Joining-galvanized steel:Challenge and Success.SOLUTIONS with Effective, Practical AdviceJoining-galvanized steel is normally done with one of a list of possible processes, depending on application requirements. The usefulness of zinc coatings for rust prevention depends on the much slower corrosion rate of zinc as compared to that of steel, and on the electrolytic protection provided by zinc because it is anodic to iron. Zinc is electrochemically more reactive than steel and therefore it oxidizes to save from rusting even bare steel spots near the coating. This characteristic is called "sacrificial" protection so that exposed spots like sheared edges of galvanized sheets, cut ends of galvanized wire and scratches on the galvanized surface result protected from rust even at a small distance from the zinc covering. Galvanizing means the application of a zinc coating on steel surfaces. Dipping the cleaned steel items into a bath of molten zinc (Hot Dip Galvanizing) is a common process. The coating thickness and its appearance depends on the details of the process. The coverage tends to appear somewhat rough and the lower edges may result thicker depending on drainage and cooling. Joining-galvanized steel is not difficult although it requires attention to the specific problems involved, as presented in this page. Relatively small parts fabricated by welding, should receive hot dip galvanizing after welding. In this case the galvanizing quality depends on the thoroughness of weld cleaning from residual slag and other contaminants. Rolled steel sheets are galvanized by a continuous electrolytic plating process. This process permits control of uniformity of plating as a thin layer. Soldering and Adhesive Joining Low temperature soldering, and adhesive joining are processes for Joining-galvanized steel that do not damage the zinc protection and should be used preferentially if the strength requirements can be provided by the construction itself. Other Welding Processes Otherwise most of the welding processes can be used for Joining-galvanized steels with minor adjustments taking into account that in and near the joints zinc is vaporized by welding. Zinc melts at 420 0C (787 0F) and boils at 906 0C (1663 0F) and is therefore removed as vapor that, when oxidized, becomes fumes at steel welding temperatures. Arc Welding Shielded Metal Arc Welding (SMAW) is applied for Joining-galvanized steel much as it is for bare steel. To avoid that zinc vapor become entrapped in the molten weld puddle one should design joints with slightly wider gaps to allow for complete fume escape. Welders should be alerted to provide sufficient heat input, to slow travel speed for degassing the molten metal and to point the electrode forward to push the zinc vapor away from the arc. The welding electrode composition used for Joining-galvanized steel should be checked to assure that the silicon content be very low (we found some disagreement on the actual recommended level) because high silicon can cause zinc to penetrate the weld metal, leading to cracking. Gas Metal Arc Welding (GMAW) and Flux Cored Arc Welding (FCAW) may require parameter adjustment to reduce spatter that tends to be higher when Joining-galvanized steel. The same recommendations as above apply. The best way to exclude zinc from possible inclusion in molten metal would be the local removal of coating before welding. Unfortunately this ultimate solution risks to be labor intensive and therefore expensive. Oxyacetylene Torch Brazing The oxyacetylene torch can used for Joining-galvanized steel by brazing with copper base filler metals. Abundant brazing flux helps reduce the zinc loss near the joint. Resistance Spot or Seam Welding Special schedules must be developed for successful resistance welding for Joining-galvanized steel. Coating affects the surface resistance and risks to contaminate the electrodes that should therefore be dressed at shorter intervals than usual. Starting from the parameters used for bare steel, one could use higher welding current, greater electrode force and shorter welding time. No zinc restoration is usually provided around the locally depleted welded spots because there is no need to do so.
Laser Welding It is reported that thorough research was involved in developing methods of Laser welding galvanized steel mainly for automotive body fabrication. It appears that the presence of voids in the welds, due to zinc vapors, cannot be completely eliminated. However it was demonstrated that the size and distribution of such voids can be contained at levels that do not reduce the design strength of the joints involved, so that manufacturers adopted the process for current production of certain models. Quality It is generally agreed that correctly performed welded joints used for Joining-galvanized steel actually present acceptable mechanical properties similar to those obtained by the same welding processes on bare metal. Zinc Fumes Zinc oxide fume generated during Joining-galvanized steel by welding, if inhaled, cause the metal fume fever. Although non toxic, the symptoms from breathing such fumes may debilitate affected persons for 24 to 48 hours. Every precautions must therefore be put in place to avoid fume inhaling. Fume extractors or source capture devices are effective if placed near the point of welding. To be effective, the flexible duct end of the vacuum fume extractor should follow the weld location if this is moving. Special welding GMAW torches with fume extractor incorporated can be used although they are somewhat bulkier and harder to manipulate than regular torches. Other than that personal masks or respirators should be worn by welders to provide positive protection while welding for Joining-galvanized steel. It is recognized that educating welders to wear effective masks may be a problem. Restoring Corrosion Resistance A new and different protection, usually by a paint containing a very large proportion of zinc particles, must be applied by brush or spray in the areas depleted of the original coating by the welding, after thorough cleaning of the surfaces.
Reference A valuable and detailed reference is available to those wishing to obtain complete information on the subject of Joining-galvanized steel. AWS WZC (D19.0-72)
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