Galvanized Steel and Stud Welding
I've heard that the stud welds on galvanized metal aren't as pure as when stud welding bare steel. Can you stud weld thru the galvanized coating? How is it done? Are there any negative repercussions to this process?
Stud welding, in the case of welding studs to a structural beam to connect a concrete deck to the beam, is typically done using the electric arc stud welding process. Electric arc stud welding was initially developed prior to WWII as a necessity for rapidly attaching wood planking to naval aircraft carriers. With this revolutionary technology, studs could now be placed on the exterior side of steel decks utilizing only one worker, circumventing the previous cumbersome and time-consuming drilling process.
Stud welding is performed with a gun attached to a power source that provides the electrical energy to generate the arc. The power source and the stud welding control system are set to control the amperage and the arc duration, which vary due to the type of stud, stud diameter, and base steel conditions. The gun is held against the steel surface and an arc is initiated between the end of the stud and the plate surface. After a predetermined time related to the power settings, the current supply is automatically switched off and, under spring pressure, the stud is forced into the molten pool of steel. The stud itself becomes part of the weld material. For hot-dip galvanized steel, the same process can be performed, but the end results are somewhat different. The presence of zinc and included zinc vapors in the weld can weaken the weld to the point of failure; however, the practice of stud welding on galvanized steel remains very common. Many times, the suggested best practice of removing zinc from the area of the weld is impractical and not feasible. Todays solution to this is to set the power to the highest setting and current to the longest duration in an attempt to fully vaporize the zinc with the arc heat. This method still results in a subpar weld with zinc included. These studs attached with weaker welds are then knocked off, leaving a spot of zinc-free metal/slag mix where the stud was removed.
The stud welding process is then repeated, positioned where the previous weld was performed; this leads to a stronger weld with less included zinc. The practice is still not perfect, as remaining slag and debris from the first weld attempt can be included in the new weld, resulting in a weld that is still inferior to one performed on a clean black steel substrate. Due to the redundancy in todays design, however, these weaker welds meet the requirements for a deck anchor. The best practice for this stud welding situation would be to either remove the zinc completely in the area of the weld by grinding or to prevent the zinc from being there in the first place by masking the top surface.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.