Mild Chemical Cleaning of Corroded Bronze Using an APP-Based Composite Solution: Improved Post-Cleaning Corrosion Behaviour and Mechanistic Interpretation

Abstract

The corrosion of copper alloys in buried or atmospheric environments is driven by the anodic dissolution of the copper matrix, a chloride-mediated autocatalytic cycle, and the accumulation of insoluble corrosion products, among which basic copper chlorides such as paratacamite (Cu2(OH)3Cl) are characteristic phases closely associated with the accelerated degradation of bronze artefacts. Here, we report a ternary aqueous cleaning system comprising ammonium polyphosphate (APP), boric acid, and the nonionic surfactant OP-10 for the removal of chlorine-containing corrosion products from simulated bronze substrates. Single-factor optimization identified the selected formulation of 4 wt% APP, 4 wt% boric acid, and 0.5 vol% OP-10, achieving an apparent corrosion-product removal efficiency of 93.7% ± 0.6% with a total colour difference (ΔE*) below 5. X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed the effective removal of the loose, chlorine-rich outer corrosion layer while preserving the compact inner layer. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements showed that the charge transfer resistance (Rct) increased and the corrosion current density (Icorr) decreased after cleaning, indicating an improvement in the short-term corrosion behaviour of the surface, which may be associated with the removal of chloride-bearing phases and/or the residual compact Cu2O layer, with a possible contribution of the adsorption of residual phosphate species on the substrate. This study provides a simple chemical route for the mild cleaning of corroded bronze surfaces and offers a preliminary exploration of polyphosphate-based systems for the conservation of copper-based cultural heritage.