Some Characteristics of Thermal Spray Aluminum Coatings

       The purity of the aluminum wire used for spraying is ≥99.9%, and the macrohardness (HV) of the coating is 41.2. Compared with zinc coating, it has higher hardness, durability and erosion resistance.



One of the differences between zinc and aluminum coatings is the nature of their corrosion products, zinc corrosion products are slightly soluble in water, while aluminum corrosion products transform into water-insoluble oxides under service conditions. Therefore, the protection period of the zinc coating depends on the thickness of the coating, and the coating is gradually thinned over time. For aluminum coating, the oxide film on its surface has self-healing properties, and the oxide film always exists on the surface. Once the pores in the coating have been closed by insoluble oxides, there is little further consumption of the coating as long as the coating is not worn away or mechanically damaged. In some corrosive environments, the aluminum coating actually plays a dual role as an anode coating and a barrier coating; the aluminum coating acts as an inert barrier anti-corrosion coating when its surface is not damaged; once it is coated The surface is damaged, it can provide active protection. Due to the presence of the oxide film, the aluminum coating dissolves in water much more slowly than would be expected from its electrochemical potential. In addition, the density of aluminum is much lower than that of zinc, and the consumption of aluminum to achieve the same anti-corrosion effect is much lower than that of zinc. Since the relative density of aluminum is one third of that of zinc, the amount of aluminum used to obtain the same thickness of aluminum coating is only 1/3 of the amount of zinc used to achieve the same purpose. In terms of corrosion protection for steel components, aluminum is similar to zinc, and its electrochemical protection of steel substrates is worse than zinc coatings. Aluminum has good corrosion resistance in neutral environments such as air and fresh water, but for steel materials, due to the high potential of aluminum at this time, it cannot play a good role in cathodic protection. In the marine environment, there are chloride ions. When present, aluminum acts as an anode relative to the steel matrix and thus provides cathodic protection to the steel. In marine environments, aluminum coatings are more protective than zinc coatings.



When aluminum is not oxidized, the electrode potential is also negative than that of the base metal, so it should have cathodic protection. Under normal circumstances, there will be a dense oxide film on the surface of aluminum, especially the inside of the coating structure and the coating obtained by thermal spraying. There is a thick Al2O3 film on the surface. The corrosion resistance of aluminum mainly depends on the shielding effect of the Al2O3 film, because the Al2O3 structure is dense and the corrosion resistance is strong, so that the annual corrosion rate of aluminum is very low, and the purpose of improving the service life is achieved. However, it should be seen that due to the low conductivity of the Al2O3 film and the positive electrode potential compared with the base metal, the cathodic protection effect of the aluminum coating for engineering is weak, that is, the base metal will be corroded when the surface layer is damaged, which gives a separate The use of aluminum-coated shields is disadvantageous. In addition, "brown spot" rust will occur when the coating is thin or not properly sealed during thermal spray application. Pores are not critical for Zn coatings of any thickness, they close the coating pores by themselves through the anodic polarization process. For Al coating, if the coating thickness is thin (less than 100pum) or the coating is rough, in a humid atmospheric environment, corrosion spots will start to appear on the interface between the coating and the substrate, that is, "brown spots". The reason for the blessing spot of the aluminum coating is due to the presence of pores in the thinner coating that are connected to the boundary. Corrosive media from the atmosphere, especially oxygen and moisture, penetrate into the interface steel matrix through these communicating pores to form brown spot-like ferrous oxides. These corrosion products run to the surface of the coating through the pores, forming visible of "brown spots". Generally speaking, these reaction products are small in number, and can close the original communicating pores and stop the corrosion, so no further serious damage will occur. Although "brown spot" does not affect the service life of the coating, it damages the aesthetic effect of the coating. In order to avoid "brown spots" on the surface of the aluminum coating, the first is to make the coating thick enough; the second is to seal the coating in time after spraying.



High temperature anti-oxidation coating is another important application of aluminum coating. Arc sprayed aluminum coating (thickness is 0.2~0.6mm) can be directly used in production without special diffusion treatment. During use, the aluminum coating melts, diffuses, and generates iron-aluminum compound reaction work. A dense Al2O3 film is formed on the outside of the iron-aluminum compound. The Al2O3 film can effectively prevent the intrusion of oxygen into the steel matrix, thereby delaying the oxidation rate of the steel workpiece. If a layer of aluminum coating is sprayed on the surface of high-carbon tool steel or high-speed tool billet before rolling, the degree of oxidation and decarburization of the billet surface during the entire manufacturing process can be controlled, resulting in considerable economic benefits. Aluminum coatings are also used for anti-oxidation applications, such as burners, heating bodies, annealing tanks, exhaust pipes, cement drying equipment, chimneys, etc.



The performance of the arc sprayed aluminum coating is better than that of the flame spraying. The fundamental reason is that the thermal energy and function provided by the flame spraying to the sprayed particles is not enough to cause a satisfactory combination of the coating, and the bonding strength of the coating is too low. During the turnover and transportation of aluminum-coated workpieces, the coating is often damaged and peeled off at the edges and protruding parts of the workpiece. When the wire is flame sprayed with aluminum coating, the requirements for surface sandblasting are very strict. As long as there is slight scale or rust that is not completely removed, the coating is difficult to form. In order to be able to obtain a more reliably bonded aluminium coating, a very high blasting grade is required. Improvements in arc spraying equipment have pushed the further broadening of the application fields of aluminum coatings.



Although arc spraying has much looser requirements on the surface blasting quality of the substrate, it is still more stringent than thermal spraying zinc coatings. After thermal spraying, the aluminum coating has a large shrinkage stress. If the sandblasting treatment of the workpiece substrate is not in place, the roughness of the surface of the substrate is not enough or the dirt on the surface is not removed cleanly, the shrinkage stress of the coating will cause the coating to foam. , even peeling off. Therefore, whether it is arc spraying or oxyacetylene flame spraying, strict on-site supervision is required during construction to ensure the quality of sandblasting, because only a good aluminum coating can protect the steel substrate for a long time. Of course, improving the level of sandblasting will greatly increase the cost of construction, because in the process of thermal spraying anti-corrosion construction, the sandblasting process is often the bottleneck process. Although rigorous blasting operations can increase construction costs, thermally sprayed aluminum coatings are still used in many applications because they maximize their longevity and are more economical.