Background There are many types of coatings that are specified as hot dip galvanised. The process involves immersing steel in molten zinc. The zinc reacts with the steel to form the galvanised coatings. The time the steel is immersed in the zinc along with post-galvanizing treatment controls the coating thickness, appearance and other characteristics.
Hot dip galvanised coatings are applied to steel to improve the anti-corrosion performance of the steel to ensure that it lasts as long as possible with a minimum of maintenance.
Standards currently being developed for the housing industry have set a benchmark of at least 50 years as the acceptable life of structural building products. Only hot dip galvanised steel products with the heaviest galvanised coatings are capable of meeting this requirement.
The Australian Standard AS 4680-1999 , Hot Dipped Galvanised Coatings on Ferrous Articles, includes galvanised coating standards on sheet, wire, tube and general articles. A great deal of confusion exists through the inclusion of galvanised coatings with significantly different coating characteristics within the same Australian Standard. Coating Thickness Counts All sheet, wire and many tube products are continuously galvanised. This means that the coating is applied at high speed and the coating thickness is controlled by the process. Immersion time in the zinc is measured in seconds. Alternatively, in the batch hot dip galvanizing process steel items are immersed for periods ranging from 3-10 minutes, depending on the mass of the items being galvanised.
These completely different methods of applying galvanised coatings produce different types of coatings. There are 4 main differences that impact on anti-corrosion performance of batch galvanised steel compared to continuously galvanised steel. These are thickness, hardness, integrity and mass.
Coating ThicknessBatch galvanised items of the same section thickness are typically at least 3 times thicker than similar continuously galvanised coatings on sheet and tube.
Coating HardnessBatch galvanised items have much thicker zinc/iron alloy layers in the coatings which gives batch galvanised items 5 times the abrasion resistance of continuously galvanised coatings.
Coating IntegrityBatch galvanised coatings apply a uniform heavy coating to all internal and external surfaces, edges and cavities. Continuously galvanised coating will always have exposed bare steel at cut edges. Continuously galvanised hollow sections are fully galvanised on the external surfaces only.
Coating MassThe cathodic protection of exposed steel by zinc depends of the mass of the zinc in relation to the area of exposed steel. Because of the drainage characteristics of batch galvanised coatings, the coating mass on batch galvanised products is significantly higher (typically 3-5 times) in proportion to thickness than continuously galvanised coatings. Hot rolled medium structural sections commonly achieve coating mass levels exceeding 1000 g/m2.
More Coating Thickness = Longer Coating Life150 years of field testing has determined that all things being equal, galvanised coating life is equivalent to galvanised coating thickness. When comparing batch galvanised coatings to continuously galvanised coating, all things are not equal.
The Cut Edge FactorAll continuously galvanised sections have exposed steel at cut edges and rely on the adjacent zinc in the coating to provide cathodic protection to the bare steel. This requirement accelerates the rate of corrosion of the galvanised coating at cut edges. The thicker the continuously galvanised section, the faster the rate of coating corrosion at cut edges because of the greater area of bare steel exposed. Even if it was possible to apply a continuously galvanised coating to a steel item to the same thickness as a batch galvanised item, the cut edge factor gives the batch galvanised coating a life typically 1.5 times greater.
Comparison of Galvanised CoatingsContinuously galvanised coatings comply very closely to their specified coating mass. Batch galvanised coatings on hot rolled steel sections almost always exceed their minimum specified coating mass.