Advanced Materials Research
ISSN: 1662-8985, Vols. 415-417, pp 1806-1809
doi:10.4028/www.scientific.net/AMR.415-417.1806
© 2012 Trans Tech Publications, Switzerland
Online: 2011-12-06
New Synthesis Method of the Lamellar Zinc Phosphate and Its
Electrochemical Corrosion Inhibitions
Dong-ping Wei1,a，Sheng-fu Wu2,b, Zeng-wei Huang 1,c，Shao-mei Ma1,d,
An-ping Liao1,e and Ai-qun Yuan1,f*
1.
College of Chemistry and Ecological Engineering, Guangxi University for Nationalities; Key
Laboratory of New Technology of Chemical and Biological Transformation Processes, Nanning,
530006, Guangxi, China
2.
College of Pre-education, Guangxi University for Nationalities, Nanning, 530006, Guangxi, China
a
[email protected],
d
[email protected],
b
[email protected], c [email protected],
e
[email protected], f [email protected]
Key word: Zinc phosphate, Corrosion inhibition, Anticorrosive pigment
Abstract. A new synthesis method namely hydrolysis precipitation was used to prepare
Zn3(PO4)2·4H2O. Structural characteristics of products were investigated by X-ray Diffraction,
scanning electron microscope and chemical analysis. The electrochemical corrosion inhibitions of
title zinc phosphate were studied by electrochemical impedance of coating immersion test. The
results show that the obtained product is a highly crystalline, micronized and lamellar
Zn3(PO4)2·4H2O. Comparing with commercial zinc phosphate, the synthesized lamellar
microcrystalline product has excellent anticorrosive property and dispersibility.
Zinc phosphate is widely used as an environmental friendly anticorrosive pigment to replace the
toxic red lead or zinc chromate due to its excellent properties such as low toxicity, poor solubility,
chemical stability as well as adhesion and impact resistance in coatings. Besides, it is also a
multi-fundamental material which can be used in surface treatment for metal, biomedical cements
and phosphor material. More and more studies found [1, 2] both the particle size and shape of zinc
phosphate are associated with its preparation approach and further affect its anticorrosive property.
The micronized zinc phosphate showed better anticorrosive performance than that of zinc chromate.
The smaller particle gives higher anticorrosive efficiency comparing with the larger one.
Micronization of the pigment particle may be an effective way to improve pigment qualities and
enhancing pigment performance.
So far, it is found that zinc phosphate has several hydrates and their general formula can be
expressed as Zn3(PO4)2·nH2O (n=0,1,2,4,8). Usually, these zinc phosphate hydrates can be prepared
by solid-liquid, solid-solid or liquid-liquid phase reaction method. Unfortunately, these methods still
lack of some shortages for example, zinc oxide used in solid-liquid method must be a high purity
raw material while purities or crystallinity of the product obtained by solid-solid or liquid phase
reaction are usually low. The ZnO-P2O5-H2O system supported the presence of the brick zinc
phosphate with a particle size about 45um, and the amorphous phase or different shape of crystal
may be obtained from liquid-liquid phase reaction [3]. Many attempts have been made to modify
the preparation or particle size of zinc phosphate, one of which is to change solid-liquid reaction to
liquid-liquid phase by adding other raw material. Zhou [4] prepared nano-zinc phosphate by firstly
adding acetic acid to zinc oxide and then adding phosphoric acid in the submerged circulating
impinging stream reactor. Our previous study [5] revealed that the method adding acetic acid or
ammonia to zinc oxide and then adding phosphoric acid really assist to decrease the particle size of
the product and improve the conversion rate of zinc oxide. However, the technological processes of
all the improving method are excessively complex and long.
In this paper, we propose a new preparation method of lamer Zn3(PO4)2·4H2O , which involve
hydrolysis precipitation of the aqueous solution between the reaction ZnO and H3PO4. This method
is characterized by high conversion rate of ZnO, less reaction time and high purity single phase
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Advanced Materials Research Vols. 415-417
1807
Zn3(PO4)2·4H2O. In addition, cheap low grade commercial ZnO can be used as a raw material
comparing with the solid-liquid reaction in ZnO-P2O5-H2O system. Then its anticorrosive
performance has been investigated by electrochemistry method [6,7] compared with the commercial
product.
Experiment
1.1 Synthesis
Zinc oxide was added to water at room temperature, and then was added to 85% (m/m)
stoichiometric amounts of orthophosphoric acid under stirring 20 minutes at 70℃. The above
solution was cooled and rapidly dumped into the water. The white precipitate was obtained and
washed to neutral and dried at 80℃ for 4 hours. The involved chemical reaction equations are
expressed as:
ZnO + 2H3PO4 →Zn(H2PO4)2 +H2O
3Zn (H2PO4)2 + 4H2O → Zn3(PO4)2·4H2O +4PO43-+12H+
1.2 Characterizations
X-ray powder diffraction (XRD) was carried out by an automated D/max-2500V diffractometer
(Japan) with Cu-Kαradiation (λ=1.54056Å) at 40kv/200mA. The diffraction patterns were
measured in the range of 0º<2θ<50º. X-650 scanning Electronic Microscope (Japan) and GENESIS
Energy Dispersive Spectrometer (EDAX Company, USA) were applied to give particle shape and
composition. Chemical analysis was determined by the common method used in pigment industry.
1.3 Coating preparation
Phenolic resin or alkyd resin, phosphate, talcum powder mixed together according to the quality of
100:15:10, ground 30 min and joined turpentine to adjust viscosity. The prepared anticorrosion
coating was besmeared brush in the tinplate specimens with coating thickness (45±10 um), dried
after sealing side, bathed in the pH=7 3.5% NaCl solution. Electrochemical parameters were
determined and surface corrosion of the coating was taken down in a different immerse time.
1.4Electrochemical technique
Measurements of the electrochemical impedance were made using CHI-660C (Chen-Hua
instrument Co. Ltd., Shanghai). The three-electrode cell consisted of painted tinplate specimens as
working electrode, a platinum counter electrode and a Saturated Calomel Electrode (SCE) as
reference.
2. Results and discussion
2.1 Chemical analysis results
The chemical analysis results of title compound were shown in Table1. The elemental analysis
results of product are 42.9 % and 42.1 % for zinc and PO4, respectively. The results are in good
agreement with the calculated values 42.7 % and 41.6 %, respectively. All the results revealed that
obtained product is in good agreement with the pigment standard for paint coating industry.
Table1 Chemical analysis results of product
Index
Product
Standard
Zn
/%
42.9
40-45
PO4
/%
42.1
40-45
Density
/g.cm-3
3.16
3-3.9
Oil absorption
/g.100-1sample
25.02
25-40
pH
6.35
6-8
Water soluble salt
/%
0.56
<1.00
2.2 XRD results
XRD pattern of the obtained product is showed in Figure1. The seven stronger peaks of 2θ at
9.78º, 19.41º, 20.15º, 26.31º, 31.36º, 46.84ºand 49.99º were indexed, and the pattern matches the
standard XRD data for α-hopeite of JCPDF 76-089. The XRD data indexed an orthorhombic
monoclinic system with space group a=10.597 Å, b=18.308 Å, c=5.0304Å, V=975.86Å3. These cell
dimensions were very similar to these of α-Zn3(PO4)2·4H2O. Both peak intensity and sharp degree
are revealed high crystallinity of the product.
1808
Advanced Materials, ICAMMP 2011
2.3. EDS and SEM results
The energy dispersive spectrometer and its analysis results are showed in Fig.2. From the mass
fraction of zinc, oxygen and phosphorus 43.19 %, 42.52 % and 13.74 % (not including carbon from
conductive adhesive used in determination), the molar ratio of these elements can be calculated as
Zn:P:O=3:2:12, which is corresponding to molecular formula of Zn3(PO4)2·4H2O. All these results
show the obtained products are pure Zn3(PO4)2·4H2O.The SEM picture (Fig.3) show the title
compound has a lamellar particle shape and a clear contour line. The particle size of blocky
granular product is about 10 um. These results state the title compound possess a highly
microcrystalline.
2000
Intensity
1500
1000
500
0
10
20
30
40
2-Theta/
50
o
Fig.1 X-ray spectrum of the title compound
Fig.2 EDS analysis of title compound
Fig.3 SEM picture of title compound
2.4 Anticorrosion Evaluation of pigment
2.4.1Electrochemical performance of coating
The phenolic coating resistance and capacitance value of two antirust pigments in NaCl solution
(pH=7) in different soaking time are shown in Fig. 4 and Fig.5.
8.4
-4.0
8.0
-2
-2
7.2
6.8
6.4
-5.2
-5.6
-6.0
6.0
-6.4
5.6
-6.8
0
5.2
0
5
10
15
20
title zinc phosphate
commercial zinc phosphate
-4.8
log(C/F.cm )
7.6
log(R/O.cm )
-4.4
title zinc phosphate
commercial zinc phosphate
25
5
10
15
20
25
t/day
t/day
Fig.4 Variation of resistance as the immersion time
Fig.5 Variation of capacitance as the immersion time
Advanced Materials Research Vols. 415-417
1809
From Fig. 4 and Fig. 5, it can be seen that each coating resistances decrease in different degrees,
while the capacitances increased in different degrees. These changes are similar to that of many
organic coating in early immersion. However, phenolic resin paint based on the title zinc phosphate
show more excellent corrosion inhibition performance because its coating resistance is the biggest
and the capacitance is the smallest. It is worth mentioning that their changes of the immersion are
less, especially all the resistance values is more than 105Ω·cm-2. The properties of commercial zinc
phosphate coating are inferior to title zinc phosphate. These results indicate the title pigment can
prevent the erosion of the corrosive medium by improving the barrier and hydrophobicity of the
coating.
2.4.2 Anticorrosive properties of coating
To evaluate anticorrosive properties of pigment, a standard coating formulation base on an alkyd
resin was used. The product was employed to prepare paint. Corrosion tests were carried out and the
results showed in Table2. From the comparison result of the corrosion test, it can be seen that
anticorrosive efficiency of the title compound in alkyd paints is superior to that of commercial zinc
phosphate. The main reasons are that this product has lamellar structure to enhance its shielding
protection effect, and micro-grade particle size is helpful to enlarge its active regions and improve
its dispersibility.
Table2 Character of zinc phosphate alcohol acid anti-paint
Index
Film appearance
Blade fineness / µ m
Adhesion/grade
Hardness /H
Impacting resistance /kg.cm-2
Film gloss /%
Resistance 3%NaCl /h
Resistance 3%NaOH / h
Resistance 3%H2SO4 / h
Bin stability 50℃, 30d
Title compound
Smooth
45
1
B
50
93
144
50
45
Slightly thick
Commercial zinc phosphate
Smooth
45
1
B
50
90
120
40
40
Colloidization
Conclusions
The title compound prepared by hydrolysis precipitation is a lamellar microcrystalline
Zn3(PO4)2·4H2O with an orthorhombic monoclinic system. Comparing with the commercial zinc
phosphate, lamellar microcrystalline Zn3(PO4)2·4H2O has excellent anticorrosive property and
dispersibility. In the coating tests, title zinc phosphate can efficiently enhance the anticorrosive
property of coating and prevent the erosion of the corrosive medium by improving the barrier and
hydrophobicity of the coating. The anticorrosive property of the title compound in alkyd paints is
superior to that of commercial zinc phosphate because this product has lamellar structure to enhance
its shielding protection effect, and micro-grade particle size is helpful to enlarge its active regions
and improve its dispersibility.
Acknowledgements
This work was supported by Guangxi Science and Technology Development Program Project
(11107013-6, 0992028-13), and Guangxi Natural Science Foundation（2011GXNSFD018015）
References
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Advanced Materials, ICAMMP 2011
10.4028/www.scientific.net/AMR.415-417
New Synthesis Method of the Lamellar Zinc Phosphate and its Electrochemical Corrosion Inhibitions
10.4028/www.scientific.net/AMR.415-417.1806
DOI References
[6] M. Beiro, A. Collazo and M. Izquierdo: Progress in Organic Coatings Vol. 46(2003), p.97.
10.1016/S0300-9440(02)00216-3