The alloying of zinc with some additional elements in specific quantities could be reliably used in the hot-dip coating process. These elements conferr substantially improved performance to the end-product specially by improving corrosion resistance.
In this paper the influence of adding different percentages of aluminum and bismuth into the zinc bath was researched.
The technological conditions for obtaining zinc-bismuth-aluminium coatings with 0.27% - 0.52% bismuth and 0.11% Al are presented. The degree of microalloying and assimilation in the zinc bath was determined by spectrometric analysis of alloy samples, taken from the melt at different time intervals.
Corrosion resistance of the layers was assessed by two types of tests: corrosion behaviour in seawater for a period of five weeks and electrochemical tests in 3% NaCl solution. Both applied corrosion tests show an increase in corrosion resistance to bismuth content increasing and metallographic analysis of samples corroded shows a general corrosion process

Anișoara CIOCAN

Gina Genoveva ISTRATE

Tamara RADU

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE
N0. 2 – 2014, ISSN 1453 – 083X
CORROSION BEHAVIOR OF ZINC ALLOY LAYERS
Tamara RADU, Anișoara CIOCAN,
Gina Genoveva ISTRATE
“Dunarea de Jos” University of Galati, Romania
e-mail: tradu@ugal.ro
ABSTRACT
The alloying of zinc with some additional elements in specific quantities could
be reliably used in the hot-dip coating process. These elements conferr substantially
improved performance to the end-product specially by improving corrosion
resistance.
In this paper the influence of adding different percentages of aluminum and
bismuth into the zinc bath was researched.
The technological conditions for obtaining zinc-bismuth-aluminium coatings
with 0.27% - 0.52% bismuth and 0.11% Al are presented. The degree of micro-
alloying and assimilation in the zinc bath was determined by spectrometric analysis
of alloy samples, taken from the melt at different time intervals.
Corrosion resistance of the layers was assessed by two types of tests:
corrosion behaviour in seawater for a period of five weeks and electrochemical
tests in 3% NaCl solution. Both applied corrosion tests show an increase in
corrosion resistance to bismuth content increasing and metallographic analysis of
samples corroded shows a general corrosion process.
KEYWORDS: coating, bismuth, aluminium, corrosion
1. Introduction
The improvement of the corrosion resistance is
the main objective in the improvement of the metallic
coatings, being the defining characteristic of the
protective coatings. Rate of corrosion can be modified
by the addition of an alloying element. The sense of
the modification will depend on the proportion of the
alloying element in the base metal, its valence, the
concentration and the nature of its characteristic
lattice defects [1].
The corrosion behavior of the coat also depends
on various factors such as: the coat chemical
composition, thickness, adherence, uniformity,
porosity [2-4].
To increase the corrosion resistance, adherence,
limiting pollution and zinc-iron reaction, it is used
more and more micro alloyed zinc melts with various
elements: aluminum, nickel, titanium, tin, bismuth,
etc. There is a great interest in carrying out research,
which explores the influence of the alloying elements
in zinc melt on coating layers characteristics and the
characteristics of the melt.
In this paper the influence of adding different
percentages of aluminum and bismuth into the zinc
bath was researched.
In the interest of environment and health safety
there is a trend in reducing or even eliminating
altogether lead from zinc baths. In North America
25% of zinc coating plants have zinc baths without
lead [5] and in Western Europe and Canada this
percent is even higher. Lead has an important role in
zinc baths [6, 7] that must be taken over by another
micro-alloying element and the research in this area
suggests the use of bismuth. The bismuth isn’t toxic
[8] like lead. The role of bismuth in zinc
coating/galvanizing baths is still controversial. Most
authors believe that bismuth increases zinc melt
fluidity, reduces surface tension and increases zinc
drainage onto the steel support [9, 10].
It may be added at a concentration of 0.1-1%
below 0.5% most likely [11] bismuth influence on the
protection layer is still questionable. It is believed to
reduce film thickness as an effect of melt fluidity
without changing the typical structure of a zinc
deposition [11].
Aluminium is added to the galvanizing bath, in a
continuous flow of the steel strips/plates, at a rate of
0.1-0.3% (optimal 0.14-0.17% [12]). Aluminium is
added primarily to act as a brake to the reaction
between zinc and steel support. Reducing the iron-
zinc reactions it results a decrease in the brittle
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE
N0. 2 – 2014, ISSN 1453 – 083X
intermetallic compound layer which has a negative
impact on the layer adhesion and plasticity.
Because, compared to zinc, it has a higher
affinity to iron, the aluminium present in the zinc bath
will quickly form the Fe2Al5-XZnX compound on the
steel surface. This ternary compound arranged in a
thin layer onto the steel support will limit the
diffusion processes leading to the formation of Fe-Zn
intermetallic compounds [13]. The aluminium content
in the layer will be much higher than that in the zinc
bath. At 0.14% Al in the bath, the entire zinc coating
(including the ternary alloy layer) contains about
0.20% Al [13].
Also, the aluminium reduces the layer thickness,
increases its brightness and increases the corrosion
resistance. The presence of aluminium in the molten
zinc leads to increased fluidity, oxidation speed
reduction and decrease iron content (and
consequently the amount of dross). Aluminium zinc
alloying to improve zinc-coating layer can reach high
percentages like galfan (5% Al) and galvalumme
(55% Al).
2. Experimental research
For the preparation of the galvanizing bath pure
zinc was used (99.996 wt.%).
Micro alloying with bismuth was made directly,
using metallic pieces, finely crushed, followed by
mechanical mixing. Laboratory experiments at the
micro alloying zinc melts were performed in the
temperature range typical galvanizing processes,
working at 450-460oC. Assimilation process of
bismuth in the melt was stable, maximum efficiency
is obtained. Aluminium alloying was made with Al-
Zn alloy with 5% Al.
The presence of aluminium in the galvanizing
bath, even at low contents (0.11% Al) leads to the
formation of Al2O3 thin films on zinc bath surface
thus preventing the formation of ash that can adhere
to the coating surface causing defects. Also in the
presence of aluminium coatings of very fine dendrites
and metallic brilliancy [14] up to lustre are obtained
(Figure 1).
Zn-Bi Zn –Bi-Al
Fig. 1. Morphology of the surfaces (bath temperature 450 ºC, immersion 7 s) x100
The determination of the chemical composition
of the obtained alloy was done using an X-ray
spectrometer on solidified samples (taken from the
melt after homogenization and at different times).
The chemical composition of the obtained alloys
is presented in Table 1.
The composition of the steel samples support is
presented in Table 2.
Table 1. The chemical composition of the Zn-Bi-Al alloys
Alloy Bi wt.[%] Al wt.% Zn wt [%] Other items wt. [%]
Zn-Bi-Al 1 0.27 0.11 99.35 Bal.
Zn-Bi-Al 2 0.36 0.11 98.75 Bal.
Zn-Bi-Al 3 0.52 0.11 99.02 Bal.
Table 2. The chemical composition of the steel straps, wt. [%]
C Si Mn P S Al As Ti V Cu Ni Cr Mo
0.025 0.015 0.210 0.013 0.010 0.046 0.004 0.002 0.001 0.005 0.008 0.025 0.001
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE
N0. 2 – 2014, ISSN 1453 – 083X
The practical cooling conditions with a cooling
speed much higher than that of equilibrium conditions
[15] together with the low content of bismuth in the
studied alloys lead to a biphasic structure formed
from polygonal grains of zinc and fine crystals of
bismuth uniformly spread in the zinc matrix (Fig. 2a).
At higher bismuth content the proportion of the
crystals dispersed in the zinc mass gets higher, and
also their dimensions are larger, as it can be seen in
Fig. 2b.
a) b)
Fig. 2. Microstructure of zinc micro-alloyed with bismuth a) 0.27% Bi; b) 0.52% Bi [11]
Micro-alloying the zinc with bismuth doesn’t
change significantly the aspect of the microstructure
of the coating layer compared to coating with pure
zinc (Fig. 3 a and b).
Bismuth yet acts on the layer made of iron-zinc
alloys in the sense of thinning it down by the
formation of a mechanical barrier between the
intermetallic compounds phase and the liquid zinc
phase. For higher bismuth content a small decrease in
iron-zinc reactions is observed, the layer of
intermetallic compounds from the steel-coating
interface is thinner compared to its thickness when it
is coated with pure zinc (Fig. 3b).
Having a higher affinity for iron than for zinc,
aluminum will form at the surface of the steel support
a thin microscopic film, out of Fe2Al5 compounds.
This layer, dense and compact, exerts a strong
braked effect on the growth of the iron-zinc
intermetallic compounds and microstructure of the
coating layer shows a fine and uniform layer of
intermetallic compounds at the interface steel/coating
(Fig. 3c).
a) Pure Zn b) Zn - 0.36%Bi c) Zn - 0.52%Bi – 0.1%Al
Fig. 3. Microstructure of the layers, (bath temperature 450 ºC, immersion 9s) x100
Regarding the thickness of the layer (as shown
in Fig. 3) bismuth and especially aluminum decrease
the coating thickness. This effect is given by the
increase in fluidity of the melt when bismuth and
aluminum are added.
Corrosion resistance of the layers was assessed
by two tests, gravimetric indices for corrosion
behaviour in seawater for a period of five weeks and
electrochemical tests in 3.5 wt% NaCl solution.
To determine the corrosion rate in sea water
samples were sampled from every type of coating
applied on the steel strip. These samples were
immersed individually in recipients containing
solution that imitates sea water: NaCl – 27 g/L;
MgCl2 – 6 g/L; CaCl2 – 1 g/L; KCl- 1g/L; pH = 6.5-
7.2, at room temperature.
The following steps have been covered: surface
degreasing, weighing, immersing into sea-water
containers, with a similar surface exposure to the
corrosive agent. After each week the samples were
washed, dried and weighed to determine the mass
variation.
50µm 50µm
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE
N0. 2 – 2014, ISSN 1453 – 083X
For the calculation of corrosion rate the
following relation was used:
][ 12  


 hmg
tS
mVcorr (1)
where:
Δm – the weight loss by corrosion, [g];
S – the surface area of the corroded surface,
[m2];
t – the corrosion time, [h].
Results are presented in Figure 4. After seven
days of exposure to corrosive environment the loss in
weight of the samples was comparable.
0 1 2 3 4 5 6
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
C
or
ro
si
on
 ra
te
, [
g/
m
2 h
]
T im e , [w eeks]
 Zn -B i-A l1
 Zn-B i-A l2
 Zn-B i-A l3
Fig. 4. The variation of the corrosion rate as a function of the exposure time
to the corrosive environment
After 14 days, the corrosion speed is modified
according to the accurrence of the corrosion products
and to their intrinsic characteristics.
Thus the Zn-Bi-Al 1 has, in this case, the highest
corrosion rate (0.0334 g/m2·h) and the best behavior
is observed at the sample coated with zinc micro-
alloyed with 0.36% Bi – 0.11% Al (0.0286 g/m2·h).
In the third week the corrosion rate significantly
decreases for all samples and decreases continuously
for the next two weeks. Generally, the corrosion
resistance increased with bismuth content.
The microscopic analysis of the surface shows
the evolution of the corrosion process in time until the
generalized corrosion of the surface at the end of the
testing period as it can be seen in Fig. 5.
Week one Week two Week five
Fig. 5. The modification of the surface during the corrosion process for sample Zn-Bi-Al 1, x100
For the electrochemical tests, the corrosive
environment used was a NaCl solution 30 g/L. A
three-electrode cell, consisting of working electrode
(Zn-Bi-Al coating samples) with an exposed area of 2
cm2, an Ag/AgCl electrode as reference electrode and
a platinum plate as a counter electrode, were used.
Potentiodynamic polarization curves were recorded in
the potential range from -1000 mV to -100 mV, at the
scan rate of 2 mV/s. All measurements were
performed at room temperature.
Before experiments samples were cleaned with
water and acetone. Fig. 6 shows the Tafel curves.
These curves were processed and the results shown in
Tab. 4.
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE
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Table 4. Electrochemical corrosion result by Tafel Method
Coating icor[μA/cm2] Rp[Ω cm2] vcor [mm/y]
Zn-Bi-Al 1 19.6300 673.46 0.208
Zn-Bi-Al 2 15.0741 918.68 0.181
Zn-Bi-Al 3 13.9972 889.23 0.166
Fig. 6. a) Tafel curve for Zn-Bi-0.36%Bi coating b) Tafel curve for Zn-Bi-0.11%Al coating
Electrochemical test shows the same trend of
decreasing corrosion rate when the bismuth content
increases.
Increased corrosion resistance can be caused by
the cumulative effects of bismuth and aluminum to
the zinc melt characteristics and their products
corrosion.
3. Conclusions
- Bismuth has a melting temperature lower than
zinc and it is easily assimilated in galvanizing bath;
- Microstructure of alloy samples, taken from
the galvanizing bath, reveals a homogeneous structure
with polygonal grains of zinc and fine crystals of
bismuth, uniformly spread in the zinc matrix;
- Layers microstructure shows a decrease in
thickness in the presence of bismuth and aluminum
especially;
- In the presence of bismuth and aluminum
reactions between iron and zinc are strongly braked;
- The corrosion rate in seawater for a period of
five weeks shows a similar variation for the three
types of layers studied;
- The most significant corrosion rate is achieved
for all the samples in the second week and then
decreases continuously in the next weeks;
- Metallographic analysis of samples corroded
shows a general corrosion process;
- Both applied corrosion tests show an increase
in corrosion resistance to bismuth content, increasing
from 0.2% to 0.52%.
References
[1]. F. C. Porter - Zinc handbook: properties, processing, and use
in design, Marcel Dekker, New York, 1991.
[2]. Khadoudj Guessoum, Delphine Veys-Renaux, Emmanuel
Rocca, Kamel Belhamel - Corrosion behaviour of zinc–cerium
alloys: Role of intermetallic phases, Corrosion Science, vol. 53,
Issue 5, May 2011, p. 1639-1645.
[3]. Lijing Yang, Yangming Zhang, Xuduo Zeng, Zhenlun Song
- Corrosion behaviour of superplastic Zn–Al alloys in simulated
acid rain, Corrosion Science, vol. 59, June 2012, p. 229-237.
[4]. Shengqiang Ma, Jiandong Xing, Dawei Yi, Hanguang Fu,
Guofeng Liu, Shengchao Ma - Microstructure and corrosion
behavior of cast Fe–B alloys dipped into liquid zinc bath, Materials
Characterization, vol. 61, Issue 9, September 2010, p. 866-872.
[5]. John Zervoudis, Graeme Anderson - A Review of Bath Alloy
Additives and their Impact on the Quality of the Galvanized
Coating, Teck Cominco Metals Ltd. 120 est, Suite 1500 Toronto,
Ontario, Canada, p. 4.
[6]. *** - Galvanising Reactive Steels, a guide for galvanizers and
specifies, International Lead Zinc Research Organisation, Research
Triangle Park, NC.
[7]. Xinhua Wang, Jintang Lu, Chunshan Che, Gang Kong -
The behavior of lead during the solidification of Zn–0.1Al–0.1Pb
coating on batch hot-dipped steel, Applied Surface Science, vol.
254, Issue 8, 15 February 2008, p. 2466-2471.
[8]. Gagne M. - Zinc Bismuth Alloys for After Fabrication Hot Dip
Galvanizing, American Galvanizers Association Tech Forum 1997,
Houston, TX.
[9]. J. Pedersen - Latest developments in nickel-zinc galvanizing,
Cominco Ltd Ontario, Canada, on line.
[10]. Pistofidis N., Vourlias G., Pavlidou E., Stergioudis G. -
Effect of Ti, Ni and Bi addition to the corrosion resistance of Zn
hot-dip galvanized coatings, Journal of optoelectronics and
advanced materials 2007, vol. 9, no. 6, p. 1653.
[11]. Tamara Radu, Florentina Potecasu, Maria Vlad -
Research on obtaining and characterization of zinc micro-alloyed
with bismuth coatings, Metalurgia International, nr. 1/2011, ISSN
1582-2214, p. 44-48.
[12]. *** - The Role of Aluminium in Continuous Hot-Dip
Galvanizing, GalvInfoNote 1.9, Rev 1.0, mar. 2009.
[13]. *** - Zinc Bath Management on Continuous Hot-Dip
Galvanizing, GalvInfoNote 2.4.1 Rev 1.0 Aug-09.
[14]. S. Sepper, P. Peetsalu, M. Saarna - Methods for evaluating
the appearance of hot dip galvanized coatings, Agronomy
Research Bio system Engineering Special Issue 1, 2011, p. 229-
236.
[15]. *** - ASM Metals Hand Book Alloy Phase Diagrams.
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Corrosion Behavior of Zinc Alloy Layers

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Corrosion Behavior of Zinc Alloy Layers

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