Operations on a Hot-dip Galvanising Line

Order of operations and main parameters

Steel structures for hot-dip galvanising pass through these operations in turn: hanging the parts on cross-beams; degreasing; pickling; rinsing in water; fluxing; drying the flux; hot-dip galvanising; cooling; quality control of the coating; release to the assembly area. The parameters below are typical; the exact values are set by the plant’s process regulations.

Process tooling: cross-beams, hooks, loops

A cross-beam is process tooling for hanging parts. Parts on one cross-beam go through the whole cycle without reloading: hanging, chemical preparation, drying, galvanising, cooling, unhanging. Parts are hung on oval holes in the angles on the underside of the cross-beam; a double-trolley overhead crane moves the cross-beam between operations.

Hooks for hanging parts come in different lengths, diameters and shapes (one-sided, two-sided); the diameter is chosen for the load capacity from a table. Instead of hooks, round steel bar 6 mm in diameter or low-carbon wire 3–4 mm in diameter is used; depending on the mass of the part it is bent into a hook or a loop. Stands 2 m high are used for loading and unloading cross-beams and a hydraulic lift for heavy parts.

The tooling is inspected regularly, with attention to the welds and load-bearing elements (lugs, loops, hooks). If wear exceeds 10%, the tooling is repaired. Cross-beams and hooks are registered in a log with a number; each device has a drawing stating its permitted load, and repairs and tests are entered in the log.

Parts are hung in two ways: vertically, by one end, or horizontally, by both ends.

Degreasing

The surface of articles usually carries grease: mineral oils or silicone lubricants. They spoil the adhesion of the zinc coating and lead to ungalvanised areas or brown stains, so the structures are acid-degreased before galvanising.

The cross-beam with the parts is dipped in the solution: at 24–26 °C degreasing takes 15–20 minutes, at 28–30 °C 10 minutes. The acidity and temperature are monitored, and an automatic system regulates the temperature continuously. Sediment like fine sand collects on the bottom of the tank: it is removed once a year or when it interferes with dipping the articles; the solution is pumped to a free tank, the sediment is removed mechanically and taken away as industrial waste. The cross-beam is then moved to the pickling tank.

Pickling

Oxides, rust and scale, are removed chemically in an aqueous solution of hydrochloric acid. The dwell time depends on the amount of rust and scale and on the temperature and concentration of the solution, but not on the thickness of the article. Pickling continues until the oxides are completely removed; the surface is checked by eye, with the articles lifted out from time to time. It takes 20–90 minutes.

Because of the risk of over-pickling, hydrochloric acid must not be used without inhibitors (acid corrosion retarders); with an inhibitor the pickling quality is good. Foaming agents are added so that the surface of the solution does not give off acid vapour. An automatic system controls the temperature continuously.

Rinsing

After pickling the articles are dipped three times in a tank of water to remove residues of the pickling solution, iron and its compounds. Iron harms the operations that follow: it contaminates the flux solution (which then has to be restored often), leaves “crumbs” on the article and forms hard zinc in the melt. A gram of iron that gets into the zinc forms 40 g of hard zinc, so 20 g of iron is enough to lose a kilogram of zinc as sediment.

The rinse water can be reused in the pickling tank: fresh pickling solution works well only when it contains a little iron. On the chemical preparation line clean water is poured only into the rinsing tanks, which cuts its consumption. The water is analysed daily for hydrochloric acid and iron; when the iron reaches 4 g/l the solution is replaced in full or in part.

Fluxing

Well-pickled and rinsed articles go to the last preparatory operation, fluxing. It ensures the right reaction with molten zinc, removes further salts and oxides not taken off in rinsing, lowers the surface tension of the zinc and improves wetting of the article. Volatile compounds formed as the flux decomposes mechanically clear contamination from the surface of the kettle and create a neutral atmosphere where the article enters.

A film of salts, zinc chloride and ammonium chloride, forms on the article. It prevents the article from oxidising in air between leaving the pickling tank and entering the zinc, and helps regulate the coating thickness (together with the dipping time and melt temperature). Ammonium chloride plays the main part in the flux: on contact with the melt it decomposes and the active chlorides react with the oxides on the kettle surface; most chlorides (of aluminium, tin, antimony) evaporate. Formation of hard zinc is linked to iron chloride, which is why good rinsing matters so much.

To improve quality, additives are put in the flux: the salt crystals come out larger and oriented in one direction, the film becomes denser, the article comes out bright, and the use of zinc and formation of ash fall.

Fluxing takes 30–45 seconds (one dip). The solution temperature is 40 °C with a drying oven and 70 °C without. When the concentration of iron salts exceeds 6 g/l, an automatic flux cleaning system corrects the solution: it lowers the acidity with aqueous ammonia and turns the dissolved iron into insoluble hydroxide with hydrogen peroxide, then filters it out.

Drying

After fluxing the cross-beam with the parts is moved into the drying oven to warm the parts and remove residual moisture. During drying a network of salt crystals forms on the surface and protects against oxidation in air; the temperature difference between the molten zinc and the article falls; and no water gets into the melt.

The drying temperature must not exceed 120 °C, and the time is until the moisture has evaporated completely (10–60 minutes). The cross-beam is then taken to the galvanising furnace.

Hot-dip galvanising

Zinc is a bluish-silvery metal that melts at 419.5 °C. In galvanising in the melt, the steel reacts with the zinc and iron–zinc compounds (phases) form on the surface of the article; the coating consists of several iron–zinc phases directly on the base metal and a layer of zinc on top. The coating thickness depends on the galvanising time, the temperature and composition of the melt and the speed at which the article is withdrawn. The coating is applied to DSTU EN ISO 1461:2024.

The process has these stages: dipping the article in the melt; holding until the flux has boiled off and the temperatures of article and melt have equalised; withdrawing from the melt; cooling.

For a ductile, corrosion-resistant, good-looking coating, and to slow the formation of iron–zinc alloys, the melt is alloyed with aluminium as a eutectic zinc–aluminium alloy in an amount of 0.05–0.2% of the zinc mass; the aluminium content of the melt is up to 0.05%. Aluminium creates a refractory film that cuts the burn-off of zinc from the kettle surface. The alloy should be melted as close to the surface of the melt as possible; the aluminium content is checked daily and recorded in a log.

To make hard zinc settle to the bottom, lead is added to the melt: it dissolves in zinc up to 1–1.3% and the rest settles, forming a layer (about 98% lead and 2% zinc) that stops dross sticking to the bottom. The layer is kept up to 100 mm and checked monthly; lead is loaded by crane as ingots in a basket, spread evenly, and after it is added the kettle is left to settle for 40 minutes.

Temperature regimes

The working melt temperature is about 450 °C; a thermocouple connected to the furnace control system monitors it automatically, and a portable thermocouple backs up the readings. To keep the melt temperature from falling below the process level, no more than 15 t of structures an hour should be immersed. A temperature above the set one shortens the life of the kettle.

Cooling, unhanging and quality control

Galvanised articles are cooled in the cooling tank to lower the temperature and keep the coating bright. The water is still and cooling is one dip; the water temperature must not exceed 80 °C and water is topped up as it evaporates. Cooling in air is allowed.

The cooled cross-beam is taken by overhead crane to the unhanging area and set on stands 2 m high (the height is chosen for the size of the parts). Large welded assemblies weighing more than 60 kg are taken off the cross-beam set on a hydraulic lift; the wire is cut with metal shears. Empty cross-beams go back to the hanging area.

Parts freed from the fixtures are stacked on racks in bundles and the quality control department inspects the zinc coating. Possible galvanising defects and their causes are covered in a separate article.