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Purpose: The purpose of this paper is to investigate the interfacial conductivity and corrosion resistance of the Ni–P/Ti4O7 composite coating that is deposited on a carbon steel substrate as bipolar plates for proton exchange membrane fuel cells. Design/methodology/approach: The Ni–P/Ti4O7 coating was prepared by electroless plating. Scanning electron microscopy, white light interference, energy dispersive spectrometry and X-ray diffraction were used, respectively, to study the surface morphology, chemical composition and phase composition of coated samples. Electrochemical impedance spectroscopy, potentiodynamic and potentiostatic polarization were used to test the electrochemical performance and corrosion behavior. The interfacial contact resistance (ICR) was measured via the standard method. Findings: The surface of the Ni–P/Ti4O7 coating is complete and dense and without obvious defects. The electrochemical test results show that the Ni–P/Ti4O7 coating provides better corrosion resistance than the Ni–P coating and substrate. Compared with the Ni–P coating, the ICR of the Ni–P/Ti4O7 coating is lower by about 82.7%. This is because the coating has more conductive contact points. The more exciting thing is that the ICR of the Ni–P/Ti4O7 coating only increases to 12.38 mΩ·cm2 after 5 h of polarization. Originality/value: This paper provides a method for achieving surface modification of metal bipolar plates. Introducing Ti4O7 particles in the Ni–P layer reduces the contact resistance before and after polarization while ensuring good corrosion resistance. [ABSTRACT FROM AUTHOR]

Abstract: Two microwave absorbing coatings (MACs) are applied onto magnesium alloy substrate and are studied using electrochemical impedance spectroscopy (EIS) during immersion in 3.5% NaCl solution. The diffusion behavior of electrolyte through the MACs was firstly studied. The result shows that electrolyte transfers significantly fast in carbonyl iron loaded coating than barium ferrite loaded coating. In order to account for the distribution of different absorbents on corrosion process, a modified equivalent circuit with a distribute transmission line model was proposed to fit EIS spectra of MACs. Simulated EIS diagrams agree very well with experimental spectra of 25% and 50% PVC MACs. Moreover, simulation data was studied to analyze degeneration of two MACs. The result shows that the corrosion of CI particles plays an important role for failure of CI coating, but the degeneration of BF coatings which were slower than CI coating was ascribed to defects within the coating. [Copyright &y& Elsevier]

Abstract: After being pre-plated a zinc layer, an amorphous Al–Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al–Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al–Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al–Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted. [Copyright &y& Elsevier]