Epoxy/Magnetite/Carbon Nanofibers Nanohybrid Coatings for Anticorrosion

The corrosion of structural metals, i.e., the alloys of iron and aluminum, usually involves the oxidation of metals and the reduction of oxygen, protons (H⁺), and/or water. As one of the most serious problems, corrosion imposes a major impact on the economics of industrial nations. Therefore, corrosion protection has become an issue of prime importance especially in the modern metallic finishing industry. Varieties of strategies have been developed to control the dynamics of corrosion (slowing down the kinetics and/or altering the corrosion mechanism). Cathodic protection (employing a sacrificial anode or an external power supply) and anodic protection (maintaining a protective passive (oxide) layer on the metal surface) have been employed to decrease the oxidation or reduction rate, respectively.

Epoxy/magnetite/carbon nanofibers nanohybrid coatings have been studied for anticorrosion purposes of stainless steel (SS). The thickness effect on the anticorrosion properties of the coatings were investigated in 3.5 wt% NaCl aqueous solution by monitoring the open circuit potential (Eocp) and tracing quasi-stationary polarization (tafel) of the coatings-coated stainless steel electrode. Electrochemical impedance spectroscopy (EIS) was also obtained to give an insight of the anticorrosion protection of the stainless steel (SS).

Experimental

The epoxy resin Epon 862 (bisphenol F epoxy) and EpiCure curing agent W were purchased from Miller-Stephenson Chemical Company, Inc. Fe₃O₄ nanoparticles (average size of 12 nm) were obtained from Nanjing Emperor Nano Material Co., Ltd.  The stainless steel used in this study is commercially stainless steel-304. All the chemicals were used as received without any further treatment. 10 wt% Fe₃O₄ nanoparticles were added in Epon 862 resin overnight so that the NPs can be totally wet by the resin, and the suspension was then sonicated (Branson 5510) for 30 min. After that, the mixture were poured into 100 mL beaker containing 2.0 wt% CNFs (named as EFCN2M10), followed by mechanical stirring for one hour (600 rpm, Heidolph, RZR 2041). All the procedures were carried out at room temperature.  The curing agent was added into the above prepared epoxy/ Fe₃O₄/CNFs nanohybrids suspension with a monomer/curing agent weight ratio of 100/26.5 for one hour mechanical stirring (200 rpm). Then the solution was mechanically stirred at 85 ^oC for 0.5 h in a water bath at the same speed (200 rpm), which is essential to remove the bubbles and to prevent the sedimentation of nanofillers during the curing process.  

 

Results and Discussion

The open circuit potential, E_ocp, indicates the thermodynamical tendency of a material to the electrochemical oxidation in a corrosive medium,¹ and can serve as an indicator of cathodic protection of the coating. Figure 1(A) depicts the open circuit potential (_Eocp) of the bare steel and ECF2M10 coated steel in 3.5 wt% NaCl solution after different immersion time. The fluctuation of E_ocp in the bare steel can be explained by  the formation at the first stage and the peeling off afterwards of the passive metal oxides at the electrode/electrolyte interface, as reported by Wessling.² The E_ocp was positively shifted to 0.2 V in the first 18 hrs after the steel surface was protected by ECF2M10 coating. The corresponding Tafel curves of the electrodes were depicted in Figure 2. The corrosion current is dramatically reduced to 6.4×10^-6 μA for the ECF2M10 protected steel, compared to 23.9 μA for the bare steel. Accordingly, the corrosion rate is calculated to be 4.234×10^-7 mm/year for the former, which is much lower than the latter (1.58 mm/year). The E_ocp shifted to more negative values with increased corrosion current for the ECF2M10 coated steel after 18 hrs, but the value is still higher than those of bare steel.   

Conclusion

Epoxy/ Fe₃O₄/CNFs nanohybrids have been employed for the first time for for protecting the stainless steel from corrosion in 3.5 wt% NaCl aqueous solution. The coating was capable of significantly reducing the corrosion current. The protection efficiency was decreased after 18 hrs but the coating is still able to slower the corrosion indicated by the relatively higher E_ocp. The diffusion of the corrosive ions through the imperfects in the thin film coating is inferred to cause the decreased protectiveness.

Figure 1 The open circuit potential (E_ocp)-time curves for (a) bare steel and (b) ECF2M10 coated steel in 3.5 wt% NaCl solution.

Figure 2 Tafel curves of the (a) SS and (b) SS/MWNTs-PU electrodes.

References

 1. Kadhum, A. A. H.; Mohamad, A. B.; Jaffar, H. D.; Yan, S. S.; Hilo, J. Int. J. Electrochem. Sci 2013, 8, 4571.

 2.  Wessling, B. Advanced Materials 1994, 6, 226.