Hot Dip Galvanizing

Hot-dip galvanization is the process of coating iron, steel or aluminum with a layer of zinc by immersing the metal in a bath of molten zinc. Galvanizing protects steel by acting as a barrier between steel and the atmosphere.

The durability of hot dip galvanized coatings reduces the overall life-cycle expenses of maintenance and coating re-application in the future.

Steel left unprotected can succumb to corrosion. For sustainability, steel must be coated to improve durability. Zinc and steel have high recycling rate, which factors into LEED ratings, and high reclamation rate.

When steel is hot dip galvanized, the zinc metallurgically bonds to steel, creating a series of zinc-iron alloy layers which protect the underlying steel from corrosion.

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Description

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Preparation

Surface preparation is the most important step in the application of any coating. In most instances where a coating fails before the end of its expected service life, it is because of incorrect or inadequate surface preparation. The galvanizing process has its own built-in means of quality control because zinc will not react with an unclean steel surface. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the zinc bath because the unclean areas will remain un-coated, and immediate corrective action can be taken.

Degreasing/Caustic Cleaning

A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag, which cannot be removed  by degreasing, must be removed before galvanizing by grit-blasting, sand-blasting,  or other mechanical means.

Pickling

A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel.

Fluxing

The final surface preparation step in the galvanizing process, a zinc ammonium chloride solution, serves two purposes. It removes any remaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc.

Galvanizing

During the true galvanizing step of the process, the material is completely immersed in a bath of molten zinc. The bath chemistry requires at least 98% pure zinc maintained at 815-850 F (435-455 C).

While immersed in the kettle, the zinc reacts with the iron in the steel to forma series of metallurgically bonded zinc-iron inter-metallic alloy layers, commonly topped by a layer of impact-resistant pure zinc.

Once the fabricated items’ coating growth is complete, it is withdrawn slowly from the galvanizing bath, and the excess zinc is removed by draining, vibrating, and/or centrifuging.

The metallurgical reaction will continue after the materials are withdrawn from the bath, as long as it remains near bath temperature. Galvanized articles are cooled either by immersion in a passivisation solution or water or by being left in open air.

Inspection

The inspection of hot-dip galvanized steel is simple and quick. The two properties of the hot-dip galvanized coating most closely scrutinized are coating thickness and appearance/surface condition. A variety of simple physical tests can be performed to determine thickness, uniformity, adherence, and appearance.