The corrosion of chromium based coatings for packaging steel

Chromium/chromium oxide based coatings, cathodically electrodeposited from either Cr (VI) or Cr (III) containing electrolytes are compared with respect to their ability to resist the corrosion driven delamination of an adherent polymer overcoat. Cathodic disbondment rates are determined using an in-situ scanning Kelvin probe technique. Anodic disbondment (filiform corrosion, FFC) rates are determined optically. The Cr (VI) derived coatings were fully resistant to corrosion driven disbondment. The Cr (III) derived coatings exhibited measurable rates of both FFC and cathodic disbondment. Disbondment kinetics are explained in relation to coating morphology, porosity and chemical composition determined using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and xray photoelectron spectroscopy (XPS).