Design Considerations

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.

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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

Protection against corrosion begins on the drawing board. No matter what corrosion protection system is used, it must be factored into the design of the product.  Once the decision has been made to use hot dip galvanizing to provide corrosion protection for steel, the design engineer should ensure that the pieces can be suitably fabricated for high quality galvanizing. Adopting the following design practices will ensure the safety of galvanizing personnel, reduce coating cost, and produce optimum quality galvanizing.

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.

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 can provide the desired information.

 

 

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 chart below shows the dimensions of work that could theoretically be galvanized by double-end dipping in (for example) a bath 8m long x 2 m deep, assuming that the width of the work also suits the bath.

Schematic indication of double-end dipping capacity of a galvanizing bath 8 metres long x 2 metres deep

3.5m = Maximum size of work which can be coated by double-end dipping for excess depth: 3.5m x 7.75m

14.3m = Maximum size of work which can be coated by double-end dipping for excess length - up to 14.3m

The chart shows that a bath nominally 8m long x 2m deep could process work 7.75m x 3.5m, or long components of up to about 14m. Note that the above chart is purely indicative and similar charts can be prepared for baths of different dimensions. The maximum sizes which a particular galvanizer can process should always be checked at the design stage.

 

Modular design

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.

The size and shape of large or unusual structures should always be checked with the galvanizer early in the design process.

 

Materials suitable for galvanizing

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.

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 stressfree 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:

1. Design for uniform section thicknesses wherever possible.

2. Use large radii at junctions with webs, fillets and raised features such as cast-in part and pattern numbers.

3. Avoid deep recesses and sharp corners.

4. Large grey iron castings should be normalised by the fabricator.

5. Castings should be abrasive blast cleaned by the fabricator to remove foundry sand and surface carbon.  Alternatively castings may be cleaned electrolytically using the Kolene process.

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.

Design for Hot-Dip Galvanizing

For a copy of the complete "Design Guide for Hot Dip Galvanizing - best practice for venting and draining" please click here to download a copy.