Molten zinc must be allowed to free flow without impediment. Hence openings must be large enough, without impairment of the structural strength for zinc to flow into the fabrication and out of it.
Strengthening gussets in the webs of columns and beams should have the internal corners cropped or holed to ensure that no zinc build up occurs in angles or corners, and that air locks and ash entrapment is eliminated. Fabricators should first check with the galvaniser before work goes to completion, to ensure that drains and vents are positioned correctly and of the appropriate size. Some long lengths of steel, not easily accommodated due to their length, may be double end dipped.
Alternatively a fabrication may be built in smaller modules that may be more easily accommodated.
For a successful chemical reaction between the molten zinc and steel surface, the surface must first be prepared to ensure there is no coating (grease, lacquer or paint) that would hinder this reaction. All steel to be galvanised is cleaned by the Process Baths before hot dipping. The baths include caustic, acid and flux mixes.
If there is a paint, lacquer or grease coating on the steel this will need to be abrasive blaseted (sandblasted) before arriving for galvanising. Note: Use only water soluble cutting fluids when drilling to avoid steel contamination that may confilct the galvanising process.
Most standard primer coatings ("mill coating") applied by steel manufacturers do not need to be sandblasted as these can be removed safely in the process baths.
Note: Pipes are often supplied with a lacquer coating, this requires blasting prior to galvanising.
The following information provides details on draining and venting, with appropriate drawings for fabrications. Contact us for experienced advice which is readily available to assist you.
All items require thorough preparation; to allow for drainage and ventilation as well as rigging the steel correctly onto jigs. It is important that specific care is taken at this stage, as not having holes in correct places is a serious potential hazard to equipment and personnel. Any pickling acids or rinse waters that may be trapped in a blind or closed joint connection will be converted to super-heated steam (can develop pressure of up to 26.2Mpa or 3800psi) when immersed in the molten zinc bath at 450 degrees celcius.
Since proper galvanising demands that inside, as well as outside, be completely cleaned and coated with zinc; air and ash must be allowed to flow upward and completely out, cleaning solutions and molten zinc must be allowed to flow through and completely cover the surface area. In all tables below of draining, venting and gusset bevel calculations, allowance has been made for the speedy and total expulsion of entrapped air and ash produced during the galvanizing process. Simply stated, the structure must be lowered into the solution without trapping any air. It must be raised from the solution without trapping any solution. Consequently, ample passageways that allow flow in and out must be designed into the assemblies. Since items to be galvanised are immersed and withdrawn at an angle, the vent holes should be located at the highest point and lowest point in each hollow member. All components of fabricated hollow sections should be inter-connected with full open tee or with mitred joints. Each closed section must be provided with a vent hole and a drain hole. Galvanising HB will visually identify the venting from the outside when assembly is received. This is necessary to check the adequacy of venting as well as to determine that it has not been omitted by mistake.