A critical assessment of the current understanding of chromium passivation treatments in tinplate

Abstract

Chromium passivation treatment on tinplate is important in stabilising the active tin surface in terms of oxidation resistance, sulphide stain resistance and ensuring good lacquer adhesion properties. Through this research, the reason was explored why dip passivation treatment, in sodium dichromate, provides superior lacquer adhesion properties on tinplate compared to electrolytic sodium dichromate (CDC) treatments. A critical assessment of the current knowledge of chromium passivation treatments on tinplate formed the basis of the experimental work. Through electrochemical and surface analytical techniques (X-Ray Photoelectron Spectroscopy and Auger Electron Spectroscopy) both dip and CDC treated tinplate surfaces were characterised in terms of surface species. It was shown through XPS, using angle resolved techniques, that no metallic chromium forms on the tinplate surface during CDC treatments. Furthermore, a difference in chromium surface species was established for the dip and CDC processes showing additional SnO and Cr(OH)3.nH2O species for the latter. Complementary to these findings, it was verified that the formation of additional chromium hydroxide species is a function of pH, governed by the applied cathodic current during the CDC process itself. The inferior adhesion properties of CDC treated tinplate were shown to be related to the difference in the respective surface species formed during dip and CDC applications. Copyright 2004, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Biermann, MC 2004, A critical assessment of the current understanding of chromium passivation treatments in tinplate, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-04172007-123305 / >