Preparation And Anticorrosion Mechanism Of High Temperature Resistant Anticorrosion Coatings For Marine Environment

In the process of promoting the"Sea-Power Strategy",marine engineering equipment and facilities have long served in harsh and corrosive marine environments and are facing severe corrosion problems.The application of anticorrosion coatings is the most commonly used,most effective and most cost-effective anticorrosion technology.In the tropical marine atmospheric corrosion environment with high humidity,high salt spray,and high radiation,general anticorrosion coatings such as epoxy coatings and polyurethane coatings are difficult to meet the protection needs under multi-factor coupled corrosion conditions.Especially in the high-temperature salt spray corrosion environment,it is difficult for conventional anticorrosion coatings to maintain their stability and long-term effectiveness,resulting in rapid coating failure.Therefore,this article aimed to develop a novel water-based phosphate ceramic anticorrosion coating to meet the high temperature corrosion protection requirements in the marine environment.The specific research content and results are as follows:1.Using aluminum dihydrogen phosphate as the main film-forming material,adding magnesium oxide as curing agent,and modified metal aluminum powder particles as functional anticorrosion fillers,a novel water-based phosphate high-temperature resistant ceramic anticorrosion coating was prepared.The curing process and film-forming mechanism of the coating were analyzed,and the composition and structure of phosphate ceramic coatings were characterized.Scanning electron microscopy（SEM）,electrochemical tests and salt spray-high temperature cyclic corrosion tests were used to characterize the effect of filler content on the morphology and high temperature corrosion resistance of phosphate ceramic coatings.The results showed that the phosphate ceramic coating with 35 wt%modified metal aluminum powder particles had a more compact structure.The coating maintained a low-frequency impedance modulus above 10⁴Ω·cm²after immersion in 3.5 wt%NaCl solution for 14 days with a high corrosion potential of-0.783 V and a low corrosion current of 1.337μA·cm^-2after 20 days of immersion.After 10 times salt spray-high temperature cycles,the coating kept the surface intact without any rust.These results implied that the coating with 35 wt%filler content possessed better high temperature corrosion resistance than coatings with higher or lower filler content.At the same time,the 1000-hour high temperature resistance test and the 1000-hour salt spray corrosion resistance test further proved that the prepared phosphate ceramic coating can meet the long-term service requirements in the harsh marine corrosive environment of high temperature.2.To analyze the anticorrosion mechanism and failure evolution mechanism of phosphate ceramic coatings,different fillers were added to the phosphate ceramic anticorrosion coatings to prepare phosphate ceramic coatings.The morphology and composition of phosphate ceramic coatings with different fillers were observed by SEM.The anticorrosion mechanism and failure evolution process of the phosphate ceramic coatings were analyzed by electrochemical test,salt spray-high temperature cycle test,and local-electrochemical test technology.The results indicated that the addition of neither ceramic particles nor primary aluminum particles could provide high temperature corrosion protection,only when the passivated metal aluminum particles could provide the best high temperature anticorrosion performance.Meanwhile,with the simulation calculation based on density functional theory（DFT）,it was found that the adsorption energy of chloride ions in the Cr₂O₃passive film on the surface of aluminum particles was lower than that in the natural oxidation product Al₂O₃of aluminum particles,while the migration energy barrier was higher.The"adsorption"and"pinning"effect of the passive film on chloride ions helped to reduce the corrosion of the substrate and enhance the high temperature corrosion resistance of the coating.3.Graphene nanosheets were added to prepare the phosphate ceramic coating.The morphology and structure of the coating were observed by SEM,and the high temperature corrosion resistance of the coating was analyzed by electrochemical test and salt spray-high temperature cycle test.The results showed that the addition of 0.1 wt%graphene could enhance the barrier effect of the coating and the conductivity of the aluminum particles inside the coating.The results show that the addition of 0.1 wt%graphene can enhance the barrier effect of the phosphate ceramic coating and the conductivity of the aluminum particles inside the coating.Therefore,the sacrificial anode efficiency of the aluminum particles would be improved and the sacrificial anode protection time of the phosphate ceramic coating would increase from a short period of several hours to more than 8 days,avoiding local failure of the coating.However,the increase in graphene content also increased the defects in the phosphate ceramic coating,while the high electrical conductivity of graphene also increased the corrosion rate of the metal substrate.Therefore,only when the graphene content was low at 0.1 wt%could help enhance the high temperature corrosion resistance and extend the service life without damaging the structure of the phosphate ceramic coating.