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 zin
c-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 interme
tallic Fe2Al5. The latter hinders alloying between the steel sheet and molt
en zinc, and is therefore referred to as the inhibition layer since it inhi
bits 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 immers
ion 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 charac
teristics. Thus, plain carbon steel sheets were galvanized with alloys A an
d B, which had different chemical compositions. The immersion time was vari
ed between 1 and 120 s. Cross-sections of samples were observed by scanning
electron microscopy. The analysis showed that, even for very short immersi
on times, samples galvanized with alloy A develop 'out-bursts' whereas thos
e with alloy B do not, even for longer immersion times. The Fe2Al5 intermet
allic structure was also investigated. It was observed that samples galvani
zed with alloy B showed, for the same immersion time, bigger grains than th
e ones galvanized with alloy A. However, for both alloys, the intermetallic
development was greater for longer immersion time. Such observations sugge
st that a small amount of titanium could serve as a catalyst for the iron-a
luminium reaction, allowing a greater development of the inhibition layer a
nd delaying growth of the Fe-Zn intermetallic. (C) 1999 Elsevier Science S.
A. All rights reserved.