Chromium/chromium oxide based coatings, cathodically electrodeposited from either Cr (VI) or Cr (III) containing electrolytes are compared with respect to their ability to resist the corrosion driven delamination of an adherent polymer overcoat. Cathodic disbondment rates are determined using an in-situ scanning Kelvin probe technique. Anodic disbondment (filiform corrosion, FFC) rates are determined optically. The Cr (VI) derived coatings were fully resistant to corrosion driven disbondment. The Cr (III) derived coatings exhibited measurable rates of both FFC and cathodic disbondment. Disbondment kinetics are explained in relation to coating morphology, porosity and chemical composition determined using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and xray photoelectron spectroscopy (XPS).

This paper describes a systematic study into the initiation and propagation of cathodic delamination on pure iron, pure tin and industrially important tin and iron-tin intermetallic (FeSn and FeSn 2 ) coatings for packaging steels, as a function of coating weight. Cathodic disbondment rates for an organic lacquer overcoat applied to the various metallic coatings are determined using an in-situ scanning Kelvin probe technique. Cathodic disbondment was not observed on pure tin and was found to propagate at reduced rates on the FeSn and FeSn 2 intermetallic coatings, relative to pure iron. An explanation of these findings is given in terms of electrocatalytic activity of various metallic surfaces for the cathodic oxygen reduction reaction. It is shown that the relative susceptibility of Fe, Sn, FeSn and FeSn 2 to cathodic disbondment increases with decreasing cathodic overpotential and is independent of metallic coating weight. Tin is traditionally used for the corrosion protection of packaging steel. Conventional tinplate is typically produced by initially electrodepositing a porous layer of pure tin (∼2.8 g.m −2 -11.2 g.m −2 ) onto the cathodic steel substrate, as shown in Figure 1a, and heating above the melting point of tin (reflowing). [1][2][3] Reflowing causes the tin layer to become fully dense (bright tin) and also allows creation of partial or complete iron-tin intermetallic, typically FeSn 2 , as shown in Figure 1b [1][2][3] .Recently the increasing price of tin in the commodities market has resulted in a need to reduce the amount of tin used in tinplate materials (tin coating weight) whilst retaining adequate corrosion resistance. One approach to doing this has been to diffusion anneal the unflowed tin (<1g.m −2 ) in a reducing atmosphere at temperatures above 500 • C 4,5 in such a way that almost all (preferably 90-98%) 6 of the free tin is converted to an iron-tin (FeSn) intermetallic as shown in Figure 1c. It has previously been found that heating to these temperatures leads to increased tin diffusion to the substrate and formation of a continuous FeSn layer which is more uniform and dense than the conventional FeSn 2 layer. 4 The principal purpose of tin in tinplate packaging materials is to provide corrosion resistance. Furthermore, in modern packaging, corrosion resistance is enhanced by using tinplate in conjuction with an organic (laquer or laminate) overcoat. Consequently mechanisms of corrosion driven coating delamination are of particular concern. The aim of the current paper is to present a detailed study of the role of free tin and iron-tin intermetallic layers in resisting atmopheric corrosion, specifically cathodic disbondment, as it affects lacquer coated packaging material.During cathodic delamination the separation of the organic coating from the metal substrate is linked to the cathodic reduction of oxygen. Within the localized corrosion cell anodic metal dissolution located in the vicinity of the defect is coupled to the cathodic delamination front by a thin (<5 μm) gel like electrol...

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E-SVET describes a novel, Environmentally controlled, Scanning Vibrating Electrode Technique which allows the measurement of localised corrosion current flux under conditions of low oxygen partial pressure. E-SVET replicates the low oxygen partial pressure conditions which occur inside a food can. SVET was utilized to investigate rapid detinning processes initiated in the presence of aerated fruit acid. In this investigation, two corrosion mechanisms were detected; de-tinning on the outer surface of the coating, followed by localised pitting on the iron-tin alloy layer. Corrosion pits were subsequently characterized using White Light Interferometry, a practical method for obtaining detailed information on the geometry of the corrosion pits.