Zinc melts at 787°F, then boils at 1663°F, becoming a vapor. As it mixes with the oxygen in the air, it changes to zinc oxide. Zinc oxide is normally visible as about 30% of the white plume rising from the welding point. Of course, proper safety precautions should be followed when welding with any zinc coated tubing.
In Allied’s process, the conversion coating over the zinc is particularly thin, and may be considered negligible as it relates to welding.
Allied tubing is additionally protected by a clear organic topcoat. In welding, this coating could generate fumes irritating to the welder if proper ventilation procedures are not followed. Removal of this coating is recommended and easily accomplished.
Restoring Corrosion Resistance
The heat from welding 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 areas will rust when exposed to the environment. There are several zinc-rich paints available which will restore corrosion protection to the weld areas.
Allied’s current touch-up paint, “Matchmaker”, is available in 12 ounce aerosol cans, packaged in cases of twelve. This touch-up paint has been specifically designed and private labeled for Allied. The paint was developed with two objectives: good corrosion resistance and custom color match.
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 by which 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 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. The corrosion resistance of the thermal spray depends on how thick the zinc coating is applied. Theoretically, there are no thickness limitations. 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.
Restoring the corrosion resistance of welds on galvanized tubing 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.
More about our coatings – Galvanized Steel Tubing
Coatings: What do they do for our products?
Coatings add to the life of our products by providing longer durability against normal environmental corrosion. Coatings improve the appearance of the tubing, and they prepare the tube surface for specific end uses.
What is so special about Allied’s coatings?
The technology and the process of carbon steel tube and pipe manufacturing are well known to the hundreds of manufacturers across this nation and in other countries. But what makes Allied so special is our ability to combine the following two functions with a high degree of success. Developing unique coating systems/formulations specially for Allied’s products and processes. Developing unconventional coatings application equipment and techniques to facilitate continuous on-line coating application and curing operations at high line speeds.
Exterior Coatings Classifications
Unless otherwise specified by a customer, almost all of our galvanized tube and pipe is triple-coated externally by using the following three sequential processes:
- Galvanizing (zinc coating)
- Conversion coating (for passivation film)
- Clear organic polymer application
- Zinc Coating
Allied uses special high-grade quality zinc for its galvanizing operation. Our zinc is 99.99% pure and contains only a trace amount of lead. Hot-dipped galvanized pipe or pipe made from pre-galvanized steel coils normally use Prime Western grade zinc and other grades which contain a high level of lead, sometimes more than 1% by weight.
The conversion coating over the galvanized surface is formed due to a chemical reaction and this film becomes an integral part of the superficial layers of the zinc coating. The film passivates the highly reactive zinc surface and retards the formation of white rust by a substantial margin. Also, the conversion film enhances bonding or adhesion of any additional paints or coatings applied over the galvanized surface. For small mechanical tube, where a silver bright type of surface is more desirable for some of our customers, a minimal amount of the conversion coating is applied for short-term indoor storage purposes. For the relatively larger diameter pipe, a heavier coat is applied.
Clear Organic Polymer Coating
The clear organic polymer coating is applied as a third coat or final finish on the exterior surface. This final layer acts as a barrier coating and further reduces the atmospheric corrosion of the product. However, there is an interesting phenomenon that occurs as a result of the combination of the second and third coating. Combined performance (corrosion protection of these two coatings) is more than the summation of the individual protection provided.
Zinc Interior Coatings
We have a number of ID (interior diameter) zinc coatings which have been specially formulated to meet specific customer fabrication requirements. We are continuously upgrading our ID coatings to maximize the overall corrosion resistance of the interior of the tube. All our ID coatings are applied after the tube is welded…thereby assuring a uniform coating on the entire interior surface. In addition, none of our ID coatings will contaminate paint or powder coat cleaning systems.