Keywords

1 Introduction

The epoxy coating of mild steel has been used in recent times to combat the high steel in concrete [1]. The good corrosion resistance, poor electrical conductivity, and ease of application are the reasons behind the usage of epoxy coating in concrete [2]. However, the properties of the coating may not withstand service conditions such as wear, marine, or seawater corrosion, as a result of the hydrophilic feature of epoxy, which weakens the coating [3, 4]. Hence, an effort has been made by the researcher to add second phase nanoparticles to overcome this great problem. Tianhao Ge et al. [5] reported on the microstructural and electrochemical behavior of an epoxy-grapheme-zinc multilayer coating. Xianming et al. [6] reported on the corrosion resistance and mechanical properties of an epoxy coating of mild steel with Fe2O3, SiO2, Zn, and clay nanoparticles. They all concluded that incorporation of nanoparticles into epoxy results in clustering and agglomeration of particles [7] and that functionalization and blending treatment of the nanoparticles can reduce the limitations [8]. For example, organic and inorganic compounds such as polyaniline emeraldine salt and aminopropyltriethoxysilane have been used to functionalize nanowhiskers/epoxy coating of mild steel by Cleide Borsoia et al. [9].

The researcher has shown that the inorganic and organic compounds are not eco-friendly and costly, which hindered the wide usage of these compounds in the functionalization of nanoparticles for mild steel. Effort was made in this work to develop a reducing agent using plant extract (orange juice) [10] because it has been reported by Aigbodion et al. [11] that orange juice contains phenolic compounds (ferulic, hydroxybenzoic, hesperidin, hydroxycinnamic, ascorbic, and citric) that can be used as functionalization of nanoparticles for epoxy coating of mild steel [12]. This work used 5 wt% potassium sulfate (K2SO4(aq), 5 wt% sodium chloride (NaCl(aq), and 1 M sulfuric acid (H2SO4(aq) for corrosion performance, as a continuation of the author’s and co-worker’s work.

2 Materials and Method

Mild steel of 0.15%C composition was grounded with a series of grit papers (240–1200PC) and inserted in a 2.9 wt.% hexafluorozircoic acid solution for a period of 1 h. The orange fruit was washed, cleaned, and pasteurized for 2 h at a temperature of 85oC, and the orange juice was then activated in steam at 65oC [12]. The sol-gel method was used in the production of the rice husk nanoparticles (RHnp) using a 35% sodium hydroxide solution. Details of the production procedure are discussed elsewhere [12]. LY556 (HERENBA BRAND) Epoxy and HY951 Hardener were used in the production of the epoxy coating. The modification of the epoxy was done using fluorinated poly (aryl ether ketone) (FPEK). The modified epoxy: hardener ratio of 2: 1 and 2%RHnp was used in the production of the epoxy composite coating. The epoxy was then mixed with 10ml of orange juice to help disperse the RHNP. A spray pyrolysis machine operating at 250rpm was used to deposit the mixture on the mild steel surface. The coated sample was cured on the laboratory floor for 24 h. The particle size of the RHnp was determined using the transmission electron microscope model (JEOL JSM840A). The microstructure was done using TESCAN SEM. A new corrosion tester model CHI604E was used for the electrochemical analysis at potentials of −1.5V to 1.5V and 0.0012V s-1. The chloride-based road salt, heavy industrial pollution conditions, and heavy rain conditions were used in this study with 5 wt% potassium sulfate (K2SO4(aq), 5 wt% sodium chloride (NaCl(aq), and 1 M sulfuric acid (H2SO4(aq). The polarization resistance was computed using Eq. 1 [2].

$$ R_{P} = \frac{{\beta_{a} \beta_{c} }}{{2.3icorr(\beta_{a} + \beta_{c} )}} $$
(1)

βa = Tafel anodic constant, βc = cathodic Tafel constant

3 Results and Discussion

Figure 1 displays the TEM image of the RHnp. It was observed that segregation and particle clustering were not seen in the TEM image. However, random distribution of fine particles was observed these were attributed to the reducing ability of the orange juice that prevents particle clusters. A similar observation was obtained in the work of [4]. An average particle size of 31.94–70.99 nm was obtained.

Fig. 1.
figure 1

TEM image of the pasteurized and activated rice husk ash nanoparticle

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Figure 2 shows the results of the corrosion analysis. It was observed that the coated samples shifted the potential to a more positive one. There is a very significant difference in the Tafel plot of the coated and that of the mild steel samples. The coated samples’ potentials were shifted to higher potentials and lower current density. The coated 5% NaCl sample has a higher potential and hence a lower corrosion rate. The mild steel samples have the lowest potential of all the samples. It was observed that the %wt%K2SO4 (aq) has a higher tendency of corrosion as a result of the lower potential, but the developed composite has a higher potential and shifted the Tafel curves to the right in the three media under investigation. The higher rate of corrosion experience for the mild steel sample could be attributed to the fact that iron oxidation and change in pH values are the major factors affecting the corrosion behavior of this cast iron. At work, corrosion rates of 3200, 2015 and 1016 mpy were obtained for the substrate, while the coated sample corrosion rates of 678, 567 and 456 mpy were obtained for K2SO4, H2SO4 and NaCl, which corresponded to 78.81, 71.86 and 55.11%.

Fig. 2.
figure 2

Tafel polarization plot of the electrochemical process

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Figure 3 displays the corroded surfaces of the samples. There was a great difference in the corroded surface of the mild steel as compared with that of the coated samples. The formation of passive films was noticed in coated samples. The passive films covered the sample surface and increased the potential of the composite sample to lower degradation by corrosion. However, the composite sample in 5wt%NaCl has the higher passive film. This is the main reason the composite sample in 5wt%NaCl has the higher corrosion resistance due to the high amount of corrosion product in the form of nodules as evidenced.

Fig. 3.
figure 3

Corroded surfaces of mild steel in (a) K2SO4, (b) H2SO4, and (c) NaCl. d) K2SO4 coated, b) H2SO4 coated, c) NaCl coated

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4 Conclusions

New insights in using orange juice as a reducing agent to functionalize the rice husk ash nanoparticle for epoxy coating of mild steel have been experimentally determined in this work. It was concluded that orange juice was able to enhance the dispersion of RHnp in the epoxy coating. 78.81, 71.86, and 55.11% corrosion resistance of the samples of K2SO4, H2SO4, and NaCl 2wt%. The work finds out that the formation of nanostructure and functionalization with orange juice helped increase the purity of the silica content of the ramp. The presence of citrate ions in the orange juice acts as a stabilizer and reducing agent, which was attributed to the fine grain size and good corrosion resistance of the composite coating.