J. E. deVries

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.

Assessment of photooxidation in multi-layer coating systems by time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to assess the formation of 18O-labeled photooxidation products in two multi-layer coating systems held in 20% 18O2/80% nitrogen atmosphere during brief Xenon arc weatherometer exposure. A four-layer model coating system without hindered amine light stabilizer or ultraviolet light absorber additives was examined first. The model was exposed for 8 days in the weatherometer, and then its TOF-SIMS-18O− response was recorded. This response tracks the known photooxidation resistance of the coating layers used to prepare the model system. Next, the technique was extended to a fully formulated six-layer ‘repair’ paint panel that had been weathered for 4 years in Florida prior to 10 additional days of weatherometer exposure in 20% 18O2/80% nitrogen atmosphere. The TOF-SIMS-18O− response observed clearly suggests that the TOF-SIMS-18O− technique can be used to assess the relative photooxidation rates of individual coating layers in fully formulated, multi-layer coating systems.

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.

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.

High-spatial and high-mass-resolution SIMS instrument for the surface analysis of chemically complex materials

A secondary ion mass spectrometer (SIMS) has been developed for the surface analysis of chemically complex samples. The instrument has an ion transmission efficiency of 12% and an abundance sensitivity of better than 107 at 500 mass resolution. A computer system has been developed to acquire, store, and process images with up to 1024×1024 pixel resolution. Complex SIMS spectra were resolved using either high‐resolution (30 000) mass analysis (HRMS) or mass spectrometry/mass spectrometry (MS/MS). Ion images of insulators or conductors were obtained with approximately 2000 A resolution. Surface studies of electronic devices and chemical sensors are presented. SIMS–MS/MS analysis was used to identify contaminants on the surface of a failed pressure sensor. The MS/MS analysis of secondary ions minimized interferences and facilitated identification of molecular species.

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.

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...

Characterization of interfacial chemistries associated with polymer systems by spatially resolved surface analytical methodologies

Abstract Two examples of recent advances in spatially resolved surface analytical characterization of paint and adhesive chemistries are presented. The examples extend previous analytical capabilities by combining the molecular specificity and bonding information of static secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) with the ability to resolve features of interest spatially. In one example, a cross-section of two paint layers, only one of which contained a photostabilizer additive, was characterized by time-of-flight (TOF)-SIMS operated in a microscope mode. Results indicate possible migration of this additive into the bulk of the adjacent paint layer. The second example presents an XPS mapping technique which was used to study the interfacial corrosion chemistry of an epoxy adhesive applied to galvanized steel. The sample was subjected to a corrosive environment while under static load. This technique afforded spatially resolved chemical information over the entire interfacial failure surfaces and allowed localized regions of corrosion activity to be imaged. Comparisons of results and techniques are made, along with discussions of the limitations of each technique.