Ray A. Dickie

The application of x-ray photo-electron spectroscopy to a study of interfacial composition in corrosion-induced paint de-adhesion

Abstract Interfacial surfaces generated by corrosion-induced de-adhesion of organic coatings on bare steel have been analyzed by X-ray photo-electron spectroscopy (XPS). XPS elemental analyses have been used to provide an initial overview of changes in composition relative to untested surfaces; high resolution XPS spectra have been used to provide more detailed, sometimes semi-quantitative, analyses of molecular functional group composition. The high resolution spectra of specimen surfaces have been compared with each other and with reference compounds qualitatively and, in more detail, by calculating difference spectra and binding energy differences. Curve resolution methods have been employed to estimate functional group compositions. Corrosion-induced de-adhesion is associated principally with cathodic corrosion reactions and reaction products, including hydroxide. Epoxy ester-based coating interfacial surfaces have been demonstrated to bear (in addition to undegraded resin) carboxylate moieties indicative of ester saponification. For model epoxy-urethane and epoxy-amine coatings, a substantial carbonate residue is deduced. This residue is attributed to degradation of urethane and urea moieties. In addition, the presence of polymer residues on the interfacial substrate surfaces is demonstrated. It is concluded that de-adhesion involves substantial cohesive failure of the coating resin in the interfacial region.

Paint adhesion, corrosion protection, and interfacial chemistry

Abstract Maintenance of adhesion under environmental exposure is a key performance parameter used to evaluate the protective capability of corrosion-protective paint systems. Interfacial chemical reactions involved in the establishment of paint adhesion, in the suppression of corrosion processes, and in the degradation and loss of paint adhesion have been extensively studied using modern surface analytical techniques. This paper reviews some of the phenomenological and mechanistic conclusions obtained from surface analytical studies. The chemistry of the interface, and the composition of the paint resin, play especially important roles in the initial stages of corrosion-induced paint adhesion loss. Physical degradation processes and their interaction with interfacial chemical processes need to be considered in evaluating the long-term durability of corrosion protective paint systems. Research needs related to paint adhesion, interfacial chemistry, and mechanisms of adhesion loss are discussed.

The Fatigue and Durability Behaviour of Automotive Adhesives. Part II: Failure Mechanisms

Abstract In part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The cyclic fatigue tests were conducted in a relatively dry environment, of 23°C and 55% RH, and in a “wet” environment, namely immersion in distilled water at 28°C. The “wet” fatigue tests clearly revealed the significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The present paper, Part II, discusses the modes and mechanisms of failure for the two adhesive systems in both the “dry” and “...

Interfacial chemistry of corrosion-induced bond degradation for epoxy/dicyandiamide adhesive bonded to cold-rolled and galvanized steels

The interfacial chemistry of a conventional epoxy/dicyandiamide adhesive formulation applied to cold-rolled and galvanized steels has been studied by X-ray photoelectron spectroscopy (XPS). In both cases, the interfacial corrosion process appears to be dominated by attack on the metal substrate, and is reminiscent of crevice corrosion phenomena. On cold-rolled steel, analysis of the interfacial bond failure surfaces suggests that the contamination layer initially present on the substrate is not disturbed during bond formation. Bond failure appears to occur between the substrate and the contamination layer; the contamination layer remains largely intact on the interfacial surface of the adhesive after bond failure. On galvanized steel, the locus of failure appears to be at the adhesive-adherend interface, within a layer of zinc corrosion products. The contamination layer appears to have been partially displaced by the adhesive during bonding.

Rapid, reliable tests of clearcoat weatherability: a proposed protocol

Abstract In this paper, a protocol is proposed for the rapid prediction of durability of automotive enamel clearcoats. The protocol is designed to minimize test time while maximizing test reliability. The protocol is based primarily on chemical measurements of degradation rates rather than observation of physical failure. There are two key aspects to the protocol. The first is the use of electron spin resonance spectroscopy (ESR) to measure the rate of formation of free radicals in the coating under near-ambient exposure conditions (photo-initiation rate). This measure is used to rapidly screen coatings under development and to monitor the weatherability of previously qualified materials. Extensive, conventional accelerated exposure tests are performed only on new materials with low photo-initiation rates. The second key aspect is the use of spectroscopic measurements of chemical degradation both to select an appropriate accelerated exposure and to determine the ratio of the rate of chemical change during accelerated exposure to that during natural exposure ( i.e. the acceleration factor). Knowing the acceleration factor and the time to failure in a valid accelerated exposure, it is possible to estimate the service life. The service life is then related to the photo-initiation rate and photo-initiation rate measurements are used in coating quality control. This report describes in detail the steps in the protocol, the kind of chemical measurements which may be required, and provides examples of its application.

Modeling paint and adhesive cure in automotive applications

Abstract In the development of thermoset coating and adhesive formulations, optimum cure conditions are defined based on measurements of physical properties, chemical resistance, and durability. The range of cure conditions over which a thermoset material exhibits acceptable performance constitutes a cure window for the material, and can be used to define acceptable limits of variability for production processes. There is variability intrinsic to the stoving of complex articles of manufacture: local variations in heat transfer and thermal mass result in substantial variations in temperature history. By applying chemical understanding of crosslinking processes and basic chemical engineering principles, cure strategies can be evaluated using a combination of heat transfer and chemical kinetic models. Examples drawn from research on automotive paints and adhesives are given.

Interfacial chemistry of epoxy-modified poly(vinyl chloride) adhesive on cold-rolled and galvanized steels

—The interfacial reactions of galvanized steel with a poly(vinyl chloride) adhesive formulated with epoxy resin and dicyandiamide have been studied using X-ray photoelectron spectroscopy (XPS). The dicyandiamide was observed to segregate partially to the adhesive/substrate interface. A small amount of amine hydrochloride, evidently formed by reaction of dicyandiamide and HCI formed by dehydrochlorination of the poly(vinyl chloride), was observed near the adhesive/ galvanized steel interface. Dehydrochlorination of poly(vinyl chloride) appears to be enhanced in the adhesive/substrate interfacial region relative to the bulk, but less degradation was observed than for unmodified poly(vinyl chloride) formulations. In parallel studies on cold-rolled steel, neither thermal degradation nor amine hydrochloride formation was observed.

Interfacial chemistry of poly(vinyl chloride) adhesive on cold-rolled and galvanized steels

—The interfacial chemistry of a model poly(vinyl chloride) adhesive formulation applied to cold-rolled and galvanized steels has been studied by X-ray photoelectron spectroscopy (XPS). Chemical changes suggesting enhanced dehydrochlorination of poly(vinyl chloride) in the polymer/ metal interfacial region were observed on both substrates. An ionic chloride species (probably zinc chloride) and an increased level of hydrocarbon were observed at the adhesive/substrate interface on galvanized steel; the total amount of chlorine present near the interface was much less than in the bulk of the adhesive. On cold-rolled steel, the amount of chloride was only slightly lower near the metal surface, and no ionic chloride was observed. Polymer degradation is suggested by an increase in hydrocarbon observed at the interface. Changes in the oxidation state of the iron surface oxide were also observed.

Molecular modeling of adhesion: the interaction of acrylate and methacrylate esters with aluminum oxide

A theoretical examination of Lewis acid-base interactions of ethyl acrylate and methyl methacrylate with a model aluminum oxide surface has been performed using semi-empirical, MNDO, molecular orbital calculations. Interactions of the carbonyl and carbon-carbon double bonds with aluminum oxide were evaluated. Overall, the structures formed by ethyl acrylate were more stable than those formed by methyl methacrylate. The highest stability was found for ethyl acrylate complexes involving the carbon-carbon double bond. These results tally with experimental studies of the adhesion of electron-beam cured acrylic coatings to aluminum, which suggests indirectly that acrylic unsaturation interacts more strongly with aluminum oxide than does methacrylic unsaturation.

Interfacial chemistry of spontaneous disbonding in stress durability testing of adhesively-bonded galvanized steel

Epoxy adhesive/galvanized steel bonds subjected to corrosion testing show a gradual loss of strength. Bonds subjected simultaneously to a static mechanical load and corrosion testing rupture spontaneously at relatively short exposure times. The differences in interfacial chemistry that accompany these exposure conditions were studied using an XPS elemental mapping technique that allowed the interfacial composition to be resolved spatially over the entire bond failure surface. An interfacial anodic process reminiscent of crevice corrosion dominated the interfacial chemistry of specimens exposed to corrosion testing without application of a static load. Bonds exposed under high loads exhibited both anodic and cathodic corrosion sites within the bond failure area. The changes in interfacial chemistry and failure mode upon application of a load are attributed to the opening of an interfacial crack at the locus of the initial corrosive attack. The ingress of electrolyte and the formation of cathodic sites adja...