Corrosion Science 87 (2014) 392–396
Contents lists available at ScienceDirect
Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
Investigation of corrosion behavior of aluminum flakes coated
by polymeric nanolayer: Effect of polymer type
Naghmeh Amirshaqaqi a,b, Mehdi Salami-Kalajahi a,b,⇑, Mohammad Mahdavian c
a
Department of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran
Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran
c
Department of Surface Coating and Corrosion, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran
b
a r t i c l e
i n f o
Article history:
Received 22 January 2014
Accepted 14 June 2014
Available online 24 June 2014
Keywords:
A. Aluminum
B. TEM
C. Acid corrosion
C. Alkaline corrosion
C. Polymer coatings
a b s t r a c t
Protection of aluminum pigments from corrosion phenomenon has been extended by an encapsulating
polystyrene (PS) and poly(acrylic acid) (PAA) nanolayers. Flakes were first coupled with 3-methacryloxypropyltrimethoxysilane (MPS) and in situ polymerizations of styrene and acrylic acid, initiating with
Azobisisobutyronitrile (AIBN) were performed. The encapsulated flakes were characterized by Fourier
transform infrared (FTIR), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscope (TEM). Also, polymer chains were analyzed by gel permeation chromatography (GPC). Subsequently the chemical stability of the pigments in alkaline and acidic aqueous media was examined.
Results indicated that polystyrene coating remarkably improved flakes’ anticorrosion property while
PAA evolved hydrogen.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Plate-like aluminum pigments (aluminum flakes) have been
widely applied in the fields of paints, inks and plastic industries
as silver bronze pigments due to their low price, metallic appearance and ‘‘flop-effect’’ [1–4]. However, when they contact with
water, acid and alkali media, they are attacked because of their
high surface area and their corrosion causes evolution of hydrogen
according to the following equations [5–11]:
2Al þ 3H2 O ! Al2 O3 þ 3H2
ð1Þ
2Al þ 6H2 O ! 2AlðOHÞ3 þ 3H2
ð2Þ
2Al þ 6HCl ! 2AlCl3 þ 3H2
ð3Þ
2Al þ 2NaOH þ 6H2 O ! 2NaAlðOHÞ4 þ 3H2
ð4Þ
Abbreviations: PS, polystyrene; PAA, poly(acrylic acid); MPS, 3-methacryloxypropyltrimethoxysilane; AIBN, Azobisisobutyronitrile; FTIR, Fourier transform
infrared; EDX, energy-dispersive X-ray spectroscopy; TEM, transmission electron
microscope; GPC, gel permeation chromatography; DMF, dimethylformamide;
DMP, 2,6-dimethylpyridine; Al, pristine aluminum flake; Al/MPS, MPS-modified Al
flakes; Al/PS, PS-encapsulated Al flakes; Al/PAA, PAA-encapsulated Al flakes.
⇑ Corresponding author at: Department of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran. Tel.: +98 411 345 9097;
fax: +98 411 344 4313.
E-mail address: [email protected] (M. Salami-Kalajahi).
http://dx.doi.org/10.1016/j.corsci.2014.06.045
0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
These results in severe deterioration of metallic luster and
dangerous pressure build up in the storage vessels [1,9]. Therefore,
inhibition of this corrosion reaction is necessary. The methods
adopted for aluminum pigments stabilization in aqueous media
can be divided into two principal categories [1–3]: the adsorption
of corrosion inhibitors on the surface such as phosphoruscontaining compounds, molybdates or vanadates [5], chromium
(VI) complexes or other heavy metal based compounds [6] and
the encapsulation of the pigment by organic or inorganic materials
[7–8,12–15].
Recent investigations reported that the encapsulation method is
more promising since the protective layer can insulate the aluminum pigments from the corrosion medium [3]. This can be done
by organic polymer layers resulted from in situ polymerization
[1,7–8,12–15]. However, there is no report about the effect of
polymer type on corrosion behavior of aluminum flakes.
In this work, aluminum pigments were encapsulated by PS and
PAA nanolayers. At first, aluminum flakes were modified using
MPS; then, in situ polymerization of styrene and acrylic acid were
performed to prepare an organic layer. The structure of the encapsulating nanolayers was characterized by means of FTIR, EDX, TEM
and GPC. Also, the hydrophilicity and hydrophobicity of the flakes
were examined by contact angle test. Furthermore, the stability of
the coated aluminum pigments in acid media of pH 1 and alkaline
media of pH 12 was also measured and effect of polymer type was
investigated.
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N. Amirshaqaqi et al. / Corrosion Science 87 (2014) 392–396
2. Experimental methods
monomer and freely-produced polymer chains. The resulting pigments (Al/PS or Al/PAA) were dried under vacuum at 40 °C for 12 h.
2.1. Materials
Blitz Aluminum paste with average particle size of 12 lm
(Benda Lutz, Batch number 2100552-2) was washed with
dimethylformamide (DMF) several times to remove the organic
compounds and then it was dried under vacuum at 100 °C for
24 h. Styrene (St, Tabriz petrochemical company, Iran) and acrylic
acid (Merck, 99%) was purified by passing through a column
packed with aluminum oxide to remove the polymerization inhibitor by adsorption. Azobisisobutyronitrile (AIBN) as an initiator
was recrystallized from methanol prior to use, 2,6-dimethylpyridine (DMP) as a catalyst, 3-methacryloxypropyltrimetoxysilane
(MPS, Merck, 98%), maleic anhydride (Merck, 99.9%), toluene
(Merck, 99%), acetone (Merck, 99.8%) and ammonia solution
(Merck, 25%) were used directly without further purification.
2.4. Instrumentation
Fourier transform infrared (FTIR) spectra were recorded on a
Bruker Tensor 27 FTIR-spectrophotometer, within a range of
500–4000 cm1 using a resolution of 4 cm1. An average of 24
scans has been carried out for each sample. The samples were prepared on a KBr pellet in vacuum desiccators under a pressure of
0.01 torr. Transmission electron microscope (TEM), Tescan Mira,
with an accelerating voltage of 100 kV was used to study the morphology of the nanocomposites; the samples of 70 nm thickness
Table 1
Results of EDX analysis of different particles.
Samples
2.2. Modification of Al flakes with MPS
Briefly, 3 g Al flakes were firstly dispersed in 100 mL acetone in
a three-necked flask equipped with a magnetic stirrer and a reflux
condenser. Then 3 mL MPS was added followed by 3 drops of DMP.
The mixture was boiled in 56 °C under a nitrogen atmosphere.
After 2 h, 0.03 g maleic anhydride diluted in 0.2 mL distilled water
and 30 mL acetone was complemented into flask and the mixture
was further refluxed in 56 °C under nitrogen atmosphere for 3
additional hours. After the reaction, the resulting product was filtered and washed several times with acetone, and finally placed
in vacuum oven at 40 °C for 12 h to obtain Al/MPS flakes.
Al
Al/MPS
Al/PS
Al/PAA
Element (wt.%)
Al
C
O
Si
90.41
85.62
67.64
70.20
3.68
6.58
26.20
14.94
5.91
6.77
5.35
14.01
–
1.03
0.81
0.85
2.3. In situ polymerization of styrene and acrylic acid
To attach polymeric chains to the surface of MPS-grafted Al
flakes, 1 g of flakes, 22 mL toluene (or 19 mL ethanol) and 22 mL
styrene (or 19 mL acrylic acid) were added into the reaction flask.
After a certain time, when all reactants had dissolved under stirring, AIBN was added to start the polymerization at 60 (or 45) °C.
The mixture was refluxed and stirred for 24 h under a nitrogen
atmosphere. After the reaction, the resulting product filtered and
the residue washed with toluene (or ethanol) to remove unreacted
Fig. 2. TEM image of (a and b) Al/PS flakes and (c) Al/PAA flakes.
Table 2
Average contact angles (over 10 samples) of test liquids on the films and samples (the
standard deviation is shown in parentheses).
Fig. 1. FTIR spectra of Al, Al/MPS, Al/PS and Al/PAA.
Sample
Distilled water
Diiodomethane
Al
Al/MPS
Al/PS
Al/PAA
80.9
72.3
126.4
61.9
11.2
20.6
0.0
24.3
(8.6)
(7.0)
(8.9)
(6.7)
(8.0)
(9.0)
(0.0)
(7.9)
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N. Amirshaqaqi et al. / Corrosion Science 87 (2014) 392–396
Fig. 3. Evolved hydrogen for 0.1 g Al content of the aluminum pigments at (a) NaOH aqueous solution (pH = 12) and (b) HCl aqueous solution (pH = 1).
were prepared by Reichert-ultramicrotome (type OMU 3). Average
molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC) technique. A
Waters 2000 ALLIANCE with a RI detector and a set of three series
columns of pore sizes of 3000, 100, and 50 Å was utilized to determine polymer average molecular weight and polydispersity index
(PDI). THF or DMF was used as the eluent at a flow rate of
1.0 mL/min, and the calibration was carried out using low polydispersity PMMA standards. Energy-dispersive X-ray spectroscopy
(EDX) was carried out on a JSM-6360LV instrument. Hydrogen
volume was used to evaluate chemical stability of samples, and
the evolved hydrogen detection was carried out as described in
the literature [16]. To normalize the evolved hydrogen volume
data, samples were exposed in 600 °C-furnace for 3 h to obtain
ash content of each sample (97.03, 92.23, 71.11 and 82.92 wt.%
for Al, Al/MPS, Al/PS and Al/PAA respectively). Then, we normalized
the data according to ash contents. Then, evolution of hydrogen for
samples was reported per 0.1 g of Al content.
3. Results and discussion
Fig. 1 exhibits the FTIR spectra of Al, Al/MPS, Al/PS and Al/PAA.
Hydroxyl absorption peak at 3430 cm1 in the curve of Al attests to
the existence of AOH groups on the surface of flaky aluminum
[13,17]. Also, the peaks at 500–600 cm1 attribute to the stretching
vibration of AlAO [18]. After modification of flakes with MPS, the
peak at 1038 cm1 attributes to the bending vibration of CAH
[19] and also the peaks at 2850 cm1 and 2930 cm1 attribute to
the stretching vibration of CAH bonds [13]. As shown in Al/PS
spectrum, the C@C stretching bands of aromatic rings appear
between 1600 and 1450 cm1 [20]. However, there is no significant
difference between Al/MPS and Al/PAA samples due to similarity in
functional groups.
In order to confirm the formation of hybrid particle, EDX analysis was employed to investigate relative elemental percentage
of surface. As shown in Table 1, no Si was observed in Al samples
while after modification with MPS, it increased to 1.03 wt.%.
N. Amirshaqaqi et al. / Corrosion Science 87 (2014) 392–396
3.68 wt.% carbon in flaky aluminum powder is due to the existence
of some organic additives on the surface of aluminum during the
manufacturing process. We tried to wash these additives completely but there are some. It is clearly observed that after surface
modification with MPS, the content of carbon increased from 3.68
to 6.58 wt.% and this continued after polymerization to 26.20 and
14.94 wt.% for styrene and acrylic acid respectively. Also, O content
decreased after styrene polymerization but it increased when PAA
was attached on the surface while the content of aluminum
becomes lower. This indicates that polymeric chains have been
grafted onto surface of flakes. Also, to determine the molecular
properties of polymer chains, samples of free chains were analyzed
by GPC test. The molecular weight (Mn) and the polydispersity
index (PDI) of PS chains were obtained about 61,000 and 2.86
respectively while there were obtained about 52,000 and 3.04 for
PAA chains.
Fig. 2 shows the TEM image of encapsulated aluminum flakes
elucidating the surface morphology of Al/PS and Al/PAA composite
particles. Congeries of attached PS are obviously seen in the Fig. 2b.
It can be seen that the polystyrene layer is continuously coated on
the particle surface, and the thickness of the polymeric layer is
estimated to be 15–20 nm. Also, it is clear that a PAA nanolayer
with the thickness of about 20–25 nm is grafted on the surface of
flakes.
To examine the hydrophilicity and hydrophobicity of the flakes,
one polar and one non-polar liquid are used as test liquids. In the
present work we used diiodomethane and distilled water as solvents. The contact angles for Al and Al/PS are 80.9° and 126.4°
respectively; it means that encapsulated flakes are more hydrophobic than bare aluminum flakes. Also aluminum flakes became
more hydrophilic after treatment reaction with MPS (see Table 2).
However, the contact angle between water and Al/PAA decreased
to 61.9° which indicates that the PAA was grafted on the surface
successfully. After modification with MPS which contains a hydrophile carbonyl moiety, if a hydrophilic solvent drop is used, the
contact angle should decrease and if a hydrophobe solvent drop
is used, the contact angle should increase. After polymerization
of hydrophile acrylic acid, again contact angle should be smaller
for hydrophile solvent drop. In the case of diiodomethane, hydrophobe PS layer decreased the contact angle significantly while
reverse trend is obvious for Al/PAA sample (see Table 2).
To assess the corrosion resistance of the pigments, 0.3 g pigments were exposed to 120 mL NaOH aqueous solution of
pH = 12 and to HCl aqueous solution of pH = 1 at 30 °C. Then, all
data were normalized to obtain evolution of hydrogen per 0.1 g
Al as described in Section 2.4. The less hydrogen volume means
the better corrosion resistance [21]. Fig. 3 represents the effect of
encapsulation the aluminum pigments on improving their chemical stability. According to the results, the hydrogen evolution starts
after a certain time from the immersion of the aluminum coupon
in the test solution. It may be believed that this time corresponded
to the period/time needed by the media to destroy the pre-immersion oxide film and is known as incubation period [22]. It can be
seen that the evolved hydrogen volume of Al/MPS particles in
alkali media decreased to 0 from 14.3 mL and it decreased to
65.5 from 104.7 mL in acid media. For polystyrene-coated flakes,
evolved hydrogen volume decreased to 0 mL for encapsulated pigments in both corrosive media. The remarkable improvement in
corrosion resistance also confirmed that polystyrene has been
formed on the surface of aluminum pigments and it has great influence on the stability of pigments in corrosive media and the corrosion resistance of composite particles has been significantly
improved by in situ polymerization. However, there is a significant
evolved hydrogen volume in Al/PAA sample (89.7 mL) while its
corrosion behavior is better than neat Al flakes. This may be attributed to the compatibility of PAA with aqueous acid media. In such
395
media, due to interaction between carboxylic acid groups of PAA
with H+ cations, acid solution can penetrate onto surface easily
and remove protective layer on the surface of flakes.
4. Conclusions
In situ polymerizations of styrene and acrylic acid were used to
encapsulation of the aluminum flakes to improve their corrosion
resistance. Although the FTIR spectra of samples gave informative
data, they were not satisfactory to prove that all steps were done
successfully. Due to this, EDX analyses were used to show the
increase in C and O contents as process proceeds. Also, GPC data
showed PS chains have the Mn and PDI of 61,000 and 2.86, respectively while these values were obtained 52,000 and 3.04 for PAA
chains. TEM image of encapsulated aluminum flakes elucidated a
nanolayer in which the thickness of Polymeric layers was estimated to be 15–20 nm for PS and 20–25 nm for PAA. Evolved
hydrogen volume tests showed that after modification of aluminum pigments, their corrosion resistance was improved and no
hydrogen volume is observed for pigments encapsulated with PS.
However, PAA nanolayer could not improve the corrosion behavior
significantly.
Acknowledgement
We are grateful for support from the Iran National Science
Foundation (INSF) (Grant No. 91002479).
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