Poly­(p-phenylene terephthalamide) (PPTA) is mostly used as a low-density polymeric fiber with high specific stiffness, strength, and thermal and chemical stability. The fiber is used as a reinforcement in composite materials in the aerospace and automobile industries, as well as in ballistic and stab-resistant articles. However, its use in composite materials is hampered by its low interfacial affinity with polymeric matrices due to its smooth and inert surface. To overcome such low interfacial interaction, various treatments have been applied to modify the aramid surface. However, it is still challenging to identify an industrially feasible process that does not negatively impact mechanical properties of the aramid fibers. The objective of this study was to investigate different ionic liquids (ILs) with suitable chemical structures as alternative compatibilizers for aramid fibers and epoxy resin. Kevlar fibers were treated with ethanolic solutions of imidazolium IL (1-n-butyl-3-methylimidazolium chloride, 1-carboxymethyl-3-methylimidazolium chloride, 1-triethyleneglycol monomethyl ether-3-methylimidazolium methanesulfonate, or 1-n-butyl-3-methylimidazolium methanesulfonate) and then analyzed by infrared spectroscopy, thermogravimetry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Fiber tensile tests, pull-out tests, and contact angle measurements were used to characterize the fiber and its interface with the epoxy resin. Treatment with all IL, except 1-carboxymethyl-3-methylimidazolium chloride, enhanced the wettability and adhesion of the fibers without imparing mechanical properties. Epoxy resin-based composites were produced using commercial fabrics before and after 1-triethyleneglycol monomethyl ether-3-methylimidazolium methanesulfonate treatment and characterized via tensile and short-beam tests. The composite produced with treated fabrics presented slightly higher tensile strength, modulus, and interfacial shear strength. This improvement can be of interest to the composite sector.

A finite element model of the protection mechanisms offered by Mg-based organic coatings was developed. The model predicted the change in the corrosion potential of AA2024-T351 as a function of pH, water layer thickness, and the inhibition of oxygen reduction reaction. The pH in the solution was calculated taking into account Mg dissolution, precipitation of Mg(OH)2, Al dissolution, and hydrolysis of Al3+ ions. The predicted critical pH value at which the corrosion potential of AA2024-T351 sharply decreases to values below pitting and pit repassivation potentials under full immersion conditions was in accordance with experimental observations. A limiting water layer thickness below which the pH-induced pit repassivation mechanism is not predicted to occur was calculated. If the inhibition of oxygen reduction reaction by Mg(OH)2 is considered, the pH-induced repassivation mechanism becomes feasible at thinner water layers. Cathodic protection offered by Mg-rich primers was modeled as a function of coating resistance, water layer thickness, and electrolyte chemistry. The magnitude of the resistance of the film in which Mg pigments are embedded mitigates the extent of the cathodic protection. The change in local pH due to corrosion reactions affected the galvanic potentials obtained. The framework developed can be used to help identify chemical inhibitors that can operate by the chemical protection mode described in this work.

The scanning vibrating electrode technique (SVET) was utilized to experimentally validate the applicability of finite element modeling (FEM) in simulating macro-galvanic-induced corrosion of AA7050 coupled to SS316, in environments representative of the boldly exposed surface of an actual fastener couple. The FEM boundary conditions were modified from the SVET environments in which the AA7050-SS316 couple sample was initially exposed, in order to better represent the steady-state corroding surface of the localized corrosion-prone AA7050. Better agreements between the SVET-derived data and the model in the case of macro-galvanic coupling behavior were achieved for near-neutral conditions, compared to acidic conditions. The current density at the electrode/electrolyte interface was determined with the validated model. In addition, the percent difference between the measured current density at the SVET probe height and that at the electrode surface was observed to scale with the magnitude of current density at the electrode surface, with the largest discrepancy seen at the galvanic couple interface. Plausible reasons for the deviation of the model predictions from the SVET-derived data are discussed.

The effect of thin film environments on the intergranular stress corrosion cracking (IG-SCC) behavior of AA5083-H131 was investigated using fracture mechanics-based testing, high-fidelity monitoring of crack growth, and electrochemical potential measurements. A protocol for conducting thin film IG-SCC fracture mechanics experiments with anodized aluminum oxide (AAO) membranes is developed and the ability to maintain films of specific thicknesses without impeding oxygen diffusion during testing is validated via EIS testing and computational modelling. The IG-SCC susceptibility was found to increase once a critical thin film thickness of 82 µm was achieved; above this thickness a duality in IG-SCC susceptibility behavior was observed. These results are analyzed in the context of a coupled anodic dissolution and hydrogen (H) embrittlement mechanism, where susceptibility is found to scale with the cathodic limitation of the governing IG-SCC mechanism. Specifically, thinner film thicknesses led to limitations on the amount of cathodic current availability, which caused a decrease in the dissolution at the crack tip, a less aggressive crack chemistry development, and thus lower levels of H production. A close correlation between the open circuit potential of the bulk surface and the crack growth kinetics was also observed, consistent with trends reported in previous IG-SCC studies on this alloy.

Metallic structures are commonly exposed to marine atmospheric conditions in real world conditions, characterized by thin water layers (WL) allowing for corrosion to occur. For austenitic stainless steels in such atmospheric environments, pitting and stress corrosion cracking (SCC) are potential degradation mechanisms. The rate and extent of corrosion and SCC on the alloy surface is dictated by the combination of environmental, physicochemical, and geometric variables, which include relative humidity, temperature, electrolyte conductivity, electrolyte film thickness, in addition to the electrochemical kinetics on the alloy surface. Due to the large potential variation in environmental conditions, modeling can serve as an important tool to evaluate environmental effects on the corrosion and SCC of alloys and galvanic couples.In this study, Finite Element Modeling approaches are used to evaluate chemical and electrochemical conditions present in a crack exposed to marine, thin film environments. A full reactive transport model, incorporating metal hydrolysis and salt formation, is built to evaluate SCC crack tip conditions. The influence of WL thickness, stress intensity level, crack length, and specimen geometry are evaluated. Specifically, crack tip pH and equilibrium potential among other variables will be compared between the various atmospheric factors both spatially and temporally. The modeling results will be compared to atmospheric SCC experiments and help give insight into critical experimental and testing phenomena. Acknowledgements Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This document is SAND2021-5051 A.