How to Prepare a Steel Structure for Hot-dip Galvanising: Checklist

At the design stage

• avoid closed cavities and pockets: every internal volume needs holes for liquids, molten zinc and gases to enter and exit — a structure with a fully closed cavity may not be galvanised (risk of explosion on immersion in the molten metal);
• round sharp corners and edges to a radius of at least 0.3 mm, unless the part’s technical requirements say otherwise;
• design welded joints as butt joints, double-sided welds, or single-sided welds with back-welding — lap joints (plate on plate without welding through the edge) cannot be galvanised;
• for hollow parts of complex shape, plan a trial galvanising of a sample before the full batch.

Choosing the steel

• most structural steels can be galvanised: carbon steels, fine-grained steels, hot- and cold-formed hollow sections; stainless steel and steels with a raised sulphur content are not suitable;
• silicon and phosphorus influence the look and thickness of the coating most: with silicon below 0.03% and phosphorus below 0.02% the coating is thin and bright, at 0.14–0.25% silicon it is bright or matt, and in the ranges 0.03–0.14% and above 0.25% it comes out thicker, darker and rougher;
• keep to a single steel grade within one structure — this gives a more even coating appearance.

Before sending for galvanising

• the surface must be free of paint, grease, metal shavings, welding slag, foundry sand and graphite residue (for cast parts) — all of this is removed before pickling, otherwise the coating may show defects;
• welds must be even, dense and continuous along their full length, without pores or inclusions;
• cast parts must have no pores or shrinkage cavities;
• include drawings or photos with your order — we check the structure for closed cavities and problem spots before galvanising.

Distortion, hanging points and delivery to the plant

• plan the hanging points: in the bath the part hangs from slings or hooks, so holes or lugs must sit where the part will not sag under its own weight;
• leave pipe branches and stubs open: a cap blocks the path of the zinc and traps air or liquid inside;
• welding, stamping and rolling leave residual stress in the metal; in the hot bath it relaxes and the part may twist, so such parts are best stress-relieved by heat treatment before galvanising;
• do not combine very thick and very thin metal: the bigger the thickness difference in a welded assembly, the more unevenly the metal heats and cools in the melt, and the higher the chance of warping; for welded assemblies we ask for a ratio of the greatest to the smallest thickness of no more than 5:1;
• tell the steel supplier that the finished structure will be galvanised — they can choose rolled steel with a suitable chemical composition;
• we unload and load vehicles with a crane or a loader; unloading by hand is not acceptable.

What happens at the plant

Degreasing, pickling, rinsing, fluxing, drying, immersion in molten zinc at ~450 °C, cooling, passivation, and a check of coating thickness and appearance. For details on every step and the full technical requirements, see the article Hot-dip Galvanising Process.

What pickling removes and what must be cleaned off before shipping

In the pickling bath the acid removes rust and mill scale (iron oxides) but does not dissolve paint, slag or thick grease. So check the surface against this list before shipping:

Residues on the surface leave uncoated areas, even when they are only barely visible films of slag or spray. Surfaces left by flame, laser or plasma cutting should be ground, and sharp edges removed, before shipping: cutting changes the composition and structure of the steel near the edge, so the minimum zinc thickness is harder to reach there and the coating may adhere less well.

Example: checking the thickness ratio in an assembly

Divide the greatest metal thickness in a welded assembly by the smallest: we ask for this ratio to be no more than 5:1. Illustrative examples:

The numbers are illustrative. The 5:1 ratio is our practical guide for welded assemblies, not a numerical requirement of any standard: the ISO 14713-2 guidance only advises avoiding large differences in thickness, for example a thin sheet welded to thick angles, and favouring symmetric sections. Even within 5:1, a thin element welded to a thick one remains an area of higher warping risk. If the ratio exceeds 5:1, consider before fabrication a more symmetric section, a smaller difference in thickness, or splitting the assembly into parts that are galvanised separately and bolted together.

What to do before galvanising and what to tell the plant

Carry out welding, cutting and hole-making (vent holes included) before galvanising: these operations damage a finished coating, and the spot would then need repair. The plant repairs uncoated areas with zinc-rich paint or paste, zinc spraying or a zinc alloy stick, but the standard limits their size: each area up to 10 cm², no more than 0.5% of the article’s surface area in total; otherwise the part is galvanised again unless agreed otherwise. More detail is in the article Zinc Coating Thickness under DSTU EN ISO 1461:2024. Together with the drawing, tell the plant:

• the steel grade and, if you have one, the quality document for the rolled steel: the appearance and thickness of the coating depend on the steel composition;
• whether the part has surfaces left by flame, laser or plasma cutting;
• the significant surfaces, meaning those where the coating matters most for service or appearance: it helps to mark them on the drawing;
• the places where hanging marks, drips and runs of zinc are unacceptable: these are discussed in advance so that the hanging points can be chosen;
• whether the part will be painted after galvanising: the surface requirements are then agreed separately;
• the three overall dimensions, flanges and brackets included: the bath is 13,000 × 1,600 × 3,200 mm (length × width × depth), and longer products are galvanised in sections or, by separate agreement, in two dips; the allowance for hanging is checked against the drawing.