Hot Dip Galvanizing Services
 

THE HOT DIP GALVANIZING PROCESS

Unlike paint systems, a hot dip galvanized coating is produced by a metallurgical reaction between iron and the coating material, molten zinc. A series of hard abrasion resistant iron/zinc alloys are formed and these are over coated with relatively pure zinc as the product is withdrawn from the galvanizing bath. The various layers all play a significant role in the provision of corrosion protection. For the coating to form, the steel surface must be completely free from all contaminants such as mill scale, rust, grease and oil.

As the name implies, the hot dip galvanizing process requires the product to be dipped into a series of cleaning and pre-treatment chemicals prior to immersion in the molten zinc. The advantage of this method is that all product surfaces are wetted uniformly, including areas which would be inaccessible for cleaning and coating by other methods. Galvanizing entails the following operations:

1. Inspection prior to galvanizing
This important pre-requisite to processing is to ensure that the design of structures, welding and fabricating standards as well as the surface condition of the material, is acceptable for galvanizing to the relevant specification. Slag from stick welding (as opposed to shielded arc welding) is not readily removed by the cleaning chemicals while the presence of some paints (e.g. enamel) are also difficult to remove except by abrasive blasting, grinding or the use of a paint remover. Water based paints or the appropriate marker should always be used for identification or marking during fabrication of products which are to be galvanized. Inspection prior to galvanizing is also necessary to ensure that products can be galvanized without the danger of an explosion. Sealed cavities where vent holes are not provided can result in the formation of superheated steam which may cause injury to personnel and damage to the product.

2. Degreasing (dirt, paint and oil removal)
Components which are appropriately suspended from material handling devices are immersed in a degreasing chemical. This is to ensure that steel surfaces are not masked from the subsequent and most important acid pickling process. If an alkaline caustic soda based degreaser is used, subsequent rinsing in water is essential in order to avoid a neutralizing effect on acid during pickling. Acid degreasers normally contain hydrochloric acid but phosphoric acid based degreasers are also effective.

3. Acid Cleaning (rust and mill-scale removal)
Hydrochloric acid is used to prepare steel for galvanizing. Hydrochloric acid is used unheated at a concentration of about 15%. In conventional plants, material is rinsed in water prior to being transferred to a flux solution. In some plants where hydrochloric acid is used, rinsing is unnecessary. The acid carried over is converted into either ammonium chloride by additions of ammonium hydroxide or zinc chloride by adding zinc dust. Other contaminants are removed thru a filtering process.

4. Flux
After pickling in acid, material is transferred to a flux solution which consists of ammonium chloride and zinc chloride. This solution is normally heated. The flux plays an important role in that it provides barrier protection to prevent flash rusting during the period between acid pickling and immersion in the zinc. It also has a final cleansing effect on steel surfaces as they enter the zinc. The flux also facilitates the formation of a uniform coating which is free from defects.

5. Hot Dip Galvanizing
After fluxing and further drying (ideally in a drying oven to avoid excessive splashing of zinc due to the presence of moisture), the products are dipped into the molten zinc which is heated to a temperature of about 835ºF. The steel is immersed at a fairly rapid speed and once the coating has formed, withdrawal is at a slow speed to ensure uniform drainage and a smooth finish. In most plants, the zinc contains small quantities of aluminum. Apart from enhancing the initial appearance of the coating, the aluminum provides added benefits which include a slight reduction in coating thickness on excessively reactive steels and a lower degree of wasteful zinc oxide formation on the molten zinc surface in the bath.

6. Quality Control and Final Inspection
In order to achieve the required coating standards, routine analysis of the various chemicals used in the process are essential. Failure to do so results not only in substandard coatings but also additional processing costs for the galvanizer. Final inspection of hot dip galvanized coatings is relatively simple. Visual examination ensures that no coating defects are present and the coating is uniform. A suitably calibrated coating thickness measurement instrument is used to ensure that the coating thickness conforms to the requirements of the relevant specification.