Introduction

Metals and alloys, being thermodynamically unstable, undergo corrosion during their use in commercial practice. An organic coating is a reasonable way to control corrosion since it precludes corrosive species from interacting directly with the metal. Pigments are incorporated into organic coatings to enhance their performance.1,2 Despite several pigments already being utilized in coatings to protect metal, they provide several obstacles, such as environmental legislation, durability, and reducing the cost of production, all of which are essential aspects in developing novel corrosion-resistant pigments used in epoxy coatings. The main challenge is identifying ecologically friendly pigments that do not jeopardize traditional pigments' established performance.3,4,5,6 Metal oxides are being used as pigments due to their chemical stability, low cost of production, and anticorrosive capabilities.7,8 Raj investigated the effect of zinc oxide nanoparticles within epoxy coatings on the corrosion resistance of mild steel in 3.5 % NaCl. The integration of nano-ZnO particles enhanced the charge transfer resistance, demonstrating improvement in barrier protection.9 Ramezanzadeh et al. incorporated nano- and micro-ZnO particles at varying weight percentages into a bisphenol A-based epoxy matrix. They discovered that employing nano- and micro-ZnO particles significantly ameliorated corrosion resistance. Nano-ZnO particles offered better corrosion protection than micro-ZnO particles in coatings. The addition of 3.5 wt.% ZnO nanoparticles enhanced the cross-link density and anticorrosive effectiveness of the coatings owing to the high surface area of the nanoparticles.10 Since nanoparticles play an extensive role as corrosion inhibitors, a small amount (1 wt.%) of ZnO nanoparticles fortified the corrosion protection behavior of epoxy coatings.11 An anticorrosive performance of jatropha oil-based epoxy acrylate resin with various weight percentages of ZnO nanoparticles painted on mild steel substrates was examined through an electrochemical test. It was found that adding 5 wt.% ZnO to the epoxy significantly improved its resistance to corrosion.12 Dhoke et al. discovered that including 0.3 wt.% nano-ZnO in a coating boosted its protective abilities by incrementing its density, consequently hindering electrolyte transport to the metal substrate.13 Madhup et al. worked on the impact of adding different nickel oxide nanoparticles to a bisphenol A-based epoxy resin on the corrosion resistance of mild steel. They postulated that incorporating nano-NiO pigments into epoxy coatings changed the coating's anticorrosive capabilities. The incorporation of 0.5 wt.% NiO nanoparticles resulted in uniform dispersion in the epoxy, which showed noticeable corrosion protection characteristics relative to other quantities of NiO nanoparticles. As the amount of NiO nanoparticles (1 wt.%) increased, the anticorrosive potential of the coating diminished owing to nanoparticle coagulation in the coating.14 Raj evaluated the impact of modified NiO nanoparticles on anticorrosiveness epoxy-coated mild steel and discovered that the accumulation of nano-NiO particles improved the coating's protective nature due to the development of corrosion products.15 Samad et al. blended various percentages of nano-ZnO particles with constant nano-SiO2 percentages in an epoxy matrix. They reported remarkable corrosion resistance due to the synergistic impact of blending 1 wt.% ZnO and 5 wt.% SiO2 nanoparticles in an epoxy matrix. Higher concentrations of nanoZnO (2 and 3 wt.%) inhibited the corrosion prevention capability of the coating due to cluster formation with nano-SiO2.16 The inhibition capabilities of ZnO and Co-ZnO nanoparticle-based epoxy coatings on mild steel (MS) substrates were quantified in 3.5 wt.% NaCl. It was pointed out that incorporating 10 wt.% Co together with ZnO nanoparticles into epoxy significantly improved its anticorrosive performance.17,18 Supplementary Table S-I (refer to online supplementary material) summarizes the studied literature. However, nano-pigments are released during service and exhibit ecotoxicity.19 In a different attempt, Vassilliou et al. observed that n-type semiconducting oxides as the pigment can retard corrosion in accordance with their semiconductivity.20 As semiconducting materials can be p-type or n-type or both, we anticipated that n-type and p-type semiconducting materials can be combined to preclude charge transfer at the coating–metal electrolyte interface and thereby prevent corrosion. In order to construct a p–n junction-based pigment, p-type NiO and n-type ZnO were selected to synthesize a two-phase mixture. To our knowledge, no studies have been published on using a ZnO/NiO micro-composite as a pigment in anticorrosive paints. We recently reported the synthesis and characterization of a ZnO/NiO pigment21 and, in the present work, its electrochemical performance is reported, and the mechanism of corrosion protection is discussed.

Experimental

Materials

ZnO (Loba Chemie; 99%) and NiO (Sigma Aldrich; 99.5%) powders were utilized as starting materials. Araldite GY 250 and Aradur 140 epoxy resins (Huntsman Advanced Materials (India), Andheri East, Mumbai, Maharashtra, India) were employed in paint formulation. In the paint composition, ingredients such as xylene, TiO2, and dioctyl phthalate (DOP) (AR grade; Loba Chemie) were utilized. MS samples were procured from a local supplier (Rajasthan Steels, Pune, India).

Pigment Synthesis

The ZnO and NiO powders were carefully weighed according to the stoichiometry composition of a 0.5 molar fraction of each and combined in ethanol to form a slurry. Zirconia balls were introduced to the slurry to grind the mixture. The slurry was ball-milled for 24 h with a ball-to-powder ratio of 5:1. The slurry was rinsed with ethanol after being removed from the ball mill, and then dried in an oven at 100°C to vaporize the ethanol. Using an agate mortar and pestle, the dry lump was pulverized, and the pulverized matter was calcined for 5 h at 600°C.

Characterization

An x-ray diffractometer (XRD; D8 Advance with Davinci Design, Bruker, Germany) was employed to analyze the phases present in the pigment with CuKα radiation of 40 kV and 40 mA. The XRD analysis was carried out in the range of 2θ = 20–90º with a scanning speed of 0.25 s/step. Rietveld refinement was used to determine the lattice parameters and weight percentages of the phases present in the synthesized pigment using TOPAS software (Bruker). The morphology of the synthesized pigment was scrutinized using field-emission scanning electron microscopy (SEM; Sigma; ZEISS, Germany). The pigment was sputtered with gold-palladium paste before SEM. The UV–visible spectroscopy (JASCO, Japan) was carried out in the range of 200–800 nm.

Paint Formulation

For the formulation of the epoxy paint, xylene (12 g), titanium dioxide (10 g), and DOP (8 g) were blended in a 70-g epoxy resin (Araldite GY250) solution. ZnO/NiO pigment (2 g), xylene (12 g), titanium dioxide (8 g), and DOP (8 g) were combined in a similar solution to formulate the ZnO/NiO-based paint. The ingredients were blended for 16 h in a jar mill (Indo-German Industries, Daman, India). TiO2 is an inert, non-inhibitive, non-toxic, and stable pigment and is conventionally used as a white pigment because of its high refractive index.22 Titanium oxide and DOP were incorporated into the epoxy resin to improve the paint's flexibility and viscosity.23 Xylene was used as a solvent for paint formulation. The hardener (Aradur 140) was introduced into the epoxy resin to accomplish cross-linking. The paint was then screened using fine cotton before being consistently coated on the MS samples with a film applicator to maintain a uniform paint thickness of 60 μm. Supplementary Figure S-1 shows digital pictures of the painted samples before and after coating.

Electrochemical Studies

The semiconducting behavior of ZnO/NiO-based painted MS was investigated using the Mott–Schottky approach, and the parameters used to execute the test are listed in Supplementary Table S-II. The open circuit potential (OCP) of bare MS, epoxy-painted, and ZnO/NiO-based painted MS samples were measured in 3.5 wt.% NaCl solution after 90 min of stabilization at various immersion times. The corrosion rate of bare, epoxy-painted, ZnO/NiO-based painted MS was calculated using Tafel extrapolation in 3.5 wt.% NaCl solution. This method generates polarization curves by supplying a voltage of 250 mV to the OCP in both the cathodic and anodic directions. Electrochemical impedance values were measured at the OCP with a 10-mV potential fluctuation over a range of frequencies of 105 Hz to 0.1 Hz. A Gamry Reference 1000 Potentiostat (Wilmington, USA) was used for all the electrochemical experiments, which were performed four times to confirm that the data were repeatable.

Results and Discussion

Structural Characterization

Figure 1 demonstrates the XRD pattern of a manufactured ZnO/NiO pigment. The hexagonal wurtzite structure of ZnO and cubic structure of NiO was confirmed with standard diffraction data of JCPDS no. 036-1451) and JCPDS no.047-1049), respectively. Table I illustrates the lattice parameters of the crystalline structures, as well as the quantification (wt.%) of phases derived by the Rietveld technique and the crystallite size estimated using the Scherrer formula.

Fig. 1
figure 1

XRD analysis of ZnO/NiO pigment calcined at 600°C.

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Table I Lattice parameters, crystallite size, and wt.% of ZnO and NiO in ZnO/NiO pigment
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Morphology and Composition Analysis of ZnO/NiO Pigment

SEM was utilized to examine the morphology of the synthesized pigment. Figure 2a shows the SEM micrograph of the ZnO/NiO pigment revealing that the particles exhibit an inconsistent appearance, with some larger particles coexisting with smaller particles, resulting in a bimodal distribution.21 The particle size is found to be in the range of 0.15–2.5 μm.

Fig. 2
figure 2

(a) SEM image at × 15 k magnification, (b) EDS spectrum of ZnO/NiO pigment calcined at 600°C.

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Figure 2b shows the EDS spectrum of the composite powder with the presence of Zn, Ni, and O. Supplementary Table S-III shows the wt.% and at.% of each element in the composite powder as observed in the EDS chemical analysis.

UV–Visible Spectroscopy

Figure 3a shows the UV–visible absorption spectra of the ZnO, NiO, and ZnO/NiO pigment samples measured in the range of 200–800 nm. The absorption spectrum of ZnO reveals that the absorption peak is strong at 368 nm owing to band gap absorption for ZnO caused by the removal of electrons from the valance band and transferred to the conduction band.

Fig. 3
figure 3

(a) UV–visible spectra and (b) reflectance spectra of ZnO, NiO, and ZnO/NiO pigment samples.

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The absorption band for the NiO sample can be observed at 331 nm. In the case of the ZnO/NiO pigment, the band edges appear at 331 nm and 368 nm which shows the existence of both NiO and ZnO in the composite sample. Figure 3b shows the UV-vis reflectance spectra of the ZnO, NiO, and ZnO/NiO pigment samples.

The band gap of the powders was calculated using the Tauc equation:

$$ \alpha h\nu = A\left( {h\nu - E_{g} } \right)^{\frac{1}{m}} $$
(1)

where A is a constant, m = 2 for the direct band gap, α = coefficient of absorption, h = Plank’s constant, υ = photon frequency, and Eg = the optical band gap.

The following equation can be used to compute the absorption coefficient (α):24

$$\alpha =\frac{{\left(1-{R}^{\prime}\right)}^{2}}{2R^{\prime}}$$
(2)

where R′ = R/100 and R is the reflectance.

The optical band gap (Eg) is evaluated by plotting (αhυ)2 versus hυ (supplementary Fig. S-2) and extrapolating the linear portion to the x-axis. The band the gap for the ZnO/NiO pigment was 3.39 eV, which is located between the band gaps of NiO (3.54 eV) and ZnO (3.34 eV) and in agreement with the reported literature.25,26 The composite powder is found to be a phase mixture of NiO and ZnO. ZnO is an inherent n-type semiconductor due to the existing zinc interstitials and electrons. Due to the existence of nickel vacancies and electron holes (Ni3+), NiO is an inherent p-type semiconductor.27

Mott–Schottky Plots

The Mott–Schottky plot (C−2 versus V) for the ZnO/NiO-based painted MS sample is depicted in Fig. 4. The positive slope of the curve in the plot depicts n-type behavior, whereas the negative slope indicates p-type behavior.28 The positive and negative slopes in the plot indicate that the ZnO/NiO-based painted MS exhibits p–n-type behavior. Since the constituent’s metal oxides are p-type NiO and n-type ZnO, it is anticipated that there should be multiple p–n heterojunctions in the synthesized pigment. The difference between these materials' band gaps and work function values (4.3 eV for ZnO and 5.3 eV for NiO) is responsible for the p–n heterojunction formation when these are in contact.29,30,31,32

Fig. 4
figure 4

Mott–Schottky plot of ZnO/NiO-based painted mild steel in 3.5 wt.% of NaCl solution.

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Open Circuit Potential Measurements

The OCP values of bare, epoxy-painted, and ZnO/NiO-based painted MS samples in 3.5 wt.% NaCl solution after 90 min of stabilization at various immersion times are shown in Fig. 5. It was found that the ZnO/NiO-based painted MS sample exhibited higher OCP values than bare steel and epoxy-painted steel. The shift in OCP values from negative to positive indicates that the ZnO/NiO-based paint has inhibited electrolyte penetration owing to the effective barrier ability of the ZnO/NiO pigments.

Fig. 5
figure 5

OCP values of bare, epoxy-painted, and ZnO/NiO-based painted mild steel samples in 3.5 wt.% NaCl solution for various immersion times.

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Potentiodynamic Polarization Study

Figure 6 depicts polarization curves created by varying the potential from Ecorr to 250 mV in both anodic and cathodic directions for bare, epoxy-painted, and ZnO/NiO-based painted MS samples in 3.5 wt.% NaCl solution. The corrosion potential, corrosion current densities, and corrosion rate values are tabulated in Table II. The corrosion potential shifted from − 626 mV for the bare steel to − 580 mV for the epoxy-painted sample, and to − 527 mV for the ZnO/NiO-based painted sample. The positive shift indicates an anodic protection mechanism in the case of painted steels. The corrosion rate for ZnO/NiO-based painted MS in 3.5 wt.% NaCl is 0.02 mpy, which is significantly lower than those of the bare steel and epoxy-painted steel samples.

Fig. 6
figure 6

Tafel plots for bare steel, epoxy-painted, and ZnO/NiO-based painted MS samples in 3.5 wt.% NaCl.

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Table II Corrosion rates for bare, epoxy-painted, and ZnO/NiO- based painted mild steel samples
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Electrochemical Impedance Spectroscopy Measurements

Electrochemical impedance spectroscopy investigations of the bare, epoxy-painted, and ZnO/NiO-based painted MS samples were carried out just after immersion and after 24, 48, 72, and 96 h of immersion of all the samples in 3.5 wt.% NaCl solution. These measurements, in terms of Bode and Nyquist plots, are depicted in Fig. 7.

Fig. 7
figure 7

Bode and Nyquist plots for (a, b) bare mild steel, (c, d) epoxy-painted mild steel, and (e, f) ZnO/NiO-based painted mild steel samples after immersion.

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The high-frequency and low-frequency portions in the Bode plots represent capacitive and resistive behavior, respectively. Zmod at 0.01 Hz is an important parameter for measuring anticorrosion efficiency.33 In the case of bare MS impedance, the (Zmod) values are seen to drop continuously from 158.3 Ω cm2 just after immersion to 129.3 Ω cm2, 107 Ω cm2, 106.3 Ω cm2, and 99.15 Ω cm2 after 24, 48, 72, and 96 h of immersion, respectively. Similarly, the epoxy-painted MS exhibited a continuous decrease in the impedance values from 21.11 kΩ cm2 just after immersion to 10.85 kΩ cm2, 7.16 kΩ cm2, 3.30 kΩ cm2, and 1.72 kΩ cm2 after 24, 48, 72, and 96 h of immersion respectively. These observations can be assigned to the corrosion of bare MS and the ingress of water into the epoxy coating and the subsequent dissolution of MS in a chloride medium. For the ZnO/NiO-based painted MS, Zmod is found to have increased from 341.5 kΩ cm2 just after immersion to 5.28 MΩ cm2 by the end of 24 h of immersion. Subsequently, it decreased to 1.35 MΩ cm2, 370 kΩ cm2, and 217.7 kΩ cm2 after 48, 72, and 96 h of immersion, respectively.

All the Nyquist plots show two semicircles. The first capacitive loop or semicircle observed in the Nyquist plot can be ascribed to the formation of corrosion products and the second one to the electrochemical processes occurring below the corrosion products. Hence, for bare MS, the circuit consisting of the charge transfer resistance, Rct, and the double-layer capacitance, Cdl, is used ,whereas for the painted MSs, the coating capacitance, Cc, the coating resistance, Rc, the double-layer capacitance, Cdl, and the charge transfer resistance, Rct, were utilized (Fig. 8; Table III).

Fig. 8
figure 8

Equivalent circuit used for modeling impedance curves: (a) bare steel, and (b) painted steel samples.

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Table III Impedance values obtained from Bode and Nyquist plots
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Also, the ZnO/NiO-based painted MS sample exhibited significantly higher Zreal values compared to the bare MS and epoxy-painted MS samples. The charge transfer resistance, Rct, values indicate the difficulty of transmitting electrons on the metal substrate, and can be compared to determine the effectiveness of various corrosion-resistant coatings.34 The corrosion resistance can be calculated from the charge transfer resistance in the case of bare MS, and from the numerical addition of charge transfer resistance and coating resistance in the case of the painted MS.35 It comes out to be 127. 2 Ω cm2, 13.63 kΩ cm2, and 171.83 kΩ cm2 for the bare MS, epoxy-painted MS, and ZnO/NiO-based painted MS samples, respectively.

The double-layer capacitance, Cdl, and coating capacitance, Cc, are also useful electrochemical parameters for corrosion protection evaluation. Corrosive species create a conductive channel for the diffusion of aggressive species. Thus, the higher the capacitance, the higher the permittivity of the solution.36 As the capacitance depends upon the area, the lowest values of coating capacitance shown by the ZnO/NiO-based painted sample compared to the epoxy-painted sample indicate less electrochemical degradation. Higher impedance Zmod and Zreal, coating resistance Rc, charge transfer resistance, Rct, values, an increase in all these values after 24 h of immersion, and the lowest values of coating capacitance in the case of the ZnO/NiO-based painted MS sample reveal its higher and long-term electrochemical resistance ability compared to the epoxy-painted MS. These findings are in agreement with the OCP and potentiodynamic polarization results (Fig. 5; Table II).

Corrosion Protection Mechanism

When steel comes into contact with the pigment, electrons are transported from the steel to NiO due to the lower work function (ϕ) of iron (4.5 eV) compared to that of NiO (5.3 eV).37,38 As the electrons are removed from the steel electrode, its potential is shifted in the positive direction. These electrons combine with the holes in the p-type NiO, resulting in a hole depletion zone in the NiO region, which appears in the downward bending of the band edges. This leads to the generation of an electric field that obstructs further electron transport.39 The majority of charge carriers in the p-type (NiO) and n-type (ZnO) regions are holes and electrons, respectively. When the p–n junction is formed, free carriers, both electrons and holes, diffuse across the junction. As the diffusion proceeds, the holes and electrons meet at the junction and recombine. The region of the junction where this recombination occurs is now depleted of charge carriers and is known as the depletion region, which appears in the bending of the band edges, as shown in supplementary Fig.S-3. At each edge of the depletion region, there is a build-up of the charge, electrons on one side and holes on the other. The built-in electric field formed in the depletion region creates a potential barrier which in turn impedes the flow of carriers across the junction.40 Charge transfer across the p–n heterojunction is restricted due to the potential barrier across the junction. Since ZnO has a higher Fermi level than the redox potential of the electrolyte, the electrons are transported from ZnO to the electrolyte.41 As a result, a positive charge remains associated with the space charge region. As the majority of charge carriers have been depleted, this region now becomes a depletion region, which appears in the upward bending of the band edges. The built-in electric field developed due to the formation of the depletion region opposes hole transport across the interface. The charge transport between the metal and the electrolyte will be restricted as a result of the arrangement presented in Fig. 9, culminating in metal corrosion prevention.

Fig. 9
figure 9

(a) Energy level diagram, and (b) corrosion protection mechanism.

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The novelty of this coating, therefore, lies in the protection of steel by the two mechanisms operating simultaneously: (1) improving the barrier effect due to the incorporation of pigments, and (2) prevention of charge transfer due to the p-type region, n-type region, and p–n junction. Since the protection mechanism depends on precluding charge transfer, it eliminates the possibility of the release of inhibiting toxic ions and corrosion products. From the commercial point of view, the novelty of the ZnO/NiO-based paint is that no primer is required, whereas all conventional paint coatings are made of a primer as a barrier coat and an inhibitive coat as the sub-coat or top coat.

Conclusion

ZnO/NiO pigment was successfully manufactured by a solid-state reaction approach and the paint containing the synthesized powder was painted on a MS substrate. XRD revealed that the pigment is composed of -ZnO (hexagonal) and NiO (cubic) phases, which were confirmed by UV–visible spectroscopy. The pigment's particle size was found from the SEM image to be in the range of 0.15–2.5 μm. The band gap determined from the Tauc plot for the ZnO/NiO pigment was 3.39 eV, which is between the band gaps of NiO (3.54 eV) and ZnO (3.34 eV). Mott–Schottky analysis revealed that the ZnO/NiO-based painted MS sample exhibited p–n-type behavior. The ZnO/NiO-based painted MS sample exhibited higher OCP values than bare MS and epoxy-painted MS, owing to the effective barrier ability of the ZnO/NiO pigments. The rate of corrosion for the ZnO/NiO-based painted MS in 3.5 wt.% NaCl was 0.02 mpy, which was significantly lower than that of the bare steel and epoxy-coated steel samples. The ZnO/NiO-based painted MS sample demonstrated the highest impedance (Zmod = 341.5 kΩ cm2, Zreal = 319.5 kΩ cm2), coating resistance (RC = 153.7 kΩ cm2), charge transfer resistance (Rct = 18.13 kΩ cm2) and lowest coating capacitance (CC = 77.54 × 10−9 F, double layer capacitance (CDL = 7.055 × 10−9 F) after immersion in 3.5 wt.% NaCl. Corrosion resistance values were 127.2 Ω cm2, 13.63 kΩ cm2, and 171.83 kΩ cm2 for the bare MS, epoxy-painted MS, and ZnO/NiO-based painted MS samples, respectively. The ZnO/NiO-based painted MS sample exhibited excellent corrosion protection due to the incorporation of ZnO/NiO pigments which improved the barrier effect and prevented charge transfer due to p–n junction formation.