Cross-sectional high-resolution microscopy of thin pretreatment layers on hot dip galvanized steel

Hexavalent chromium containing pretreatments and primers for coil coating are soon to be entirely prohibited, which sets new demands for Cr-free alternatives. Most of the presently used Cr-free pretreatment layers operate predominantly via barrier formation and adhesion promotion mechanisms and lack the self-healing effect typical for Cr6+-pretreatments. This sets new demands also for the formation and monitoring of these layers. The barrier thickness and chemical composition of Cr-free pretreatment layers on hot dip galvanized steel were studied using cross sections from broad ion beam (BIB) sample preparation and ultramicrotome cutting. BIB milling provided finely polished cross sections of pretreated samples. Film thicknesses of 20-50 nm were accurately determined for Cr-free pretreatments containing 4-10mg Ti/m(2) using BIB milling and scanning electron microscopy imaging. Scanning transmission electron microscopy, integrated with aberration correctors and X-ray energy dispersive spectrometry, of an ultramicrotome cut pretreated and painted samples provided detailed chemical information. Metal complexes were detected close to the pretreatment/zinc interface, while the polymeric part of the pretreatment layer prevailed closer to paint. Copyright (C) 2014 John Wiley & Sons, Ltd.