Design new epoxy nanocomposite coatings based on metal vanadium oxy-phosphate M0.5VOPO4 for anti-corrosion applications

Epoxy nanocomposite coatings are an essential way to protect petroleum storage tanks from corrosion. For this purpose, the new nanocomposite epoxy coatings (P-M/epoxy composites) have been successfully designed. The P-M/epoxy composites are based on the metal vanadium oxy-phosphate M0.5VOPO4 (where M = Mg, Ni, and Zn). The function of P-M/epoxy composites as anti-corrosion coatings was explored using electrochemical and mechanical tests. Using electrochemical impedance spectroscopy (EIS), it has been noticed that the pore resistance and polarization resistance of the P-M/epoxy composites remain higher as compared to the neat epoxy. The P-M/epoxy composites have the greatest impact on the cathodic dis-bonded area and water absorption. Besides, P-M/epoxy composites exhibit a very high order of mechanical properties. Further, Mg0.5VOPO4 has the greatest effect on the anti-corrosion properties of epoxy coating followed by Zn0.5VOPO4 and Ni0.5VOPO4. All these properties lead to developing effective anti-corrosion coatings. Thus, the net result from this research work is highly promising and provides a potential for future works on the anti-corrosion coating.

Preparation of nanocomposite coatings and coated electrodes. The nanocomposite coatings (i.e. P-Ni/epoxy, P-Zn/epoxy and P-Mg/epoxy nanocomposites) were prepared by blending epoxy resin (type Bisphenol-based-Ciba Co.), poly-amidoamine hardener (Arkema Co.), xylene and 1.0% of M 0.5 VOPO 4 (where M = Mg, Ni and Zn). All the ingredients were homogenized using a speed mixer for 3.0 h. The final formula was grounded for 2.0 h to achieve adequate fineness.
Carbon steel sheets (from petroleum storage tank source) were utilized as coated working electrodes. The electrode dimension is 12 mm × 16 mm × 0.50 mm). The preparation of working electrodes before the coating was conducted using the standard method ASTM G1-03 24,25 . The film applicator was used to apply a very thin layer on the steel surface. The coated electrodes were placed in the oven at 333 K to get a complete cure coating surface. The coating micro-meter (Mitutoyo) was used to measure the coating layer thickness. It was approximately 38 ± 5 μm.
Electrochemical and mechanical experiments. EIS measurements were used to explore the anticorrosion performance of new nanocomposite coatings. The adequate 3-electrodes (i.e. working, calomel electrode (SCE) and Pt electrodes) glass cell was used for EIS measurements. All experiments were conducted using Potentiostat//Galvanostat system type Gill-AC-947.
Water absorption (Ø%) of the nanocomposite coatings was calculated using the coating capacitance from EIS experiments at initial (C 0 ) and after 7 days (C t ) of the immersion time. Brasher-Kingsbury relation was used to get Ø% for different nanocomposite coatings 26,27 .

Results and discussion
XRD pattern of M 0.5 VOPO 4 . The M 0.5 VOPO 4 (where M = Mg, Ni and Zn) compounds were analyzed using the powder X-Ray diffraction technique. The diffractogram of the materials was recorded in the 2-theta range of 10°-80° as illustrated in Fig. 1. The XRD pattern confirms the high purity of the synthesized M 0.5 VOPO 4 materials without the presence of any crystallized impurities.
Anti-corrosion properties of P-M/epoxy composites. The influence of new synthesis phosphate compounds M 0.5 VOPO 4 on the anti-corrosion properties of the epoxy coating was confirmed by the EIS studies. The Nyquist plots for carbon steel electrodes coated with neat epoxy, P-Ni/epoxy nanocomposite, P-Zn/epoxy nanocomposite and P-Mg/epoxy nanocomposite in 3.5% NaCl solution at 303 K are presented in Fig. 2. This figure demonstrates that the Nyquist plots for all coated electrodes have the two-time constants with the exception of P-Mg/epoxy nanocomposite which show one time constant. The appearance of a first peak at the high frequency for neat epoxy, P-Ni/epoxy nanocomposite and P-Zn/epoxy nanocomposite is attributed mainly to the coating layer 33 . While the second peak at the low frequency is due to the corrosion process under the coating layer 34 .
Generally, most epoxy coatings deteriorate with time, causing more complicated impedance behavior than the excellent coating. Over time, the corrosive solution (i.e. 3.5% NaCl solution) penetrates the coating texture and forms solution/metal interface under the coating 35 . This leads to steel corrosion process at the liquid/metal interface (Fe(s) = Fe(aq) 2+ + 2e) 36,37 . According to this situation, the most suitable equivalent electric circuit that www.nature.com/scientificreports/ fits the Nyquist plots for neat epoxy, P-Ni/epoxy nanocomposite and P-Zn/epoxy nanocomposite is shown in Fig. 3a. The elements of Fig. 3a are the capacitance of the epoxy coating (C c ), pore resistance (R po ), polarization resistance (R p ), solution resistance (R s ) and the capacitance of double layer (C dl ). All these elements are listed in Table 1. We observed that both R po and R p values were significantly increased by using P-Ni/epoxy and P-Zn/epoxy composites comparing with R po and R p values in the case of pure epoxy. Interestingly, P-Mg/ epoxy composite is able to heal the coating defect and form one time constant. In this case, the second peak at  www.nature.com/scientificreports/ the low-frequency disappears (see Fig. 2) and the corresponding equivalent electric circuit is shown in Fig. 3b. Moreover, P-Mg/epoxy composite shows the highest R po value (see Table 1). Following the same EIS data, it was clear that C c and C dl are linked to the barrier performance of composites coatings. The low C c and C dl values of new P-M/epoxy composites indicate their good barrier performance against corrosive solution 38,39 . It is worth noting that Mg 0.5 VOPO 4 has the greatest effect on the anti-corrosion properties of epoxy coating followed by Zn 0.5 VOPO 4 and Ni 0.5 VOPO 4 .
The cathodic de-lamination tests for the new nanocomposite coatings are critical to investigate the strength of coatings adhesion with metal subtract 40 . The cathodic de-lamination data for neat epoxy, P-Ni/epoxy, P-Zn/ epoxy and P-Mg/epoxy nanocomposites in 3.5% NaCl solution at 303 K are exhibited in Fig. 4. We noted that the incorporation of Ni 0.5 VOPO 4 into the epoxy resin has a slight impact on the cathodic disbonded area. On the other side, the incorporation of Mg 0.5 VOPO 4 and Zn 0.5 VOPO 4 into the epoxy resin has a great impact on the cathodic disbonded area. This means that the new synthesis phosphate compounds could result in a strong adhesion between the epoxy resin and the steel substrate. Our study also confirms that Mg 0.5 VOPO 4 has the greatest impact on the cathodic disbonded area followed by Zn 0.5 VOPO 4 .
The electrolyte absorption by coating layer is the main factor in the quality of new coatings synthesis. Where the coating layer that absorbs less amount of corrosion electrolyte is characterized by a good barrier layer. According to this parameter, the results of the water absorption Ø% (see Eq. (1)) for neat epoxy, P-Ni/epoxy, P-Zn/ epoxy and P-Mg/epoxy nanocomposites are presented in Fig. 5. In the case of neat epoxy, the water absorption Ø% was very high comparing with the P-M/epoxy composite. This indicates that the new P-M/epoxy composites are able to prevent the passage of the electrolyte inside the coating matrix. It is noting also that P-Mg/epoxy has the lowest Ø% followed by P-Zn/epoxy and P-Ni/epoxy. This confirms that Mg 0.5 VOPO 4 plays a great role in the decline in water absorption by epoxy coating.
Mechanical properties of P-M/epoxy composites. The mechanical tests (i.e. cross-cut adhesion, impact resistance, bend test and contact angle) further reveal the various mechanical features of epoxy coating acquired by incorporation by new synthesis phosphate compounds M 0.5 VOPO 4 . As illustrated in Table 2, in contrast to pure epoxy coating, cross-cut adhesion and bend tests are pass for all P-M/epoxy composites. We also observed a significant increase in the impact resistance of the coatings from 65 kg cm −2 in the case of pure epoxy to 85, 88 and 93 kg cm −2 in the cases of P-Ni/epoxy, P-Zn/epoxy and P-Mg/epoxy composites, respectively (see Table 2). This means that the presence of new synthesis phosphate compounds inside the epoxy matrix improves both the adhesion and the degree of the coating flexibility 41,42 . Moreover, the contact angle became wider with the addition of phosphate compounds from 61° in the case of pure epoxy to 88°, 89° and 89° in the cases of P-Ni/  www.nature.com/scientificreports/ epoxy, P-Zn/epoxy and P-Mg/epoxy composites, respectively (see Table 2). The wider contact angles in the presence of phosphate compound mean that the new epoxy nanocomposites absorb less amount of corrosive solution, which confirms the anti-corrosion performance of new epoxy nanocomposites.
The anticorrosive mechanism of P-M/epoxy composites. Epoxy coating permeability represents the vital defect in the coating layer leading to the failure in preventing the corrosive ions from transferring causing metal surface corrosion 43,44 .   www.nature.com/scientificreports/ Here, the incorporation of a small size of M 0.5 VOPO 4 inside the epoxy matrix is able to heal the epoxy coating layer. According to the above data, the phosphate particles M 0.5 VOPO 4 were distributed inside the pore of the epoxy matrix, leading to very low pore size. This action makes the zigzagging route for moving the corrosive ions is longer 45 , leading to the low possibility of the corrosion of steel surface and formation of iron oxide 46 .
The new epoxy nanocomposites are characterized by very good mechanical properties comparing with pure epoxy. The main reasons for this behavior are the improvement in the cross-linking of the epoxy matrix and the prevention in the epoxy layer disaggregation by the phosphate particles M 0.5 VOPO 4 47 . DSC curves (see Fig. 6) support this statement. Where the incorporation of the small size of M 0.5 VOPO 4 inside the epoxy matrix led to the increase in T g from 91.4 °C for pure epoxy to 104.3 °C for P-Ni/epoxy, 105.7 °C for P-Zn/epoxy, and 107.2 °C for P-Mg/epoxy. This shifting in the T g values is due to the increase in the cross-linking density of epoxy resin 48,49 . This behavior is responsible for the excellent mechanical properties of epoxy resin in the presence of phosphate particles M 0.5 VOPO 4 .
The type of metal atoms M = Mg, Ni and Zn in the structure of phosphate particles M 0.5 VOPO 4 is the main factor in determining the anti-corrosion performance difference between new epoxy nanocomposites. Where the reduction electrode potential values of metal increase in the sequence: Mg < Zn < Ni 50 . Moreover, metals Mg and Zn have the ability to lose electrons more than iron atoms. This means that Mg and Zn can supply cathodic protection for steel surfaces. This leads to an additional anti-corrosion effect for epoxy nanocomposites besides their physical barrier against the corrosive electrolyte. On other hand, Ni is less active than the iron atom. This explains why P-Ni/epoxy nanocomposite is the lowest anti-corrosion performance. Also, Mg exhibits a very electronegative potential (i.e. − 1.75 V) comparing with Zn (− 1.1 V) 51 . This higher electronegative potential supplies more cathodic protection for steel surfaces resulting in higher anticorrosion properties.

Conclusions
In this study, the new P-M/epoxy composites based on the vanadium oxy-phosphate M 0.5 VOPO 4 (M = Mg, Ni and Zn) was successfully developed the anti-corrosion properties of epoxy coating nanocomposites were clearly investigated by electrochemical and mechanical measurements. In summary, the anti-corrosion properties of epoxy were improved by incorporating vanadophosphates inside the epoxy resin. This was clearly detected from the high values of pore resistance and polarization resistance. Our study also confirms that Mg 0.5 VOPO 4 has the greatest impact on the cathodic disbonded area followed by Zn 0.5 VOPO 4 . The formation of epoxy nanocomposites containing vanadium oxy-phosphate was decisive for achieving excellent mechanical properties such as crosscut adhesion, impact resistance, bend test and contact angle). The changing of the metal atoms M = Mg, Ni and Zn in the structure of M 0.5 VOPO 4 particles is the main factor in determining the anti-corrosion performance difference between epoxy nanocomposites. This work establishes the great potential of the vanadophosphates/ epoxy nanocomposites for the development of high-performance anti-corrosion coatings. www.nature.com/scientificreports/