Technical Information
Steel Preparation & Design
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
Surface Preparation
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.
Drilling & Venting
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.
Drain & Vent Hole Sizes (min) for Hollow Sections
1. The drain and vent hole size represent the minimum acceptable and it is preferred that the full tube cross section be provided.
2. The table is also applicable to hollow sections fabricated from channels and angles, etc. Use the table with the appropriate outside dimensions of the boxed sections.
3. For larger sections use the associated table for tanks.
4. Lifting eyes to be provided for the hanging of steel work (adjacent same side as vent hole) or provide holes for lifting wires.
Positions of Drain & Vent Holes for Hollow Sections
When both internal and external surfaces are to be galvanized at least one filling and draining hole must be provided, with a vent diagonally opposite to allow the exit of air during immersion. Internal baffles should be cropped as illustrated. Manholes should finish flush inside to prevent trapping excess zinc.
1. Drain and vent holes are to be placed at diagonally opposiste locations typically as shown in Fig 1, 2 & 3 to facilitate complete venting and draining of the vessel.Each mitred corner or scallop in internal baffle plates must be equal; in area to that of the drain holes Fig 1, 2 & 3.
2. Provide suitable lifting lugs Fig 3.
3. Drain and vent holes must be flush internally and be located as close to corner welds as possible. Fig 4. NB Figs. 5 & 6 unacceptable.
Should above drain and vent hole locations be difficult to include during fabrication contact us for further advice.
Gussets & Web
Welded gussets and webs on columns, beams, and channel sections should have corners cropped or holed,
1. To prevent the entrapment of air in pockets and corners,
2. To faclitate drainage during withdrawal from the galvanising bath.
Welding: Use continuous welding all round to avoid moisture traps.
Drilling: Use only water soluble cutting fluids.
End Plates: Provide holes in end plates for venting and draining or crop section webs.
Overlapping Surfaces
Avoid narrow gaps between plates, overlapping surfaces, and back-to-back angles and channels. When small overlaps are unavoidable, seal edges by welding. When left unsealed, small overlapping areas may trap pickle acid that can later escape to colour or damage the galvanised coating. The pickling acids or rinse waters are able to get inbetween smaller spaces than where zinc can easily flow, which means any trapping (often occuring with welded togther plates) can trap these liquids and cause seeping of those trapped liquids after galvanising.
If contacting surfaces cannot be avoided, a hole 6mm in diameter for every 0.01m2 of overlap area should be placed in one of the members, and the perimeter of the contacting area should be continuously welded.
Bevel Sizes: Guildeline
Bevel cuts for angles and channels = 25% of flange width.
Bevel cuts for I Beams and columns = 25% of half the flange width.
Bevel cuts for RHS End plates = 25% of half the widest side.
Bevel Example:
1. Bevel sizes are 45 degrees (e.g. 40mm bevel = 40 x 40 x45 degress).
2. Gusset plates or abutments in channels and beams have both ends bevelled.
Design and Fabrication of Steel Plate Structures for HDG
1. One hole or lifting lug required in plates where detailed in Fig. 1, 2 & 3.
2. Two holes or lifting lugs required in plates where detailed in Fig 4 (2 holes or lugs when over 6m).
3. Hole Sizes:
a) Plates up to and including 10mm thick require 8mm - 10mm dia. hole(s).
b) Plates over 10mm thick and up to and including 16mm thick require 16mm dia. hole(s).
c) Plates over 16mm thick require larger diameter holes or lifting lugs (talk to us for guidelines).
4. Gauge or centers of holes/lifting lugs should be 1.5 diameter of holes required for lifting from edge of plate. (Fig 5).
5. Lifting Lugs:
a) Lifting lugs should be welded to plates in posistions detailed.
b) Lug thickness should equal thickness of plate being galvanized.
c) Lugs should have sufficient weld to support plate during the galvanizing process.
6. Stiffeners or attachments welded to plates at lifting points may be utilised as lifting lugs.
7. It is preferred that lugs be welded to the side of plates where their removal after galvanizing is not necessary. (e.g. floor plates).
8. Cutting, Shaping and Welding Plates: If possible plates should be cut from one sheet to eliminate or minimise butt welds and stresses.
9. Platform Plate: Welds should be kept to a minimum when welding plates to heavy subframes to minimise weld stresses that cause distortion during galvanizing.
Bolting or screwing subframes is preferable. It is preferable that fittings or attachments on plates be designed for bolted connection to plates to minimise weld stresses. When cutting plates to shape it is preferred that all operations be as uniform as possible. If plates exceed maximum sizes shown in Fig 4 talk to Galvanising HB for further guidance. If large quantities of plates or flats of the same shape and size are to be galvanized, talk to Galvanising HB and maybe holes can be eliminated or altered through jigging techniques.
Drain & Vent Holes for Various Tank Capacities
1. Calculate cylindrical tank capacity as follows:
Capacity (M cubed) = 0.785 x D squared x L (dia. & length in metres).
2. For tanks smaller than listed above the min. size for:
drain hole = 50mm dia.
vent hole = 30mm dia.
Clearance for Moving Parts
Drop handles, hinges, shafts, and spindles require provision of minimum adial clearances as detailed in the table above, to allow for the thickness of the galvanizing coating.
Shaft or spindle size, Minimum hole clearance
Up to 10mm dia. = 1mm
10 to 30mm dia. = 2mm
Over 30mm dia. = 2 to 2.5mm
Australian standard 1214 specifies the following oversize tapping allowances:
Nominal internal thread diameters up to and including M24: 0.4mm allowance.
Nominal internal thread diameters over M24 up to M36: 0.5mm allowance.
Galvanising Standards
Suited to your Needs
Galvanising (HB) Ltd complies with the New Zealand standard AS/NZS 4680. The coating thickness guide can be found on the Galvanizing Association of New Zealand website. The service life of galvanizing is directly proportional to the coating thickness.
Zinc coating on steel: 1g/m2 = 0.14µm. A influencing factor that will alter the life of galvanised steel is the environmental exposure (e.g. by the sea, by other chemical atmospheres).
Galvanising (HB) Ltd can work to accommodate your requirements. We are able to provide (upon request prior to galvanising) a Quality Assurance Certification Report.
After HDG Painting Requirements: The final process bath is where items are cooled in after the zinc bath dipping. It iincludes chemicals that helps to prevent any white rust occuring on the galvanised steel. In some cases the chemicals from the cooling (quench) tank make the surface more reactive, which can effect the painting if immediately after galvanising. We are able to offer a different method of cooling in cases where requested, which does not go through the chemical cooling, but rather it is be air cooled.
Process Baths
Chemical Details
A breakdown of the Process Baths used at Galvanising HB Limited are as follows.
1) Caustic Tank: Removes primer or grease prior to pickling.
2) Rinse Tank: Between each stage a water tank is used to rinse any left over chemicals before being moved to the next stage.
3) Acid Tanks: HCL Acid 'pickles' steel prior to galvansing, cleaning the steel surface right back to clean steel.
4) Flux Tank: A tank mostly consisting of zinc amonium chlorides which helps the bond of the zinc to the steel.
5) Zinc Bath: Molten zinc which bonds to steel that is submerged creating several layers of zinc alloys.
6) Quench Tank: Sodium Dichromate mixture which helps prevents galvanised steel from reacting with the oxygen and forming a 'white rust' layer. This also helps remove ash (from any remaining contanimants left on steel) and provide a shiny silver finish on relevant items.
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