What makes epoxy-phenolic coatings on metals ubiquitous: Surface energetics and molecular adhesion characteristics

Microstructural Defects of Epoxy–Phenolic Polymers on Metal Substrates during Acidic Corrosion

“…These effects unfortunately cannot be completely decoupled in a model system that retains some industrial relevance. Wetting energy is one of the parameters in controlling the evenness of coatings, and indeed we found in an earlier study that type 7 coatings appear to be more evenly applied onto TFS substrates, also validated here by a relative lack of defect sites from optical microscopy. The wetting difference of the uncured polymers on metals is due to the difference in interfacial free energy, the density of −OH groups on the exposed metal surfaces, and the epoxide-to-hydroxyl ratio of side groups within the polymer backbone.…”

“…Results demonstrate the effectiveness of CLSM in evaluating the microscopic degradation of polymer coatings applied to metal substrates. Combined with chemical analysis of the coatings here and of the surfaces in an earlier study from our group, the results suggest that both substrate chemistry and surface morphology are of critical importance in delaying the onset of corrosion in metals protected by an organic coating.…”

“…It is likely that the relative lack of defect sites on the freshly deposited coatings is offset by the inherently rougher nature (Table ) and increased iron content of the underlying TFS metal substrate, which creates many shortened pathways that promote oxygen diffusion to the metal surface. Earlier work by a number of researchers show that the delamination of polymer coatings from metal surfaces proceeds by oxygen diffusion through the coatings and ion transport along defect sites. , At rough spots on the metal surfaces in our study, shortened barrier pathways caused by the raised asperities serve as localized cathodic and anodic sites for redox reactions that result in regions of high alkalinity, cause blister formation along the coating–metal interface, and lead to eventual detachment of the coating from the metal surface .…”

“…A passivation layer consisting of chromium oxide is often applied to the metal substrate due to its superior ability to enhance the binding strength . The morphology of the metal substrate alters the interfacial energy of the coatings and is an important consideration in interfacial wetting and adhesion studies . Higher substrate roughness can increase the effectiveness of coating adhesion by providing a larger interfacial area but simultaneously decrease it in the long term through the presence of discrete locations where the coating is effectively thinner and the underlying substrate more vulnerable to corrosion .…”

“…9 The morphology of the metal substrate alters the interfacial energy of the coatings and is an important consideration in interfacial wetting and adhesion studies. 10 Higher substrate roughness can increase the effectiveness of coating adhesion by providing a larger interfacial area but simultaneously decrease it in the long term through the presence of discrete locations where the coating is effectively thinner and the underlying substrate more vulnerable to corrosion. 11 The goal of this study is to quantify how metal surface roughness, metal composition, and epoxy molecular weight affect the degradation of epoxy−phenolic coatings submerged in acidic liquids.…”