Evolution of the rubber–brass adhesion interphase during humidity aging. Part I: combined effects of brass copper content and cobalt salt

The chemical evolution of the rubber–brass-plated steel cord interface during vulcanization and environmental aging is governed by complex interactions among the brass coating composition, rubber formulation, and interfacial reaction chemistry. In this study, the evolution of the interfacial region formed between rubber compounds and brass-plated steel cords was systematically investigated as a function of brass Cu/Zn composition, cobalt salt addition, and humidity aging. Three raw steel cords with different nominal Cu contents of 51.4, 64.2, and 75.5% were first characterized by Auger electron spectroscopy (AES) depth profiling to establish their initial surface and subsurface chemical structures. The raw cords exhibited a carbon- and oxygen-containing outer surface, followed by a Cu–Zn-rich brass coating and a gradual transition toward the Fe-rich steel substrate. Increasing the nominal Cu content resulted in progressive Cu enrichment of the brass-rich region and a corresponding decrease in the relative Zn contribution. Following rubber encapsulation and vulcanization, the interfacial region underwent substantial chemical modification, characterized by the redistribution of brass-derived Cu and Zn species and the development of sulfur-containing reaction products. The addition of cobalt salt further modified the interfacial chemical evolution, indicating its involvement in the formation and stabilization of the vulcanized rubber–brass interphase. During humidity aging at 85 °C and 85% relative humidity, progressive changes in the distributions of Cu, Zn, Fe, O, C and S were observed, demonstrating that moisture exposure induced chemical redistribution and degradation of the interfacial reaction layer. The extent and characteristics of these changes depended on the initial Cu/Zn composition of the brass coating and the presence of cobalt salt. The results demonstrate that the rubber–brass-plated steel cord interface is a chemically dynamic region whose structure evolves through coupled brass dissolution or redistribution, sulfur-mediated reactions, oxidation, and moisture-induced degradation. These findings provide a mechanistic understanding of the formation and environmental evolution of the rubber–brass interphase and establish a fundamental basis for designing more chemically stable brass coatings and rubber formulations for durable steel cord–rubber composite systems.