Study of water permeation dynamics and anti-corrosion mechanism of graphene/zinc coatings

Abstract

In this paper, graphene/zinc-containing coatings were prepared and the mechanism of the action of graphene was discussed. The change in corrosion potential of the coating in artificial seawater was recorded in order to clarify the evolution in coating state. The electrochemical impedance spectroscopy (EIS) was measured and used to discuss the electrochemical structure of the coating in detail, and by the fitting of EIS together with cathodic protection current, the influence mechanism of graphene on the coating performance was discussed. SEM and micro-domain Raman spectroscopy were used to validate inferences about diffusion kinetics. EIS of dry coating and cathodic protection current were carried out to observe the cathodic protection. Depending on the coating state reflected by the corrosion potential, the coating process was divided into the stages of initial shielding, fluctuation, cathodic protection, shielding and failure. The electrochemical structures of the coatings and equivalent circuits at each stage were established. In the first two phases, the coatings undergone initial infiltration of corrosive media and activation of zinc particles. In the cathodic protection stage, the anode sacrificial reaction of zinc powder was the main process. During the shielding and failure stage, the steel substrate started to corrode. The EIS of the coatings with insufficient graphene shown the characteristic of capacitance of zinc corrosion product, since no coherent electron transport channels were formed. The characteristic of graphene that was first used was conductivity, which improved electrical contact between zinc particles as well as zinc and iron inferred from the EIS of dry coatings, and broken the block of non-conductive zinc corrosion products on the further anode sacrifices of zinc concluded from EIS of immersion test. This was advantageous for improving the utilization of zinc, increasing cathodic protection current and enhancing the protective properties of the coating. The dispersed and layered structure of the graphene sheet also improved the shielding performance of the coating, which was reflected in the initial shielding stage prior to cathodic protection relative to the non-graphene coatings and the reduced water diffusion coefficient in graphene-containing coatings. The tracking of the corrosive media using micro-Raman spectroscopy confirmed the enhanced barrier of the graphene coating. The process of water penetration into the coating was mainly Fick diffusion, which described by the equation of ϕ(z,t)=1+4/π∑n=1+∞[(−1)n/(2n−1)]cos[(z/L)(n−1/2)π]exp(−(Dt/L2)(n−1/2)2π2).