Fabrication of biochar-based superhydrophobic coating on steel substrate and its UV resistance, anti-scaling, and corrosion resistance performance

In this study, we report an eco-friendly and facile process for the synthesis of biochar, BC, and a cobalt-biochar nanocomposite, Co-BC, using rice straw biomass. We constructed two superhydrophobic coatings on steel substrates using potentiostatic electrodeposition of nickel-modified biochar, Ni@BC, and nickel modified by cobalt-biochar nanocomposite, Ni@Co-BC, then, these coatings were soaked in an ethanolic stearic acid solution. Fourier transform infrared spectroscopy showed that the stearic acid-grafted Ni@BC coating, Ni@BC@SA, and the stearic acid-grafted Ni@Co-BC composite, Ni@Co-BC@SA, were well grafted on the steel surface. Scanning electron microscopy revealed that the superhydrophobic coatings have nanoscale features. Atomic force microscopy results showed that the Ni@Co-BC@SA coat had higher roughness than Ni@BC@SA, resulting in higher superhydrophobicity. The water contact angles for Ni@BC@SA and Ni@Co-BC@SA coatings were 161° and 165°, respectively, while the values of water sliding angles for both coatings were 3.0° and 1.0°, respectively. Quantitative estimation of the scale inhibition efficiency revealed that the Ni@Co-BC@SA coating exhibited greater efficiency compared to the Ni@BC@SA coating. Additionally, the Ni@Co-BC@SA coating demonstrated improved corrosion resistance, UV resistance, mechanical abrasion resistance, and chemical stability compared to the Ni@BC@SA coating. These results highlight the superior performance of the Ni@Co-BC@SA coating and its potential as a highly effective and durable superhydrophobic coating for steel substrates.

Biochar synthesis from rice straw.The process for manufacturing BC involved thoroughly washing the rice straw to get rid of any impurities, then drying it in the air before putting it in an oven for 24 h at 60 °C.The clean, dried rice straw was then processed via a mixer to create a fine powder.The BC was then produced by pyrolyzing ten grams of the fine powder for three hours at 600 °C in a muffle furnace.
CoSO4⋅7H 2 O (4.8 g) was added to 100 mL of deionized water containing 10 g of the rice straw fine powder with a wt% ratio of 1:10 for Co: rice straw fine powder.The mixture was sonicated for 30 min and then stirred for 1.0 h.After that, the mixture was oven-dried at 60 °C overnight.After that, it was pyrolyzed in a muffle furnace at 600 °C for three hours to obtain the cobalt-modified biochar material, Co-BC.

Superhydrophobic coating construction.
Before electrodeposition, emery paper of various classes was used to mechanically polish the steel substrate, starting with coarse (grade 120) and working up to the finest (800 grade).After being degreased in a solution of soap for ten minutes, the substrate was then immersed in 2.0 M H 2 SO 4 for one minute, washed with distilled water, and then put straight into the electrodeposition bath.Table 1 shows the electrodeposition considerations for creating a Ni@BC and Ni@Co-BC coating on the steel substrate.The platinum sheet was utilized as the anode and was placed apart from the steel substrate, acting as the cathode, by a spacing of 2.0 cm.Ni@BC and Ni@Co-BC coatings that had been made were washed with distilled water and allowed to dry at room temperature overnight.Then, the substrates were placed in ethanolic solutions containing 1 × 10 -2 M stearic acid, SA, for 15 min before being dried at room condition for 24 h.Different characterization and evaluation procedures were applied to the Ni@BC coating modified with stearic acid, Ni@BC@SA, and the Ni@Co-BC coating modified with stearic acid.Ni@Co-BC@SA.
Surface characterization.Utilizing a Fourier transform infrared spectrophotometer, FTIR, the surface's chemical composition was examined (model: Bruker Tensor 37 FTIR).An X-ray diffractometer was used to conduct an X-ray diffraction analysis using monochromatic Cu K radiation (Bruker D2 phaser).The surface topography of the created SHP coatings was examined utilizing a scanning electron microscope, SEM (model JSM-200 IT, JEOL).The atomic force microscopy, AFM, was accomplished by Scanning Probe Microscope (SPM9600-Shimadzu Japan).Utilizing an optical contact angle goniometer, the WCA and WSA were calculated using 5 µL water droplets (Rame-hart CA instrument, model 190-F2).The WCAs and WSAs values that are displayed are the averages of three measurements made at various substrate locations.

UV resistance tests.
The wettability of the prepared SHP surface at various time intervals under UV irradiation (λ = 365 nm, 300 W) was used to test its UV resistance.Every two hours, the WCA values were determined.The UV lamp and SHP coating remain spaced apart by about 10 cm.By analyzing the same sample at five separate sites, the average values were calculated.

Results and discussion
Composition of the fabricated coatings.The Fourier transform infrared spectrophotometer, FTIR, was used to analyze the surface's chemical composition of the fabricated coats.Figure 1 displays the FTIR spectra of coated steel with Ni@BC@SA and Ni@Co-BC@SA.The spectra of steel processed with Ni@BC@SA has large absorption peaks at 3278 cm −1 , which are the stretching vibration modes of the -OH group 52 .The asymmetry and symmetry vibration of the stearic acid's -CH 2 -is attributed to the peaks at 2856 cm −1 and 2922 cm −1 , respectively 53 .The shoulder near 1579 cm −1 matched the stretching vibration of C=O, and C=C 36 .The O-H bending or C-O stretching vibration of phenol is responsible for the peak at 1342 cm −138 .The bending vibration of -C-OH is responsible for the peak at 1083 cm −138 .Ni(OH) 2 corresponds to the peak at 716 cm −132 .The spectra of steel coated with Ni@Co-BC@SA exhibit the same peaks as those of Ni@BC@SA with an additional peak at 471 cm −1   The XRD technique was used to determine the crystal orientation and composition of steel grafted by Ni@ BC@SA and Ni@Co-BC@SA SHP coatings.Figure 2 displays the XRD patterns of various prepared coatings.The Ni @BC@SA coat exhibits 5 diffraction peaks in its XRD pattern.The four peaks at 2Ɵ values equal 42.8°, 53.1°, 73.3°, and 89.9° are related to the faced cubic centered, fcc, of Ni (JCPDS NO. 04-0831).The XRD peak at 2Ɵ values equals 28.9° is corresponds to biochar 56,57 .
Table 1.Bath constituents and working environments for electrodeposition of Ni@BC@SA and Ni@Co-BC@ SA coating on the steel surface.While the XRD pattern of the Ni@Co-BC@SA composite demonstrated the same peaks of the Ni @BC@SA with a lower intensity and other new peaks at 46.2°, 56.4°, and 78.5° which are referring to face-centered-cubic cobalt 58 .The XRD peaks of steel coated with Ni@Co-BC@SA are broad, showing that the deposited structures have small-sized particles.
Morphology and wettability of the fabricated coatings.One of the most crucial things to take into account while analyzing SHP features is surface morphology, so the topography of the produced SHP coatings on the steel substrate has been investigated using the SEM technique.A micrograph of coated steel with Ni@ BC@SA is shown in Fig. 3a, it is obvious that the formed structures contain particles with a diameter of only a few nanometers.Some of the nanoparticles produce larger aggregated particles.Figure 3b shows a micrograph of steel that has been grafted with Ni@Co-BC@SA film.The Figure demonstrates that the deposited structures contain smaller, circular-like nanoparticles than Ni@BC@SA film.Apparently, the Co might act as a nucleation site and speed up the nucleation process rather than crystal growth, which is why the Ni@Co-BC@SA coating  contains smaller nanoparticles 59,60 .The Ni@Co-BC@SA hence exhibits stronger superhydrophobicity due to its higher surface roughness.The transparent flakes of BC layers are clearly seen especially in the case of Ni@ Co-BC@SA.The WCAs and WSAs were measured to ascertain the wettability behavior of the constructed SHP coatings.Ni@BC@SA and Ni@Co-BC@SA films have WCA values of 161° and 165°, respectively, while both films have WSA values of 3.0° and 1.0°.These findings suggest that Co promotes superhydrophobicity and roughness.The air that can be stored in the nanostructures effectively prevents water from touching the surface 61 .Additionally, Ni@Co-BC@SA coated steel has wettability that is advanced to numerous earlier documented values [62][63][64][65] .

Roughness of the fabricated coatings.
The surface roughness of the bare and SHP coated steel was further characterized using the AFM.According to the 3D AFM image, Fig. 4a, the bare steel's arithmetic average roughness, Ra, was 0.34 µm.For SHP coated steel by Ni@BC@SA, the Ra value increased to 1.60 µm, showing that the deposited coat increases the steel surface roughness, Fig. 4b. Figure 4c shows that the Ra value for SHP coated steel by Ni@Co-BC@SA increased to 2.21 µm, which can be attributed to the BC doping with cobalt, which significantly increases the roughness of the steel surface.

Chemical stability.
To demonstrate that the created SHP film may be utilized in the industrial sector, a chemical stability test must be carried out.The correlations between pH and the WCAs and WSAs of water droplets on the produced SHP coatings are displayed in Fig. 5. Figure 5a shows that the Ni@BC@SA films are SHP between pH values of 3 and 11, while Fig. 5b shows that the Ni@Co-BC@SA films are SHP between pH values of 2 and 12, where the WCAs are frequently greater than 150° and the WSAs are less than 10°.As a result, incorporating Co to BC enhances the SHP coating's chemical stability in both basic and acidic environments.To investigate the effect of prolonged immersion time of the coating on its superhydrophobicity, we measure the WCA of a coating in different pH values (3, 7, and 11) at different immersion times 0.5, 2, 4, and 6 h, Fig. 6.The results show that, at pH 7 the coated steel with Ni@BC@SA retains its superhydrophobic characteristics at all examined immersion periods where the WCA is always greater than 150°, while for pH 3, the coat retain superhydrophobicity until immersion time of 2 h, and finally for pH 11, the coat retains superhydrophobicity until www.nature.com/scientificreports/immersion time of 0.5 h.While the coated steel with Ni@Co-BC@SA retains its superhydrophobic characteristics at the different pH until immersion time of 6 h (the maximum examined immersion time).The SHP coated steel with Ni@Co-BC@SA has greater chemical stability than several previously known values 62,66 .
Mechanical stability.Mechanical abrasion can damage the SHP surfaces.Even when touched with the finger, some SHP surfaces can crack 67 .The primary focus now is on enhancing the abrasion resistance of SHP coatings so can use in the industrial sector 68 .The prepared SHP films were subjected to abrasion tests to determine their resistance to mechanical abrasion.Figure 7 depicts how the abrasion length affects the changes in WCAs and WSAs of the prepared SHP films.The prepared Ni@BC@SA SHP film retains its SHP property until a 500 mm abrasion length.While the prepared Ni@Co-BC@SA SHP film retains its SHP property until a 900 mm abrasion length.These findings showed that adding Co to the developed SHP BC-based film greatly increased its mechanical stability.The coated steel with Ni@Co-BC@SA has greater abrasion resistance than several previously known values [69][70][71][72] .
The durability of the SHP sample is investigated via storage in an ambient atmosphere.After three months of storage in air, the values of the WCAs of Ni@BC@SA and Ni@Co-BC@SA films are 151° and 158°, and the WSAs are 9° and 5°, respectively.These findings show that the produced SHP films demonstrate long-term stability and durability and are stable in air.

Corrosion resistance behaviour. PDP results.
The PDP technique has been used to examine the corrosion behavior of bare and SHP coated steel by Ni@BC@SA and Ni@Co-BC@SA.Figure 8 displays the PDP plots of uncoated and SHP coated steel in a 0.5 M NaCl aqueous solution.The oxygen reduction reaction is represented by limiting diffusion currents in the cathodic polarization curves, Eq. (1) 73 .
Therefore, the cathodic process is controlled by the oxygen gas diffusion from the bulk to the surface of the electrode.The development of an ideal anodic Tafel area is hindered by the rapid generation of corrosion products on the electrode surface for the bare steel, or by the development of a passive layer when the steel is treated with a SHP coating 74,75 .
The PDP parameters including corrosion potential (E corr ), corrosion current density (i corr ), and protection efficiency (%P) of bare and SHP coated steel are displayed in Table 2.The protection efficiency was calculated using Eq.(2) 76 .
(1) where, i o. and i are the corrosion current density of the bare and SHP coated steel.Because of the coated steel's SHP properties, the i corr .value for coated steel with Ni @BC@SA is smaller than that for bare steel.The SHP coating microstructures' trapped air can lower the contact area between the steel and solution, which causes a more rapid decrease in the i corr value 77 .The doping of biochar with cobalt enhances the SHP coating property and leads to a higher reduction in the contact area of the medium and steel.Therefore, steel coated with Ni@ Co-BC@SA has a greater protection efficiency than coated steel with Ni @BC@SA.
EIS results.Figure 9 displays the Nyquist and Bode plots of uncoated and SHP coated steel in a 0.5 M NaCl solution.The Nyquist plots, Fig. 9a, exhibit a depressed capacitive semicircle at high frequency and a diffusion tail at low frequency.The interfacial charge transfer reaction is what causes the depressed capacitive semicircle of the Nyquist plots at high frequencies 78,79 .Mass transport is responsible for the diffusion tail at low frequencies.These results indicate that the existence of a protective SHP layer is the reason why the steel coated with Ni@BC@SA exhibits superior charge transfer resistance than bare steel.The coated steel with Ni@Co-BC@SA displays the highest capacitive semicircle, indicating that it offers the greatest degree of protection.The doping of biochar with cobalt enhances the superhydrophobicity and so the Ni@Co-BC@SA coat becomes more effective to restrict the transfer of corrosive species like Cl -and H 2 O into the surface of the steel metal.The manufactured SHP coated steel in 0.5 M NaCl solution exhibits greater impedance at the low frequency on the Bode plots than bare steel, as shown in Fig. 9b.This confirms that the steel substrate is being protected by the created SHP coatings.Two-time constants are shown at low and intermediate frequencies in the phase angle plot, Fig. 9c.The unprotective corrosion products of bare steel or the protective SHP coating were responsible for the time constant that appeared in the low-frequency region.The electrical double layer was responsible for the time constant that appeared at the moderate frequency [80][81][82] .
The equivalent circuit depicted in Fig. 10 was employed to fit the EIS experimental results and the Zsimpwin program was used to determine the impedance parameters.The components of the equivalent circuit are; charge transfer resistance, R ct , double-layer constant phase element, CPE dl , solution resistance, R s , and Warburg element.W. Equation ( 3) was used to determine the protection efficiency 76 :     3.As it is obvious, each of Rct and %P of the bare steel < steel + Ni@BC@ SA < steel + Ni@Co-BC@SA, and so the corrosion resistance increases in the same order.The corrosion resistance of the SHP coated steel by Ni@Co-BC@SA is greater than numerous formerly recorded values [83][84][85] .
Mechanism of anti-corrosion performance.On the bare steel surface, the water molecules can readily adsorb.Uncoated steel can also suffer severe corrosion from chloride ions adhering to the surface and forming [FeClOH] − .As a result, the corrosion process can simply be initiated when Cl -ions and water come into contact with the metal surface 86 .
On the other hand, steel that has been coated with SHP films has a nanostructure that has hydrophobic material adsorbed to it.The holes between the peaks of the rough surface are easily filled with air.Due to the obstructive effect of trapped air, aggressive ion species in corrosive environments, such as Cl − , may rarely attack the underlying surface 10,86,87 .The SHP surface in neutral solutions was also shown to be negatively charged.According to reports, the presence of electronegative functional groups gives biochar a negative zeta potential value [88][89][90] .A biochar-based SHP surface's negative charge led to a drop in the amount of Cl − anion present close to a solid surface, increasing corrosion resistance 10 .It is also reported that the cobalt oxide nanoparticles have a negative zeta potential value [91][92][93] .So, the steel coated with Ni@Co-BC@SA has enhanced corrosion resistance than the SHP Ni@BC@SA coating.
Anti-scaling performance.The weight gained of CaCO 3 at the substrate surface is used to test the ability of a given substrate to suppress scale formation and adherence to it.Figure 11 shows the increase in weight of CaCO 3 (mg/cm 2 ) of the bare steel and SHP coated steel every 2 h until 20 h of immersion in a solution of 0.01 M NaHCO 3 and 0.01 M CaCl 2 at 60 °C.The Figure shows that the bare steel has a higher weight gain value than steel coated by Ni@BC@SA.So, the prepared SHP coated steel has a lower rate of scale formation due to the intrinsically low surface energy of stearic acid as well as the air pockets between the nano structures 94 .The steel coated by Ni-Co @BC@SA has the lowest weight gain value due to its higher superhydrophobicity, a higher amount of air is trapped between the nanostructures.The weight gain in all cases at low immersion time is linearly increased with immersion time but at high immersion time, a plateau is reached.To calculate the scale inhibition efficiency (% SI), Eq. ( 4) was used: where W o and W are the weight gained by the bare and SHP coated substrates.Table 4 displays the values of W, W o , and % SI for the bare and SHP coated steel by Ni @BC@SA and Ni@Co-BC@SA.The values of Wo and W were taken at 20 h of immersion.
SEM was utilized to examine the morphology of CaCO 3 crystallization on the surface of uncoated and SHP coated steel.According to Fig. 12, rhombic crystals on bare steel mostly depicted the shape of the CaCO 3 scale, this is consistent with the relatively stable form of ordinary calcite CaCO 3 95 .However, on the produced SHP coating, the scale's shape clearly changed from rhombic crystals to needle-like structures, which are less stable and adhere to surfaces poorly 95 .
The formed scales on the bare steel and SHP coated steel was isolated and their crystal structure was examined using XRD technique, Fig. 13.The results show that the scale formed on the bare steel substrate is composed mainly of calcite, as indicated by the presence of peaks at 2θ equals to 23.5, 29.4,35.7, 39.2, 42.9, 46.9, 47.8, 56.7,  59.8, and 63.5 degrees.This is consistent with the formation of calcium carbonate scales under typical corrosion  conditions.On the other hand, the scale formed on the superhydrophobic coated steel by Ni@BC@SA composed mainly of vaterite, as indicated by the presence of peaks at 2θ equals to 20.3, 24.7, 26.6, 32.6, 38.9, 43.8, 50.1, 55.9, 60.4, and 63.6 degrees.Vaterite is a less stable form of calcium carbonate compared to calcite.The presence of vaterite suggests that the superhydrophobic coating may have influenced the nucleation and growth of the calcium carbonate scales on the surface of the coated steel.The XRD peaks observed for the superhydrophobic coated steel by Ni@Co-BC@SA showed similar peaks to those observed for the superhydrophobic coated steel by Ni@BC@SA but with lower intensity.This suggests that the addition of cobalt to the biochar-based coating did not significantly alter the composition of the calcium carbonate scales formed on the surface of the coated steel but decreases the rate of scale formation [96][97][98] .
Ultraviolet stability.The intrinsic property of the SHP materials themselves mostly determines UV resistance 99 .The manufacture of coatings with UV resilience is a crucial concern for outdoor applications.A SHP surface can have a long UV stability without losing SHP property when a proper choice of materials is made.Figure 14 demonstrates the impact of UV-irradiation time on the WCA of the SHP coated steel by Ni@ BC@SA and Ni@Co-BC@SA.The SHP-coated steel by Ni@BC@SA has UV stability of up to 65 h while the steel-coated steel by Ni@Co-BC@SA has UV stability of up to 95 h.The SHP coated steel by Ni@Co-BC@SA has greater UV stability than several previously known values [100][101][102][103] .

Conclusion
1. Biochar was produced using rice straw, an environmentally friendly biomass source, and used to create superhydrophobic coatings of Ni@BC@SA and Ni@Co-BC@SA on a steel substrate.The doping of biochar with cobalt increases the coating superhydrophobicity. 2. The Ni@BC@SA coating retains superhydrophobicity in the pH range of 3-11, whereas the Ni@Co-BC@SA coating retains superhydrophobicity in the pH range of 2-12.Additionally, the created Ni@BC@SA coating demonstrates superhydrophobicity until an abrasion length of 500 mm, while the Ni@Co-BC@SA coating exhibits superhydrophobicity until an abrasion length of 900 mm. 3.According to the PDP findings, the corrosion current density is significantly reduced when steel is coated with a SHP coating, which also results in a significantly reduced corrosion rate.This is further confirmed by the EIS results.The scale inhibition efficiency for coated steel with Ni@BC@SA and Ni@Co-BC@SA is 30.26Table 4.The weight gain and %SI for the bare and SHP coated steel by Ni @BC@SA and Ni@Co-BC@SA.www.nature.com/scientificreports/and 51.32%, respectively.In terms of UV stability, the coated-steel with Ni@Co-BC@SA remains stable for up to 95 h, whereas the SHP coated-steel with Ni@BC@SA remains stable for up to 65 h.

Figure 1 .
Figure1.FTIR spectra of coated steel by Ni @BC@SA and Ni@Co-BC@SA.

Figure 2 .
Figure 2. XRD patterns of the SHP coated steel with Ni @BC@SA and Ni@Co-BC@SA.

Figure 3 .
Figure 3. SEM micrographs of the SHP coated steel with (a) Ni @BC@SA and (b) Ni@Co-BC@SA.

Figure 4 .
Figure 4. 3D AFM topography images of the surface of (a) bare and SHP coated steel with (b) Ni @BC@SA and (c) Ni@Co-BC@SA.

Figure 5 .
Figure 5.The change in solution pH with the water contact angle and water sliding angle of the coated steel by (a) Ni@BC@SA, and (b) Ni@Co-BC@SA.

Figure 6 .Figure 7 .
Figure 6.The change in solution pH with the water contact angle at different immersion time of coated steel by (a) Ni@BC@SA, and (b) Ni@Co-BC@SA.

Figure 8 .
Figure 8.The PDP plots for the bare and the SHP coated-steel in 0.5 M NaCl solution.

Figure 9 .
Figure 9. (a) Nyquist, (b) Bode and (c) Theta plots of bare and SHP coated steel in 0.5 M NaCl solution.

Figure 12 .Figure 13 .
Figure 12.Morphology of scales on the (a) bare steel and SHP coated steel by (b) Ni@BC@SA, and (c) Ni@ Co-BC@SA.

Figure 14 .
Figure 14.Effect of UV-irradiation time on WCA of the SHP coated-steel by Ni@BC@SA and Ni@Co-BC@SA.
, which is attributed to Co 3 O 4

Table 2 .
The PDP parameters for the bare and the SHP coated-steel in 0.5 M NaCl solution.

Table 3 .
The EIS parameters for the bare and SHP coated steel in 0.5 M NaCl solution.