Control of the growth of zinc-iron phases in the hot-dip galvanizing process

Zinc-iron phases may develop at the steel substrate/zinc coating interface during the hot-dip galvanizing process. These phases are hard and brittle, and make the material unsuitable for the forming process. Growth of the zinc-iron phases could be controlled adding 0.18 to 0.30% of aluminium to the galvanizing bath, which reacts with iron to produce a thin layer of intermetallic Fe2Al5. The latter hinders alloying between the steel sheet and molten zinc, and is therefore referred to as the inhibition layer since it inhibits or retards the formation of Fe-Zn phases. Nevertheless, this layer is unstable and local growth of Fe-Zn phases ('out-burst') is found at longer immersion times. This 'out-burst' phenomenon depends on many factors, such as the chemical compositions of both the bath and the steel, and the immersion time. The aim of the work was to investigate the influence of both immersion time and a small addition of titanium to the galvanizing bath on coating characteristics. Thus, plain carbon steel sheets were galvanized with alloys A and B, which had different chemical compositions. The immersion time was varied between 1 and 120 s. Cross-sections of samples were observed by scanning electron microscopy. The analysis showed that, even for very short immersion times, samples galvanized with alloy A develop 'out-bursts' whereas those with alloy B do not, even for longer immersion times. The Fe2Al5 intermetallic structure was also investigated. It was observed that samples galvanized with alloy B showed, for the same immersion time, bigger grains than the ones galvanized with alloy A. However, for both alloys, the intermetallic development was greater for longer immersion time. Such observations suggest that a small amount of titanium could serve as a catalyst for the iron-aluminium reaction, allowing a greater development of the inhibition layer and delaying growth of the Fe-Zn intermetallic. (C) 1999 Elsevier Science S.A. All rights reserved.