Fabrication and evaluation of Rb 2 Co(H 2 P 2 O 7 ) 2 $ 2H 2 O/waterborne polyurethane nanocomposite coating for corrosion protection aspects

Here we investigate the in ﬂ uence of new acidic pyrophosphate (Rb 2 Co(H 2 P 2 O 7 ) 2 $ 2H 2 O) (DP) incorporation in waterborne polyurethane (WBPU) coatings on the corrosion protection e ﬃ ciency of WBPU coatings for carbon steel in 3.5% NaCl solution. We used potentiodynamic polarization, electrochemical frequency modulation (EFM) and pull-o ﬀ adhesion and O 2 gas permeability measurements to measure the corrosion protection e ﬃ ciency of the WBPU coating. The dispersion of the DP nano-particles into the WBPU matrix was characterized via TEM observations. We ﬁ nd that the corrosion resistance and mechanical properties of the WBPU coating increase with incorporation of DP nano-particles into the WBPU coating.


Introduction
Carbon steel is the most important alloy used in petroleum pipelines. However, its main drawback is corrosion. 1 The integration between cathodic protection and organic coatings can establish long-term protection for petroleum pipelines. 2 Polyurethane coatings are used in a broad range of construction applications under different conditions, especially on cathodically protected surfaces. 3,4 Waterborne polyurethane coatings are being progressively used instead of coating with organic solvent to minimize the release of volatile organic solvents. 5 Waterborne polyurethane has been vastly used as an effective coating for protecting metallic surfaces from corrosion. 6 Polyurethane has hydroscopic susceptibility and lacunar structure which make its coating permeable to aggressive agents. 7 These drawbacks accelerate the corrosion of the metallic substrate over time. 8 In this status, the incorporation of nanoparticles in organic coatings is a favorable approach for enhancing corrosion resistance of polyurethane coatings. 9,10 The improvement of the corrosion resistance and mechanical properties of organic coating using inorganic materials is a promising approach for coating system. [11][12][13] Recent works showed that inorganic materials improves the corrosion resistance of organic coatings by enlarging the route of corrosive ions and water in the coating to reach the metal/ coating interface and enhance the barrier effects of coating matrix. 14,15 In our previous works the effects of new inorganic materials on corrosion resistance and mechanical properties of organic coating were studied. [16][17][18][19][20] We proved in these works that the nanoparticles of inorganic materials improve the corrosion resistance and mechanical properties of organic coating. As to continue these investigations, the current work intends to study the effect of the new acidic pyrophosphate Rb 2 Co(H 2 P 2 O 7 ) 2 $2H 2 O (DP) incorporation in waterborne polyurethane (WBPU) coatings on the corrosion protection efficiency of WBPU coating.
Acidic metal pyrophosphates are characterized by their biological uses. Moreover, they are used as additive in fertilizers, or even in medicine. 21 In acidic pyrophosphates, various oxoanions (HP 2 O 7 ) 3À , (H 2 P 2 O 7 ) 2À and (H 3 P 2 O 7 ) À are involved in the molecular structure. These oxoanions are interconnected by strong hydrogen bonds, leading to various geometries (i.e. chains, ribbons, layers or three-dimensional network). 21 The practical tests have been conducted on samples of carbon steel immersed in salt solution (3.5% NaCl) and measured by means of electrochemical and mechanical methods.

Materials and chemicals
A carbon steel panel with the following chemical composition: (wt%); 0.06 C; 0.02 Cu; 0.7 Mn; 0.005 P; 0.012 Ni; 0.06 Si; 0.015 Cr; 0.004 Mo; 0.001 S; 0.002 V and Fe (bal.), was used as a metallic substrate. The metallic substrates used in the electrochemical experiment were mechanically cut into 3.5 cm Â 2.5 cm Â 0.3 cm dimensions, then abraded with emery papers up to 1200 grit, and washed with absolute ethanol and acetone. 22 The clean metallic substrates were dried at room temperature and stored in a moisture free desiccator before coating.
Waterborne polyurethane (the solid content is 37 wt%) and isocyanate based hardener were purchased from DOW Chemical Company and Bayer Company, respectively.
The aggressive environment used was 3.5 wt% NaCl solution. All chemicals were of AR grade and the solutions were prepared using deionized water.

Preparation of WBPU/DP nano-particles coating
Crystals of the DP were thoroughly grounded and sieved as to get nanoparticles, these were added into the waterborne polyurethane in different concentrations to obtain 0.2%, 0.5% and 1.0% by weight in the nal coating formula. DP nano-particles were completely dispersed in WBPU coatings by mechanical stirring at 1500 rpm for 40 min, followed by ultrasonication process for about 15 min. Isocyanate based hardener was added to the coating formula at 1 : 3 ratio to prepare a homogeneous and stable coating formula.
There are limitations of adding nano-particles above 1.0%, because of homogeneity and to slow down nano-particles reaggregation aer mechanical stirring removal.

Characterization
A single crystal was selected and X-ray diffraction (XRD) data were collected on an Oxford Diffraction XCALIBUR four-circles X-ray diffractometer using graphite monochromatized MoKa radiation (l ¼ 0.7173Å) equipped with a SAPPHIR CCD twodimensional detector.
A transmission electron microscope (Model: Jeol-Jem 1200EX II equipped with TEM) was used to determine the size of DP nano-particles and to evaluate the dispersion and distribution of DP nano-particles in WBPU.

Preparation of the coated substrates
The prepared coatings were applied on clean carbon steel panels by conventional spraying process. The thickness of dry coating lm (measured by hand held micrometer, B.C. Ames Co.) was in the range of 65 AE 10 mm. The coated substrates were stored in a moisture free desiccator before testing.

Electrochemical experiments
The Corrosion resistance property of coatings was conducted using potentiodynamic polarization and EFM measurements.
The electrochemical set-up composed of three electrodes layout in a glass cell, with a Pt counter electrode and a saturated calomel reference electrode (SCE). The experiments were conducted in a 100 ml volume cell at 298 K using a temperature control water bath.
All electrochemical measurements were carried out with ACM instruments Potentiostat/Galvanostat (Gill AC Serial no. 947).
For potentiodynamic polarization experiments, open circuit potential (OCP) of system aer immersion for 2 h and 7 days in corrosive solution was recorded. Then, the Tafel plot was plotted by sweeping the applied potential from À0.25 to +0.25 V with respect to OCP with a constant sweep rate of 1.0 mV s À1 . EFM measurements were implemented by applying potential perturbation signal with amplitude of 10 mV with two sine waves of 2 and 5 Hz.
All electrochemical experiments were performed in triplicate to guarantee reproducibility of the results and the mean values were recorded.

Pull-off adhesion experiments
Pull-off adhesion testers (PAT model GM01/6.3 kN) was used to determine the adhesion strength of WBPU coatings coatings aer 10 days of immersion in 3.5 wt% NaCl solution. All the experiments were repeated three times, and good reproducibility of the results was observed.

O 2 gas barrier property experiments
O 2 gas barrier property of prepared coating was determined at 298 K using Oxygen Permeation Analyzers (model 8501, Illinois Instruments, Inc.). The measurements were occurred according ASTM D3985 standard test methods.

TEM observation
The TEM image of DP nano-particles is shown in Fig. 3. The TEM analysis demonstrates that the particle size of DP nanoparticles was found to be around 70 nm.
The TEM image of WBPU/DP nano-particles nanocomposite (see Fig. 4) conrms that the DP nano-particles were uniformly dispersed in the WBPU matrix.      the electrochemical parameters (corrosion potential E corr and corrosion current density j corr ) obtained from polarization curves are presented in Table 1.
The E corr of bare carbon steel is about À0.676 with j corr of about 63.09 mA cm À2 . In the case of neat WBPU coating, E corr shis to more positive potentials (À0.411 V) and j corr decreases to 13.08 mA cm À2 , conrming that the coated carbon steel substrate by neat WBPU coating is still susceptible to corrosion to some extent. On the other hand, WBPU + DP coated substrate exhibits more positive E corr with a remarkable decrease of j corr ( Table 1). The noble shi of E corr accompanying with decrease of j corr , suggest that the incorporation of different concentrations of DP nano-particles into WBPU coating improves the corrosion resistance of the mild carbon steel substrate remarkably. 23 The protection efficiency (h j %) of the coatings was dened as: 24,25 where j o corr and j c corr are the corrosion current densities in the absence and presence of coatings, respectively.
The calculated h j % values obtained from polarization measurements are listed in Table 1. The protection efficiency h j % of the WBPU + DP coatings was found to be signicantly increased as compared to neat WBPU ( Table 1). The h j % of the WBPU + DP coatings was increased as the percentage of DP nano-particles increased. Coated carbon steel sample with WBPU + 1.0% DP, showed the highest the corrosion resistance (h j % ¼ 99.60).
In addition, coatings have been modied with nano-particles to achieve desirable properties such as increased adhesion and cohesion. 26 Here, cohesion reects the strength and fracture resistance of the coating material, while adhesion indicates the interfacial bond strength of coatings or adhesives. 27 This improvement in the corrosion resistance of WBPU coating in the presence of DP nano-particles is attributed to; DP nanoparticles are well dispersed in WBPU coating, which restrict the diffusion of the corrosive ions and water through the WBPU coating lm. 28,29 Moreover, the large surface area and the small size of DP nano-particles, absorb more WBPU coating on its surface which reinforces the density of the WBPU coating lms, that way the transport paths for the corrosive ions and water to pass through the WBPU coating become longer and consequently inhibiting the corrosion process. 30 The effect of long time immersion of WBPU + 1.0% DP coating was shown in Fig. 6. It is observed that aer immersion of the coated carbon steel by WBPU + 1.0% DP, there is a slightly increase in the magnitude of the corrosion current (from 0.25 mA cm À2 to 2.9 mA cm À2 ) with increasing the immersion time from 2 h to 7 days. This is referring to the high corrosion resistance nature of the WBPU + 1.0% DP coating.
3.3.2 Electrochemical frequency modulation. Electrochemical Frequency Modulation (EFM) is a powerful nondestructive method for measuring the corrosion rate of coating system. The EFM technique was used for monitoring of the corrosion rate of various types of corroding systems within reasonable time without prior knowledge of Tafel parameters by applying a small polarizing signal.
The EFM intermodulation spectra of uncoated and coated carbon steel by WBPU coating containing DP nano-particles immersed in 3.5% NaCl solution at 298 K are shown in Fig. 7. The parameters obtained from EFM spectra (j corr(EFM) and the causality factors CF2, CF3) are listed in Table 2.
It seems that the values of the causality factors CF2, CF3 for uncoated and coated carbon steel substrates are around the theoretical values (2 and 3), signifying the accuracy of the obtained results. 31 According to Table 2, all coated carbon steel show higher j corr(EFM) than uncoated substrates; meanwhile, WBPU coatings containing DP nano-particles show signicantly greater j corr(EFM) values. From EFM data, the protection efficiency (h EFM %) of the coatings can be calculated using eqn (2): 32 where j o corr(EFM) and j c corr(EFM) are the corrosion current densities obtained by EFM data in the absence and presence of coatings respectively. The calculated h EFM % values are listed in Table 2. According to these results, DP nano-particles tend to agglomerate in WBPU matrix by increasing DP wt%. The WBPU coating with 0.1 wt% DP nano-particles shows the best corrosion protection performance (97.12%). This is consistent with the nding that obtained from polarization measurements.

Mechanical property of WBPU coating in the presence of
The mechanical property of WBPU coating in the absence and presence of DP nano-particles is conducted using pull-off adhesion measurements and the results are presented in Fig. 8. The average pull-off adhesion strength of WBPU coatings enhances by incorporation of different amount of DP nanoparticles. The average adhesion strength of the WBPU coating without DP nano-particles was 3.2 MPa, while average adhesion strengths of 6.6 MPa, 8.8 and 10.5 MPa were obtained at 0.2 wt%, 0.5% wt% and 1.0 wt% of DP nano-particles, respectively.
The adhesion strength of WBPU highly depends on the chain extenders and their content. 33 Neat WBPU coating adheres to carbon steel surface via chemical adhesion supported by mechanical interlocking adhesion. The reactive functional groups of WBPU coating such as NH and CO groups tend to adhere to carbon steel surface via formation of chemically bond. 34 In these applications the formation of covalent chemical bonds occurs across the interface. Coating systems containing reactive functional groups tend to adhere more strongly to metal surface containing similar reactive functional groups. 23 The incorporation of DP nano-particles is also believed to diminish the porosity of the WBPU coating. 35,36 Moreover, the DP nano-particles can suppresses the coating cracking when the WBPU coating is exposed to an external stress and it will result in the improvement of the coating exibility. This causes better adhesion of WBPU coatings containing DP nano-particles than neat WBPU.
3.5. O 2 gas barrier property of WBPU coating in the presence of Rb 2 Co(H 2 P 2 O 7 ) 2 $2H 2 O O 2 gas permeability of WBPU coating in the absence and presence of DP nano-particles at 298 K was presented in Table 3. The   incorporation of DP nano-particles into WBPU coating causes a signicant decrease in the permeation of oxygen across the coating thickness. The best barrier performance was recorded for the coating with the highest concentration of Rb 2 Co(H 2 P 2 -O 7 ) 2 $2H 2 O (i.e. WBPU + 1.0% DP). It has been explained on the basis that the incorporation of DP nano-particles into WBPU coating can create a tortuous path for O 2 gas molecules. This causes an increase of the total path of the gas. 37 This results support the corrosion protection data.

Conclusions
The effect of nano-particles of new acidic pyrophosphates (Rb 2 Co(H 2 P 2 O 7 ) 2 $2H 2 O) on the corrosion protection efficiency of WBPU coatings for carbon steel in saline solution (3.5% NaCl) was assessed by electrochemical and mechanical methods. Potentiodynamic polarization and EFM curves proved that the addition of DP nano-particles had improved the corrosion protection performance of WBPU coating. Increment of the DP nano-particles concentration exhibited a signicant effect on the coating performance. The superior the corrosion protection efficiency of the WBPU coating was achieved by the addition of 1.0 wt% of DP nano-particles. Further, WBPU coating containing DP nano-particles showed the highest pulloff adhesion strength, demonstrating the enhanced mechanical performance of WBPU coating with the addition of DP nano-particles.

Conflicts of interest
There are no conicts to declare.