Evolution of the rubber–brass adhesion interphase during humidity aging. Part II: combined effects of brass coating weight and cobalt salt

The influence of brass coating weight on the formation and stability of the bonding interphase between brass-coated steel cord and cobalt-containing rubber was systematically investigated. Steel cords with three different brass coating weights (D code: 2.62 g kg−1, A code: 3.88 g kg−1, and E code: 5.01 g kg−1) were vulcanized with a rubber compound containing 0.86 phr cobalt salt, and the interfacial characteristics were evaluated before and after humidity-aging treatment. The interfacial microstructure and chemical composition were analyzed using Auger electron spectroscopy (AES) depth profiling. The AES depth profiles revealed that increasing brass coating weight increased the effective brass layer thickness and delayed exposure of the steel substrate. However, the intermediate brass coating weight (A code) exhibited the most homogeneous Cu–Zn distribution throughout the coating, whereas the D code possessed an insufficient brass layer and the E code showed compositional heterogeneity. AES analysis demonstrated that the stability of the Cu–S and Zn-O containing reaction products formed during vulcanization was strongly dependent on brass coating weight. These interfacial characteristics were closely correlated with the adhesion results, particularly after humidity aging. The A code maintained the most stable bonding interphase and exhibited superior adhesion durability, while both lower and higher brass coating weights resulted in reduced interfacial stability during aging. The results demonstrate that optimization of brass coating weight is essential for controlling the interfacial reaction between brass and rubber. An appropriate coating thickness promotes the formation of a uniform and chemically stable interphase, leading to enhanced long-term adhesion durability.