Preparation and properties of natural gallic acid based rust conversion emulsion

Abstract

Purpose The paper developed a novel gallic acid-based rust conversion emulsion (RCE) that is applied in the treatment of rusted steels. The purpose of this paper is to investigate the methods for the synthesis of RCE and study the mechanism of rust conversion. Design/methodology/approach Conversion emulsion was prepared using styrene, acrylate and self-developed gallic acid (GA)-based rust converter (GRC) via seed emulsion polymerisation. The polymerisable GRC was synthesised by the ring-opening reaction of glycidyl methacrylate with natural GA. The effects of the GRC dosage and its feeding modes on the RCE synthesis were analysed. The corrosion resistance, surface morphology, composition and mechanism of rust conversion coatings were studied using electrochemical tests, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Findings The results showed that conversion coating on rusted steels treated with RCE, with various dosages of GRC (weight per cent), synthesised using seed emulsion polymerisation, exhibited the best adhesion and corrosion resistance. Raman spectroscopy revealed that RCE converted the original multiphase rust into stable crystalline phases of α-Fe2O3 and Fe3O4. According to XPS and energy dispersive X-ray analysis, the phenolic hydroxyl groups of RCE were proposed to chelate with Fe ions to form macromolecular ferrum compounds. Research limitations/implications The pre-rusted steels demonstrated a better corrosion resistance than rust-free steels after treatment with RCE. Practical implications The paper developed a novel GA-based RCE with high efficiency and environment-friendly method. Originality/value This work is expected to replace the conventional rust conversion paints and cause a significant impact on extending the service life of rusted steels.