In-situ scanning electrochemical microscopy visualization of self-passivation in galvanized steel under aggressive environments

Ensuring the long-term durability and safety of galvanized steel structures in industrial and environmental applications requires a deep understanding of their corrosion resistance, particularly at defects such as cut edges and scratches. These sites often initiate localized corrosion due to micro-environmental inhomogeneities. This study investigates the dynamic corrosion behavior of galvanized Zn/Q235 steel couples by integrating macro-electrochemical testing with in-situ high-resolution scanning electrochemical microscopy (SECM) to reveal the underlying interfacial physicochemical mechanisms. While macro-electrochemical measurements indicated significant passivation, with the corrosion current density decreasing by over an order of magnitude to 1.00 × 10⁻ 6 A·cm⁻ 2 after 24   h, micro-scale SECM mapping revealed a more complex mechanism. As corrosion progressed, oxygen reduction reactions became increasingly concentrated at the Zn/Q235 interface, where localized pH stabilized near 8. This remarkable pH buffering is attributed to the continuous precipitation of zinc corrosion products, an interfacial process that consumes both Zn 2+ and OH⁻ to form a passivating layer. At simulated scratch defects, an initially aggressive galvanic coupling with steep pH gradients was gradually suppressed as these precipitates formed, demonstrating a self-passivation effect. These findings elucidate a locally driven, interfacial chemical buffering mechanism, governed by precipitate formation, that controls passive film development and slows degradation. The results establish direct links between interfacial pH regulation, corrosion-product formation, and self-passivation at defects, advancing the interface science underpinning environmentally resilient galvanized coatings.