The primary reason for vent and drain holes is to allow air to be evacuated, permitting the object to be completely immersed into cleaning solutions and molten zinc. Proper sizing and location make it safer to galvanize and provide the optimal finish.
The secondary reason is to prevent damage to the parts. Any pickling solutions or rinse waters that might be trapped in a blind or closed joining connection will be converted to superheated steam or gas and can develop a pressure of up to 3600 psi (1100 MPa) when immersed in molten zinc. Not only is there risk of damage to the fabrication being galvanized, but there also is risk of serious hazard to galvanizing personnel and equipment.
Air and frothy fluxes must be allowed to flow upward and completely out. Cleaning solutions and molten zinc must be allowed to flow in and completely wet the surfaces. Proper galvanizing results when the inside and outside of a product are completely cleaned and zinc-coated.
The structure must be lowered into the solution without trapping any air, then raised from the bath without trapping any solution. Consequently, ample passageways allowing unimpeded flow into and out of the part must be designed into assemblies.
Because items to be galvanized are immersed and withdrawn at an angle, the vent holes should be located at the highest point and drain holes at the lowest.
All sections of fabricated pipe-work should be interconnected with full open-tee or miter joints. Each enclosed section must be provided with a vent hole at each end.
Most galvanizers prefer to visually identify the venting from the outside. This is necessary to verify the adequacy of the venting as well as to determine that venting has not been mistakenly omitted. Some galvanizers may hesitate to process complicated pipe assemblies unless all venting is visible on the outside and readily accessible for inspection.
Base-plates and end-plates must be designed to facilitate venting and draining. Fully cutting the plate provides minimum obstruction to a full, free flow into and out of the pipe. Since this is not always possible, using vent holes in the plate often provides the solution.
Vent holes are frequently left open but can be closed with drive caps or plugs after galvanizing.
Various methods of venting are acceptable, but the subsequent plugging of these holes should be kept in mind, where necessary or desired.
It is recommended that tubular structures be completely submerged in one dip into the galvanizing kettle. This minimizes potential internal coating problems that, because of the size and shape of the item, may be difficult to discover during inspection.
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