Zinc thermal spraying is a process where zinc or zinc alloys are melted and then sprayed onto a prepared substrate, creating a layered coating. Thermal spraying is a highly effective and proven method of corrosion prevention, giving galvanic as well as barrier coating protection to iron and steel.
As with any coating process, proper surface preparation is essential. Grit blasting prior to coating application produces a properly clean and roughened surface, upon which the molten sprayed zinc anchors with bond strengths in the range of several thousand pounds per square inch.
Zinc Metal Coatings
The main sprayed metal coatings are either pure zinc or an alloy of zinc and 15% aluminum. Sprayed coatings of pure zinc have been available since the 1920’s and are typically applied with a thickness of 100 microns or more. If not further coated, the pure zinc coating will form a stable, passive film of zinc corrosion products with a predictable weathering rate. In many environments, the sprayed zinc alone can provide sufficient service life. The zinc 15%-aluminum thermal spray coating was introduced in the late 1970’s and is about the highest aluminum composition in zinc that can be industrially made into wire. Because the micro-structure of each of the droplets of this sprayed coating consists of both a zinc-rich and aluminum- rich phase, Zn-15%Al offers the optimum corrosion protection to steel in aggressive atmospheric conditions such as marine environments.
Method of Application
Zinc is thermal sprayed using either an electric arc or a combustion flame process. The electric arc process, which allows higher deposition rates with improved economy, uses two consumable zinc wire electrodes. The wires given opposite electric charges and are fed close together to maintain an electric arc, causing the wires to melt. The molten material is sprayed onto the prepared surface with compressed air. In the combustion process, a single zinc wire, or a source of zinc powder, is fed into an oxy-fuel flame, which melts the zinc which is then atomized by compressed air applications of zinc and zinc alloy thermal sprayed coatings can be easily controlled by the equipment operator to provide a thickness of 50 – 500 microns (.002” to .020”). This is important when considering atmospheric conditions that will dictate the protective thickness required as a very long life or highly corrosive conditions necessitate a thicker coating.
Edge coating, a normally difficult task for all coating systems, is easily and efficiently achieved using the zinc thermal spraying process. The use of sealers and/or topcoats over a zinc metalized coating will further protect the surface and usually provide an even longer life of the coating system. Again, environmental conditions will determine the use and type of sealers.