The protective performance, in conditions of total immersion, of an acrylic water-based paint applied to rusty steel, has been studied using electrochemical techniques. There was no rust, blister, crack, or flake that occurred on coating after 500 h immersion. The data obtained have enabled the protective mechanism to be proposed. The specific pigments utilized in the formulation of the paint studied can release phosphates to form a protective layer on metal substrate, which can impede the access of aggressive species to substrate surface. The coatings performed electrochemical activity in the beginning of immersion; then the layer formed and resistance of coating increased.
The corrosion of iron and its alloys gives rise to a yearly loss of billions of dollars. Approximately 90% of all metallic surfaces are protected with organic coatings [
Now, government pays more and more attention to personnel healthy and environmental protection. So, composition of paints and their applications are facing more stringent requirements. However, traditional paints are facing adverse condition, because of those utilized volatile organic compounds (VOCs) as solvents and toxic chemical as anticorrosive pigments. Except for a good anticorrosive performance, excellent quality paint shall be equipped with environmental friendliness and easy construction performance. In order to better meet the requirements of industry development, such as aviation and shipbuilding, low VOC, nontoxic, and poor surface preparations are the development direction.
However, due to the environmental and safety issues, considerable research activities have conducted to enhance the increasing demand to reduce volatile organic compounds (VOCs) and hazardous air pollutants emissions, increasing the efforts to formulate waterborne systems for use as coatings [
Surface preparation is a key factor prior to painting and the success of the protective coating system depends on its correct execution. Traditional theory believes that poor surface preparation followed by a good coating system usually brings worse results than the use of low quality products on a well prepared surface. Rusts and oxides on the metal surface influence negatively the behavior of a coating system [
Now, our team has excogitated a new paint that employs noncontaminating inhibitors, water as solvent, and can meet poor surface preparation. In order to better demonstrate the protective function of coating, the behavior of this acrylic water-based paint applied to rusty steel is studied by electrochemical techniques.
The behavior of a new acrylic paint produced by our lab has been studied. Table
Technical sheet of studied paint from our lab.
Product description | Water-based acrylic primer |
Intended uses | For use at new steel structure and maintenance and repair |
Color | Gray |
Volume solids | 61 ± 2% (ISO 3233:1998) |
Typical film thickness | 100 |
Method of application | Airless spray, brush, roller |
EIS measurements were carried out in the solution of 3.5 wt% NaCl with a PARSTAT 2273 system, over the frequency range from 105 Hz to 10−2 Hz at open circuit potential, with a 20 mV potential perturbation. The internal parallel capacitance of the measuring machine was smaller than 5 pF. A three-electrode arrangement was used, consisting of a saturated calomel electrode (SCE) as reference electrode, a platinum electrode as counter electrode, and the coated sample as the working electrode. For the working electrode the exposing area was 3.14 cm2, which would decrease the magnitude of the measured impedance and avoid hitting the limit of the measurement instrumentation, especially at moderate and high frequencies. The Pt electrode area was nearly the size of the working electrode, about 4 cm2. Fitting of the impedance spectra was made using ZsimpWin software.
The LEIS measurements were performed on coating specimens that immersed in 3.5% NaCl solution through a PAR Model 370 Scanning Electrochemical Workstation. Thus, the test solution for LEIS measurements was 0.001 M NaCl solution. The microprobe was stepped over a designated area of the electrode surface. The scanning took the form of a raster in
Figure
Micrographs of the studied painted samples before and after immersion test. (a) The studied painted samples before immersion. (b) The studied painted samples after immersion.
In Table
Immersion testing results for scratched coating samples after different stage.
Epoxy antirust paint | Studied paint | |
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0 h |
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24 h |
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8 d |
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From the above results, the studied coating shows the function of inhibition to 3.5% NaCl solution after immersion. Particularly, compared to epoxy antirust coating, it is easy to see that the studied coating is provided with self-healing ability. This will be discussed in detail in the following sections.
Figure
Impedance spectra of painted samples without scratch after different immersion time in 3.5 wt% NaCl solution: (a) Nyquist plots from 1 h to 12 h, (b) Nyquist plots from 1 d to 21 d, (c) Bode plots from 1 h to 12 h, and (d) Bode plots from 1 d to 21 d.
Figure
Impedance spectra of painted samples with scratch after different immersion time in 3.5 wt% NaCl solution: (a) Nyquist plots and (b) Bode plots.
Impedance spectra of epoxy antirust painted samples with scratch after different immersion time in 3.5 wt% NaCl solution: (a) Nyquist plots and (b) Bode plots.
Impedance spectra of epoxy antirust painted samples and painted samples with scratch after 8-day immersion times in 3.5 wt% NaCl solution.
In order to guarantee the poor surface preparation and a good anticorrosive performance of our paint, many specific pigments are added in it. These specific pigments are chemical active, so the modulus of coating is at a low level. The specific pigment utilized in the formulation of the studied coating is aluminum triphosphate. The mechanism of actuation of this compound has not been clearly established.
In last decades, phosphate-based pigments are frequently applied in coatings to improve their corrosion resistance [
At 1 h, the Nyquist diagrams of the system consisted of one half of capacitive arc at high frequencies and another half of capacitive arc at low frequencies. The one at high frequencies can be attributed to the reaction between water and the large amount of aluminum triphosphate well dispersed in the coating matrix. The equivalent circuit shown in Figure
Equivalent circuit representing the coating system.
Study of the evolution of the diagrams of EIS with time of immersion enables an analysis to be made about the variation of the protective capacity of painted samples. In our case, from the fit of the experimental diagrams to the equivalent circuit proposed, the values of the capacity,
Impedance value of painted samples without scratch calculated from EIS spectra.
1 h | 2 h | 4 h | 8 h | 12 h | 1 d | |
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274.6 | 357.6 | 274.9 | 246.9 | 244.2 | 236.4 |
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3.26 × 10−4 | 3.43 × 10−4 | 2.85 × 10−4 | 2.05 × 10−4 | 1.73 × 10−4 | 1.53 × 10−4 |
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0.4926 | 0.6544 | 0.6686 | 0.6743 | 0.6837 | 0.7029 |
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1.58 × 104 | 4776 | 3818 | 7051 | 1.61 × 104 | 3.29 × 104 |
Time evolution of
Figure
Impedance value of painted samples with scratch calculated from EIS spectra.
0 h | 8 h | 24 h | 96 h | 192 h | |
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243.5 | 356.7 | 286.4 | 258.9 | 259 |
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1.91 × 10−4 | 2.10 × 10−4 | 1.75 × 10−4 | 1.20 × 10−4 | 1.04 × 10−4 |
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0.7724 | 0.6473 | 0.7035 | 0.7698 | 0.7913 |
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4.83 × 104 | 2.02 × 104 | 2.62 × 104 | 3.30 × 104 | 3.73 × 104 |
Time evolution of
For the epoxy antirust painted samples with scratch, the Nyquist diagrams present a single capacitive arch too (Figure
Impedance value of epoxy antirust painted samples with scratch calculated from EIS spectra.
0 h | 8 h | 24 h | 96 h | 192 h | |
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278.2 | 296.4 | 380.1 | 382.9 | 341.3 |
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5.92 × 10−7 | 4.09 × 10−7 | 6.32 × 10−7 | 9.90 × 10−7 | 3.83 × 10−7 |
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277.8 | 283.1 | 358.8 | 333.9 | 267.4 |
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6.52 × 10−5 | 8.35 × 10−5 | 6.986 × 10−5 | 7.17 × 10−5 | 8.56 × 10−5 |
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0.65 | 0.7302 | 0.7465 | 0.7209 | 0.6377 |
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1.35 × 105 | 1.83 × 105 | 1.163 × 105 | 4.1 × 104 | 1.74 × 104 |
As indicated in the LEIS projection of epoxy antirust coating (Figure
Time dependence of LEIS profiles and their projections of epoxy antirust painted samples.
The present work (Figure
Time dependence of LEIS profiles and their projections of studied painted samples.
Micrographs of the epoxy antirust painted samples (a) and studied painted samples (b) with defect after immersion test.
The behavior, in conditions of total immersion, of an acrylic water-based paint applied to rusty steel, has been studied using electrochemical techniques. The set of data obtained has enabled a mechanism for the anticorrosive performance of the coating.
This coating had a good anticorrosive performance. After 21 days of total immersion, there was no rust, blister, crack, or flake that occurred on coating. Compared to the epoxy antirust painted samples, the studied coatings exhibited better self-healing and anticorrosive feature. The electrochemical results show that the specific pigments utilized in the formulation of the paint studied caused the electrochemical activity of the coating. When the water has penetrated into the coating, the pigments based on phosphate anions can release phosphates to form a protective layer on the metal substrate. The layer prevents the access of water and corrosion reaction to protect substrate.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (no. 51071027).