Study On Corrosion Behavior Of Zn-Al Coating In Typical Simulated Environment

With the development and utilization of marine resources, a large number of maritime infrastructure constructions and metal materials have been constracted and put into use and exposed steel structures are corroded seriously by electrochemical reactions with surrounding medium in Marine environment. Zinc aluminum alloy coating is a widely used and good protection effect. As a two-phase structure of coating, its performances in many aspects are better than that of pure aluminum and pure zinc coating.The ocean is a complex corrosive environment. The corrosion resistance and service life of the same material is not the same in different regions of corrosive environments, even different temperature and salinity will have a great impact on the service life of metal materials. In order to meet the applications in complex marine corrosive environments, flame spray, electric arc spray and low pressure cold spray are used for Zn-Al coating preparation to study the effects of splash corrosive environment, seawater tidal zone, marine atmosphere and temperature and salinity of immersion zone on corrosion resistance of the coating.SEM, EDS, XRD, three-dimensional video microscopy and tensile testing machine are used to study the micro morphology, composition and physical properties of the coating.The results showed that the surfaces of flame spraying and electric arc spraying Zinc-aluminum coatings are rough, and there is a certain amount of pores and pits; surface of cold spray zinc-aluminum coating is relatively smooth,and there is no obvious defects such as pores and pits; the results of tensile test show that the bonding strength of low pressure cold spray zinc-aluminum coating is significantly higher than the flame spraying and electric arc spraying zinc-aluminum coating.Analyse the corrosion behavior of coating by neutral salt spray test and electrochemical methods, the results showed that there is no red rust on the surfaces of the three Zn-Al coatings after neutral salt spray test which lasted for 150 days, indicating that the coating can continue to protect the metal base.The weight gain and changes in the bonding strength of the coating showed that flame sprayed Zn-Al coating was affected by salt spray corrosion worst, followed by arc spraying Zn-Al coating, in the long term, cold spray coating has more long-term protective effect. In the splash corrosion tests, the impedance of cold spray coating is significantly greater than two thermal spray coatings, and the resistance of flame spraying and arc spraying coatings are similar. Due to the intermittently scouring of corrosive medium, fluctuation of the impedance is more apparent than that of total immersion test. Protective performance of the coating is superior in alternating wet and dry conditions than that of full immersion condition because the ratio of wet and dry is 12-12 h, drying time is so long that electrolyte ion in anode and cathode region can't pass through and interface corrosion is stopped. When the coating is immersed in corrosive media, corrosion process restarted. Therefore, long time drying in alternating wet and dry condition slowed corrosive media penetrate the surface of the coating.Immersion test, the corrosion rate of the three zinc-aluminum coatings in different concentrations of corrosive media is almost 3.5% <7% <5%. The impedance of the cold spray sample soaked in 7% NaCl increase more, which means that in harsh environmental conditions, cold spray zinc-aluminum coating can fully play its protective role; as the temperature increases, the corrosion rate of the three zinc-aluminum coating increases. Compared to thermal spraying zinc-aluminum alloy coating, the corrosion current density of cold spray coating is smaller and cold spray coating is affected less by temperature. long-term immersion shows that cold spray zinc-aluminum coating has long-lasting protective effect.