Effect of sizing materials on corrosion resistance of basalt fibers in various exposure environments

This study investigated the influence of sizing materials on the corrosion resistance of basalt fibers exposed to 16 controlled environments combining pH 4–13 and salinity from 0 to 20,000 ppm as chloride. Fibers were exposed to 60°C for 600 days in an environmental chamber, and degradation mechanisms were evaluated using SEM-EDX (scanning electron microscopy and energy-dispersive X-ray spectroscopy) analysis and chemical analysis of exposure solutions. Sized fibers consistently exhibited less degradation than unsized fibers due to the protective sizing layer. Fiber deterioration intensified under highly alkaline and saline conditions, with seawater causing the most damage because of the combined effects of chloride and sulphate ions. Sulphate, together with multivalent cations (Mg ²⁺ , Ca ²⁺ , Fe ²⁺ , Al ³⁺ ), promoted thicker corrosive layers via insoluble precipitates (e.g., gypsum) and adsorption on Fe-Al hydroxide crusts. Solution chemistry indicated leaching of Al, Ca, Fe and K from the fibers, while Mg and Na showed inverse trends. Hydroxide and chloride attack on the silica network further accelerated deterioration. Although the sizing layer gradually dissolved through salt precipitation and organic leaching, it provided measurable early-stage protection in aggressive environments which emphasised the need for more durable coatings to ensure long-term performance in harsh marine and alkaline environments.