Facile Formulation of New Innovative Eco-Friendly Hybrid Protective Coating for Mild Steel in Acidic Media

Abstract

This research deals with the formulation, characterization, and evaluation of new anticorrosive protective coatings. The study objective is to protect mild carbon steel in acidic media by adherent nonporous polymeric coatings formulated from polystyrene and shrimp shells. Solid wastes of shrimp shells are dried into a fine powder and sonicated in toluene. The obtained suspension is refluxed with polystyrene. The hot-melt coatings are applied to the metal surface by the hot dipping technique. The shrimp shells improve the performance of polystyrene. These eco-friendly, low-cost anticorrosive coatings are formulated from solid waste (SW) of shrimp shells and polystyrene (PS) with no aiding additives. Intense vibrational bands in the infrared spectra and the high thermal stability of the coating samples confirm the compatibility of the coating constituents. The results of the evaluation of coating performance by electrochemical impedance spectroscopy and potentiodynamic polarization techniques show that the coating is protective for mild steel in the aggressive acidic media of 1.0 M HCl. The coating protects the metal surface without affecting the corrosion mechanism. Polarization curves show that the coating film retards both the anodic metal dissolution reaction and the cathodic hydrogen evolution reaction, acting as mixed-type inhibitors. The percent protection (%P) increases with the increasing weight percent (wt.%) of PS and the SW of shrimp shells. A %P up to 99% is achieved for the coating composition of 2.0 g/L PS + 0.02 g/L SW. The %P obtained by impedance and polarization measurements are in good agreement. The prepared multi-functional polymeric coating forms an adherent nonporous coating film on the metal surface. Impedance plots show that the coating samples are insulating dielectric coatings that electrically insulate the metal surface from the aggressive acidic media. The coating protects the metal surface by the adsorption mechanism. Shrimp shells fill the pores and increase the stiffness of the polymeric coating film of polystyrene. The obtained results in this study will be useful for all industrial sectors and academic research in the field of corrosion control of metals and alloys.

1. Introduction

Corrosion is a serious problem that affects various engineering applications such as buildings, chemical plants, automobiles and mechatronics, as well as the metallurgical and medical sectors. Corrosion cost can be reduced by twenty percent if a low-cost effective corrosion control method is applied. Corrosion is affected by: the presence of inclusions or other foreign matters on the metal surface; the homogeneity of the metal surface; the nature of the corrosive environment; incidental environmental factors such as variations in dissolved oxygen, temperature, and the movement velocity of either the environment or the metallic system itself; continuous or broken oxide scales; the presence of deposits on the metal surface; galvanic effects between dissimilar metals; and the occasional presence of stray electrical currents from external sources [1,2,3].

Metal and alloys corrode in aqueous corrosive surroundings (air, moisture, or soil) through chemical or electrochemical reactions [1]. Corrosion forms and mechanisms include pitting corrosion: localized attack causing deep holes; galvanic corrosion: electrons flow between two dissimilar metals in corrosive media resulting from the potential difference between the two metals differing in position in terms of the electromotive force series (the noble metal is the anode, and the active metal is the cathode); uniform corrosion occurs over the entire metal surface with the same corrosion rate. In crevice corrosion, the corrosive liquid is captured between metal gaps forming differential concentrations of corrosion cells; erosion corrosion is caused by high-velocity corrosive fluid. Stress corrosion cracking is caused by mechanical tensile stress form a fracture in the metallic structure exposed to corrosive media [1]. Intergranular corrosion occurs at the grain boundaries of the metal alloy; corrosion fatigue is an erosion corrosion caused by metal fretting and a corrosive medium. Flow-assisted corrosion (FAC) occurs when the protective oxide layer on a metal surface is liquefied or removed by water or wind, causing deterioration and corrosion of the metal surface [1].

Developing corrosion control methods is a necessity for petrochemical and all industrial companies, and the engineering sector. The common corrosion control methods are protective coatings and linings, cathodic/anodic protection, and corrosion inhibitors (CIs). Material selection is applied to low-cost metals and alloys having excellent mechanical strength and high corrosion resistance.

Protective anticorrosive coating is the best corrosion control method, which includes cathodic and anodic protection, material selection, modifying the environment, metallic and non-metallic linings such as fiberglass, glass flakes, etc. Corrosion inhibitors (CI) possess adsorption centers such as oxygen, nitrogen, sulfur and phosphorus (heteroatom with lone-pair electrons), and multiple double and triple bonds and aromatic rings in the molecular structure adsorb on the metal surface, forming a protective layer that reduces the corrosion rate.

Examples of CIs are free radicals and aromatics containing functional groups such as −OH, −Si−O, −C=C−, −COOH, −NH₂, −SH, −S−S−, and −C=O; for example, Schiff bases, hydroxyl quinolines, fluorescein, oxines, bromothymol blue and 7-amino-4-methyl coumarin [4].

An anticorrosive protective coating must be: adherent to the metal surface; durable for twenty years of service; resist moisture, pressure and microorganisms; and form a protective physical coating film barrier at the metal/solution interface to isolate the metal surface from the corrosive environment [5]. While various CIs have been reported for MS in aqueous media, the eco-friendly anticorrosive hybrid coatings for MS in acidic media are inadequate [6].

The protective coatings include: a metallic coating such as zinc electrodeposited on the steel surface; non-metallic organic coatings such as paints, lacquers, coal tar; and inorganic coatings such as porcelain enamels, chemical-setting silicate cement linings, glass coatings, and linings [7].

Various reported eco-friendly hybrid organic–inorganic coatings are limited to protecting carbon steel in neutral media [8,9]. These include: a flame-resistant coating incorporating a P-atom linked to the NH₂ and OH of chitosan (CT); a composite of CT/epoxy oleic acid and graphene oxide filler; CT-grafted stearic acid silica; a CT–silica–Na₃PO₄ composite; propionated chitin; CT–SiO₂/polydimethyl siloxane; and an epoxy resin di-glycidyl ether bisphenol reinforced with CT and silica. Eco-friendly hybrid coatings are suitable replacements for Zn, Al, or Al-Zn coatings used for galvanizing steel sheets, fabricated parts, nuts, bolts, and fasteners.

For steel, different plant extracts can act as eco-friendly CIs in coating formulations [10]: The berberine alkaloid in mustard seeds, which contains long-chain aromatic N-rings and branched C-chains with multiple methoxy functional groups; gum, radish seeds, tannin extracts of the chamaerops humilis plant, emilia sonchifolia, and Baphia nitida leaf extract; green leafy vegetable extracts petroselinum crispum, Eruca sativa and anethum graveolens; roasted coffee; melanoidins, from gorse; phenanthroline; prunus persica, from olive; the ginger rhizome; Dillenia pentagyna fruits; and Senecio anteuphorbium. These plant extracts have been evaluated in aqueous solution only.

Carbon steel (CS) is the preferred engineering alloy (due to its mechanical strength and relative low cost) in the industrial arena, including oil and gas transportation pipelines, down-hole casing, reinforced concrete, vehicles, refining, and chemical processing, despite its corrosion tendency, particularly in acidic media, especially hydrochloric acid (HCl). This aggressive acid is utilized for the chemical cleaning of boilers, descaling, oil-well acidification, pickling, and in other industrial fluids for the removal of deposited rust, iron oxide scales, and processing [11]. Mild steel (MS) alloy is a low-C steel and low-cost common construction alloy in all chemical, petrochemical, and applied industries dealing with aqueous solutions of acids, alkalis, and salts. It is active in both galvanic series and electromotive force series, contains less than 0.4% C and 0.5% Mn, has average mechanical properties and a tensile strength up to 300 MPa, and can be easily welded but corrodes in acidic media. The carbon content slightly affects the corrosion resistance, but improves the hardness and mechanical strength of MS [11].

MS is widely used by the industry in Riyadh, manufactured by SABIC Co. in the Saudi Arabia kingdom, and primarily consumed locally and internationally as construction alloys of underground pipelines for crude oil and water services. Corroded pipelines are difficult to be replaced. Protecting pipelines against corrosion with an environmentally eco-friendly anticorrosive coating formulation is a must [1,2,3,10].

The formulation of a protective coating from PS and shrimp shells is a new research point. Shrimps are among the globally significant fish products, including in Saudi Arabia, which are exported as frozen after disposing of the head and shells, causing environmental pollution and aesthetic damage. Shrimp shell solid waste (SW) represents about forty-five percent of treated seafood wastes disposed in landfills [12]. The nitrogenous polysaccharide chitin biopolymer is abundant in the external and internal structures of invertebrates and shrimp shells. Shellfish crab–shrimp waste contains 30–40 wt.% protein, 30–50 wt.% calcium carbonate and phosphate minerals, and 20–30 wt.% chitin [13]. The chemical composition and extraction of chitin that can be de-acylated to yield chitosan biopolymer are shown in the Supplementary Information, Figure S1. The SW of shrimp shells is cheap, natural, environmentally acceptable, abundant, available, and contains an effective CI represented by inorganic metal salts and the functional hydroxyl and acetyl groups of chitin.

Polystyrene (PS) is a thermoplastic organic polymer. The chemistry and characteristics of PS are clarified in Figure S1 [14]. Up to now, there has been no reported protective hybrid coating of PS intercalated with the chemical components of shrimp shells.

Coating obstacles [15] include the photo-degradation of organic coatings, oxygen for durability finishes, and the gelling and alkaline hydrolysis of ester primers. A single-layer organic coating is less protective, has low mechanical strength, and thermally degrades above 150 °C. Inorganic coatings exhibit residual porosity and stress-induced cracks, allowing the diffusion of corrosive species on the metal surface. Metallic coatings such as chromium, zinc, nickel, Al, and copper are toxic pollutants. The goal and the aim of this study is the formulation of a new protective anticorrosive coating as the best corrosion control method for metals and alloys. The designed anticorrosive hybrid coating will encounter many electron-donor groups such as –OH, −NH₂, −C=O in shrimp shells and localized double bonds on polystyrene. This goal will be achieved by grafting available cost-effective PS with eco-friendly shrimp shell solid waste, which is naturally biodegradable, available, and low-cost, to formulate a new, cheap, and safe anticorrosive coating (not toxic to the environment, animal life or aquatic life).

The novelty of this work is that it is the first formulation of such a low-cost durable coating without issues such as photo- and thermal degradation in organic coatings, the oxygen requirement for durability, the gelling and alkaline hydrolysis of ester coatings, low mechanical strength, residual porosity and stress-induced cracks. It is an environmentally friendly protective coating that could replace the expensive chemical CIs that are costly and seriously damage the environment. The availability of shrimp shell SW and PS enabled the extrapolation of the obtained lab results into the commercial-scale production of such a protective coating using scale-up technologies and techno-economic analysis. The protective coating will be applied to protect: the main engineering and construction alloy MS in 1.0 M HCl to avoid utilizing the traditional hazardous protective coatings for this valuable metal alloy against moisture, pressure and microorganisms. Insulating properties of the formulated coating could be achieved as the phosphate group in shrimp shells will form a double bond with NH₂ and link to the OH of chitin and chitosan biopolymers encountered in the shrimp shell.

2. Experimental

2.1. Methods and Materials

Double-distilled water was used to clean glassware and prepare stock and test solutions. The chemicals used in this study were all of certified analytical grade from Sigma Aldrich Co., St. Louis, MO, USA: 37% hydrochloric acid (HCl), toluene organic solvent, and synthetic PS were provided by Saudi Basic Industries Corporation, SABIC Co., Riyadh, Saudi Arabia.

2.2. Formulation, Characterization, and Application of Coating Samples

Shrimp shells were processed into nanoparticles as follows: Natural shrimp shell samples (heads and scales) were collected from the central fish market in Riyadh, Saudi Arabia, washed with tap water, double-distilled water, and dried in an electric oven at a temperature of 120 °C for 2.0 h as shown in the Chart S1 [16].

Seven coating samples were formulated: First, 0.01 g or 0.02 g SW powder was suspended in 10 mL toluene and sonicated under ultrasound waves for 1.0 h. Then, 1.0 g or 2.0 g PS were added gradually to the SW suspension. The mixture was stirred at 50 rpm under a conventional condenser at 60 °C for 6.0 h until complete homogeneity. Coating samples were characterized as described in Chart S1.

Before the coating application, the metal surface was pretreated [17,18]. The hot-melt coating mixture was poured into a small, clean, dry, cold glass beaker. The cleaned metal sample was dipped in the hot-melt coating in a batch-mode process three consecutive times (three layers) so that the interval between two successive layers was 10 s. The hot dipping coating enabled the ease of the coating application. Coated samples were held on a horizontal wood rack by thick cotton strings for curing at the room temperature. The solvent evaporated and the coating cured after one day, leaving the dry rigid adherent coating film on the metal surface [19].

2.3. Corrosion Rate Determination of MS Samples in 1.0 M HCl

The corrosion rate (CR) of bare and coated MS was determined by electrochemical techniques such as DC potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) [5]. The working electrode (WE; with bare or coated metal surface) was constructed by Hadeed, Saudi Arabia, in MS-SABIC circular disc samples with 1.0 cm diameter. The chemical composition (wt.%) consisted of 0.37% C, 1.21% Mn, 0.23% Si, 0.021% P, 0.017% S, 0.016% Cu, 0.02% Ni, 0.02% Cr, 0.003% V and the remainder wt.% was iron.

All electrochemical measurements were carried out in a three-electrode electrochemical cell containing MS as the WE sample with an exposed surface area of 1.01 cm², a saturated calomel reference electrode (RE), and Pt. auxiliary counter electrode (CE). The surface of the bare WE was treated through mechanical polishing by utilizing a polishing machine (Amazon Co.UK: ENEACRO manufacturer, Guangdong, Guangdong, China) consecutively loaded with different grades (320, 600, 800, 1000, 1200) of emery papers, starting from the coarse emery papers of grade 320 and proceeding to the finest grade 1200. The MS sample was washed several times with double-distilled water followed by acetone. The metal surface was rubbed with a soft cloth added to alumina Al₂O₃ paste to obtain a mirror-finish metal surface, then it was washed with double-distilled water, then degreased with acetone and finally dried just before use [20,21]. Coated metal surfaces were used with no pretreatment.

A stock solution of 4.0 M HCl was prepared in double-distilled water from which 1.0 M HCl test solution was prepared using the appropriate dilution. The cell and its components (three electrodes and test solution) were connected to an ACM potentiostat (Gill AC serial No.631) core running with pilot integration controlled by Gill AC serial No.631 core running sequencer software program in order to carry out impedance and polarization measurements. The ACM potentiostat was computerized for data logging and analysis. The electrochemical cell was maintained at 30 ± 0.2 °C. The experimental setup in this study is represented in Chart 1.

The steady-state open-circuit potential (E_OCP) for either the bare or coated metal WE was attained for half an hour for reliable impedance and polarization measurement. EIS spectra were recorded at E_OCP potential in the frequency range: 30 kHz–0.1 Hz.

Impedance parameters: The charge transfer resistance (R_ct), solution resistance (R_s), and double-layer capacitance (Cdl) were calculated via nonlinear fitting of Nyquist plots to a simple Randles theoretical equivalent circuit [5].

DC potentiodynamic polarization curves were obtained by polarizing the WE surface by an overpotential of ±250 mV around the E_OCP and a scan rate 30 mV·min·⁻¹ [5]. The corrosion current density (i_corr) is found at the intersection points of Tafel lines at the corrosion potential (E_corr).

CR is expressed as corrosion current density (i_corr) and the reciprocal (1/Rct). Percent protection (%P) was calculated using Equation (1).

$$\mathrm{Percent}\mathrm{protection},\%\mathrm{P}=\frac{\mathrm{Rct}-\mathrm{Rcto}}{\mathrm{Rct}}\times 100=\frac{\mathrm{io}-\mathrm{i}}{\mathrm{i}}\times 100$$

(1)

where i_o, i, Rct_o and i, $\mathrm{Rct}$ are the corrosion current density and charge transfer resistance of the bare and coated metal surface, respectively.

3. Results and Discussion

3.1. Characterization of SW–PS Coating Samples

The functional groups of the SW solid powder, PS, and SW–PS are shown by FTIR spectra recorded in the absorbance mode in Figure 1a,b.

All labeled FTIR spectra confirmed the successful bonding of the SW to PS, in which the SW–PS intercalation affected the vibrational frequency of the functional groups of both PS and the SW.

For the shrimp SW sample, the vibrational bands in Figure 2 at the characteristic wavenumber $\overline{\upsilon },$ cm⁻¹ are assigned as follows: 3471 cm⁻¹, amine NH₂; 2986 cm⁻¹: OH¯ of chitin; 1650 cm⁻¹: CH₂, CH₃, aliphatic groups; 1424 cm⁻¹: bending vibrations (NH₂ and OH in CH₂OH groups); 1657 cm⁻¹: amide CONH₂ group; 2329.39 cm⁻¹: asymmetric bonding of NH₃⁺, the protonated NH₂ group; 2079.18 cm⁻¹: peptide bonds of amino acids in the cell wall proteins of shrimp shells; 1787.90 cm⁻¹: ester and amide carbonyl in the esters of fatty acids, alcohol, and amide carbonyl; 1626.73 cm⁻¹: bending of NH₂N-H, C-N; 1078.52 cm⁻¹: bending of OH¯ in the alcoholic OH¯ hydroxyl group [22,23].

Intense vibrational bands at 1500 and 1000 cm⁻¹ confirmed the presence of a high percentage of calcium phosphate Ca₃(PO₄)₂ minerals in the shrimp shell SW. This finding confirmed that this SW is rich in different inorganic minerals of calcium salts such as calcium carbonate, sulfate, and phosphate [12,13].

Figure 3a–c include SEM micrographs showing the different surface crystalline morphologies of the coating constituents. Figure 3a shows the inorganic calcium salt minerals in shrimp shells. Figure 3b SEM micrograph shows that the native PS polymeric chains assume a planar horizontal orientation. Figure 3c shows that the intercalation between the SW and PS is significantly modified after grafting PS to SW shrimp shells [24].

Figure 4 shows PXRD patterns at 2-theta 10–80° for the SW and SW–PS coating [14].

The broad PXRD patterns of the fine-powder shrimp shell samples are characteristic of nano-scale powder. The intense diffraction peaks for the coating sample indicated that the intercalation of rigid, thermoplastic glassy PS improved the crystallinity of the fine SW powder [25]. The SW–PS coating consists of microcrystalline thermoplastic elastomeric resins that could be used in a hot-melt adhesive coating to promote an adhesive, hard, tough, and glossy coating film on the metal surface.

TGA confirmed that the SW–PS coating is more thermally stable than SW and PS. The higher thermal stability of the coating sample confirmed the efficient intercalation of the minerals and chitin biopolymers in the shrimp shells with PS (Figure 5).

Figure 6a,b show DTA thermograms of the SW and coating samples, respectively.

The TGA thermogram confirmed that the SW–PS coating is more thermally stable than SW and PS. This behavior could be explained as: The main chemical constituents of shrimp shell are 18.1% chitin, 33.3% calcium carbonate (CaCO₃), and 38.6% protein. Chitin is a biopolymer rich in primary hydroxyl CH₂OH and acetamide CH₃CONH groups. Chitin chains can form intramolecular hydrogen bonding (HB). In addition, chitin could form an intermolecular bond with both the NH₂ and COOH functional groups of the amino acids of proteins encountered in the SW of shrimp shells.

Calcium carbonate (CaCO₃) act as a filler for the polymeric matrix of chitin and polystyrene by reducing the surface tension, water absorption, adsorption of chemical species and friction of the polymer melt; it disperses the polymer melt and improves free-flowing properties, and prevents the absorption of moisture during storage or agglomeration during mixing, which causes a loss of gloss and scratches or grooves on the metal surface. Additionally, CaCO₃ increases both the impact strength and elasticity modulus of the polymeric coating film [26].

The DTA thermogram of the shrimp shell SW showed only one endothermic peak, indicating bond breaking. The DTA thermogram of the coating sample showed an endothermic peak for bond breaking and an intense exothermic peak at 120.16 °C, indicating bond formation. This finding is strong evidence of the compatibility and strong bonding between the coating constituents. The chitin biopolymer in shrimp shells was grafted and cross-linked with PS [27]. The grafting of PS with the SW of shrimp shells is modified by the various amino acids of proteins in the SW. The number of C=O, C−O, −OH, and NH₂ functional groups in the coating exceeded that of PS (which contains only double bonds and delocalized π-electrons in aromatic ring). The shrimp SW enhanced the hydrophilicity of the PS polymeric film, which is required for wetting the metal surface to achieve adherence [28].

3.2. Evaluation of SW–PS Coating for Mild Steel in 1.0 M HC1 at 30 °C

Figure 7 shows the open-circuit potential, E_OCP (versus standard calomel reference electrode) of MS in 1.0 M HCl due to the electrical double layer at the metal/solution interface. A steady-state equilibrium or E_OCP was attained after 30 min. The coating samples S1–S7 shifted the equilibrium potential of MS to a more noble value, indicating the formation of an insulating coating film on the metal surface [1].

Figure 8 shows the impedance plots represented as Nyquist plots.

The complex Nyquist impedance plots of all bare and coated MS samples in 1.0 M HCl consisted of depressed capacitive semicircles with negligible diffusion tails, indicating that corrosion was under activation control. The same locus of the Nyquist plots for MS for the bare and coated samples revealed that all the tested samples were corroded by the same corrosion mechanism [1,10].

The semicircle diameter (which equals charge transfer resistance, R_ct) increased with the increasing weight of PS and the shrimp SW in the coating formulation. The reciprocal, 1/R_ct, represents the corrosion rate [16].

The presence of only one time constant in the Nyquist plots confirmed the capacitive behavior of the metal surface at the peak frequency of the half-circle (f_max.) [21]. The impedance parameters were calculated by nonlinearly fitting the Nyquist plots of the bare and coated metal surfaces to the theoretical equivalent circuit model (Figure 9) with a negligible fitting error. The suitability of this equivalent circuit to the appropriate impedance plots was confirmed by the coincidence of experimental and theoretical data points [1].

In this equivalent circuit model, due to the heterogeneity of the corrosion system, the capacitances of the electrical double layer (EDL) and coating film were implemented as constant phase elements, Q_dl, and Q_f, of the impedance and are represented by Equation (2).

ZCPE = Q⁻¹ (iω)⁻ⁿ

(2)

where I = (−1)1/2, ω is the frequency. For rad s-1, ω = 2πf, and f is the linear frequency (Hz).

The values Q and n define the impedance, Z, of Q_dl as represented by Equation (3) [10]:

If n equals one, then Q_dl resembles a capacitor of impedance (ZC):

ZC = (iωC)⁻¹

(3)

where C is the ideal capacitance. For a heterogeneous solid–liquid corrosion system, n ranges from 0.9–1 [1]. R_s ($\mathsf{\Omega }$·cm²) represents the solution resistance between the WE and RE. The parameter R_ct ($\mathsf{\Omega }$·cm²) is the charge transfer resistance connected in parallel to the constant phase element Q_dl and represents the double-layer capacitance at the metal/solution interface.

The elements R_f and Q_f represent the resistance and constant phase element of the coating surface film, respectively. The parameters n and n_f describe the heterogeneity of the corrosion system [1,10]. Element W represents Warburg’s impedance. Negligible Warburg diffusion tails were observed at the low-frequency region in the Nyquist plots because the hydrogen evolution reaction is the primary cathodic reaction for steel in acidic media, and the oxygen reduction reaction is negligible [1].

The impedance parameters of the nonlinear fitting of Nyquist plots to the equivalent circuit model are collected in

Table 1. Impedance parameters for mild steel in 1.0 M HCl at 30 °C.

Sample	RsOhm·cm−2	nf	Rf	QdlµF·cm−2	nEdl	Rct, Ohm·cm−2	W, Ohm·cm−2	%P
S0: bare metal	2.1	0.9	5.3	1520	0.8	10	0.023	-
S1: SW	2.9	0.9	7.24	1430	0.7	13.4	0.020	25
S2: 1.0 g PS	4.3	0.9	7.94	1470	0.6	36.3	0.018	72
S3: 2.0 g PS	4.2	0.8	6.47	734	0.7	48.2	0.015	79
S4: 1.0 g PS, 0.01 g SW	3.1	0.8	8.54	625	0.6	250	0.013	96
S5: 1.0 g PS, 0.02 g SW	3.7	0.7	8.88	421	0.6	300	0.012	97
S6: 2.0 g PS, 0.01 g SW	3.6	0.8	8.84	330	0.7	520	0.010	98
S7: 2.0 g PS, 0.02 g SW	3.7	0.7	8.81	230	0.8	800	0.009	99

.

The small values of solution resistance (Rs) reflect the good conductivity of the test solution and the external electric circuit, as well as the appropriate geometry of the electrochemical cell and the absence of Ohmic (IR) overpotential [3]. The increase in the film resistance R_f for the coating samples confirmed the isolation of the metal surface from the corrosive 1.0 M HCl.

The values of the parameters n_f and n_Edl were unequal, confirming the heterogeneity of the corrosion system at the metal/solution interface. The decrease in Q_dl proved the adsorption of the active coating ingredients on the metal surface. The hydrophobic, polymeric PS matrix confirmed the electrical and heat-insulation properties of the formulated coating. The coating layer acted as an insulating parallel plate capacitor (dielectric coatings), isolating the metal surface from the corrosive environment. The functional groups of chitin and the π-electrons adsorbed on the active sites of the metal surface inhibited corrosion. The SW increased the chelating ability of PS [20].

The increase in R_ct is attributed to the formation of a protective adherent hydrophobic coating film on the metal surface, blocking the active sites on the metal surface [1,18], increasing the electrical resistance of the metal surface, and inhibiting electron transfer from anodic sites to the cathodic sites across the metal surface.

The impedance behavior of the bare and coated samples in the high-frequency region was clarified by Bode plots (Figure 10). The impedance of the metal surface to electron transfer increased with the increasing wt.% of PS and SW and decreased with the increasing applied frequency [21].

The polarization curves of the bare and coated metal MS samples in 1.0 M HCl are shown in Figure 11.

The polarization curves showed Tafel behavior, indicating that the corrosion reaction was under activation control.

The SW–PS polymeric coat shifted both the anodic and cathodic polarization curves to higher overvoltages with the increasing wt.% of SW and PS. The coatings were mixed-type CIs, which retarded the anodic redox metal dissolution reaction and the cathodic hydrogen gas evolution reaction. The coating functional groups adsorbed on metal surfaces blocked both anodic and cathodic active sites [1,10].

The coating shifted the corrosion potential (E_corr) of MS/0.1M HCl to a more noble direction in agreement with the time–potential curve, indicating a compact, adherent, polymeric coating film on the metal surface [1].

The polarization parameters were obtained by applying the Tafel extrapolation method to the tangent lines of both the anodic and cathodic polarizations at ±50 mV around E_corr or E_rest. These parameters included the corrosion potential (E_corr) corrosion current density (i_corr, mA·cm⁻²), anodic and cathodic Tafel slopes (β_a and β_c), and are collected in

Table 2. Polarization parameters for different MS samples in 1.0 M HCl.

wt.% (g)	−Ecorr (mV)	icorr (mA·cm−2)	βa	−βc	%P
			mV·dec−1
S0: bare metal surface	471	1.61	100	79	-
S1: SW	461	0.65	124	98	25
S2: 1.0 g PS	456	0.62	138	99	61
S3: 2.0 g PS	451	0.27	140	98	83
S4: 1.0 g PS-0.01 g SW	446	0.11	147	89	93
S5: 1.0 g PS-0.02 g SW	440	0.08	139	97	95
S6: 2.0 g PS-0.01 g SW	432	0.06	136	96	96
S7: 2.0 g PS-0.02 g SW	430	0.05	137	95	97

. Percent protection (%P) was calculated using the values of i_corr [18].

The higher cathodic Tafel slope (β_c) compared to the anodic Tafel slope (β_a) for the coated samples indicated that this coating formulation filled all the pores on the metal surface, increasing the overvoltage of the metal dissolution and hydrogen evolution reactions. The decrease in i_corr and increase in %P with the increasing wt.% of SW and PS in the coating indicated that these coating were anticorrosive for MS in 1.0 M HCl [18].

Figure 12 represents the histogram of %P from the impedance and polarization measurements [1].

%P was increased with the increase in the active constituents of the coating film, which increased the fractional surface that was covered by the extra number of CI molecules encountered in the coating formulations.

The coating sample of composition 2 g PS + 0.02 g SW had the highest %P. The chemical constituents of shrimp SW decreased the hydrophobicity of PS and enabled the wettability of the metal surface before forming a hydrophobic, adherent, thin coating film. %P increased with the increasing wt.% of the PS and SW coating constituents. The slight differences in %P obtained from the impedance and polarization measurements are attributed to the theory behind each technique [20].

Chitin, an abundant biopolymer in shrimp shell SW, is a biodegradable, linear amino-polysaccharide copolymer (1–4)-2-amido-2-deoxy glucosamine, and is more ordered and rigid than synthetic polymers. Cross-linking, grafting, and copolymerization decrease the solubility and increase the chemical stability. CaCO₃ filled the pores in the PS and cross-linked the PS and chitin [29].

The hydrophobic coating improved the corrosion resistance of MS due to electron-donating functional groups with delocalized electron density covering the metal surface. The corrosion resistance of the coated MS sample is attributed to the adsorption on the metal surface. Physisorption involves the electrostatic interaction between the oppositely charged metal surface and inhibitor molecules. Chemisorption involves a strong covalent coordinate bond due to electron charge sharing between inhibitor molecules and the metal surface [16,17,18,19,20,21].

3.3. Surface Analysis of Mild Steel Samples

Figure 13a,b shows the surface morphology of the bare and coated MS samples as SEM micrographs after immersion in 1.0 M HCl for two months. The SEM micrographs show that the bare metal surface was rough due to the high corrosion rate in 1.0 M HCl.

The presence of SW inorganic salts increased the adhesion of the protective coating film on the metal surface.

Figure 14a–e show SEM micrographs of coated MS after immersion in 1.0 M HCl for two weeks compared to a blank, clean, bare metal surface. The SEM micrograph in Figure 14a shows the clean metal surface with a regular atomic array and no corrosion features.

Figure 14b shows that the coating sample composed of 1.0 g PS + 0.01 g SW formed an incomplete monolayer, and some corroded areas appeared.

The coating morphology was improved by grafting PS with 0.02 g SW. SEM micrographs in Figure 14c–e show a smooth metal surface covered by a protective and adherent coating film that uniformly covered the whole metal surface (coating sample containing 1.0 g or 2.0 g PS). However, the continuity of the coating film was improved by increasing the wt.% of both PS and SW.

The continuous, thin SW–PS coating film protected the MS against the aggressive 1.0 M HCl. The coating containing a phosphate group acted as a natural primer, fixing the first and subsequent coating layers to the metal surface. No gelation was observed, and the impermeable protective layer exhibited consistent performance. These hybrid organic–inorganic materials provided a moisture barrier against erosion and atmospheric corrosion.

Figure 15a–c show SEM micrographs confirming the durability and outstanding performance of the coating sample; the 2 g PS + 0.02 g SW formulation yielded the best coating sample after aging in storage for one year.

The coating sample was stable against gelling in storage [21]. Figure 15a shows the aged, coated metal after immersion in 1.0 M HCl for a month.

The coating formed an adherent, continuous coating film on the metal surface without using any primer, undercoat, or topcoat. This performance ensured that adherent coatings had enhanced resistance, durability, and chemical resistance, as well as good surface morphology and impermeability to moisture and corrosive media.

Figure 15b,c shows the freshly prepared coating sample and another coating sample stored in a sealed, transparent glass bottles at ambient temperature for a year.

The colloidal stability of the coating sample was slightly changed in storage. The aged coating sample particle size was maintained without significant coagulation, agglomeration or gelation. The slight change in the aged SW–PS sample compared to the freshly prepared sample reflected its good colloidal stability against coagulation from aging [18].

3.4. Evaluation of Physical Parameters of Coating Samples

According to ASTM standards, hot-melt coating is characterized by its mechanical and technological properties. The effect of the wt.% of PS and SW, in g/L, on the coating properties is illustrated in

Table 3. Some physical properties of SW–PS (Formulas 1–4).

Method	Sample No.				Parameter
	1	1	2	1	wt.% PS
ASTM	0.02	0.01	0.02	0.01	wt.% SW
ASTM D-938	67	63	65	63	Congealing point, CP °C
ASTM D-36	101	78	85	79	Softening point, SP °C
ASTM D-127	79	77	77	76	Drop m.p., °C
ASTM-638	11	9	10	8	Penetration at 25 °C, dmm
ASTM D-445	482	521	526	560	Dynamic viscosity at 140 °C, cp
ASTM-638	846	813	839	809	Tensile strength at break, 25 °C, psi
ASTM-D638	8.67	8.77	9.09	9.25	%Elongation at break, 25 °C
-	5B	5B	5B	5B	Cross-cutter tester

.

The coating showed acceptable melting behavior (CP, SP, and drop m.p.), dynamic viscosity, hardness (low penetration value), and mechanical strength (tensile strength and elongation), enabling the protection of mild steel in 1.0 M HCl [30].

The SW–PS coating was shown to be a temperature-responsive, self-gelling polymeric coating. The inorganic mineral salts in shrimp shell SW intercalated with PS, forming a resin with SP 101 °C. The coating could tolerate applied temperatures of 20 °C less than the SP [30]. The main factor affecting the physical and mechanical properties was the wt.% of both PS and SW. A high wt.% of SW increased the CP (the coat resists coagulation in storage). A high SP and m.p. indicated a good hardness to tolerate heat tempering due to the enhanced hydrophobicity. The decrease in elongation at the breaks reflected a rigid polymeric coating film. The cross-cut coating samples (5B) were classified according to ISO class—0/ASTM class 5B—the cut edges on the coating film surface were smooth and none of the squares of the lattice coating film were detached, indicating a continuous, adherent protective film [31].

4. Conclusions

Shrimp shell–polystyrene blends can be protective hybrid coatings for MS in a corrosive aqueous solution of 1.0 M HCl. The low-cost ingredients of SW improved the performance of PS in the coating formulations. Shrimp reinforced the PS’s mechanical and thermal properties, acting as acrylic and stiffening agent. The chemical components of shrimp SW are biodegradable, yielding no toxic byproducts. The SW improved the interfacial bonding between PS and the hydrophilic metal surface. A hot-melt coating is recommended to protect MS.

Polystyrene was grafted with traces of shrimp shell solid waste. No chemicals such as chromate, nitrate, or persulphate were used in the coating formulations. The coating formula can cover a wide range of products as a hot-melt coating for protecting mild steel in acidic media. The durability of the coating during aging reflected its resistance to wear corrosion by physical causes.