p-n junction for such coatings due to the presence of an iron oxide layer on top of a chromium oxide one, which increased the total amount of point defects (charge carrier density) and drastically affected to their corrosion resistance (the polarisation resistance decreased by one order of magnitude and their oxide layer showed slower kinetics and a higher passivation current). In contrast, coatings with a single chromium oxide layer showed a p-type semiconducting behaviour as well as the best corrosion performance.