Influence of pH values on the electrochemical performance of low carbon steel coated by plasma thin SiO_xC_y films

2.1 Materials

Carbon steel samples were obtained from Central Metallurgical Research Institute in Egypt and was analyzed by FOUNDRY-MASTER Optical Emission Spectrometer. The composition of it (in wt%) as follow: 0.271% C, 0.214% Si, 0.829% Mn, 0.0305% P, 0.0156% S, 0.0122% Cr, 0.0187% Ni, 0.0033% Al, 0.0041% Co, 0.0071% Cu, 0.0001% Ti, 0.0028 % V, 0.0095 % W, 0.0095% Pb, 0.0108% As, 0.0003% B and 98.57% Fe. Tetraethyl orthosilicate (TEOS) (Si(OC₂H₅)₄ with molar mass 208 g mol⁻¹ was acquired from Sigma-Aldrich with analytical grade 99%.

2.2 Preparation of SiOx-like thin film

The disc-shaped steel samples (25 mm in diameter and 10 mm in thickness) were polished with silicon carbide emery papers with different grades (60, 80, 120, 400, 800, 1000, and 2000) to get a smooth surface. Afterwards, the samples were washed in acetone and dried at room temperature. As shown in Fig. 1 the electrical electrodes are faced to each other and separated by a distance, this distance is called the gap distance between the two electrodes (interelectrode distance in the plasma reactor).

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Fig. 1

Schematic diagram for the discharge cell.

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The steel specimen was then placed in the plasma chamber at room temperature and the chamber was pumped down to 300 mTorr using a vacuum pump. After that, the argon gas was introduced into the bubbler system containing (TEOS) liquid and carried the TEOS vapor into the plasma reactor until the pressure reached 1 Torr. Thereafter, the rf power was switched on and fixed at 100 W for 5 min. SiO_x layers were synthesized by the plasma generator (RFG 1 K-13) with 13.6 MHz and 1000 W power. The schematic representation of the experimental setup is shown in Fig. 1.

2.3 Corrosive solutions

The electrochemical measurements of treated steel were carried out in a 3.5% NaCl solution (pH 6.5) at room temperature. Furthermore, the 1 cm gap distance sample (best condition) was evaluated in a wide range of pH values (1.0, 3.0, 5.0, 9.0, and 12.0) to provide a clear understanding of the corrosion behavior of the plasma-treated samples. The solutions with different pH were prepared using a phthalate buffer system as shown in the supplementary (Table S1) (Elsayed et al., 2016).

The analytical grades of the chemicals used were: HCl (37%), NaOH (99.9%), KCl (99.9%), borax (99.9%), NaCl (99.9%), and potassium hydrogen phthalate (99.9%).

2.4 Electrochemical measurements

Electrochemical experiments were carried out with a Potentiostat /Galvanostat /FRA analyzer (Autolab PGSTAT 30), using a classical three-electrode set up with a saturated calomel reference electrode (SCE) and a platinum grid as a counter electrode. The surface area of the working electrode was 1 cm².

Potentiodynamic polarization tests were performed by scanning an applied potential from −0.3 to 1 mV at a scan rate of 1 mV s⁻¹. The corrosion rate k_corr in (mmpy) was obtained from the I_corr values by the following equation (Fahmy et al., 2021).

The inhibition efficiency ( ${\eta }_{\mathrm{p}\mathrm{o}\mathrm{l}}$ ) was calculated from the potentiodynamic parameter Eq. (2).

Electrochemical impedance spectroscopy (EIS) was done by applying an AC excitation of 10 mV amplitude (peak-to-zero) of sinusoidal voltage in the frequency range from 100 kHz to 10 mHz at open circuit potential (OCP). Impedance data were modeled using ZSim 3.30d software to choose proper electrochemical equivalent circuits and calculate EIS parameters. Also, the inhibition efficiency ( ${\eta }_{\mathrm{E}\mathrm{I}\mathrm{S}}$ ) was calculated using Eq. (3).

2.5 Investigation of the deposited films

3.1 Electrochemical investigations

3.1.1

3.1.1 Potentiodynamic polarization measurements

The impact of plasma deposited silicon oxycarbide thin films on the electrochemical properties of low carbon steel in 3.5% NaCl aqueous solutions was demonstrated using potentiodynamic polarization curves. Both the cathodic and anodic polarization curves of low carbon steel in 3.5% NaCl at 30 °C (room temperature) dependent on the gap distance of electrodes are seen in Fig. 2.

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Fig. 2

Potentiodynamic polarization curves of untreated and plasma-treated steel at different gap distances of the electrodes in 3.5% NaCl at room temperature (30°) using a scan rate of 1 mV s⁻¹.

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The enhancement of both cathodic and anodic reactions of treated steel compared to the untreated ones were recorded a more pronounced enhancement was observed in the anodic reaction compared to the cathodic one. The associated electrochemical parameters such as corrosion current density ( $I_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$ ), corrosion potential (E_corr), corrosion rate (K_corr), corrosion inhibition efficiency ( ${\eta }_{\mathrm{p}\mathrm{o}\mathrm{l}}$ %), anodic, and cathodic Tafel slope ( ${\beta }_a$ , ${\beta }_c$ ) are shown in Table 1. The $I_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$ values are significantly decreased by reducing the gap distance and reaches a minimum at 1 cm gap distance. The corrosion current ( $I_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$ ) is reduced from 12 µA/cm² for the blank sample to 2.942 and 0.714 µA/cm² for treated samples at gap distances 2, and 1 cm, respectively. The sharp decrease of the corrosion current density indicates a large inhibition of localized corrosion, such as pitting corrosion. This means the protective coating blocks the defects in the steel substrate and prevents the penetration of Cl⁻ ions through it.

Table 1 Potentiodynamic polarization associated parameters of low carbon steel coated by plasma deposited thin SiO_xC_y films at different gap distances in the aqueous solution of 3.5% NaCl at room temperature.

Gap distance	Icorr (µA/cm2)	Ecorr (mV VS SCE)	βa (mV.dec−1)	βc (mV.dec−1)	K (mm/year)	Ƞpol %
blank	12.001	−809.3	115.00	−203	0.08981	ــــــــــــ
3.0 cm	07.351	−711.4	101.00	−325	0.05501	38.74
2.0 cm	02.942	−624.3	064.75	−310	0.02201	75.48
1.0 cm	00.714	−534.7	067.54	−316	0.00534	94.05
0.5 cm	06.264	−767.1	102.00	−224	0.04688	47.80

The protective efficiency reaches to 94.05 and 75.48% for 1 and 2 cm, respectively. Moreover, the protective coatings increase the corrosion potential of the steel substrate from −0.8093 V to −0.5347 V, −0.6243 V, and −0.7114 V vs. SCE, for 1, 2 and 3 cm gap distance, respectively. The difference in E_corr between the plasma deposited samples and the blank are more than 85 mV and shifted to more positive values. This is an indication that the silicon oxycarbide thin film highly retarded the anodic dissolution of steel.

Potentiodynamic results indicated that silicon oxycarbide thin films provide effective protection with efficiency up to 94.05 and 75.48% for 1 and 2 cm gap distance, respectively. The protective action of such film might be attributed to its chemical structure (SiO₂-like) which is mainly composed of the Si-O-C and Si-O-Si backbone, both of which are an essential key for metal protection (Saloum et al., 2018).

3.1.2

3.1.2 Electrochemical impedance spectroscopy (EIS)

The corrosion behavior of plasma-treated samples was also measured using EIS in a 3.5% NaCl solution at 30 °C and the results are explained in terms of Bode and Nyquist plots. According to the Nyquist plot Fig. 3a and Bode plot Fig. 3b and c, all plasma-treated samples exhibit two capacitive loops which account for two different time constants. The first loop observed at high frequencies correspond to the properties of the protective silicon oxycarbide layer and the second one at lower frequencies is characteristic for the steel substrate-electrolyte interaction (Anagri et al., 2019). The impedance spectra were analyzed by fitting the experimental data to an equivalent circuit model composed of two-time constants as shown in Fig. 3d, and the different EIS parameters such as film resistance (R_f), double-layer capacitance (C_dl), charge transfer resistance (R_ct), constant phase element (Y₀ and n) and efficiency of inhibition ( ${\eta }_{\mathrm{E}\mathrm{I}\mathrm{S}}$ %) are listed in Table 2. The parameters in Table 2 were obtained by fitting the experimental EIS data using the fitting tools in ZSim 3.30d software.

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Fig. 3

EIS analysis of base material and plasma-treated samples in 3.5% NaCl at room temperature (30°). (a) Nyquist plot, (b and c) Bode plots, and (d) the electrical equivalent circuit used to fit EIS data.

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Table 2 EIS parameters of low carbon steel coated by thin SiO_xC_y films based on gap distance in the aqueous solution of 3.5% NaCl at room temperature.

Gap distance	Rs (Ω cm2)	Rct (Ω cm2)	CPE		Rf (Ω cm2)	Cf		Cdl (μF cm−2)	Chi- square (χ2)	ηEIS %
			Yo (μ Ω −1 sn cm−2)	n		Yo (μ Ω−1 sn cm−2)	n
Blank	9.6	200.5	1200.00	0.73	-----	-----	-----	708.50	1.4 × 10−3	-----
0.5 cm	27.3	319.2	0.05	0.92	727.2	620.0	0.73	0.02	4.3 × 10−3	37.18
1 cm	89.5	3565.5	09.80	0.53	6300	88.0	0.61	0.50	1.1 × 10−3	94.37
2 cm	25.43	1739.6	62.30	0.73	3152	37.7	0.72	27.30	5.2 × 10−4	88.47
3 cm	26.2	387.0	300	0.62	10.54	300	0.79	80.15	3.3 × 10−3	48.19

The fitted data indicate that the chi-square (χ²) values are around 10⁻³. The lower value of χ² indicates a good agreement between the fitted data and the experimental results.

Constant phase element (CPE) was used in order to deal with the non-ideal capacitance response. The impedance of the constant phase element (ZCPE) and the double-layer capacitance (C_dl) are calculated by using Eqs. (4) and (5):

(4)

$$Z\mathrm{C}\mathrm{P}\mathrm{E}=[{{Y_0(\mathrm{j}\omega )}^n]}^{-1}$$

(5)

$$C_{\mathrm{d}\mathrm{l}}={({Y_{0}Rc\mathrm{t}}^{1-n})}^{1/n}$$ where Y₀ = magnitude of the CPE, n = CPE exponent, ω = angular frequency and j =  ${(-1)}^{1/2}$ .

Nonetheless, (n) determines diverse physical phenomena such as surface heterogeneity results from surface roughness, inhibitor porous layer formation and degree of polycrystallinity. The values of n merely reflect the surface homogeneity of the Si-O-C layer.

The double-layer capacitance (C_dl) is significantly decreased after plasma deposition because of the nature of the organosilicon layer which acts as a barrier layer and slows down the interaction between the electrolyte and steel substrate. C_dl values were decreased due to a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer.

Furthermore, the Bode plot obtained for untreated steel showed a capacitive loop in the low-frequency region. After the film deposition, the low-frequency maximum is still detected but a more intense component was developed in the high-frequency region. The second maximum correspond to electrolyte-film interactions and the presence of the two components suggests simultaneous reactions on the film and substrate surfaces takes place. The presence of small pores in the protective film that allow electrolytes to reach the steel substrate, and the onset of a low-frequency time constant might be the reason.

Moreover, the growth in the high-frequency (HF) maxima is accompanied by a drop in the low-frequency one, reveals a better performance of the system under the corrosive medium (Smith et al., 2015; Li et al., 2010). EIS results revealed good corrosion behavior of carbon steel coated with silicon oxycarbide thin films, which is in good agreement with potentiodynamic polarization analysis.

In conclusion, the 1 cm gap distance is the optimum gap distance for the deposition of an effective film against corrosion.

3.1.3

3.1.3 Influence of pH values on the corrosion performance

In order to get a more in-depth electrochemical investigation, a new series of samples were prepared using TEOS/ plasma and the gap distance was fixed at 1 cm. The electrochemical properties of newly prepared samples were inspected in various buffer solutions with different pH values. The electrochemical behavior of plasma-treated steel was tested in solutions with different pH values by electrochemical impedance spectroscopy (EIS). Fig. 4 displays the Bode plots of treated samples in comparison to the untreated ones in different pH solutions (1, 3, 5, 9, and 12). The untreated sample exhibits almost a similar behavior in all buffer solutions, which give a concavity shape with a maximum around 10¹ Hz.

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Fig. 4

Examples of EIS plots of plasma-treated samples in phthalate buffer solutions with different pH as: Nqusit (a, c, and e), and Bode (b, d and f) for pH 3, 9 and 12 at room temperature (30°).

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For the plasma-treated sample, the HF loop was detected in all pH solutions and in the solution with pH 9 indicating a great improvement in the corrosion resistance of the treated carbon steel in this medium (Rangel et al., 2019). Moreover, the presence of only one concavity at the high-frequency region suggests that there is no interaction between the solution and steel substrate. Therefore, the film entirely isolates the steel substrate from the corrosive electrolyte (Barranco et al., 2004; Yadav et al., 2019) which will be further confirmed in a later section through SEM investigations.

Also, it is worth noting that the phase angle was elevated to be ∼ 80° and 90° in the case of pH 9 and 12, respectively as presented in Fig. 4 d and f. This variation reveals the capacitive behavior of the coatings which retard the flow of oxidative species through it (Rangel et al., 2019).

The Bode plots also demonstrate the impedance as a function of the frequency where the impedance increases with decreasing frequency achieving the highest value at the low-frequency extreme. The total resistance of system R_t (R_t = R_ct + R_s) was derived from the impedance/frequency curve at the lowest frequency extreme (10⁻¹ Hz) (Rangel et al., 2020).

The obtained impedance was presented in Table 3 and it was seen that the highest impedance was observed in alkaline solutions with inhibition efficiency 99.58 and 93.49% for pH 9 and 12, respectively. However, the highly acidic solutions are aggressive towards coating causing cracks, pinholes, or other kinds of defects that may originate from the partial rupture of films allowing the electrolyte to diffuse through the film reaching the substrate.

Table 3 EIS parameters of plasma coated steel with 1 cm gap distance between electrodes in aqueous phthalate buffer solutions with different pH at room temperature.

pH	Sample	Rs (Ω cm2)	Rct (Ω cm2)	CPE Yo (μ Ω −1 sn cm−2)		Rf	Cf Yo (μ Ω −1 sn cm−2)		Cdl (μF cm−2)	Chi- square v	ηEIS %
1	Blank	46.70	269.6	110.00	0.80	-----	-----	-----	45.60	1.9 × 10−4	-----
	1 cm	46.08	584.2	57.00	0.87	18.25	5400	0.66	34.20	6.7 × 10−5	53.83
3	Blank	300.90	415.5	190.00	0.76	-----	-----	-----	85.20	7.0 × 10−5	-----
	1 cm	278.66	996.5	0.065	0.86	1089	45	0.78	0.01	1.4 × 10−4	58.30
5	Blank	180.70	56.8	1.17	0.93	-----	-----	-----	0.56	1.5 × 10−3	-----
	1 cm	177.38	182.0	0.027	0.78	160.9	6.14	0.88	8.5 × 10−4	1.3 × 10−3	68.79
9	Blank	151.50	7772.0	240.00	0.71	-----	-----	-----	309	6.5 × 10−5	-----
	1 cm	167.30	1.89 x106	1.7 x10−4	0.89	16,000	1.6 × 10−3	0.76	6.2 × 10−5	1.1 × 10−1	99.58
12	Blank	14.40	566.3	43.00	0.67	-----	-----	-----	6.89	1.1 × 10−3	-----
	1 cm	14.93	8705.2	5.40	0.80	3500	53	0.44	2.51	8.0 × 10−4	93.49

The electrochemical performance of silicon oxycarbide films in the alkaline solution can be explained through the following statement.

The strong alkaline solutions (pH more than 9) are OH-rich which can break the polarized Si-O-Si bond in the amorphous silicon oxycarbide matrix (Sorarù et al., 2002).

3.2 Surface morphology

The surface of the steel before plasma treatment was smooth and crack-free as shown in Fig. 5a. Moreover, the impact of plasma treatment on the morphological features of steel was studied for a gap distance of 1 cm between plasma electrodes as an example (optimum gap distance) using the SEM as presented in Fig. 5c and show that the mild steel was covered by a homogeneous layer.

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Fig. 5

SEM image of carbon steel sample: blank (untreated) (a), blank sample after 24-hour immersion in 3.5% NaCl solution (b), the deposited sample with a gap distance 1 cm as an example (c), the deposited sample with a gap distance 1 cm after 24-hour immersion in 3.5% NaCl solution (d), cross-section of the treated sample with a gap distance 1 cm (e), cross-section of the treated sample with a gap distance 1 cm after 24-hour immersion in 3.5% NaCl (f).

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Fig. 5b reveals the surface morphology of the untreated sample after 24-hour dipping in a 3.5% NaCl solution. The steel surface was damaged by pitting corrosion caused by chloride ions attacked the steel surface. Conversely, the plasma-treated samples after dipping show considerable improvement as presented in Fig. 5d. The corrosive medium caused some of the ruptures on the protective layer, but the steel below it was protected, which confirms that plasma/TEOS treatments provide a good protection for the steel against corrosion.

Moreover, the cross-section view of the coating (Fig. 5e) shows that the thickness of the deposited layer was in the range from ∼ 1.3 to 1.8 µm. The cross-section view after 24-hour immersion in 3.5% NaCl was revealed in Fig. 5f and displays a strong superficial damage of the protective layer but still covers the steel sample and isolates it from corrosive solution. Additionally, the SEM images of 2 and 3 cm gap distance are presented in Fig. S1 and show that the homogeneity of the deposited film gets worse by increasing the gap distance between two electrodes.

The effect of pH of corrosive solution on the morphological feature of plasma-treated carbon steel was revealed in Fig. 6a and 6b. The corrosive medium (pH 1) caused strong pitting on the protective layer, but the film still appears united and shows a spongy-like structure (Fig. 6a). Contrastingly, the surface of SiOC layer exhibits a very protected surface after 24 h immersing in pH 9 solution and appeared homogeneously structured, with little pits and cracks (Fig. 6b). This can potentially account for unique electrochemical results of plasma-treated samples in this medium.

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Fig. 6

SEM images of plasma-treated steel samples with 1 cm gap distance after 24-hour immersion in solution with pH: (a) 1, (b) 9 at 30°.

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Finally, the plasma-treated samples display a great stability in neutral and alkaline mediums confirming the plasma/TEOS treatments afford good protection for the steel against corrosion in these mediums and agreeing well with the electrochemical measurements results.

3.3 Elemental analysis using energy dispersive analysis of X-ray (EDX)

The elemental composition of the deposited film was estimated using EDX analysis, the obtained results were revealed in Table 4.

Silicon, oxygen, and carbon were detected in all of the treated samples. Moreover, it was found that the atomic percentages (At%) of previous elements had a regular trend in dependence on the gap distance between the two electrodes indicating the gap distance is an influential parameter in the plasma deposition. However, the sample with 1 cm gap distance contained the highest Si, O, and C (At%), and the lowest Fe (At%) in the limit of the EDX. Conversely, a 3 cm gap distance sample holds the lowest Si and O content while the carbon content is undetectable. The enhancement of the deposition process with the reduction of the gap distance may be attributed to the fact that the TEOS molecules were fragmented in the vapor-plasma phase generating highly reactive species. These fragments are immediately randomly recombined with each other (Fahmy et al., 2012). This condensation is taking place on the steel electrode surface forming a highly cross-linked interfacial layer which may provide good corrosion protection (Vassallo et al., 2006). Furthermore, the species leading to film growth are the radicals rather than the ions. The ions which are accelerated in the sheaths mainly contribute to ion bombardment and structural rearrangement of the growing layer. As results, the films consist of various silicon oxycarbide, silicon carbide, or silicon oxide matrix (SiO₂ like) based on the gap distance between the electrodes. When the two electrodes get closer to each other (less than 1 cm), fragment ions were deposited on the substrate. In this case, the plasma sheath on the two electrodes is interfered and minimized the deposition rate of films. In addition to the fragmentation of the monomer and poly-recombination, the area of plasma energy, in such case, is very small leading to etching of the layers that are already deposited (Scheme. 1) (Fahmy and Friedrich, 2013).

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Scheme 1

Suggested reactions mechanisms of TEOS in plasma phase.

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TEOS (theoretically) consists of Si (07.7), O (30.8), C (61.5) atomic percent, therefore, it can be noted that the composition of the deposited layer might be mainly silicon oxycarbide (Si-O-C This suggestion is proven later by FTIR measurements.

Table 5 presents the elemental percentages (At%) of thin films deposited at optimum gap distance (1 cm), after 24 h immersion in various pH solutions. The obtained results reveal that silicon oxycarbide has a great stability in the neutral and alkaline mediums.

3.4 Chemical bonding of the deposited layer

The structure of the deposited films was investigated using FTIR in ATR mode as mentioned previously. The absorbance bands of deposited films with 1 and 2 cm gap distance (as examples) were selected and presented in Fig. 7. All treated samples have similar peaks, confirming that they have the same film composition however, the intensity of the peaks diminishes as the gap distance increases. The presence of absorption bands in the range from 440 to 1340 cm⁻¹ can be referred to as different vibration modes of the oxygen– silicon bridge (Si-O-Si) (Kakiuchi et al., 2012). Rocking, bending, and stretching of the Si-O bond in Si-O-Si are revealed by the bands in ∼ 440, 810, and 1050 cm⁻¹, respectively. Furthermore, the absorption bands ranging from 1375 to 1444, 1520–1830, and 2200 cm⁻¹ correspond to CH_x bending vibrations, (C = C; C = O), and Si–H, respectively (Pfuch and Cihar, 2004; Cho and Boo, 2012).

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Fig. 7

FTIR spectra of plasma deposited films based on the gap distance 1 and 2 cm compared to the blank sample.

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The inset of Fig. 7 shows the spectra of the deposited film with a 1 cm gap distance in the range from 1250 to 910 cm⁻¹ to quantitatively contribute to the embedded silicon content using Gaussian functions in Origin software, version 2018. Wavenumbers were given along with the calculated intensities in percentages based on the area under the peak and the regression coefficient was better than 0.999.

Peaks at 1050–1150 cm⁻¹ are characteristic of Si–O–Si, Si–O–C, and C–O–C stretching. The broad peak at 1095 cm⁻¹ correspond to the C–O stretching vibration (Fahmy et al., 2011) while the detected band at 1040 cm⁻¹ confirms the formation of Si–O bonds results from the Si-O–C covalent bond while the peak that monitored at ∼ 1200 cm⁻¹ is corresponding to C-O-C (Xie et al., 2011). The C–O and Si–O stretches are predictable at 1095–1150 cm⁻¹ (The inset of Fig. 7), therefore, named as antisymmetric and symmetric Si–O–C stretches (Fahmy et al., 2020).

Thus, it can be assumed that the deposited film composes mainly of a silicon-oxycarbide matrix. In detail, silicon oxycarbide is a crosslinked material that has a structure resembling a silicon dioxide network (SiO₂) containing some carbon atoms instead of the oxygen in the tetrahedral network and is generally described as (SiO_xC_y) (Vassallo et al., 2006). Also, carbon can exist as a free carbon phase form (graphite-like phase) (Yang et al., 2005; Sen et al., 2013). The silicon oxycarbide network is classified into SiO₂- like or SiC- like constituents based on the amount of incorporated carbon in the network (Niemiec et al., 2018). Since the FTIR data does not specify any distinct peaks of Si-C, this meaning that not much carbon has been incorporated into the silicon oxycarbide matrix which confirms that the deposited film is SiO₂ like.

3.5 Crystallinity

The crystalline structure of untreated / treated steel samples was examined by XRD and is presented in Fig. 8. In the XRD survey all treated samples (based on the gap distance) exhibit the same diffraction patterns compared to the one obtained with the untreated sample. Two θ (°) 44.3 and 64.8° for reflection from plans 220 and 400, respectively were observed (Anantharaman and Al-Omari, 2012). Treated samples with gap distances 0.5 and 3 cm show peaks (44.3 and 64.8°) in the similar position of the untreated sample and lower intensities compared to the blank ones. While the treated samples with gap distances 1 and 2 cm are shifted to lower 2θ. This provides strong evidence that the silicon compound incorporates as an interstitial impurity in the steel lattices causing expansion due to such incorporation (Sethi and Bhat, 2019). It can be concluded that the deposited films in case of gap distance 1 and 2 cm are chemically bonded to the mild steel substrate.

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Fig. 8

XRD patterns of treated samples at different gap distances between the two electrodes.

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