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.
Highlights
The current paper focuses on one important mode by which such corrosion-driven failure can occur, namely cathodic delamination
In the case of cathodic delamination it is the cathodic oxygen reduction reaction (ORR) which is responsible for the disbondment of the organic coating from the metal substrate
Anodic metal dissolution occurring in the defect region is coupled to a cathodic delamination front via a thin (
Summary
SKP has been widely used as means of following the progress of organic coating delamination from a variety of metallic substrates,[6,7,8,9,10,15,24,25] including galvanized steel[8,9,10] and pure phase MgZn2,1–4 and recent work has highlighted its role in providing mechanistic information by measuring the characteristic potentials associated with different portions of a localized corrosion cell.[26] the high cathodic delamination resistance observed in the case of organic coated MgZn2 has been attributed to an inversion in the normal polarity of the localized corrosion cell evolving between an intact organic-coated surface and MgZn2 exposed at a penetrative organic coating defect.[1,2,3,4] Here we show that, by arranging the geometry of the localized corrosion cell in such a way that cathodic delamination occurs in a direction normal to the gradient of the original (PVD)Mg wedge deposit, it is possible to characterize simultaneously the full range of Mg coating weights present in a single-substrate combinatorial library. This process was completed to reduce the complexity of the profiles and to aid in determining the location of the cathodic delamination front
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