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Surface Finishing Tutorial

#075 Corrosion Protection Measures - Metal Coating - 3

Category : Corrosion - Corrosion Protection
April 8, 2011

(1) Metal Thermal Spraying

This is called Metal Thermal Spraying where thermally liquefied metals using gas or electrical power are spray coated onto subject surfaces by clean air. This method using thermal spray guns can relatively easily apply metal coatings on wide variety of objects as large as gigantic bridges to as small as dentures.

Wire Melt Thermal Spray Gun

Not only the metals with low melting points such as Zn and AL, but high melting point metals such as stainless steels and Ni alloys can be used for the thermal spraying process. Further, by spraying materials of high hardness such as ceramics, hard surfaces with excellent wear characteristics can be obtained.

However, the coating surfaces obtained by the thermal spraying processes are porous and not appropriately corrosion resistant. The coated surfaces will need to be either re-liquefied or other proper processes will need to be applied to seal the open pores. Re-liquefaction process is where the coated surfaces are reheated with flames and such to fill all the open gaps. Another sealing method is painting. Paint sealing on particularly porous coatings improves the metal coating adhesion by penetrating the pores acting as anchor to the base.

For corrosion protection of steel structures, highly corrosion resistant zinc, aluminum, AN-AL alloys are used for spraying. These coating materials are generally pore-sealed and painted as bases for final paint coatings. The ZN thermal spray coating thicknesses of 80, 120, 160, 200μm, AL thicknesses of 80, 120, 160, 200, 300μm, and Zn-AL alloy thicknesses of 80, 100, 120μm are generally used for industrial purposes. The wire melt thermal spray type is the frequently used method but an arc melt spray is also used in some applications.

Wire Melt Thermal Spray System

Arc Thermal Spray System