The following article will cover the process of welding and restoring corrosion resistance.
Restoring Corrosion Resistance to Welds and Zinc Rich Coatings
The heat generated from the welding process vaporizes the protective zinc coating near the weld. Even though the remaining zinc continues to provide some protection to the zinc-free areas, the appearance is poor and the zinc-free area will rust when exposed to the environment. The same applies to the interior (ID) of the tube, except exposure to the environment is minimal.
There are several zinc rich paints available which will restore corrosion protection to the weld areas on both the ID and OD, with varied application methods.
Note: It is necessary to take the proper steps in cleaning the surface of weld slag and oil to insure a strong mechanical bond.
Some of the coatings are for use “as is” and some are for use as a base coat in powder coating applications.
When coating the weld area of a tube to be used in the shiny galvanized state, a standard cold galvanizing compound should be used. There are numerous manufacturers and various brand names commercially available. The key to choosing a corrosion resistant cold galvanizing compound is the percentage of zinc dust present in the compound. Some of the recommended compounds are listed in the table below.
Recommended Cold Galvanizing and Color Match Coatings
|Galvilite® Cold Galvanizing Compound||95%||Spray or brush||ZRC Worldwide||Stock|
|Matchmaker||0%||Spray||Seymour of Sycamore||Stock|
Salt spray tests have shown that painting or powder coating over zinc increases the rust-free product life. For customers who powder coat, and are concerned about additional corrosion protection in the weld zones, cold galvanizing compounds are available that are heat resistant (up to 750° F), and act as a primer for powder coating. We suggest the two manufacturers in the table below. For those manufacturers who paint the tubing, all of the cold galvanizing compounds in both tables will provide corrosion protection in the weld zone where the zinc has been vaporized.
Heat-Resistant Cold Galvanizing for Powder Coating Applications
145 Enterprise Drive
Marshfield, MA 02050
M.A. Bruder & Sons
600 Reed Road
Broomail, PA 19008
*Galvilite is available directly from Allied Tube & Conduit®. Galvilite is a registered trademark of ZRC Worldwide.
Re-coating the ID of a tube to prevent corrosion where the weld zone heat has deteriorated the coating is quite similar to the method used in rust proofing a car. A spray gun with an extension is inserted, typically through a hole, and the coating is sprayed in a pattern covering 360 degrees of the interior.
All of the cold galvanizing compounds described in the OD Applications section of this bulletin can be used for re-coating of the ID surface.
Thermal Spray Process
Thermal spraying of zinc will completely restore the corrosion resistance of galvanized tube where welding has burned the zinc off the surface of the tube and where the weld metal is exposed. The resulting sprayed fabrication will have the same or better corrosion resistance in the thermally sprayed weld area as it does in the unaffected tube.
Thermal spraying is a simple process where zinc wire is continuously melted and compressed air atomizes the molten metal and projects it onto a prepared surface.
Conceptually, it is similar to spray painting, except the paint is liquid zinc. Like painting the atomized metal hits the surface of the part and sticks. Unlike welding there is no metallurgical bond occurring between the liquid zinc and metal surface. The zinc bonds mechanically to the metal surface.
Wire Flame Spray
The Wire Flame Spray Process applies coatings of any metal that can be drawn into a wire and has a melting point below that of a combustion flame.
Drive rolls feed the wire through a flame spray gun to its nozzle. There it is continuously melted in an oxygen and fuel-gas flame. Compressed air atomizes the molten metal and projects it onto a prepared surface. This method is the best choice for all-purpose spraying. The zinc coating can be applied fast and at a low cost.
The Electric Arc Process
Electric Arc Spraying applies zinc in wire form. Push/pull motors feed two electrically charged wires through the arc gun to contact tips at the gun head. An arc is created that melts the wires at temperatures higher than 7200º F. Like the wire flame process, compressed air atomizes the molten zinc and projects it onto a prepared surface. This process is excellent for applications where wide, large surfaces must be sprayed. The arc system can produce a spray pattern ranging from 2 to 12 inches and can spray at extremely high speeds.
The most important aspect of thermal spraying is surface preparation. Proper surface preparation requires removal of any foreign material and roughening of the surface. The recommended preparation is to grit blast the surface. This is highly effective in removing foreign material and roughening the surface.
Grit blasting is fast, although it does require containment of the blast stream in either a blast cabinet or by use of a “Vacu-Blast” unit, which captures most of the abrasive and recycles it. In typical galvanized tube fabrication only small sections around weld areas need be cleaned; therefore, the “Vacu-Blast” system is a likely candidate for surface preparation.
The corrosion resistance of thermal spray obviously depends on how thick the zinc coating is applied. Theoretically there is no thickness limitation. Practically, there are reasonable limits that vary depending on the application. In a very broad sense, coatings can be applied to a thickness greater than .100 and as thin as .002 to .003 inches. The average thickness for zinc thermal spray is .003 to .006 inches.
Allied´s coating process uses 99.9% pure zinc. The zinc wire used in the thermal spray process is 99.9% pure. As a result the combination process is much purer than other galvanizing methods.
Thermal spraying does generate particles of zinc oxide which must be captured. Therefore, spraying must be done while pointing the spray gun in the general direction of a dust recovery unit. The dust recovery unit has to be of the dry cartridge type. Several recovery systems are available and selection depends on the size of the recovery area needed.
In a typical manufacturing process, after fabrication the welded areas and non-coated components would be grit blasted and thermal sprayed. Due to the secondary processing and equipment it may be necessary to create an additional work area.
Restoring the corrosion resistance of welds on galvanized tube using thermally sprayed zinc will restore the full corrosion resistance of a welded structure to the same level as Allied´s galvanized tube itself. Thermally sprayed restoration of the zinc coating results in a superior product when compared to the same product when restoration of corrosion resistance is achieved by using “cold galvanizing” inorganic zinc coatings.
It should be noted that since there are no organic solvents associated with thermal spraying the coating is suitable for immediate powder coating without fear of bubbling or blistering due to solvent evolution.