Hot-Dip Galvanising of Steel: A Guide

What hot-dip galvanising is and what it gives the steel

A part or a whole structure is lowered into a bath of molten zinc, and the steel comes out wrapped in a zinc layer. The layer guards the metal against corrosion, and its requirements are set by DSTU EN ISO 1461:2024. In the bath the zinc diffuses into the surface layer of the iron, so the coating does not simply sit on the steel but grows into it.

The protection works twice. The zinc layer keeps moisture and oxygen away from the metal, and if the coating is scratched, the zinc corrodes instead of the steel. That is why hot-dip galvanising is chosen for structures that stand outdoors for decades: poles, masts, gantries, fences and frames.

We galvanise in our own bath, 13 000 × 1 600 × 3 200 mm (length × width × depth). There is no minimum order. More about the service is on the hot-dip galvanising page.

How the process goes

Articles are hung on crossbeams and taken through the tanks in turn: degreasing, pickling, rinsing and fluxing. Then come drying, immersion in molten zinc (about 450 °C), cooling, passivation and inspection. The chemical preparation is needed because zinc will not bond to metal that still carries scale, rust or grease.

The requirements for articles and the effect of steel composition are covered in the article on the galvanising process. The settings of every plant operation (times, temperatures, solution composition) are in the article on line operations, and the plant itself is described in the article on plant equipment. Lines and installations are also the subject of the galvanising line page.

How it differs from electro- and cold galvanising

Electro-galvanising deposits zinc by electrolysis, giving a thin, even layer. It is used for small fasteners and parts with close tolerances. Cold galvanising is a paint with a high zinc dust content, applied by brush, roller or spray, and it lasts less than a metallic coating.

Large structures that work outdoors get hot-dip galvanising. Cold galvanising is for repairs: welds and damage after erection. The comparison table and an answer on what to pick in your case are in the article Hot-dip, electro- or cold galvanising.

Coating thickness and service life

The standard ties the minimum coating thickness to the thickness of the steel: the thicker the article, the higher the threshold. Steel composition, mainly silicon and phosphorus, also affects how the layer looks and how thick it grows. The table of thresholds, the measuring routine and the acceptance rules are in the article on coating thickness.

Service life depends on the environment: in the countryside zinc is used up slowly, in an industrial zone or by the sea faster. ISO 9223 sorts atmospheres into corrosivity categories C1–C5 and CX (extreme). An indicative calculation for each category, the duplex system (zinc plus paint) and maintenance tips are gathered in the article on service life.

How to design and prepare the article

Most coating problems are born on the drawing. A closed cavity leaves a surface without zinc, and if there is liquid inside, it boils in the melt and the article may distort or burst. Hollow members therefore need vent and drain holes, and lap joints are vented according to the overlap area.

Also think about internal stresses after welding, differences in thickness within a joint, and lifting points. The rules, with hole sizing and examples, are in the article on designing for hot-dip galvanising. Before sending anything to the plant, go through the preparation checklist: a surface free of paint and slag, continuous welds, one steel grade.

Not sure the structure is ready? Send a drawing or a photo and we will check it for closed cavities before galvanising.

What coating defects there are and where they come from

Grey coating, runs, black spots, blisters, flaking, uncoated areas: each defect has its own cause, and not all of them arise in the bath. Some come from the steel composition and the design of the article, others from surface preparation and the immersion routine.

Defects of the base metal, the design and corrosion in storage are described in the article Defects of hot-dip galvanising. Defects from preparation, the bath and handling after immersion are in the article Zinc coating defects. Small uncoated areas can be repaired, and the repair rules are given in the article on thickness.

What the cost depends on

The first price factor is weight, since galvanising is charged per kilogram. Then come dimensions (the article must fit in the bath), the complexity of the shape and the batch size. Closed cavities, thin-walled members and narrow gaps need extra preparation.

The price per kilogram and per tonne and a worked example for a batch of poles are in the article on the cost of hot-dip galvanising. A quick estimate by weight is available in the calculator, and delivery around Kyiv and the region is covered on the page about price and delivery. Prices on the site are indicative; we name the final sum after reviewing your list of articles.

What to send so we can price the galvanising

For a quote we need:

• a list of articles with dimensions, weight and quantity;
• drawings or photos;
• service conditions: the region, and whether the sea or a road treated with de-icing agents is nearby;
• the delivery address, if delivery is needed.

Leave blank whatever you do not know and we will clarify it. From these data we calculate the cost and name the lead time.