Thermal spraying is a process wherein a feed stock material is heated and propelled as individual particles or droplets on to a surface.
The thermal spray gun generates the necessary heat by using an electric arc.
As the materials are heated they are changed to a molten state and are confined and accelerated by a compressed air stream to the substrate.
The particles strike the substrate, flatten, and form thin platelets (splats) that conform and adhere to the irregularities of the prepared substrate and to each other.
As the sprayed particles impinge upon the surface, they cool and build up, splat by splat, into a laminar structure forming the thermal spray coating.
Figure illustrates a typical coating cross section of the laminar structure of oxides and inclusions. The coating that is formed is not homogeneous and typically contains a certain degree of porosity, and, in the case of sprayed metals, the coating will contain oxides of the metal.
The bond between the substrate and the coating is that of mechanical.
The properties of the applied coating are dependent on the feed stock material, the thermal spray process and application parameters, and post treatment of the applied coating.
Hardness, density, and porosity
The thermal spray coatings are often used because of their high degree of hardness relative to paint coatings.
Their hardness and erosion resistance make them especially valuable in high-wear applications.
In the case of thermal spray metallic coatings the hardness and density of the coatings depend on the thermal spray material, type of thermal spray equipment and the spray parameters.
In general, the higher the particle velocity, the harder and denser the coating.
The porosity of the coating depends also on the thermal spray process, application parameters, and thermal spray material.
Metallic (Zn/Al) thermal spray coatings are anodic to the underlying metal (cs) substrate.
Because corrosion occurs at the anode, anodic coatings will corrode in corrosive environments and the cathode will not.
Anodic coatings will corrode or sacrifice to protect the substrate.
Thermal spray (Arc Method ) coatings may have very high adhesion.
The coatings applied by thermal spray (Arc Method ) process with high particle velocity can have tensile adhesions of greater than 3000 psi as measured by ISO 4624/ ASTM D 4541.
Zinc Aluminium, and Zinc - Aluminium alloy coatings are important anticorrosive coatings because they are anodic to steel.
In other Words, they corrode preferentially to steel , acting sacrificial coatings preventing the corrosion of the underlying steel substrate. Zinc is a much more Active metal in the respect then aluminium
On the other hand , aluminium coatings are harder ,have better adhesion, and form a protective oxide layer that prevents self-corrosion.
Alloys of the two metals have properties somewhere in between, depending on the ratio of zinc to aluminium. An 85-15 (percent by weight) alloy of zinc and aluminium is a widely used thermal spray coating material because it is thought to have the best combination of attributes from both metals.
Arc spraying : Arc spraying uis generally the most economical thermal spray method for applying corrosion resistant metal coatings, including Zinc, Aluminium, and their alloys.
Energy costs are lower and production rates are higher than they are with competing methods such as wire flame spray.
Arc spraying may be used to apply electrically conductive materials including metals, alloys.
In arc spraying, an arc between two wires is used melt the coating material. Compressed gas, usually air, is used to atomize and propel the molten material to the substrate. The two wires are continuously fed to the gun at a uniform speed. A low voltage ( 18 to 40 volts) direct current(DC) power supply is used, with one wire serving as the cathode and the other as the anode.
Figure shows a typical arc spray system comprised of a DC power supply, a wire feed system, a compressed-air supply, controls, and an arc spray gun. Figure 3 shows the components of a typical arc spray gun, including wire guides, gin housing, and gas nozzle. Coating quality and properties can be controlled by varying the automization pressure, air nozzle shape, power wire feed rate, traverse speed, and standoff distance. Arc sprayed coatings exhibit excellent adhesive and cohesive strength.
Thermal spray is used for a wide variety of applications.
The primary use of thermal spray coatings is for corrosion protection.
Coatings of zinc, aluminium, and their alloys are anodic to steel and iron and will prevent corrosion in a variety of service environments including atmospheric, buried, salt- and fresh water immersion, and high temperature applications.
Coatings of aluminium are frequently used in marine environments.
The marine industry / the installations near sea shore uses aluminium coatings for corrosion protection of many ship components/their structures.
Because these materials are anodic to steel, their porosity does not impair their ability to protect the ferrous metal substrate.
Zinc, zinc-aluminium alloy coatings may corrode at an accelerated rate in severe industrial atmospheres or in chemical environments where the pH is either low or high.
For this reason these materials are typically sealed and painted to improve their performance.
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