Intermittent Welding Prior to HDG

Industry best practices for venting and drainage are addressed in ASTM A385 Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip), ensuring both galvanizing operator safety and the quality, consistency, and appearance of the HDG coating.

To prepare fabrications for hot-dip galvanizing, ASTM A385 explains it is best practice to avoid overlapping surfaces welded together with narrow gaps. When gaps are less than 3/32 in. between the overlapping surfaces, molten zinc from the galvanizing kettle cannot enter the gap to coat the interior area. When overlapping surfaces are unavoidable, special fabrication measures must be taken to ensure structure integrity, galvanizing quality, and aesthetics near the overlapped area. The available methods are to:

• Ensure minimum 3/32 in. gap between plates such as when intermittent welding.
• Perform seal welding and provide sufficient vent holes or a length of unwelded area as required to prevent weld blowout based on steel thickness and size of overlap area, as detailed in ASTM A385 Tables 1 & 2.
• Ensure sufficient venting when gap is less than 3/32 in. and apply caulking to seal the joint after galvanizing and mitigate the appearance of a weeping weld.

The AGA is often asked about maintaining a minimum 3/32 in. gap when intermittent welding is specified. Fabricators may insert small shims or welding wire to ensure the gap. Shims can be removed or pulled from the joint after welding, but in practice they can be difficult to extract from the joint and are sometimes left in place. Some fabricators prefer to use welding wire as the shim material because the reduced contact between the shim material and the plates allows for easier extraction. Shims are most practical for smaller overlapping areas. Some fabricators report challenges achieving uniform gap spacing when using shims for larger overlapping areas or heavier sections.

More information is available in AGA Publications Welding & Hot-Dip Galvanizing and Design of Products to be Hot-Dip Galvanized After Fabrication.