Editorial Feature

Galvanizing - The Effects of Coating Thickness

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There are many types of coatings that are referred to as hot-dip galvanized. The hot-dip galvanization process necessitates the immersion of steel in molten zinc, which reacts with the steel to create a galvanized coating. The time period from when the steel is soaked in the zinc to the post-galvanizing treatment governs features such as thickness and appearance of the coating.

Steel is hot-dip galvanized to improve its anti-corrosion performance, thus guaranteeing that it lasts for as long as possible, with very little maintenance.

Now, the standards being established for the housing sector, have set a benchmark of a minimum of 50 years as the acceptable life of structural building products. This condition can be matched only by hot-dip galvanized steel products, with the thickest galvanized coatings.

The Australian Standard AS 4680-1999, Hot Dipped Galvanized Coatings on Ferrous Articles, comprises galvanized coating standards on tube, sheet, wire, and general products. There exists considerable misinterpretation because of the incorporation of galvanized coatings with prominently distinct coating characteristics within the same Australian Standard.

Coating Thickness Counts

All wire, sheet, and tube products are continually galvanized. This means that the coating is applied at high speed, and the thickness of the coating is controlled by the process. The time for immersion in zinc is usually measured in seconds, but in the batch hot-dip galvanizing process, steel products are immersed for a time period of 3 to 10 minutes based on the mass of the items that are being galvanized.

These completely different methods of applying galvanized coatings produce different types of coatings. When compared to continuously galvanized steel, there are four key differences that have an impact on the anti-corrosion performance of batch galvanized steel—thickness, hardness, integrity, and mass.

Coating Thickness

The thickness of batch-galvanized products with the same section thickness is often at least three times that of similar continuously galvanized coatings on sheet and tube.

Coating Hardness

The thickness of zinc/iron alloy layers in the coatings of batch-galvanized products is greater, thus offering these products five times the abrasion resistance of continuously galvanized coatings.

Coating Integrity

Batch galvanization applies an even, heavy coating to all inner and outer surfaces, edges, and cavities. At cut edges, continuously galvanized coatings will have exposed bare steel. Furthermore, continuously galvanized hollow sections are fully galvanized only on the outer surfaces.

Coating Mass

The cathodic protection of exposed steel using zinc is dependent on the ratio of mass of the zinc and the area of exposed steel. The drainage features of batch galvanized coatings make the coating mass on batch galvanized products significantly higher (often three to five times) in ratio to thickness than continuously galvanized coatings. Hot-rolled medium structural sections typically attain coating mass levels exceeding 1000 g/m2.

More Coating Thickness = Longer Coating Life

About 150 years of field-testing has established that all things are equal; hence, the service life of galvanized coatings is directly proportional to the thickness of the coatings. However, this does not hold good when comparing batch galvanized coatings to continuously galvanized coatings.

The Cut Edge Factor

All continuously galvanized sections consist of exposed steel at cut edges and rely on the zinc in the vicinity of the coating to provide cathodic protection to the exposed steel. This leads to acceleration of the corrosion rate of galvanized coatings at cut edges. When the continuously galvanized section is thicker, the coating corrosion rate at cut edges is faster because of the greater area of exposed steel.

Even if it was feasible to apply a continuously galvanized coating to a steel article with the same thickness as a batch galvanized article, due to the cut edge factor, the batch galvanized coating typically has a 1.5 times greater service life.

Comparison of Galvanized Coatings

Continuously galvanized coatings stick very accurately to their specified coating mass. Batch galvanized coatings on hot-rolled steel sections almost always exceed their minimum specified coating mass.

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