1 Introduction

Metal structures’ deterioration is caused by their interaction with the environment, known as corrosion [1,2,3,4,5]. A broad variety of items, including pipelines, buildings, and other objects, are made from metal [6,7,8,9,10]. A key concern in the chemical industry is corrosion, which causes various problems in production lines and is generally the source of faults and production interruption [11,12,13,14,15,16]. Damage from corrosive units pollutes the environment and puts human health at risk. Gas, oil, and chemical industries employ steel alloys frequently because of its mechanical properties [17,18,19,20,21,22]. Steel is also used in industry because it is less expensive than anticorrosive metals or alloys [23,24,25,26,27,28]. Today, mild steel is utilized in various technical applications, including manufacturing vehicle components, structural forms, and sheets used in pipelines, buildings, plants, bridges, etc. Because of its resistance to general corrosion, mild steel is one of the most frequently utilized metals in corrosive settings [29,30,31,32,33,34,35]. However, some environments, such as hydrochloric acid, are very aggressive to steel alloys. Hydrochloric acid is used widely in the processes of pickling, acid cleaning, and many other industrial applications. The corrosion rate of metallurgy and alloys is an essential property of the ease with which these metals react to a particular environment [36,37,38,39,40]. Sintering activity in nanoparticles is high, and they are significant because of their unique characteristics, which might lead to new and interesting uses [41,42,43,44,45]. In high-temperature settings, nanoparticle coatings have strong thermal and electrical characteristics and are resistant to oxidation, corrosion, erosion, and wear [46,47,48,49,50,51,52]. These characteristics are critical in applications, such as pipes, castings, and the automobile sector.

Any substance with unique or innovative characteristics due to its nanoscale is referred to as a nanomaterial, NPs, (1–100 nm). In addition to strong catalytic activity, structuring nanomaterials have intriguing electrical, optical, and magnetic characteristics [53,54,55,56,57,58,59]. The structural dimensionality of nanomaterials has a major impact on photocatalytic activity. However, the structural dimensionality of nanomaterials has a major influence on their characteristics. Spherical NPs with zero dimensionality, for example, have a large specific surface area, leading to a faster photocatalytic breakdown of organic contaminants [60,61,62,63,64,65,66]. Surface characteristics like adhesion, wettability, corrosion resistance, and wear resistance can all be improved by functional coatings. In other situations, such as semiconductor device production (when the substrate is a wafer), the coating contributes a new attribute to the completed product, such as magnetic response or electrical conductivity, and is an integral component. In the context of corrosion, coating refers to thin layers of a covering substance deposited or put on the surface of any item to improve critical qualities and create a protective barrier against the surface deteriorating owing to its reaction with its environment [67,68,69]. Titanium dioxide (TiO2) is now a popular material for adsorbents, catalytic, super capacitors, lithium-ion batteries, and gas sensors due to its exceptional optical, electrical, photocatalytic, and thermal properties. The hue of pure TiO2 is almost white. Due to the presence of other metals, it appears in different hues, such as yellow and brown [70, 71]. The three main phases of TiO2 crystals are anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). The physical characteristics of each polymorph are very special. The rutile phase is, under environmental conditions, the most stable of the three polymorphs. Anatase and brookite may, however, be created and become rutile if the temperature increases. The scale of particles affects the stability of successive TiO2 phases. Anatase is thermodynamically stable when the nanoparticles are less than 11 nm. The enhanced surface characteristics of TiO2 nanosheets (NSs) are thought to make them more active than nanoparticles. This distinctive nanostructure included a significantly larger surface area, a greater number of open access sites, and enhanced durability, all of which are necessary for an effective coating [72, 73].

Many studies in the field of coating science are focused either on synthesis and diagnosis or on the testing of coating performance. Limited works take into considerations a comprehensive views of the synthesis, diagnosis, and testing via different techniques. The purpose of this study is to synthesize and characterize rutile TiO2 phase nanosheets. The synthesis process is achieved using the deposition electrochemical method. The characterization is carried out using XRD, SEM, TEM, and FTIR. The nano-synthesized rutile TiO2 nanosheets have been used for mild steel coating. Thermometric, weight loss, polarization measurements, and electrochemical impedance microscopy were used to investigate the corrosion protection of mild steel in 1 M HCl in the presence and absence of TiO2.

2 Experimental Work

2.1 Materials and Preparations

Mild steel was used as a test sample. It has the following chemical compositions (wt%): 0.4 C, 0.44 Mn, 0.038 S, 0.34 Si, 0.041 P, 0.35 Cr, 0.37 Ni, 0.09 Mo, and the balance is Fe. An electrolysis method was used to synthesize pure rutile TiO2 nanosheets using 120 mL of 0.1 M sodium hydroxide (NaOH) at 293 K as the electrolyte. A rectangular titanium sheet (40 mm × 20 mm × 1 mm) is utilized as the anode and counter electrode cathode. Before installing the substrates in the cell, they were cleaned acoustically using ethyl alcohol and deionized water in a 10-min sequence. For 3 h, the applied direct current voltage between the electrodes was 3 V with a current density of 9.2 × 10–3 mA/cm2. A white powder was formed, and the producing material was filtered with deionized water before drying for subsequent analysis overnights. The following is a proposal for an electrodeposition reaction route to produce TiO2 [24]:

$${\text{Ti}} \to {\text{Ti}}^{{{4} + }} + { 4}e^{ - } \left( {\text{at anode}} \right),$$
$${\text{Ti}}^{{{4} + }} + { 4}e^{ - } \to^{{}} {\text{Ti}}^{* } \left( {\text{at cathode}} \right).$$

Here, Ti* is an active form of Ti

$${\text{Ti}}^{{{4} + }} + {\text{ 4OH}}^{ - } \to {\text{Ti }}\left( {{\text{OH}}} \right)_{{4}},$$
$$\mathrm{Ti}{\left(\mathrm{OH}\right)}_{4}\stackrel{{\mathrm{Ti}}^{*}}{\to }{\mathrm{TiO}}_{2} \left(\mathrm{nanosheets}\right)+2{\mathrm{H}}_{2}\mathrm{O} \quad (\mathrm{at pH}=12).$$

Emulsion solution was prepared by adding different pure rutile TiO2 nanosheets powder concentrations to 100 mL of (acetic acid\water 1:4) as solvent. Then, the solutions were mixed using the ultrasonic technique method. A deposition cell device has been used to coat pure rutile TiO2 nanosheets on mild steel surface. The working electrode (mild steel) and small platinum sheet were inserted in the electrolyte as a counter electrode. The electrophoretic deposition of TiO2 nanosheet on the mild steel was carried out at room temperature with a direct current (DC) of 10 V and 45 min. Then, specimens were then cleaned and dried.

2.2 Methods

In the mylius tube, a piece of mild steel with a dimension of (2 cm × 0.5 cm × 0.01 cm) was employed with 15 mL of HCl solutions. Then the mylius tube was installed in a Dewar flask and sealed with a stopper and then drilled to insert the digital thermometer sensor. First, the temperature of the HCl solution was gradually raised until it reached a constant value. Next, the experiment was repeated with several concentrations of TiO2 nanosheet (25–200 ppm) [1,2,3,4,5].

For weight loss method, in a beaker, 50 mL of 1 M HCl solution was added, and mild steel (M.S) with a dimension of 2.5 cm × 0.8 cm × 0.03 cm (about 4.2 cm2) was suspended before being submerged in HCl solution. The weight of the sample was determined before and after immersion at different periods. Weight losses were determined for all samples of mild steel (25–200 ppm) and uncoated mild steel. All samples were washed with deionized water and acetone before being weighed and then, the weight was assessed. For purposes of accuracy, all tests were repeated twice and the average value was taken. The whole experimental work is explained in a flowchart as shown in Fig. 1.

Fig. 1
figure 1

Flowchart of experimental work

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In electrochemical analysis, a three-electrode assembly of 1 cm2 area mild steel as the working electrode, Pt as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode were utilized. During potentiodynamic polarization measurements, the Vertex One–Netherlands equipment was used. The Tafel graphs were recorded at a scanning rate of 0.5 mV/sec at a potential range of 2.5 V relative to OCP. From the liner region of the Tafel, its parameters are determined. Electrochemical Impedance Spectroscopy (EIS) measurements were taken in the 2500–1-Hz frequency range at OCP and a 10-mV AC potential. All electrochemical experiments for corrosion of mild steel in corrosive environments were conducted with and without the coating.

3 Result and Discussion

3.1 Diagnosis and Morphological Studies

XRD diagnosed the nature of produced powders. In Fig. 2, the TiO2 crystal structure was observed at diffraction angles between 20° and 80°, by XRD. Rutile TiO2 can be ascribed to the high peaks at the Muller values of 27.46°, 36.09°, and 54.34°, respectively, corresponding to 110, 101, and 211. With the formula of Scherrer (Eq. 1) [24], the dimensions of the crystallite were obtained. The predicted crystal size is approximately 24.6 nm.

Fig. 2
figure 2

XRD pattern of titanium oxide nanosheets

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$${D}_{\mathrm{av}}=\frac{\kappa \lambda }{\beta \mathrm{cos}{\theta }_{\mathrm{B}}},$$
(1)

where κ is the shape factor, λ represents the wavelength of the X-ray falling on the sample, β is the full width at half maximum, and θB is the Bragg’s diffraction angle.

SEM was used to determine the shape and size of the main nanosheets [11, 14], as illustrated in Fig. 3. The results reveal that the nanosheets are highly crystalline and exhibit distinct two-dimensional properties. They appear to have a rectangular form and nanoscale thickness. A few quantities of TiO2 nanoparticles are also shown with the produced nanosheets in the pictures. Their average size was estimated to be around 13 nm. In addition, a TEM examination of produced powders was performed. Aggregation occurs during the creation of nanosheets, as seen in Fig. 4. At the nanoscale, the flakes are revealed to have a thin thickness. The nanosheets’ large surface area necessitates research into their coating behavior [12].

Fig. 3
figure 3

SEM images of titanium oxide nanosheets

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Fig. 4
figure 4

TEM images of titanium oxide nanosheets

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Figure 5 shows the FTIR range of TiO2 nanosheets. The band, 500–600 cm−1, is a portion of the vibration bending (Ti–O–Ti) in the TiO2 matrix. The peak 1653 cm−1 concerning Ti–OH water bending modes. The broadest band, seen with 3373–3500 cm−1, is the extended vibration of the hydroxyl group O–H in TiO2 nanosheets [24].

Fig. 5
figure 5

FTIR spectrum of titanium oxide nanosheets

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3.2 Thermometric Studies

This method is considered one of the most delicate corrosion monitoring techniques. However, this approach is quite useful in analyzing the corrosion behavior of different metals and alloys in various conditions of corrosion mediums. Results of this technique were corroborated by well-established procedures, including weight loss and hydrogen evolution measurements. Mylius early documented and detailed the reaction vessel and process for assessing corrosion behavior using this method. Uncoated and coated mild steel surface with different concentrations of TiO2 nanosheets are shown in Fig. 6 at different time intervals (0–40 min). For the sample as uncoated (blank), the temperature was 17.9 °C at 40 min. Then, when coating the sample with 25-ppm TiO2 nanosheets, the temperature was decreased to 16.8 °C at 40 min. Finally, as a 50 ppm, when adding TiO2 nanosheets, the temperature dropped further to 15.0 °C at 40 min. However, these results agree with many researchers [9,10,11,12,13,14,15,16,17,18,19,20,21,22]. Temperature against time plots for blank and 25–200 ppm indicates the effect of (HCl) on corrosion of mild steel. The results can be evaluated as a function of reaction number (RN). It can be calculated and illustrated as shown in Eq. 2 and Fig. 7, respectively [12,13,14,15,16,17,18,19,20,21,22,23,24,25].

Fig. 6
figure 6

Relation between temperatures of the acid solution vs time for samples

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Fig. 7
figure 7

The values of reaction number (RN) and corrosion protection (%Pth) as a function of coating concentration

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$$\mathrm{Reaction\, Number } \, \left(\mathrm{R}.\mathrm{N}\right)=\frac{{T}_{\mathrm{m}}-{T}_{\mathrm{i}}}{t}.$$
(2)

Tm and Ti denote the highest and starting temperatures, respectively, while t denotes the time to achieve Tm. Furthermore, corrosion protection percent can be determined by thermometric method (%Pth) using the following equation [15,16,17,18,19,20,21,22,23,24,25].

$$\mathrm{Corrosion \,Protection } \, \left({\mathrm{\%P}}_{\text{th}}\right)=\frac{\mathrm{RN \,free}-\mathrm{RN \,coated}}{\mathrm{RN \,free}}\times 100.$$
(3)

It can be observed when adding TiO2 nanosheets, the RN decreases, and the corrosion protection %Pth increases to approach 78% at 200 ppm. However, this result emphasizes that metal surface is protected and isolated from corrosive environments. This result is in good agreement with many researchers [13,14,15,16,17,18,19,20].

3.3 Weight Loss Method and Kinetics Studies

It is clear from these experiments that weight loss increases with increasing exposure duration time and decreases when metal is coated in both the presence and absence of nanosheets. The weight loss of coated and uncoated mild steel in HCl solutions can be determined and shown in Fig. 8A. Furthermore, the relationship between the percentage inhibition efficiency of TiO2 nanosheet and time is shown in Fig. 8B. The inhibition efficiency was calculated by dividing the weight difference before and after immersion divided by the original weight. It is clear that the inhibition efficiency was high during the first hour, approaching 97% in the presence of 200-ppm TiO2 nanosheet and then it decreased with time. This indicates a weak stability of TiO2 nanosheet in corrosive HCl solution during the long term of exposure (more than 3 h). However, corrosion inhibitors and coating materials are used to protect steel alloys against corrosion in aggressive solutions, especially during pickling and acid cleaning processes. Most pickling processes using HCl are achieved within 10–20 min, which makes TiO2 nanosheet a practical inhibition material within this period [23].

Fig. 8
figure 8

A Relation between weights loss and concentration at different times at 20 °C and B relation between inhibition efficiency of TiO2 and time at different times at 20 °C

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The weight loss of mild steel per samples surface area (m2) per time (days) is shown in Fig. 9, which represents the corrosion rate of mild steel. The corrosion response was greater for uncoated mild steel. The lowest corrosion rate was at 200 ppm, where a dissolving piece of metal decreased. However, this is in good agreement with many researchers [23,24,25,26,27,28,29,30,31,32,33,34,35,36]. The corrosion protection percentage (%PCR) is determined using the corrosion rate of coated and uncoated metal surface using the following equation [34,35,36].

Fig. 9
figure 9

The values of corrosion rate (CR) and the Corrosion Protection efficiency (%PCR) against coating concentration at 20 °C

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$$\mathrm{Corrosion \,Protection } \, \left(\mathrm{\%}{\mathrm{P}}_{\text{CR}}\right)=\frac{\mathrm{C.R} \,{\text{uncoated}}-\mathrm{C.R} \,{\mathrm{coated}}}{\mathrm{C.R} \,{\mathrm{uncoated}}}\times 100.$$
(4)

The results showed that in the sample with a concentration of 200 ppm, the coating protection reached 75.93%. This is in good agreement with many researchers [48,49,50,51,52,53,54,55,56, 66,67,68,69,70,71,72,73].

Three models were suggested according to reaction order. Zero, first, and second order of reaction are shown in Eqs. 5 to 8.

$$\text{Weight loss}={k}_{o}t,$$
(5)
$$\text{ln}(\text{weight loss})={k}_{1}t,$$
(6)
$$\frac{1}{\text{weight loss}}={k}_{2}t,$$
(7)

where weight loss represents the difference between initial and final weight loss of mild steel after specific time. ko, k1, and k2 represent the rate constant of zero, first, and second order of reaction, respectively. After graphical analysis of these three models, the best fitting was obtained via zero-order model. Table 1 collects all kinetics data. The zero-order reaction provided the higher correlation coefficients (R2).

Table 1 Reaction equations data and correlation coefficients
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3.4 Electrochemical Studies

An efficient dynamic polarization technique was conducted for 30 min using a mild steel sample immersed in a 1 M hydrochloric acid medium and various inhibitor doses (25–200 ppm). Mild steel, standard calomel electrode (SCE), and platinum electrode make up the electrode fitting. Polarization investigations were used to identify polarization curvatures. Cathodic and anodic curves can be used to determine current corrosion density (icorr) and corrosion potential (Ecorr). The following equation is used to calculate the protection efficiency from polarization data (%PPO) [25,26,27,28,29,30,31,32,33,34,35].

$$\mathrm{\%}{P}_{\text{PO}}=\frac{ {\left({i}_{\mathrm{corr}}\right)}_{\mathrm{w}}-{\left({i}_{\mathrm{corr}}\right)}_{\mathrm{i}}}{{\left({i}_{\mathrm{corr}}\right)}_{\mathrm{w}} } \times 100,$$
(8)

where (icorr)w and (icorr)i denote the corrosion current densities without and with inhibitor, respectively. Figure 10A shows the changes in the anodic and cathodic polarization curves. As the concentration of the nano-inhibitor increases, the polarization curve changes and through it, the corrosion current and corrosion voltage decrease at a temperature of 20 °C. Therefore, the titanium oxide nano-inhibitor has a good effect on coating processes (Table 2) [30,31,32,33,34,35,36,37,38,39,40]. The cathodic and anodic slopes of the linear Tafel reign are evaluated and listed in Table 2. The slight changes in the values of ba besides the displacement in Ecorr (maximum 196 mV at 200-ppm TiO2), which is higher than 85 mV, confirm that the used TiO2 nanosheet is considered as an anodic-type inhibitor [74]. The values of bc are approximately constant in the absence and presence of the inhibitor. Figure 10B displays the results of open-circuit potential (OCP) experiments on mild steel corrosion in 1 M HCl in the absence and presence of various concentrations of TiO2 nanosheet. In all cases, a relatively stable potential was established after approximately one thousand seconds of immersion. The potential for mild steel corrosion initially increased in the blank system. This was attributed to the production of insoluble iron oxides or corrosion products on the surface of mild steel, which rendered the steel more passive [34]. In the presence of TiO2 nanosheets, the nature of the OCP was comparable to that of the blank. In general, the OCP values of coated systems were greater than those of uncoated systems. This can be due to the protective layer formed by the TiO2 nanosheets on the surface of the steel and suggests that anodic dissolution of the steel is inhibited. In addition, the OCP values increase when TiO2 nanosheet concentrations rise [36].

Fig. 10
figure 10

A Polarization curve and B OCP vs. time at 20 °C

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Table 2 Electrochemical data for coated TiO2 Nanosheets
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A.C impedance measurements for mild steel corrosion in 1 M HCl after a half-hour immersion were taken at 20 °C. Figure 11 shows the Nyquist plots for mild steel in unrestrained acid, as well as the inhibitor concentrations, and data are collected in Table 2. Electrochemical impedance spectroscopy analysis reveals the kinetics of electrode activities and the electrochemical system’s surface characteristics. The observed rise in impedance characteristics in inhibited solutions is connected to the inhibitor’s corrosion-inhibiting effect. The Nyquist plots for all systems generally assume the shape of one depressed semicircle, corresponding to one time constant, despite the presence of a tiny hint of low-frequency inductive activity. The capability of the semicircle dips with its center below the actual axis to show that capacitance is scattered across the electrode surface inhomogeneity. As a result, Nyquist plots were fitted to the equivalent circuit model, which consist of solution resistance (Rs), capacitance of the double layer (Cdl), and charge transfer resistance (Rct). It was previously used to study mild steel/acid solution interaction. When an inhibitor is added to an acid, Rct rises, and Cdl falls, indicating that the corrosion process is delayed. Cdl values decline due to inhibitor adsorption on the metal/electrolyte interface, caused by a reduction in the dielectric constant and a rise in the double-layer thickness. This indicates that the inhibitors reduce the mild steel specimen’s corrosion rate by adsorption on the already formed metal/electrolyte contact. To quantify the protective effect from electrochemical impedance data (%PEI), the charge transfer resistance values in the inhibitor’s absence (Rct) and presence (Rctinh) may be compared [75,76,77].

Fig. 11
figure 11

Nyquist plots and equivalent circuit for specimens in the presence and absence of coating

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$$\%{P}_{\text{EI}}=\left[1-\left(\frac{{R}_{\mathrm{ct}}}{{R}_{\mathrm{ct}}^{\mathrm{inh}}}\right)\right]\times 100,$$
(9)

where Rct and Rctinh are the charge transfer resistances for inhibited and uninhibited systems, respectively. Charge transfer resistance is calculated using the difference in impedance on the real axis at high and low frequencies. At the frequency where the imaginary component of the impedance is highest, the capacity Cdl is determined using Eqs. 10 and 11 [74,75,76,77].

$$f{\left(-{Z}_{\mathrm{max}}\right)}_{\mathrm{img}}=\frac{1}{2\pi {C}_{\mathrm{dl}}{R}_{\mathrm{ct}}},$$
(10)
$${C}_{\mathrm{dl}}=\frac{1}{2\pi {f}_{\mathrm{max}}{R}_{\mathrm{ct}}}.$$
(11)

Because the bigger adsorption layer acts as a dielectric constant, low double-layer capacitance suggests that less electric charge is stored. The increase in values in inhibited systems that correspond to an increase in the diameter of the Nyquist semicircle confirms that the corrosion-inhibiting effect is much stronger in the latter system, implying that the inhibitors’ corrosion-inhibiting effect is synergistically enhanced in the latter system. The Simplified Randles Cell is one of the most used cell models. A charge transfer, a solution resistance, and a double-layer capacitor are all incorporated (or polarization resistance). The double-layer capacitance is parallel to the charge transfer resistance. The Simplified Randles Cell is a useful model in and of itself and a foundation for more complex models. The comparable circuit for a Simplified Randles Cell is shown in Fig. 11. The charge transfer resistance (Rct) of mild steel in uninhibited HCl changes dramatically with the presence of an inhibitor. The value of charge transfer resistance grows as concentration rises. The fact that inhibitor efficiency has risen backs up this conclusion. The semicircular form of all Nyquist graphs implies that the charge transfer mechanism controls mild steel corrosion [74,75,76,77].

4 Conclusion

In conclusion, the findings of XRD, SEM, FTIR, and TEM techniques showed the formation of rutile nanosheets of TiO2 with an average size of 13 nm. Nanoparticles were used as a coating for steel surface in aggressive solution. The performance of nano-titanium oxide was 79.1, 80.1, 78.5, and 79.8% using thermometric, weight loss, polarization measurements, and electrochemical impedance microscopy, respectively, at 200 ppm of nanoparticles and 20 °C. Furthermore, polarization outcomes showed that the magnitude of corrosion current densities was reduced with the addition of nanosheets TiO2. While electrochemical impedance microscopy outcomes showed that the metal resistance was enhanced due to the formation of a coating layer on the surface. Reaction kinetics showed that the corrosion process was according to the zero-order model. However, more work may be recommended for the doping of TiO2 coating with traces of inorganic metal ions, such as zinc, samarium, strontium, and barium, which may enhance the performance of the protective layer.