Synergistic modification of phytic acid conversion coatings with tannic acid and cerium salts for improved corrosion protection of FeCo alloys

FeCo alloys are highly regarded for their excellent soft magnetic properties, yet their susceptibility to corrosion in chloride-rich environments hinders their widespread application. To overcome this limitation, we have developed a novel composite coating on the FeCo alloy surface by synergistically modifying a phytic acid conversion coating (PACC) with tannic acid (TA) and cerium salts. This was achieved using a simple, ethanol-assisted layer-by-layer self-assembly method. Its structure and performance were systematically evaluated using multiscale characterization techniques and electrochemical testing. Our results demonstrate that this synergistic modification effectively repairs the intrinsic porosity of the PACC, yielding a denser and more uniform film. The corrosion resistance of the modified composite layer significantly surpasses that of both the bare alloy and the single-layer PACC, with the PACC-Ce-5TA sample exhibiting the best protective performance. At the initial corrosion stage, its polarization resistance increases by two orders of magnitude compared with the substrate, achieving a protection efficiency of up to 98.72%. More importantly, dynamic electrochemical impedance spectroscopy (EIS) and wire beam electrode (WBE) visualization indicate that the composite layer exhibits remarkable self-healing behavior. The superior protection is attributed to a multi-faceted mechanism: a robust physical barrier formed by a sacrificial outer TA layer and a densified inner PACC layer, supplemented by the cathodic inhibition of Ce3+ ions and an active self-healing function, where TA molecules chelate with metal ions at defect sites. This study provides a green and effective strategy for durable corrosion protection of FeCo alloys and related metallic materials. • A novel eco-friendly TA-Ce synergistically modified phytic acid coating is constructed on FeCo alloy. • The synergy of Ce3+ and TA effectively repairs defects in the PACC, forming a dense barrier. • The modified coating exhibits excellent corrosion resistance with up to 98.72% protection efficiency. • Dynamic electrochemical tests reveal remarkable self-healing behavior of the coating. • The self-healing mechanism arises from Ce3+ cathodic inhibition and TA-metal chelation repair.