Facile route for processing natural polymers for the formulation of new low-cost hydrophobic protective hybrid coatings for carbon steel in petroleum industry

Abstract

This research helps with the creation, assessment, and characterization of a new hybrid protective coating for carbon steel alloy in acid conditions. The findings of this study will be useful for both chemical and petrochemical companies as well as scientists. This study aims to protect C-steel in acid pickling solution 1.0M HCl and formulate new hydrophobic protective hybrid organic–inorganic coatings from biopolymers chitosan and plant resin guar gum. Eight coating samples of chitosan in the absence and the presence of guar gum, silica and two heterocyclic compounds are prepared at feasible operational conditions using hot melt method. The aiding additives improved compatibility between coating constituents as confirmed by using different methods of analysis. This new processing approach has addressed the problems of using chitosan in corrosion control such as solubility in acid media and low mechanical strength. Coating samples of chitosan and its composites with the heterocyclic compounds (2-Hydrazinyl-6-methyl-4, 5-dihydro pyrimidine-4-on) or (2-Hydrazinyl-6-phenyl-4, 5-dihydro pyrimidine-4-on) are potent biocides. Coating shifts corrosion potential of carbon steel by 30 mV to more noble direction relative to the active potential 520 mV of bare carbon steel surface. Impedance and polarization measurements indicate that coating samples protect metal surface as mixed-type inhibitor by adsorption mechanism. There is a good agreement between percentages protection %P of coating calculated using the values of charge transfer resistance, R_ct, and corrosion current density, i_corr. All %P values are above 99% for all coating samples. Guar gum plant resin increases gloss of the coating film. Silica fills the pores in the polymeric film and increases the stuffiness of the polymeric coating film by modifying the particle size. All coated samples have high contact angle ranging from 150° to 165° indicating low wettability and high hydrophobicity of coating film on the metal surface.

Introduction

The best corrosion control method is protective anticorrosive coating, and it is more superior compared to other corrosion control methods such as cathodic and anodic protection, material selection, modifying environment, metallic, and non-metallic linings fiberglass, glass flake [1, 2]. Coating encountered corrosion inhibitor (C.I.) possesses adsorption centers electron-donor groups with lone-pair electrons and multiple bonds such as free radicals, aromatics, –OH, –Si–O, –C=C–, –COOH, –NH₂, –SH, –S–S–, and –C=O. Many corrsion inhibitors were evaluated, for eaxmples: Schiff bases, hydroxyl quinolines, fluorescein, oxines, bromothymol blue and 7-amino-4-methylcoumarin [3].

Coating obstacles are: photo-degradation in organic coatings, required oxygen for durability finishes, ester primers may experience gelling on aging and alkaline hydrolysis of ester groups unless the required precautions are taken [4]. Less protective single-layer organic coating has low mechanical strength and thermally degrades above the temperature 150 °C [5]. Protective inorganic coating exhibits residual porosity, and stress-induced cracks allow diffusion of corrosive species to metal surface. Metallic coatings such as chromium Cr, zinc Zn, nickel Ni, aluminum Al, and copper Cu involved toxic pollutants [6].

Chitosan (CT) is biodegradable linear poly 2-amino-2-deoxy-d-glucopyranose, poly 1, 4-ß-d-glucopyranosamine or partially N-de-acetylated chitin with free NH₂. It contains 8.7% N. It is extracted from exoskeleton of crustaceans like shrimp and crabs; it is abundant, ecofriendly, antibacterial agent and is more ordered and rigid than many synthetic polymers [7]. It is inefficient corrosion inhibitor for C-steel in acid media due to solubility and gelation at pH below 3 [8]. CT coatings have low mechanical strength and inconsistent performance [9]. Crosslinking and grafting decrease solubility of CT, increase chemical stability of coating film and enhance mechanical strength. Chitosan was grafted by acyl, alkyl, hydroxyl-alkyl, carboxyl-alkyl, glycerol phosphate and acrylamide groups or modified by quaternization, thiolation, phosphorylation, sulfation, and copolymerization. Inorganic atoms P, Si, B, and N fill polymer pores. Coating bonded metal surface via O, N and C=C double bond. Grafted CT is a primer hydrophobic coating [10]. Cross-linkers such as benzyl, glyoxal, and glutaraldehyde linked to –NH₂ favored adsorption site of CT [11]. Mono-functional epichlorohydrin cross-linker binds only to OH⁻ groups [11].

Protective coating adheres to metal surface durable for twenty years’ of service, resist moisture, pressure and microorganisms [12]. Reported hybrid organic inorganic coatings are inadequate. Flame-resistant electrically insulating coating incorporated P atom forming double bond with NH₂ and linked to OH of CT [13]. Low efficient CT/epoxy composite for C-steel/35% sodium chloride required modification of chitosan by chemical additives [14]. Hydrophobic coating CT grafted-stearic acid silica resin resists photodegradation. CT/0.8 wt% silica Na₃PO₄ composite is insulated mild steel in neutral media. Hydrophobic propionated chitin transparent coating film has excellent mechanical properties [15].

Hydrophobic CT-SiO₂/poly dimethyl siloxane protected steel in 3.5 wt% NaCl [16]. Hydrophobic durable coating Al–PO₄ adhesive@polydi-CH₃-siloxane silica/halloysite composites modified by perfluoro-decyl-tri-ethoxysilane. Silica enhances coating performance and sustainable release of C.I. [17]. Epoxy resin di-glycidyl ether bisphenol is reinforced by CT and silica [18]. CT loaded by silica coupled 3-glycydoxy-tri-methoxysilane with tetra-ethoxy silane and silver nanoparticles are biocides coatings for titanium alloy implants [19].

In the next section, we consider all aspects of preparation, application, characterization and evaluation of new hybrid coating, Sect. “Results and discussion” shows the results and discussion of this study. In Sect. “Conclusion,” we present the finding of the study future directions for research and recommendations.

Experimental

Material and methods

All chemicals in this study are of analytical grades obtained from Sigma-Aldrich Co. used as received without any further purification. Chitosan (CT) (molecular weight, Mw. 300 k Da, 75% deacetylation degree (DD); guar gum (GG) for improving viscosity of coating. Tri phenyl phosphonium bromide (TPPB), phosphorous acid (H₃PO₃), silicate glass (SG: chemical composition (wt%) 46% SiO₂, 24.5% Na₂O, 24.5% CaO, 5% P₂O₅), HCl, NaOH, citric acid, sodium dodecyl sulfate (SDS) and two heterocyclic compounds, Cpd1 (2-Hydrazinyl-6-methyl-4, 5-dihydro pyrimidine-4-on) and Cpd2 (2-Hydrazinyl-6-phenyl-4, 5-dihydro pyrimidine-4-on), as corrosion inhibitors (C.I.) have the same chemical structure and differ only in that methyl group of compound Cpd1 replaced by phenyl group in Cpd2.

Formulation of the hydrophobic coating (hybrid coat formulation)

Hydrochloric acid, HCl, Merck, 37%, Mw.36.47 g mol⁻¹, sp.gr. 1.2 g mL⁻¹ at 25 °C is used for preparation 1.0M HCl test solution. Eight coating samples S_o–S₇ described in Table 1 are prepared. Sample S_o contains CT plus all additives. S₁ (CT-GG), S₂–S₅ contain same % GG and variable % CT and %SG.

Table 1 Chemical constituents of coating samples, wt%

3 mL H₃PO₃ gives phosphate PO₄³⁻ group for film elasticity and improving adherence of primer coating. 20 mL 0.5M NaOH alkaline media enhancing film forming by CT. 10 mL of 10⁻⁴ M either Cpd₁ or Cpd₂ dissolved in ethanol. A volume 10 mL of 0.5M SDS anionic surfactant solubilize constituents in water green solvent in spherical micelles formed above critical micelle concentration.

Hot melt coatings are prepared at operational conditions of 100 rpm, melting temperature of 110 °C for duration time of 1.5 h to ensure complete solubility and compatible constituents. CT is chemically grafted at primary –OH and NH₂. Coating is applied to metal surface via hot dipping.

Characterization techniques are carried out at central Lab. Faculty of Science, Alexandria University to ensure compatible constituents. Chemical compositions of the coating samples are confirmed as reported elsewhere [20,21,22]. Functional groups are determined using Fourier transformer infrared (FTIR) using Bruker TENSOR 37 spectrophotometer 1430 calibrated at wavenumber, $\overline{\upsilon }$ range 4000–450 cm⁻¹ and ambient temperature. Sample is finely grounded with infrared grade potassium bromide and compacted into pellet disk. Topography of coated metal sample is evaluated by measuring roughness and analyzing by scanning electron (SEM) micrographs using JSM-IT200 SEM [22].

Thermogravimetric analysis (TGA) and differential thermal analysis (DSC), at heating rate 10 °C min⁻¹ using de-aerated platinum cell under nitrogen flow 20 mL min⁻¹ are recorded using Shimadzu DTA/TGA-50. Particle size distributions of some diluted suspended coating samples in double distilled water samples are determined in triplicates using NanoZS/ZEN3600 Zetasizer and photon spectroscopy of non-invasive light backscattering at angle 173°, 25.0 ± 0.1 °C [22].

CT and CT composites with Cpd₁ and Cpd₂ are evaluated as biocides for MIC for different bacteria strains [23], supplementary information SI.1. 30 mL 100 ppm 10%CT is chemically grafted at primary OH, NH₂ by 10 mL of 10⁻⁴M Cpd₁ or Cpd₂. The mixture is agitated applying sound wave of frequency > 20 kHz) ultrasonic bath for one hour, then magnetically stirred at 50 rpm under reflux at 50 °C for 1.0 h. till complete homogeneity. Total bacterial count tests are described in SI.1.

Coating is evaluated in 1.0M HCl using C-steel corrosion coupons, dimensions 7.6 × 1.2 × 0.12 cm. The scanned cross-sectional area equals 18.88 cm². The chemical composition wt%: 0.18%C, 84%Mn, 0.01%P, 0.005%S, 0.02%Cu, 0.02%Cr and Fe remainder [24]. Coupon surface is hand polished using 320, 600 and 1000 mesh size grades emery papers starting by coarse one and proceeding to finer grade till mirror finish. Polished clean sample is rinsed thoroughly with double-distilled water, absolute ethanol and dried just before hot dipping in hot melt coating and cured at room temperature after one day [23].

The details of corrosion rate, C.R determination by electrochemical impedance spectroscopy, EIS and DC potentiodynamic polarization techniques are described in SI.2. C.R. is expressed by corrosion current density i and reciprocal $1/\text{Rct}$ [24].

$${\text{Percent}}\,{\text{protection}},{\text{ \% }}P = \frac{{{\text{Rct}} - R_{{{\text{cto}}}} }}{{{\text{Rct}}}} \times 100 = \frac{{i_{{\text{o}}} - i}}{i}{ } \times 100{ }$$

(1)

where the parameters i_o, R_cto and i, $\text{Rct}$ are corrosion current density and charge transfer resistance of bare and coated sample, respectively.

Statistical micro-calculation origin 8.0 is used for data analysis. Contact angle is measured using static method and direct measurement of the tangent angle at the three-phase contact point on a sessile small-volume liquid drops five micro-liters profile [10].

Results and discussion

FTIR spectra confirmed successful bonding intercalation among coating constituents. Characteristic vibrational absorption bands were shown in supplementary information Fig. SI.1. All bands at characteristic $\overline{\upsilon }$, cm⁻¹ are assigned to functional groups [25]. 3471(NH₂, OH̅), 2986 cm⁻¹CH₂, CH₃, 1650 cm⁻¹ bending NH₂, 1424 cm⁻¹, 1078.52 alcoholic CH₂OH, 1657: CONH₂ amide. 466 cm^−1–588 cm⁻¹ O–Si–O bending; Si–O–Si 1033 cm⁻¹stretching; 925 cm⁻¹− 948 cm⁻¹Si–O₂ nonbonding orbitals, 478 cm⁻¹− 486 cm⁻¹Si–O–Si rocking; 771 cm⁻¹− 779 cm⁻¹Si–O–Si bending, 2329.39Asym. NH₃⁺, 1762 cm⁻¹ester, 1787.90 amide, esters, carbonyl, alcoholic group, Intra-, and inter-molecular hydrogen bonds, H.B. at 1073, 1028, 952, 897, 1070 cm⁻¹− 1076 cm⁻¹ PO₄ ⁻³, 470.19 cm⁻¹Fe–O, 1158, 1074, 1029, 894 cm⁻¹ polysaccharide units. Intense broad band at 3700–3000 cm⁻¹ due to stretched O–H bond overlapped with NH₂ and inter- or intra-molecular H.B. between OH, NH₂ of CT and GG [7].

Functional groups of CT, GG and SG are maintained in coatings.

Asymmetric stretching –CH,-CH₂ in CT hydrophobic pyranose rings at 2924, 2864 cm⁻¹, 1658 cm⁻¹ and 1593 cm⁻¹ new amide bond. 1600 cm⁻¹ deformed overlapped NH₂, stretching intense band amide at 1654.48 cm⁻¹, CT Schiff’s base, 3052 cm⁻¹, 3027 cm⁻¹ (C–H, phenyl group); 1691 cm⁻¹ C=N, 1600, 1575, 1493 and 1454 cm⁻¹C=C of phenyl group; 757, 692 cm⁻¹ (benzyl group) of Cpd₂, 1250 cm⁻¹ (epoxide moieties), 1000 cm⁻¹ C–H stretching. Stretching C=O Cpd₁ overlapped with amide of CT at 1657 cm⁻¹. At 2207, 2162 cm⁻¹ C≡N, 1641 cm⁻¹C=N, benzyl ring at 757 cm⁻¹, 692 cm⁻¹ [25].

SEM micrographs of CT@GG coating showed smoother surface topography than either CT or SG, Fig. 1. Chitosan appeared as rigid polymeric chains with regular vertical alignment.

Fig. 1

a-c Comparative SEM micrographs for the main coating constituents: a chitosan, b CT@GG and c ceramic SG

After exposure to 1.0M HCl for 3 days, outstanding and durability of coated sample relative to bare metal surface are confirmed from SEM micrographs, Fig. 2.

Fig. 2

SEM micrographs: a bare metal sample and b coated metal sample

SEM micrographs of S₄ hybrid coating loaded by Cpd₂ confirmed good surface morphology due to synergetic inhibiting by phenyl group of Cpd₂. SG fill pores and reinforced coating. [10]. Coating formulations contain multi- functional groups: NH₂, OH⁻ from CT and functional groups of Cpd₁ or Cpd₂.

Biocide activities of CT and CT composites with Cpd₁ and Cpd₂ are shown in Table 2.

Table 2 Minimum inhibitory concentrations (MIC) of tested samples as biocides

The low MIC of CT and its composites for different tested bacteria and fungi species suggested applicability of these composites coating antimicrobial biocides for mitigating microbiologically induced corrosion (MIC) encountered in petroleum industries [23].

Cpd₁ and Cpd₂ have the same chemical structure, but differ in the presence of methyl or phenyl or substituent can chemically grafted CT via NH, NH₂ giving CT-Schiff’s base.

Equilibrium steady open circuit potential E_OCP of C-steel is attained after 20 min before impedance and polarization measurement, Fig. 3. All E_OCP are shifted to more noble compared to bare steel (520 mV versus SCE) [24].

Fig. 3

Potential-time curves for coated sample C-steel in 1.0MHCl for sample So to S7

Figure 4 showed Nyquist impedance plots for coated C-steel sample in 1.0M HCl have capacitive semicircles confirmed dielectric impermeable film coatings insulator and isolated metal surface. No diffusion tail observed at low frequency region indicating that corrosion is under charge transfer control. This finding is confirmed from Bode impedance Z at low frequency region plots, Fig. 5 [24].

Fig. 4

Nyquist plots for coated (S1–S7) and bare (CT, CT-GG) C-steel coupons in 1.0M HCl

Fig. 5

Bode impedance plots for coated C-steel samples in 1.0MHCl

The negative phase shift angle (θ = 80°) for some representative coating samples shown in Fig. 6 (indicated that alternating current I(ω) proceeds the alternating voltage V(ω) and confirmed coating adherence [26]. The θ value and the coating performance had been increased in the order: S₇ > S₆ > S₄.

Fig. 6

Representative phase shift plots of some coated samples: a S₄, b S₆ and c S₇

The presence of phenyl group in Cpd₂ (S₇) increased protection efficiency of coating. The inset in Fig. 4 is an equivalent circuit model simulated heterogeneous metal solution/interface used in nonlinear fitting of Nyquist plots with negligible error. Elements constant phase elements Q_dl and Q_f are capacitances of electrical double layer and coating film, respectively. The parameter R_s resistance of solution between working electrode and reference electrode; R_f is the resistance of coating film; R_ct is the charge transfer resistance across metal surface [24]. All impedance parameters including the heterogeneity constant (n) were collected in Table 3.

Table 3 Impedance parameters for coated samples in 1.0M HCl

The parameter (n) ranged from 0.9 to 1 confirmed heterogeneity of corrosion system [25]. The decrease in Q_dl approved adsorption of active ingredients of coating on metal surface.

Figure 7 showed representative polarization curves for coated samples C-steel in 1.0M HCl. Both the cathodic and anodic polarization curves are shifted to higher over-potentials indicated that all the coated samples act as mixed-type inhibitors [24]. Tafel behavior indicated that corrosion of steel in HCl is under activation control. Steel corrodes in HCl given soluble ferrous chloride, FeC1_2(aq.) and hydrogen gas.

Fig. 7

Polarization curves for coated C-steel samples (S₃–S₇)

Coating inhibited rates of both anodic oxidation and cathodic reductions. Polarization parameters: Corrosion potential, E_corr., corrosion current density, i_corr., anodic-, and cathodic Tafel slops β_a, β_c, respectively, for coated C-steel samples are obtained using Tafel extrapolation method at ± 50 mV around E_corr., Table 4 [24,25,26].

Table 4 Polarization parameters C-steel in 1.0M HCl

Coating samples shift E_corr. of steel to more noble potential relative to bare steel in 1.0M HCl.

Figure 8 showed comparative %P of coating sample calculated from impedance and polarization measurements.

Fig. 8

Comparative %P of coated samples

%P from of coating samples from both techniques are in good agreement followed the order: S₇ > S₆ > S₄ > S₅ > S₃ > S₂ > S₁ > S_o.

Cpd₂ gave more dispersed coating film than Cpd₁ due to more delocalized electrons on phenyl group. The optimum 3.0% SG showed sufficient due to increased stiffness of coating film. Large 4% SG declined polymer flexibility and decreased %P. SG sealed pores caused by aggressive chloride in pitting corrosion [26].

Functional N and O⁻ groups of coating adsorbed on metal surface and protected it against corrosion. NH₂, OH⁻ donate free lone pairs of electrons to the vacant orbitals in steel surface form type coordinate.

NaOH cleaved hydrophobic ring of CT giving open structure C=N imine enhancing film formation. Micelles of SDS solubilized coating constituents in water.

This trend is confirmed by immersion a coated sample (CT, S₄) in 1.0M HCl for successive three days. After Fig. 9, coating showed gloss film that enhanced by GG.

Fig. 9

SEM micrograph for coated C-steel samples: a CT only and b S₄ immersed in 1.0M HCl for 3 days

Adsorption–desorption isotherms of N_2(g) on some coated samples are shown in Fig. 10.

Fig. 10

Adsorption–desorption isotherms of N_{2 (g)} on steel surface: bare S_o and coated: S₄, S6, S7

All adsorption–desorption isotherms are type IV according to IUPAC classification confirmed modified adsorption capacities of CT on SG loading in S₄, S₆, S₇ [27]. Monolayer completed at knee point B. Hysteresis loops confirmed desorption of N_2(g) from tubular capillary pores on deceasing pressure. Developed mesopores in coating sample S_o (CT + additive except silica) decreased on SG loading and developed into micropores. Maximum adsorbed volume, CC.g⁻¹ followed the order: S₁ (320 CC) > S₄ (17CC) > S₆ (13CC) > S₇ (12CC).

This trend confirmed filling polymer pores by SG. Difference among S₄, S₆ and S₇ confirmed successful loading of Cpd₁ and Cpd₂ on polymer matrix [15].

BET for infinite adsorbed infinite multilayers approached Langmuir adsorption isotherm on solid surfaces at low P/P^o. Volume of adsorbed gas increased with increasing gas pressure, P [27].

$$V\, = \,V_{{\text{m}}} \cdot \, a \cdot P$$

(2)

At high pressure, limited monolayer coverage V_m reached. Isotherm at intermediate pressure depends on constant a related to temperature. Textural properties of coatings are determined by applying BET assuming equal evaporation and condensation rates of adsorbed N_2(g).

$$\frac{P}{{V \left( {P^\circ - P} \right)}} = \frac{1}{{V_{m} C}} + \frac{{\left( {C - 1} \right)}}{{V_{m} C}} \cdot \frac{P}{P^\circ }$$

(3)

where P^° and V_m are the saturation vapor pressure and volume corresponding to monolayer of nitrogen respectively. The adsorption constant (C) depends on the temperature, related to the heat of adsorption in monolayer and reflect gas–solid interaction [27].

$${\text{C}} \cong {\text{exp }}[(\Delta H_{{1}} - \Delta H_{{\text{L}}} )/{\text{RT}}]$$

(4)

$\text{Heat }\Delta$H_adsorption, J.mol⁻¹ = ${\Delta H}_{\text{adsorbed monolayer}} , \Delta$H₁− ${\Delta H}_{\text{liquefied monolayer}}, \Delta$H_L.

The total pore volume and the surface area $({a_S}, \text{BET})$ and the pore diameter of coating samples were decreased on loading silica glass (SG) on CT as shown in Table 5 because SG filled the meso pores of CT binder (mean pore diameter: 2–50 nm according to IUPAC classification giving micro pores.

Table 5 Pore characterization of coated samples

The best linear fit of BET plot including Knee point B: at plot ${\raise0.7ex\hbox{$P$} \!\mathord{\left/ {\vphantom {P {V_{{\text{a}}} \left( {P^\circ - P} \right)}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${V_{{\text{a}}} \left( {P^\circ - P} \right)}$}}$ versus P/P^o at P/P^o (0.05–0.35), Fig. SI.2 enabled calculation of V_m, C and specific BET surface area of coatings, a_{S, BET.}

$$a_{{S, {\text{BET}}}} = \frac{{V_{{\text{m}}} \alpha N_{{\text{A}}} }}{{m V_{o} }}$$

(5)

where $\alpha$ molecular cross section area 16.6 Å² molecule⁻¹, N_A Avogadro’s number 6.023 × 10²³ molecules mol⁻¹, m adsorbent mass, g, V_o molar volume 22.414 L mol⁻¹ [27].

Positive adsorption constant (C) indicates exothermic adsorption process. Heat of adsorption of CT is higher than $\Delta$H_ads for S₄ in agreement with V_m trend. Cpd₂ (contains phenyl group) improved film continuity than Cpd₁. SG network is inorganic mineral element reinforced adsorption of functional groups. Both porosity and BET surface area decreased on intercalation with SG [15, 17].

Sample S₄, 3.0% SG optimum reinforced coating adhesion. SG binder fill polymeric pores enhanced mechanical properties and polymer stiffness. 4.0% SG declined polymer flexibility.

Figure 11 showed modified particle size distribution of coating of S₄ and S₅ coating samples.

Fig. 11

Particle size distribution of S₄ and S₅ coating samples

Multi-functional coating constituents functional groups C=O, C–O,–OH, NH₂, double bonds adsorbed on metal surface [27].

Intercalation of Cpd₂ onto CT matrix is confirmed by comparative thermal gravimetric analysis TGA, Fig. 12.

Fig. 12

Comparative thermograms: CT, CT/NaOH, and CT/NaOH/Cpd₂

The constituent of hybrid coating could be represented as shown in Fig. 13.

Fig. 13

Chemical constituents of hybrid coating (chitosan and guar gum were represented by monomeric unit)

The mechanism behind coating formulation could be schematically represented PP as shown in Fig. 14.

Fig. 14

Representative pathway for blending chitosan by Cpd1 or Cpd2

NaOH opened the hydrophobic ring CT giving imine C=NH bond that intercalated by Cpd₁ or Cpd₂ [27]. The grafted CT is thermally stable organic coating.

DSC thermogram of the most protective sample S₇ is shown in Fig. 15.

Fig. 15

DSC thermogram of S₇ coating sample

DSC thermogram confirmed thermal stability of coating sample that showed only three endothermic peaks representing thermal energy required for bond breaking between coating constituent. Broad intense peak at temperature range 31.7 °C–295.1 °C confirmed strong binding between polymeric matrix in coatings. Less intense peak at 320.3 °C–515.7 °C represented heat of decomposition of ceramic SG. The last peak at 579.9 °C–698.8 °C represented breaking of metal-coating bonds.

All coated samples have high water contact angles (WCA) fluctuated in the range 150° (S_o)0.1–165°0.4 (S₇) indicated low wettability and adequate hydrophobicity and water resistance of coating samples. WCA is regularly increased with sample number except S₄ showed higher WCA 161.2° than S₅ (158.0 °C) due to declining in polymer flexibility at large 4% SG.

Protection mechanism could be explored as coating adhesion prevents penetration of localized corrosive electrolyte pockets under coat. Metal/polymeric coating interface involves physical and chemical adsorption of molecules of polymers and corrosion inhibitor; chemical reaction between both polymer and C.I. with metal surface; mechanical interlocking polymer molecules into pores on metal surface; electrostatic interaction of functional groups C=O, C–O, –OH, NH₂, COOH and oppositely charged metal surface. Each mechanism operates under certain conditions. Adsorption predominates where polymer adheres to metal surface or metal oxide surface film via Lewis acid–base interaction. Free NH₂, OH chelates metal surface.

10⁻⁶M TPPB provided polarizable phosphorous atom (forming P = N with NH₂ of CT giving primer coating) and three hydrophobic phenyl groups for extensive delocalized π-electron density. SG was a reinforcing agent binding CT chains via oxygen atom. Low electron negative Si increased delocalized electron density. Silica regulated polymer (CT) chains enhancing thermal, mechanical strength, hardness, toughness and stiffness of polymeric coating by filling pores in polymeric matrix as well as improved rheology and corrosion resistance epoxy coatings [20, 26]. Traces aiding additives are SDS (wetting and dispersion agent), NaOH improved CT film. H₃PO₃ stabilizer and plasticizer [21]. Citric acid cross-linker contains three planar carboxylic acid, COOH⁻, and OH giving ester group with OH of CT leaving NH₂ free to bind metal surface. Ceramic inorganic SG regulates CT chains and covalently linked amino NH₂ and hydroxyl OH groups. Synergetic protection is achieved by adjusting nature, proportion, compatible constituents for homogeneous dispersion structure, functionality giving pure homogeneous, structurally tunable transparent insulating protective coating film.

Conclusion

This study provided new protective hydrophobic hot melt coating of chitosan with guar gum and SG for acid pickling of C-steel in 1.0M HCl acid, and it is maximized at S₇ (62 wt% CT, 3.0 wt% SG, 35 wt% GG + 1 × 10⁻⁴M Cpd₂). The high %P confirmed compatibility of coating constituents. 4.0% SG decreases flexibility of polymer chains. Along with exposure to two days immersion in HCl at same experimental conditions, %P 95% of S₇ remains constant. Coatings with barrier impermeability ensure long-term anticorrosive stability. CT in alkali NaOH formed coating film. Coating constituents are compatible green anticorrosive coatings. SG fill polymeric pores enhanced thermal stability, mechanical strength and polymer-stiffness as well as chemical resistance. Coatings adhered to metal surface, durable resist microorganisms. Grafted CT is a primer enhanced coating, adhered to metal surface and prevented water penetration as well as enhanced flexibility and hydrophobicity of hot melts adhesives coatings. Incorporated Si–O bond in CT enhanced thermal stability, strength, and adherence. Polymer content controlled micro-topography, wettability, roughness and wear resistance. The formulation methodology of this type of hybrid coating is a facile low-cost approach with no limitations. The future directions for this research will be focused to protect carbon steel in other corrosive media such as marine and atmospheric environments. This hybrid coatings prepared in this study can be recommended for commercial production and application in prototype scale for protection of steel during acid pickling.