Metallurgical aspects of steel galvanization Jul14

Metallurgical aspects of steel galvanization...

Metallurgical aspects of steel galvanization Steel articles which are not protected on the surface can get serious damages because of various environmental conditions such as rain, snow, wind and extreme temperatures. These adverse environmental conditions convert iron into iron oxide and corrode steel with consequent increase in volume and decrease in strength. To avoid environment conditions acting on the steel surface various protective surface coatings are used. Out of the different types of surface coatings, galvanizing is most popular and reliable. Process of galvanization Galvanization is the process of applying a protective zinc (Zn) coating to steel. It can be done by hot dip, electro chemical and electro deposition processes. Hot dip galvanization process is most popular method presently being used for galvanizing. The process flow of galvanization process is in Fig 1. Fig 1 Process flow of galvanization process The galvanizing process consists of following three basic steps. Surface preparation – The surface preparation consists of three steps namely (i) caustic cleaning (ii) pickling and (iii) fluxing. Fluxing is done with zinc ammonium chloride. Surface preparation is an important step in galvanizing since zinc simply will not metallurgically react with steel surface if it is not perfectly clean. Any failure or inadequacies in surface preparation will immediately show after the steel is withdrawn from the molten zinc bath since the unclean areas will remain uncoated. Galvanizing – After surface preparation steel is immersed in the molten zinc bath where the steel surface is wetted by the molten zinc. The steel temperature in the bath should be in the range of 445 deg C to 465 deg C to have proper coating. The metallurgical reactions that result in the formation and structure of the zinc/iron alloy layers continue after the steel is withdrawn from...