Twin-Chain PVA Gels with Organic Acids for the Selective Removal of Iron Oxyhydroxides from Corroded Iron Surfaces

Corrosion of historic wrought-iron artifacts often leads to the formation of unstable iron oxyhydroxides, whose selective removal should minimize alterations to the forging black patina. This work introduces twin-chain polymer network (TCPN) gels based on functionalized poly(vinyl alcohol) (PVA) with succinic acid, loaded with bio-derived organic acid chelators (succinic acid, SUA; citric acid, CITA) for the selective, controlled removal of corrosion layers from historic iron surfaces. The gels, prepared through a sustainable freeze−thaw process yielding a porous network, were characterized by confocal laser scanning microscopy and oscillatory rheology. Gel cleaning performance was evaluated on laboratory mock-ups reproducing different corrosion typologies using optical coherence tomography, colorimetry, and micro-Raman spectroscopy before being validated on a real 16th−17th century steel gauntlet (Gauntlet C71, Museo Luigi Marzoli, Brescia, Italy). TCPN-CITA gels achieved more complete removal of iron oxyhydroxides, while TCPN−SUA gels performed comparably on more loosely bound, poorly crystalline corrosion products; in both cases, the forging patina was preserved. The chelating strength and Fe3+ affinity are discussed as the main drivers of this selectivity. These findings support the potential of bio-based TCPN−PVA gels as a tool for the conservation of corroded metal cultural heritage.