Combinatorial Studies into the Effect of Thermally Inter-Diffused Magnesium on the Kinetics of Organic Coating Cathodic Delamination from Zinc Galvanized Steel

Abstract

This paper describes a high-throughput study into the role of Mg in preventing corrosion driven coating disbondment of organic coatings from Zn-Mg alloy galvanized steel. A graded Mg wedge is applied to a hot-dip zinc galvanised steel substrate using physical vapour deposition, and subsequently annealed to produce metallic inter-diffusion and formation of Mg2Zn11 intermetallic. An overcoat of electrically insulating polyvinyl butyral (PVB) is applied and corrosion is initiated from a penetrative coating defect using an aqueous electrolyte. The variation in Mg coating weight across the wedge facilitates a systematic investigation of the effect of Mg on Volta potential and the rate of corrosion driven cathodic coating disbondment using scanning Kelvin probe (SKP) potentiometry. The rate of cathodic disbondment is shown to decrease rapidly even at very low Mg coating weight (corresponding to 25 nm thickness before annealing). The results are explained in terms of the galvanic polarity of the corrosion cell formed between Zn exposed at the defect site, and the intact Zn-Mg layer at the metal-organic coating interface.

The Use of Chromium and Chromium (III) Oxide PVD Coatings to Resist the Corrosion Driven Coating Delamination of Organically Coated Packaging Steel

“…This finding is in direct contradistinction to that previously reported in the case of MgZn "wedge" combinatorial libraries on a zinc substrate, in which delamination rate decreased dramatically as surface Mg 2 Zn 11 intermetallic layer thickness increased. 35 In the case of both Cr(III) oxide and Mg 2 Zn 11 intermetallic surface layers, the local rate of cathodic O 2 reduction and therefore organic coating delamination rate is expected to decrease as layer thickness increases. One possible explanation is that the invariance of delamination rate on iron results from a lateral (direction normal to the direction of the delamination) diffusion of electrolyte ions (most importantly OH -) in the thin layer of catholyte that forms beneath the delaminated PVB coating.…”

“…Thin metallic layers were sputtered on to the steel substrate using a Kurt J Lesker PVD75 Physical Vapour Deposition system following a procedure described previously to apply aluminium 34 and magnesium 35 PVD wedge coatings. Samples were attached to the sample holder using countersink head machine screws in order to keep them flat.…”

“…The intention of the current work is therefore to build upon the findings obtained that showed that corrosion driven cathodic disbondment of a PVB coating, from production line chromium (III) metal-oxide-carbide coated steel, decreased with increased chromium oxide thickness. It does so by following a methodology used previously for aluminium 34 and magnesium based coatings, 35 to apply a chromium and chromium (III) oxide wedge of graded thickness to a steel substrate using PVD. This allows one to determine the specific role that either the metallic chromium layer thickness (t Cr ), or 2.)…”

Investigation of properties of epoxy coating on X80 steelunder cathodic polarization conditions

“…Paper is available on the website: www.idk.org.rs/journal of the surface [7], the presence of soluble salts on the surface [8], the level of the cathodic polarization potential, and the size of the gap between the exfoliated coating and the steel [9], which affects the change in corrosion and electrochemical conditions on the surface of the pipe near the area of disbondment.…”