Hot dip galvanizing - Principle

Hot-dip galvanizing in accordance with EN ISO 1461 is a highly effective process for protecting steel against corrosion. It is used in many sectors, mainly construction, but also for steel components exposed to the atmosphere: transmission towers and pylons, street furniture, automotive and industrial parts, etc.

The manufactured part is immersed in liquid zinc, with a zinc content of over 98.50%, at a temperature of between 440 and 460°C (and sometimes, for very limited production, above 530°C). Compared to other solutions for protecting steel against corrosion, the coating layers obtained are significantly thicker. Combined with the cathodic protection provided by zinc to steel, the barrier provided by galvanization is the most effective anti-corrosion process.

To meet the requirements of different applications, it is important to precisely adjust the process parameters.

With regard to steels, we consider non-reactive steels, which form coating layers of moderate thickness, and reactive steels, which form thick layers. In this context, certain elements added to steels, such as Si or P, play a decisive role in the formation of coating layers. These effects are discussed in this article.

Since the intermetallic layers formed during galvanization are relatively fragile, galvanization coating thicknesses should ideally not be too great. Adding certain elements to the zinc bath, such as Al or Ni, can limit the growth of Zn-Fe intermetallics. The compositions of zinc alloys that have been of some importance in industry are described.

The purpose of this article is to emphasize that hot-dip galvanizing is not simply a matter of depositing zinc on steel. Knowledge of the influence of various parameters on the outcome is necessary to avoid or minimize potential defects.