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

Abstract

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 Cr 6+ -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 4-10 mg 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.

Corrosion mechanisms of Al‐alloyed hot‐dipped zinc coatings in wet supercritical carbon dioxide

“…[19] Broad ion beam (BIB) milling technique has lately become more popular in preparation of high-quality cross-sections of delicate samples. [20][21][22] Although the BIB technique has been widely available already for longer than a decade, it has not been extensively utilized in corrosion studies. A recent development of a more surface sensitive energy-dispersive spectroscopy (EDS) system for scanning electron microscopes allows utilization of lower accelerating voltages, thus narrowing the spatial resolution in element mapping well below 100 nm [23,24] and providing a lucrative platform for closer investigation of BIB-polished specimens.…”

Assessment of pitting corrosion in bare and passivated (wet scCO2‐induced patination and chemical passivation) hot‐dip galvanized steel samples with SVET, FTIR, and SEM (EDS)

“…[ 9 ] Argon ion beam milling is capable of producing high‐quality cross‐sections of very thin layers (<100 nm) and abrasion‐sensitive deposits on zinc coatings. [ 10–12 ]…”