synergistic corrosion resistance of cerium film and silane film on hot-dip galvanized steel
TRANSCRIPT
Synergistic Corrosion Resistance of Cerium Film and Silane Film on
Hot-dip Galvanized Steel
WU Haijianga, YANG Feiyingb
School of Electromechanical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Keywords: Galvanized steel, Cerium conversion film, Silane film, Corrosion resistance
Abstract. This work aims at developing a new environmental-friendly treatment for hot-dip
galvanized (HDG) steel as alternative to the classic systems based on chromates. A double layer film
on the HDG steel sheets was prepared by immersing the sheets in 5 g/L Ce(NO3)3 aqueous solution
and 5 vol.% silane solution in turn. The morphology of the cerium conversion film was analyzed
using scanning electron microscopy (SEM). The corrosion resistance of the films was investigated by
linear polarization (LPR) and natural salt spray (NSS) tests. The results show that the surface
morphology of cerium conversion film appears on a “dry-mud” structure, which is favorable to
enhance the combined strength between the cerium conversion and the silane film. The corrosion
protection efficiency of the double layer films increases greatly, especially both the anodic and
cathodic processes of zinc corrosion on the samples are suppressed conspicuously, and the synergistic
protection effect of the single cerium film and the single silane film is evident.
Introduction
Hot dip galvanizing is an effective protection measure for steel against atmospheric corrosion. And it
is widely used in the constructions, communication, appliance and automotive industries. But it is
necessary to provide the system with an additional corrosion resistance because galvanized steel is
very prone to corrosion, specially “white rust” that appears during storage and transportation under
very aggressive conditions[1]
. For a long time, chromate compounds Cr(VI) have been used as
effective and inexpensive corrosion inhibitors for zinc and been widely acknowledged. However, due
to the Cr(VI) toxicity[2]
, the researchers all over the world are searching for environmentally friendly
alternative treatments.
The majority of the researchers are already working with new physical or chemical processes to
produce uniform, pore-free, mechanically suitable and corrosion protecting thin layers, including
molybdate[3,4]
, silicate[5]
, rare earth salts[6~10]
and kinds of organic compounds[11~19]
treatments. These
inhibitors or passivators can protect the zinc coating from corrosion to a certain degree. Besides, it is
very promising for the applications of rare earth salts and silanes[13~19]
.
However, compared with the traditional chromate passivation process, the single film obtained by
the above treatment process is always unpleasurable. The objective of this work is to develop a new
two-step treatment processes for hot-dip galvanized steel using a combination of Ce(NO3)3 and
vinyltrimethoxysilane, aimed at improving corrosion resistance and adhesion during exposure to
sodium chloride aqueous solution and the salt spray test, respectively. The surface morphology of the
films was observed by scanning electron microscopy.
1 Experimental
Cold-rolled steel sheets Q235 of 40 mm×30 mm×2 mm were used as the substrate material, the
chemical composition is listed in Table 1. The samples were degreased, pickled, fluxed in a mixed
solution with 30 g/L NH4Cl and 20 g/L ZnCl2 at 60°C, dried and dipped in a molten zinc bath at
450°C for 1 min, then withdrawn slowly and quenched in water immediately. The thickness of the
Applied Mechanics and Materials Vols. 34-35 (2010) pp 2021-2025Online available since 2010/Oct/25 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.34-35.2021
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galvanized layer measured by STH-1 pachometer was about 50µm that was composed of 30µm Fe-Zn
alloy layer and 20µm free zinc layer.
Table 1 Chemical composition of Q235 steel (wt.%)
Elements C Si Mn P S
Content 0.13 0.02 0.38 0.014 0.031
A solution of Ce(NO3)3 was prepared in a salt concentration of 5 g/L. A 5% vinyltrimethoxysilane
(H2C=CH-Si(OCH3)3) solution was prepared by adding the silane into a mixture of distilled water and
ethanol. The ratio of silane/ethanol/distilled water was 5/5/90 (v/v/v). The solution was stirred for 1~2
hours to ensure that the silane was sufficiently hydrolyzed and silanols groups were obtained for the
subsequent condensation reactions. The pH value of the solution was modified to 4.1 by adding
1mol/L acetic acid or 1mol/L NH3·H2O to enhance the solution stability. All reagents are reagent
grade quality.
Hot-dip galvanized steel sheets sample without any post treatment, denoted as HDG. Besides, the
substrates were treated at ambient temperature according to the procedures described below: (1) HDG
was immersed in the Ce(NO3)3 solution for 5 min, and then dried in air, denoted as Cer. (2) HDG was
immersed in the prepared silane solution for 2 min, and then dried in air, denoted as Sil. (3) Cer was
immersed in the prepared silane solution for 2 min, and then dried in air, denoted as Cer+sil. This
procedure is referred to as the two-step treatment.
The Neutral salt spray (NSS) test was undertaken in a salt spray test chamber (model YL-40C) in
accordance with ASTM B117-85 (Standard Method of Salt Spray (Fog) Testing). The corroded
solution was 5% sodium chloride solution with pH 6.5-7.0 at (35±2) °C. The samples were placed
perpendicularly with an angle of 30°. The salt spray was kept continuously for 2, 4, 6 and 8 hours
respectively. According to the white rust area (the average values of three duplicate samples)
generated on the samples, the corrosion protection effect of various films was evaluated.
The potentiodynamic linear polarization tests were carried out using a CHI750A electrochemical
measurement station produced by CH Instruments, Inc. A three-electrode electrochemical cell
arrangement was used to evaluate the corrosion rate of the samples, consisting on the working
electrode (1 cm2 of exposed area), saturated calomel electrode (SCE) as reference electrode, and a Pt
foil as counter electrode. The corrosive media was 5% (mass fraction) NaCl solution at ambient
temperature, prepared from reagent grade NaCl and distilled water. The scan rate was 1 mV/s. The
acquired measurement data were analyzed by using the software packages of CHI750A.
Scanning electron microscopy (SEM) was used to observe the microstructure and surface
morphology of the samples. The SEM analyses were performed with a XL-30FEG scanning electron
microscope (Philips Co. Ltd., The Netherlands).
2 Results
2.1 Linear polarization results
The potentiodynamic polarization curves of the various samples in 5% NaCl aqueous solution are
shown in Fig. 1, and the corresponding electrochemical parameters obtained are represented in
Table 2.
Fig. 1 shows that for the pre-treated samples the cathodic branch and the anodic branch of their
polarization curves displaced towards lower current density, i.e., both the anodic processes and the
cathodic processes were suppressed. The current density decreases following the order: HDG>Cer
only>Sil only>Cer+sil. Therefore the Cer+sil film obtained by the two-step pre-treating provided the
strongest reduction of current density.
It is seen in Table 2, the open corrosion potentials (Ecorr) of the sample Cer, Sil, Cer+sil and HDG
were approximately -993mV, -991mV, -994mV and -1002mV respectively. It is obvious that the
pre-treatment made the Ecorr displace toward the positive direction slightly, however, the corrosion
2022 Mechanical Engineering and Green Manufacturing
current density (Icorr) varied significantly for the different sample. The Icorr of Cer+sil was
0.655µA•cm-2
, which was much lower than 4.32µA•cm-2
of Cer, 2.94µA•cm-2
of Sil and 11.9µA•cm-2
of HDG. Moreover, the corrosion resistance of the whole system can be evaluated in terms of the
polarization resistance Rp. As seen from Table 2, all the treatments supply effective protection and the
Rp of the treated samples was at least multiple of that of the HDG sample, the Rp of Cer+sil was
increased 47 times in comparison with that of HDG. Comparing the Icorr and Rp of all samples, it
seems that the effect of the two-step treatment is not simply the sum of the effect of two single
treatments, the two-step treatment brought a synergetic effect, by which the corrosion resistance of
galvanized steel is improved greatly. The two-step treatment is much more effective than the single
treatments in enhancing the corrosion resistance for HDG steel.
-8 -7 -6 -5 -4 -3 -2 -1-1.3
-1.2
-1.1
-1.0
-0.9
-0.8anodic branch
cathodic
branch
4
3
2
1
E/V
(SC
E)
log i/A·cm-2
1:HDG
2:Cer
3:Sil
4:Cer+sil
1
234
Fig. 1 Polarization curves of various samples immersed in 5% NaCl solution
Table 2 Electrochemical parameters of various samples
Samples Ecorr (mV vs. SCE) icorr (µA•cm-2
) Rp (kΩ•cm2)
HDG -1002 11.9 0.702
Cer -993 4.32 3.77
Sil -991 2.94 5.97
Cer+sil -994 0.655 33.4
2.2 NSS results
The results of NSS are summarized as follows (shown in Fig. 2). After spraying for 2 h, the corroded
area of HDG sample was up to 75%, in contrast with that, the surfaces of Cer, Sil and Cer+sil samples
were not corroded. Moreover, after spraying for 8h, the HDG sample was corroded seriously and
entirely, meanwhile the white rust areas generated on the surfaces of Cer, Sil and Cer+sil samples
were 40%, 43%, 3% respectively. Hence three kinds of different films (Cer, Sil and Cer+sil) could
delay the formation and growth of the white rust, especially for the Cer+sil layer, the formation of the
white rust was remarkably delayed.
2 3 4 5 6 7 8
0
20
40
60
80
100
Co
rro
sion
are
a/%
Corrosion time/h
HDG
Cer
Sil
Cer+sil
Fig. 2 Results of NSS test for various samples
Applied Mechanics and Materials Vols. 34-35 2023
2.3 Appearance of the cerium conversion film
The surface morphology of Cer can be observed in the SEM micrograph given in Fig. 3. The
cerium-based conversion layer is characterized by a “dry-mud” structure, which is typical for thick
conversion film and is a result of the stresses induced in the film during the drying process. This kind
of “dry-mud” morphology is favorable to enhance the combined strength between the cerium
conversion and the silane film.
Fig. 3 SEM micrographs of cerium conversion coating on HDG steel
3 Discussion
Immersion of the hot-dip galvanized steel test panels in the cerium nitrate solution produced an
immediate, spontaneous reaction on the sample surfaces. The substrate underwent a noticeable color
change to yellow-gold as the coating time increased up to approximately 5 min. No noticeable color
changes occurred for immersion times of more than 5 min, when the test coupons were removed from
the coating solution, or during storage. In anodic area, zinc has a high solubility and dissolves with
formation of Zn2+
ions, while the local pH increase at the zinc surface proximity leads to cerium
hydroxide and zinc hydroxide precipitation on the zinc surface due to the oxygen reduction reaction in
cathodic area. Ce(OH)3 and Zn(OH)2 may further be changed to Ce2O3 and ZnO. When the Ce(III)
film becomes reasonably thick, Ce(III) can be oxidized to Ce(IV) at the surface, which makes
enrichment in Ce(IV)[7]
. At last a cerium conversion film, which is mainly composed of Ce(III),
Ce(IV) and Zn oxide/hydroxide, is formed on the surface of hot-dip galvanized steel[7~10]
. Especially
the surface morphology of the cerium conversion film appears on a “dry-mud” structure.
Alkoxy groups of the silane molecules tested in this work may convert to hydrophilic silanols
(Si-OH). The stability of the silane layer depends on the formation of Si-O-Metal bonds, whose
stability is probably affected by the condition of the surface metal oxide/hydroxide. Immersing Cer in
the prepared silane solution, Si-OH groups in the solution will be adsorbed spontaneously onto the
surface of cerium conversion film through hydrogen bonds between Si-OH group and surface metal
oxide/hydroxide[17]
and subsequently react with cerium and zinc oxide/hydroxide in cerium
conversion film, leading to the formation of a covalent metallo-siloxane bond (Si-O-Zn or probably
Si-O-Ce type bond). Subsequently on the interaction between silanol groups of adjacent molecules to
form siloxane (Si-O-Si) reticulation that mainly forms the more external part of the whole film on the
substrates[13~19]
. Thus it is expected that the films have double layers structure which is probably
consisted of the inner layer mixed with cerium and zinc oxide/hydroxide mainly and the outer silane
layer, while owing to the reactivity of silanol groups towards a “dry-mud” structural basic
oxide/hydroxide, the silane film should be strongly bounded to the substrate. In the duration of
corrosion, the persistent protective effect of the substrate may be provided by the physical barrier
action of the silanic coating and the cerium conversion film. Besides, it seems to bring out some
degree of the synergetic effect between these two layers, which results in the excellent corrosion
resistance of the whole layers.
2024 Mechanical Engineering and Green Manufacturing
4 Conclusions
A double layer film probably consisted of the inner cerium conversion film and the outer silane layer
was obtained on the surface HDG steel by two-step treatment.
Linear polarization and NSS tests show that the double layer film could simultaneously restrain the
anodic and the cathodic process of the corrosion reaction, and the corrosion protection efficiency
increases greatly. Besides, the synergistic corrosion protection effect of the single cerium film and the
single silane film is exerted.
SEM results reveal that the surface morphology of cerium conversion film appears on a “dry-mud”
structure, which is favorable to enhance the combined strength between the cerium conversion and
the silane film.
Acknowledgment
This work is A Project Supported by Scientific Research Fund of Hunan Provincial Education
Department under the grant number 08C330 and Doctor Start-up Research Fund of Hunan University
of Science and Technology under the grant number E50837.
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Applied Mechanics and Materials Vols. 34-35 2025
Mechanical Engineering and Green Manufacturing 10.4028/www.scientific.net/AMM.34-35 Synergistic Corrosion Resistance of Cerium Film and Silane Film on Hot-Dip Galvanized Steel 10.4028/www.scientific.net/AMM.34-35.2021
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