Hot-dip galvanization

Galvanized fumes are released when the galvanized metal reaches a certain temperature. This temperature varies by the galvanization process used. In long-term, continuous exposure, the recommended maximum temperature for hot-dip galvanized steel is 200 °C (392 °F), according to the American Galvanizers Association. The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter-metallic layer.

Process

The process of hot-dip galvanizing results in a metallurgical bond between zinc and steel, with a series of distinct iron-zinc alloys. The resulting coated steel can be used in much the same way as uncoated.

A typical hot-dip galvanizing line operates as follows:

• Steel is cleaned using a caustic solution. This removes oil, grease, dirt, and paint.
• The caustic cleaning solution is rinsed off.
• The steel is pickled in an acidic solution to remove mill scale.
• The pickling solution is rinsed off.
• A flux, often zinc ammonium chloride is applied to the steel to inhibit oxidation of the cleaned surface upon exposure to air. The flux is allowed to dry on the steel and aids in the process of the liquid zinc wetting and adhering to the steel.
• The steel is dipped into the molten zinc bath and held there until the temperature of the steel equals that of the bath.
• The steel is cooled in a quench tank to reduce its temperature and inhibit undesirable reactions of the newly formed coating with the atmosphere.

Lead is often added to the molten zinc bath to improve the fluidity of the bath (thus limiting excess zinc on the dipped product by improved drainage properties), help prevent floating dross, make dross recycling easier and protect the kettle from uneven heat distribution from the burners. Environmental regulations in the United States disapprove of lead in the kettle bath. Lead is either added to primary Z1 grade zinc or already contained in used secondary zinc. A third, declining method is to use low Z5 grade zinc.

Steel strip can be hot-dip galvanized in a continuous line. Hot-dip galvanized steel strip (also sometimes loosely referred to as galvanized iron) is extensively used for applications requiring the strength of steel combined with the resistance to corrosion of zinc, such as roofing and walling, safety barriers, handrails, consumer appliances and automotive body parts. One common use is in metal pails. Galvanised steel is also used in most heating and cooling duct systems in buildings

Individual metal articles, such as steel girders or wrought iron gates, can be hot-dip galvanized by a process called batch galvanizing. Other modern techniques have largely replaced hot-dip for these sorts of roles. This includes electrogalvanizing, which deposits the layer of zinc from an aqueous electrolyte by electroplating, forming a thinner and much stronger bond.

In some cases, it may be desirable to have designated parts of the metal as non-galvanized. This is often desired when metal will be welded after galvanization. To accomplish this, a galvanizer will typically use a masking compound to coat the areas that will not be galvanized during the hot dip process.