Welding galvanized tube is done exactly the same way as welding the same bare steel tube; the same welding processes, volts, amps, travel speed, etc. can be used without modification when the switch is made to galvanized tube. A competent welding engineer should, however, review the welding electrodes which are being used for excessive silicon content, which can cause zinc penetration of the weld metal. The silicon in the welding electrode should not exceed 0.04%.
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. If a customer is interested in purchasing the Matchmaker paint in bulk for spray bottle applications, they may buy directly from the manufacturer, Seymour of Sycamore, (815) 895-9101. The part number is AE- 1. MSDS sheets on these paints are available directly from each manufacturer; see the following page for a list of addresses and phone numbers.
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. The two methods of thermal spraying most applicable to tube fabrication are Wire Flame Spray and the Electric Arc Process. The initial equipment cost for an integrated thermal spray, material preparation, and dust recovery system depends on the specific application. A range of costs for the individual components are:
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.
Welders should be taught to keep their heads out of the fume plume and to position themselves relative to the air flow around themselves so zinc oxide dust does not collect inside their welding shields. If a welder finds white dust inside his welding shield, he is not positioning himself properly. To complement proper positioning, a fully effective method for preventing inhalation of zinc fumes is to wear a suitable mask. These masks are similar to a painter’s mask; although there are many larger and more complicated masks, these are effective with minimal interference or discomfort to the welder. The higher priced masks contain activated charcoal which removes some odors as well as the zinc oxide; welders who use these masks frequently wear them even when they are not welding galvanized steel, since they make the air smell better.
The other approach to removing the air from the welder’s breathing space is to capture it so the fumes never rise into the welder’s face. This can be done using local ventilation to capture the fumes, then to either exhaust the air or filter it. The fumes can be captured either at the source or by general ventilation. Source capture devices are usually flexible ducts attached to the exhaust or filter system. One difficulty with “source capture” devices is that their range is limited to less than a foot from the end of the flexible duct; this means that the welder has to move the duct if he moves outside its capture range. Another type of “source capture” device that can be used when “MIG” welding is a welding gun that has a vacuum nozzle attached directly to the welding gun. All “MIG” welding gun manufacturers, including Tweco, Lincoln, Hobart, and Binzel make these modified guns and filter units. Their primary disadvantage is that they are slightly bulkier than guns without vacuum attachments. This can make it more difficult for the welder.
The optimum method for capturing welding fumes over a large area is a downdraft work table. This is because the fumes are drawn downward away from the welder’s breathing zone. Interestingly, the effective capture distance of a downdraft table can be easily extended to over a foot simply by the addition of an overhead fan. Downdraft tables are available from Weldsale Company, (215) 739-7474. Downdraft work stations will cost approximately $10,000 for a large (5′ x 8′) work station. Downdraft ventilation is not only better than overhead ventilation, but it is usually less expensive, since many of the components are off-the-shelf items, and the ventilation system is integrated into a convenient working table.
Welding fumes and zinc oxide dust can be removed by general ventilation; however, American National Standards Institute (ANSI) Z49.1 limits zinc to 5mg/cubic meter. General ventilation or tall ceilings (over 16 feet) are needed for all welding operations to ensure adequate dilution of smoke and other pollutants associated with welding. Overhead exhaust systems can be designed and engineered to remove these pollutants and zinc oxide dust from the air. These systems can exhaust to the outside atmosphere or they can be recirculating systems.