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Joining-galvanized steel:Challenge and Success.SOLUTIONS with Effective, Practical Advice
Joining-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. Furthermore zinc provides electrolytic protection because it is anodic to iron. Zinc is electrochemically more reactive than steel. Therefore it oxidizes, saving from rusting even bare steel spots near the coating. This characteristic is called "sacrificial" protection.
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 coverage tends to appear somewhat rough and the lower edges may result thicker, depending on drainage and cooling. The coating thickness and appearance depend on the details of the process. 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 a uniform plating as a thin layer.
Low temperature soldering, and adhesive joining are processes for Joining-galvanized steel that do not damage the zinc protection. They should be used preferentially if the strength requirements can be provided by the construction design itself. Otherwise most of the welding processes can be used for Joining-galvanized steels with minor adjustments. One should take into account that zinc is vaporized by the welding heat, in and near the joints. Zinc melts at 420 °C (787 °F) and boils at 906 °C (1663 °F) and is therefore removed as vapor. When oxidized, zinc vapor becomes fumes at steel welding temperatures. Shielded Metal Arc Welding (SMAW) is applied for Joining-galvanized steel much as it is for bare steel. One should avoid that zinc vapor become entrapped in the molten weld puddle. Joints must be designed with slightly wider gaps to allow for complete fume escape. Welders should be alerted
The welding electrode composition used for Joining-galvanized steel should be checked to assure that the silicon content be very low. In fact we found some disagreement on the actual recommended low level: 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. This would 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. 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. Special schedules must be developed for successful resistance welding for Joining-galvanized steel. Coating affects the surface resistance. It risks to contaminate the electrodes: those 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. An Article on Inserted strips in Resistance Spot Welding was published (3) in Issue 136 of Practical Welding Letter for December 2014. 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. Therefore manufacturers adopted the process for current production of certain models. It is generally agreed that correctly performed welded joints used for Joining-galvanized steel actually present acceptable mechanical properties. These result similar to those obtained by the same welding processes on bare metal. 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. A new and different protection should be applied cold, by brush or spray, in the areas depleted of the original coating. All the surfaces at and around the welding should undergo thorough cleaning. Then a paint is used, containing a very large proportion of zinc particles. An Article on Zinc Fumes was published (3) in Issue 55 of Practical Welding Letter for March 2008. An Article on Welding Galvanized Steel was published (7) in Issue 99 of Practical Welding Letter for November 2011. An Article on Braze Welding Galvanized Steel was published (3) in Issue 115 of Practical Welding Letter for March 2013. An Article on Cored Wire for Welding Galvanized Steel was published (4) in Issue 135 of Practical Welding Letter for November 2014. 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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