Study on failure behavior of conductive and corrosion-resistant conversion film on magnesium alloy in damp-heat environment

This study investigated the failure behavior and mechanism of conductive and corrosion-resistant film on magnesium alloy under “double 85” test conditions. The surface morphology, microstructure, and composition of ITO composite conversion film (ITO film) at different damp-heat test times were analyzed using X-ray photoelectron spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy, and the corrosion resistance was investigated using an electrochemical workstation in contact with a 3.5 wt.% NaCl solution. The electrical contact resistance (ECR) was measured using the four-wire Kelvin detection technique, and the bandgap was determined using an ultraviolet diffuse reflectance spectrometer. The results demonstrated that the ECR of the ITO film increased from 10 −1 Ω in −2 to 10 6 Ω in −2 , and the bandgap increased from 2.363 to 2.777 eV after the damp-heat test. In addition, the impedance arc radius of the film decreased to approximately one-fifth of its original value, and the impedance modulus value decreased to less than 10 4 . These findings indicated that exposure to a damp-heat environment resulted in an increase in the electrochemically active points within the film participating in the electrochemical reactions and expansion of the electrochemical reaction area. This exacerbated the corrosion of the magnesium substrate because it resulted in the destruction of the film, leading to an increase in the number of pores and defects, which heightened the exposure of the substrate to corrosive media. Furthermore, Cl − ions could easily penetrate through cracks in the film, which promoted charge transfer processes and accelerated the electrochemical corrosion reactions. In summary, these factors collectively resulted in the failure of the film’s conductivity and corrosion resistance. Graphical abstract