Effect of in situ cold spray/micro-arc oxidation composite coating on the galvanic corrosion of steel/aluminium laser-MIG hybrid fusion-brazed joints

Abstract

To leverage the benefits of steel and aluminium while mitigating the risk of galvanic corrosion and improving corrosion resistance, an in situ cold spray/micro arc oxidation (MAO) composite coating was prepared on the steel/aluminium laser-MIG hybrid fusion-brazed joint. The micro-structural morphology, composition, surface Scanning Kelvin Probe (SKP) potential variation and corrosion resistance of the cold spray/MAO composite coating were investigated via scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, SKP technique and electrochemical testing. Direct MAO treatment produced a much thinner film (~5 μm) than cold spraying followed by MAO treatment on the steel/aluminium welded joint (~ 32.45 μm). The cold spray/MAO-treatment also reduced the SKP potential difference from 1500 mV in the untreated sample to 640 mV in the treated sample. The spectral noise resistance R_sn of the coating-metal galvanic couple was 4 orders of magnitude higher than that of the 6061–304 galvanic couple. SKP potential and electrochemical noise analyses revealed that introducing an intermediate ceramic transition layer on the surface of steel/aluminium welded joint effectively alleviated its tendency to undergo galvanic corrosion. Electrochemical impedance spectroscopy and potentiodynamic polarization results demonstrated the excellent corrosion resistance of the cold spray/MAO composite coating, in particular, the I_corr of the coating (62.68 × 10⁻⁶ A/m²) was lower and the E_corr (−0.33 V) and R_p (390.9 Ω·m²) values were higher than those of the film formed on the weld seam via the direct MAO treatment.