| Steel structures are widely faced up with corrosion problems.Among the anticorrosion methods,zinc-rich epoxy coatings(ZREs)are under most concern for the corrosion protection of steel in marine environments since ZREs can provide both cathodic protection and self-healing effects.However,high zinc content is necessary for the cathodic protection of the coating to the steel substrate,while high zinc contents lead to low barrier effects and poor mechanical properties.Besides,only a small part of zinc,which is reported as less than 25%,can be utilized.In the present study,two methods were used for the fabrication of ZREs with high application properties.In one method,graphene,as one kind of two-dimensional sheet with high electrical conductivity,has been utilized as fillers in ZREs.The influence of graphene on the corrosion protection performance of ZREs with 80 wt.%of high zinc content was investigated,and the cathodic protection performance of ZREs with zinc powders partially replaced by graphene was investigated.In the other method,a one-step method was induced to fabricate ZREs with gradient-dispersed zinc powders to realize the assembly of ZRE with low zinc contents that could provide enough cathodic protection.The research firstly analyzed the influence of graphene on the cathodic protection and corrosion protection performance by electrochemical measurements.Furtherly,the dispersion of graphene and the influence of graphene on the micro-structure were investigated by scanning electronic measurements and mercury intrusion porosimetry.,while the corrosion rate of zinc powders was measured with hydrogen evolution and differential scanning calorimetry.Based on the former results,the microscopic influence of graphene was discussed.Secondly,electrochemical measurements were employed to evaluate the cathodic protection and corrosion protection performance of ZRE of gradient-dispersed zinc powders of which the amount was as low as 30 wt.%.With the help of scanning electronic measurements,the influence of the gradient dispersion of zinc powders was investigated.Then,the influence of zinc contents on the corrosion protection of ZREs with gradient-dispersed zinc powders.Results are as follows.Firstly,the influence of graphene on the corrosion properties of ZREs with 80 wt.%of zinc content was investigated.Results showed that graphene could enhance the barrier effects and mechanical properties of the coating,and would not deteriorate the cathodic protection of the coating.Besides,with proper graphene content,the cathodic protection duration of ZREs could be prolonged.Further investigation showed that the influence of graphene included that graphene could fill in pores in the coating,and that graphene could involve in the galvanic corrosion between zinc powders and steel substrate.The protection potential of the coating to the substrate was increased by graphene as the cathode,but would not surpass the potential range for the cathodic protection on condition that graphene was uniformly dispersed,thus ZREs could still provide cathodic protection to the substrate.The influence of graphene on the cathodic protection duration relied on the graphene contents.With graphene content lower than 0.3 wt.%,graphene could decrease the porosity of the coating,leading to a decrease in the contact area between zinc powders and the solution.As a result,the corrosion rate of zinc powders was decreased and the cathodic protection duration of ZREs with graphene was prolonged.With graphene content higher than 0.3 wt.%,the main influence of graphene was the galvanic corrosion process,leading to an increase in the corrosion rate of zinc powders and a decrease in the cathodic protection duration of the coating.Conditioning the influence of graphene in the corrosion protection,adhesion force,and tensile strength of ZREs,the optimal graphene content in ZREs is 0.3 wt.%.Secondly,graphene was added in ZREs to partially replace the zinc powders with the maximum 6 wt.%amount of graphene that can be dispersed uniformly in epoxy resin,and if the ZREs with zinc powders partially replaced by graphene could provide cathodic protection to the substrate was investigated.Results showed that the electrical contact between zinc powders and steel substrate could be satisfied with the zinc content in the range from 40 wt.%to 70 wt.%.However,even with zinc content as high as 70 wt.%,ZREs could not provide cathodic protection to the substrate.Further research showed that during the diffusion process of the solution,the galvanic corrosion process between graphene and zinc powders accelerated the formation of zinc corrosion products,leading to a deterioration of the electrical contact between zinc powders and the substrate.As a result,there were not enough zinc powders that could provide cathodic protection to the substrate.Besides,for ZREs with a low amount of zinc which could not provide cathodic protection,graphene could enhance the barrier effects of the coating.On condition that graphene was uniformly dispersed,barrier effects of ZREs would be improved with graphene content increased.Finally,by adjusting the curing duration of ZRE and settling rate of zinc powders,a gradient dispersion of zinc powders was realized in ZREs with zinc content lower than 50 wt.%.The mechanical properties of ZRE with gradient-dispersed zinc powders of 30 wt.%content was much better than that of ZRE with an 80 wt.%high zinc content,and the coating could provide enough cathodic protection to the substrate with prolonged cathodic protection duration.Mechanism results showed that by gradient dispersion of zinc powders,the zinc content near the coating/substrate interface was increased to as high as 73 wt.%,which ensured enough cathodic protection to the steel substrate.Besides,at the bottom of the coating,zinc powders led to an increase in the barrier effect of the coating.As a result,the corrosion rate of zinc powders was decreased,and the cathodic protection duration was prolonged.By further research,the optimal zinc content was found to be around 40 wt.%for ZREs with gradient dispersed zinc powders.With the most zinc content at the coating/substrate interface,the coating could provide enough cathodic protection performance.Besides,the zinc content near the interface was nearer to critical pigment volume concentration than that of ZREs with 30 wt.%of gradient dispersed zinc powders,thus the barrier effect was better and so was the cathodic protection duration. |
