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In Situ Scanning Kelvin Probe Study of Cathodic Disbondment on Trivalent Chromium Coated Steel

Paper No. 91
Mr James Edy1, Professor Hamilton Neil McMurray1, Doctor Arnoud deVooys2 – 1Swansea University , Swansea, United Kingdom, 2TATA Steel Europe, Ijmuiden, The Netherlands

 

Abstract

Electrocoated Chromium packaging Steel (ECCS) originally developed as an alternative to tinplate, has excellent resistance to corrosion driven coating disbondment. ECCS is deposited from plating electrolyte which contains hexavalent chromium, and poses a health risk to plant workers due to its carcinogenic properties. European Legislation will ban the use of hexavalent chromium for such purposes in 2017. A mixed chromium-metal-oxide-carbide coating, deposited from trivalent chromium, has been developed by TATA Steel Europe to produce Trivalent Chromium Coated Technology steel (TCCT).

Ongoing studies at Swansea University in collaboration with TATA Steel show that a range of trivalent chromium coatings demonstrate inhibition of corrosion-driven disbondment. In-situ scanning Kelvin Probe (SKP) technique has demonstrated the ability of TCCT to inhibit cathodic disbondment. TCCT of varying coating thicknesses, overcoated with a model coating of Poly(vinyl) butyral (PVB) were studied.

Resistance to cathodic disbondment increased as TCCT coating thickness increased. Longer initiation times to delamination were recorded above a threshold chromium metal content. Delamination rates decreased in line with increases in chromium oxide content. Delamination kinetics were determined to be linear and thus under electron transfer control. It is proposed that increases in overall coating thickness decreases porosity in the chromium coating, reducing potential cathodic sites for the cathodic front to advance over. It is further proposed that chromium oxide, due to its electrical insulating properties, inhibits electron transfer control and thus reduces delamination rate. Characterisation of the surface was performed using SEM and XPS.