Protection against corrosion begins in the design phase and galvanized coatings provide outstanding corrosion protection for steel.
Once the decision has been made to use hot dip galvanizing, the designer should ensure that the pieces can be suitably fabricated for high quality galvanizing. Treatment of design details in accordance with good corrosion design practice will further increase the life of galvanized steel fabrications. AS/NZS 2312.2 provides excellent design advice and we have expanded on the information available in that Standard in this section.
If in doubt
Most galvanizers have seen your problem before. If you have any doubts about designing for galvanizing, talk to your galvanizer. They can provide the most help. A list of leading Australian galvanizers is available here. If you prefer, you can also talk to the technical staff at the Galvanizers Association of Australia.
Consistently good galvanized steel products will be produced when the essential requirements listed are incorporated at the design and fabrication stages of production.
Design features should be discussed with the galvanizer. Close liaison between design engineer, materials engineer, specifier, fabricator and galvanizer will ensure high quality galvanized products, minimum cost and faster delivery.
Molten zinc and all processing solutions must be able to enter and drain from fabrications without difficulty and this may require additional holes for venting and draining.
Steel items to be processed must fit into the preparation tanks and galvanizing bath. This is not limited to the physical vessel dimensions but that space which results from double end dipping techniques, which normally meet the permissible road transport dimensions. The size of the dipping tanks in Australia are listed for every GAA memberhere.
Adopting the following design practices will ensure the safety of galvanizing personnel, reduce coating cost, and produce optimum quality galvanizing.
MATERIALS SUITABLE FOR GALVANIZING
Steels. Most ferrous materials can be galvanized. Mild and low alloy steels and iron and steel castings are all regularly and successfully galvanized. Steel fabrications which incorporate stainless steel parts and fittings are also readily galvanized. Detailed information is available by contacting us, and AS/NZS 2312.2 also provides a list of materials and their Australian Standards which are suitable for galvanizing.
Soft-soldered assemblies or those with aluminium rivets cannot be galvanized. Brazed assemblies may be galvanized, but the galvanizer should be consulted at the design stage.
Castings. The galvanizing of sound stress-free castings with good surface finish will produce high quality galvanized coatings. The following rules should be applied in the design and preparation of castings for galvanizing:
Design for uniform section thicknesses wherever possible.
Use large radii at junctions with webs, fillets and raised features such as cast-in part and pattern numbers.
Avoid deep recesses and sharp corners.
Large grey iron castings should be normalised by the manufacturer.
Castings should be abrasive blast cleaned by the manufacturer to remove foundry sand and surface carbon. Alternatively, castings may be cleaned electrolytically using the Kolene process.
SIZE AND SHAPE
Facilities exist to galvanize components of virtually any size and shape, depending on the handling facilities and layout of the galvanizing plant. Large cylindrical objects can often be galvanized by progressive dipping.
The size and shape of large or unusual structures should always be checked with the galvanizer early in the design process.
Double end or depth dipping is a term used to describe the process of galvanizing an item which is longer or deeper than available bath dimensions. In this procedure the item is lowered into the bath so that half or more of its length or depth is immersed in the zinc bath. When the zinc coating has been achieved, the item is raised from the bath and adjusted in handling so that the ungalvanized part can be immersed in the bath. It should be noted that in this procedure an overlap of zinc coating will occur, and this may have to be addressed in the case of visually obvious structural elements that require an aesthetic finish.
Guidance in these cases should be sought from the galvanizer or the Galvanizers Association of Australia.
Large structures are also galvanized by designing in modules for later assembly by bolting or welding. Modular design techniques often produce economies in manufacture and assembly through simplified handling and transport.
Weld areas in structures assembled by welding after galvanizing must be repaired to give corrosion protection equivalent to the galvanized coating as described under Repairing Hot Dip Galvanizing.
GENERAL FABRICATION GUIDELINES
Fabricated assemblies should be designed to eliminate undrained areas which will collect water and detritus in service resulting in localised corrosion damage. Good venting and draining design will ensure good drainage of water from the structure in service and optimise the durability of the fabrication.
Weld areas. Due to the silicon content of some welding rods, weld areas may produce localised grey coatings when galvanized. The galvanized coating is likely to be slightly thicker in these areas and will have no detrimental effect on coating life.
The development of grey coatings due to silicon steels is entirely related to steel composition and cannot be controlled by the galvanizer. Even when these weld areas are ground flush prior to galvanizing, heavier grey coatings may still result. Low silicon welding rods can be used to reduce this effect.
Welding slags. Arc welding slags are chemically inert in acid cleaning solutions and must be mechanically removed before articles are delivered to the galvanizer. The fabricator should remove these by chipping, wire brushing, flame cleaning, grinding or abrasive blast cleaning.
Welding electrode manufacturers supply general purpose electrodes coated with fluxes which produce virtually self-detaching slags and their use is recommended.
Good joint design with adequate access facilitates the welding process to produce sound continuous welds, avoiding locked-in slag, and easing slag removal.
You can galvanize all mild and low alloy steels, and most iron and steel castings.
You can galvanize a wide variety of shapes and sizes and weights. But consult your galvanizer if you have an unusual or complex shape.
Follow the guidance about vent and drain holes for hollow sections (see the poster below).
Avoid large overlapping surfaces if possible. If unavoidable, make sure you follow the venting guidance.
Use butt welds in preference to lap welds.
Where lap welds are used, face the joints downwards to avoid collection of moisture and detritus.
Do not use silicon-containing anti-splatter paints. They cannot be removed in the general pre-treatment process and will result in no galvanized coating formation in those areas.
Provide about 1mm extra clearance on all mating surfaces.
Do not attempt to use low melting point soldered joints. They will collapse when galvanized.
Remove moulding sand from castings by grit blasting. Galvanizing will not form on sand.
Remember the galvanizer must lift your fabrication or item. Make sure you provide lifting points for them to use.
Do not ask your galvanizer to galvanize fabrications which are very stiff in one plane but not in another. They will almost certainly distort.
Avoid large unsupported areas of thin gauge sheet. They will probably distort.
The GAA has produced a poster that covers the basic fabrication requirements. Click hereto download a pdf of the poster, or order it as an A3 chart for your workshop here.
GENERAL DESIGN FOR DURABILITY
Here are some general rules to be used in the design of items to be hot dip galvanized to maximise the durability of the items.
Avoid the use of horizontal boxed sections, ledges, seams and flat undrained areas.
Use rounded internal corners rather than squared corners in vessels and containers to avoid build-up of detritus.
Design to eliminate crevices and unnecessary openings.