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

Abstract

This study uses confocal microscopy and image processing to investigate the microstructural changes of coating–metal systems immersed in a heated acidic bath. Unlike standard optical microscopy techniques, 3D confocal microscopy images can quantitatively reveal microscopic defects formed at early stages of cathodic delamination. The coatings are made of fluorescent epoxy–phenolic resins cured at high temperatures onto tinplate (T23) and tin-free steel (TFS) substrates. When the coated metal substrates are immersed in acetic acid, a series of microscopic corrosion events occur at the polymer–metal interface. These events are quantified by changes in the thickness distribution of the degraded samples relative to that of intact coatings. The degradation rate is highest for epoxy–phenolic polymers on TFS substrates, represented by multiple orders of magnitude increase in the number density of defect sites. Higher molecular weight coatings provide slightly better resistance against delamination. The coating thickness dictates the rate of oxygen diffusion and ion transport along the polymer–coating interface, where raised asperities serve as localized sites for metal oxidation and formation of alkaline species, leading to subsequent delamination of the cured polymers from the surfaces. The results show that the metal surface topology is as important as chemistry when designing the corrosion resistance of products containing acidic liquids, and that confocal microscopy is useful in quality control through early detection of mesoscale polymer failure.

Isothermal Titration Calorimetry Reveals Entropy-Driven Bisphenol A Epoxy Resin Adhesion to Metal Oxide Surfaces

“…Epoxy resin formulations comprise approximately 60% w/w of an organic solvent and are typically cast on metal substrates with wet film applicators, such as a Mayer rod setup prior to curing. ,, This initial wetting or adhesion of the uncured resin governs the intermolecular forces that directly affect the practical adhesion of the cured epoxies . When the epoxy resin is initially brought in contact with the metal oxide surface, the effect on adhesion of a large abundance of solvent molecules that are present at the interface is often neglected .…”

“…The coating minimizes food interaction with metal surfaces to prevent spoilage from corrosion and ensure long-term durability. Epoxy coatings synthesized from diglycidyl ether of bisphenol A (DGEBA) are commonly used in food and beverage packaging, , as well as in microelectronics and aircrafts − due to their superior barrier property, thermal stability, and excellent adhesion to metal substrates. ,,− Recent efforts to develop alternative coating chemistries with similar resilience as DGEBA epoxies have, however, intensified due to toxicological and health concerns regarding bisphenol A (BPA) (Figure A), a precursor to DGEBA. ,,, Coatings developed to replace DGEBA nevertheless fail more readily because the physicochemical interactions that give rise to the ultrastrong epoxy-metal adhesion remain poorly understood. , The 1–10 μm thick polymer coating masks the polymer–metal interface, thereby limiting information regarding the interfacial interactions that govern adhesion using conventional surface analytical techniques, such as X-ray photoelectron spectroscopy and infrared spectroscopy. , …”

Reflective microscopy for mechanistic insights in corrosion research

“…A range of optical microscopies (e.g. fluorescence microscopy, [10] confocal laser microscopy, [11] etc.) were proposed to address these issues.…”

Molecular structure effects on the mechanisms of corrosion protection of model epoxy coatings on metals

“…40 We also show how the substrate topology is a critical factor in the progression of the corrosion of polymer-coated metals. 41 In this paper, we extend the fundamental study and analysis of epoxy coatings by investigating the corrosion protection of coatings having the same backbone structure with different molecular weights, crosslinked with a single phenolic resole curing agent undergoing monitored accelerated acidic degradation. To probe the effect of metal pretreatment on the corrosion protection, we study the behavior of coatings on tinplated and chromium-coated carbon steel.…”