A Novel Silver-polystyrene Microemulsion for the Control of Microbiologically Influenced Corrosion in Cooling Water Systems

Materials

The rods of silver electrodes (100% purity, two gauge, 12-inch) that are ideal for colloidal silver generators were purchased from Copper Co., Alexandria, Egypt. The anode:cathode area ratio was adjusted to 1:10 to maximize the anodic current density.Polystyrene-2% divinylbenzene co-polymer was obtained from Alibaba Suppliers, Egypt.

Methods

Preparation of the microemulsion

The 30% w/v of polystyrene-divinylbenzene co-polymer was dissolved in 30 mL of toluene. The solution was agitated at 1000 rpm and then degassed for 10 min. 10 mL of 60 ppm concentration SNPs was added dropwise, and the mixture was stirred at 1600 rpm for 1h, followed by degassing for 30 min. The tween 80 (surfactant-emulsifying agent) and 3.0 mL absolute ethanol co-surfactant were added to ensure the compatibility between its microemulsion constituents. The mixture was centrifuged at 5000 rpm for 1 h to remove excess surfactants and the unreacted materials.

Figure 1a and b shows the electrolytic cell used in the preparation of SNPs and the co-polymer micro beds as a solid support encapsulated SNP.^[14]

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Figure 1: (a) Schematic representation of the formulation of the microemulsion. (b) Visualize the orange color confirmed SNPs. (AgNPs or SNPs: Silver nanoparticles)

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During electroysthesis, Ag⁺ ions were reduced into Ag⁰. The small nuclei of SNPs were grown by further reduction and deposition of Ag atoms. The alkaline water, as a dispersion medium and stabilizer, prevented aggregation and controlled the particle size. SNPs were kept at room temperature in brown bottles to avoid photo-degradation reaction.

Evolution of the microemulsion as a corrosion inhibitor.

The tested C-steel sample has the characteristics as collected in Table 1.

Table 1: Chemical composition of the C-steel sample in weight percentage.

Material	C	Mn	P	S	Cu	Cr	Fe
Percentage %	0.3	0.84	0.01	0.005	0.02	0.02	98.805

Before each experiment, the corrosion coupon was polished with different grades (320 – 1000) emery papers, starting from the coarse 320 and proceeding to the finest grade 1000. The coupon was washed with de-ionized water, degreased with acetone, then dried and stored in a desiccator.^[17,18]

Weight loss chemical method

The gravimetric experiments were carried out in 350 mL jars containing 300 mL industrial cooling waters from recirculating cooling systems from Egyptian Petrochemical Company, Alexandria, containing different ppm concentrations of the microemulsion. The jars containing cooling water (blank or mixed with SNPs or different concentrations of the microemulsion) were thermostatted at 25°C, simulating the actual temperature in utilities units. Each clean coupon was weighed accurately, fixed by Teflon wire at the jar cover, and immersed in cooling water. The coupons were then removed from the jars, washed thoroughly with double-distilled water, immersed in (15% HCl for 5-10 s, followed by 20 Wt.% Na₂CO₃ for 30 s.), dried, and reweighed. The weight differences between the initial weight and the final weight of the coupons (weight loss, Δm) were recorded. Equation 1 was used to determine the corrosion rate (CR) in mills per year (mpy).

(1)

$\begin{array}{c}CR\left(mpy\right)=\frac{\text{mass}\text{loss}\left(\Delta \text{m}\right)\text{in}\text{gram}}{\text{d}\times \text{A}\times \text{t}}\\ =0.807\times \frac{\Delta m\left(mg\right)}{t\left(days\right)}\end{array}$

Where d: density (g cm^-3) of steel, A: exposed area of the corrosion coupon (cm²), and t is the immersion time (days).^[17]

The protection efficiency for the metal surface was calculated using equation 2.

(2)

$Inhibitorefficiency\left(\%IE\right)=\left(\left(\frac{C{R}_o-CR}{C{R}_o}\right)\times 100\right)$

Where CR_o and CR are the corrosion rate (mpy) in the uninhibited and the inhibited solution.

Potentiodynamic polarization technique

The potentiodynamic polarization was carried out on a polished, cleaned, and dry working electrode in a three-neck 100 mL glass electrochemical cell containing a working electrode (WE), auxiliary (AE), and reference electrodes (RE). The WE was the carbon steel electrode with a 1.28 cm² surface area. Saturated calomel electrode (SCE, Hg/Hg₂Cl_2(s), KCl _(saturated)) electrode (RE) has 0.242V potential (E_calomel) at 25^oC was periodically calibrated to avoid the change of E_calomel with the temperature according to equation 3.

(3)

${\text{E}}_{\text{calomel}}=0.\text{242}–0.000\text{78}\left({\text{t}}^{\text{o}}\text{C}\right)$

The auxiliary counter electrode (CE) was an inert Pt wire, completing the electrical circuit, allowing current flow. The SCE was inserted into the Luggin capillary positioned closely to the WE surface for minimizing Ohmic IR resistance. The cell and its components were carefully cleaned after each experiment using chromic acid-H₂SO₄ mixture, washed with tap water, and double-distilled water. Each experiment was carried out with a new metal surface in a 50 mL test solution. Cell was thermostated at 25^oC until thermal equilibrium and connected to computerized Metrohm Autolab B.V.PGSTAT128N 12 V/800 mA Potentiostat AUT86580, Utrecht, Netherlands. Nova 1.11.0 software was used for data analysis. Each electrode was connected to the appropriate joints [Figure 2].

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Figure 2: Schematic representation of cell connection during polarization. (SCE: Saturated calomel electrode; DC: Direct current (3V))

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After each experiment, the cell and its components were cleaned with water (tap, double distilled), a small volume of test solution to remove the polarization products. The open circuit rest steady state potential of WE (E_OCP or E_rest) was established for 15 min. WE were polarized within ± 300 mV around E_OCP at the sweeping rate 10 mV.min^-1.^[17,18]

(4)

$\text{Inhibition}\text{efficiency}\text{(\%IE)}=\frac{{\text{i}}_{\text{o}}-\text{i}}{\text{i}}\times 100$

Where i_o, and i, are the corrosion current density in the absence and the presence of different concentrations of the microemulsion.

Characterization of SNPs by transmission electron microscopy (TEM) and optical activity

Figure 3a shows the TEM micrograph and Figure 3b shows UV-Vis. Electronic absorbnce of SNPs in UV-Vis. region of electromagnetic radiation.

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Figure 3: (a) TEM micrograph and (b) UV-Vis. Spectra of SNPs. (SNPs: Silver nanoparticles; TEM: Transmission electron microscopy; UV–Vis: Ultraviolet–visible Spectroscopy)

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In TEM, the colloidal solution of SNPs was visualized as spherical-shaped particles of a uniform particle size. The characteristic sharp UV-Vis absorbance band at 430 nm confirmed the metallic Ag⁰ surrounded by surface plasmon as a sea of free electrons.^[12]

The stability of SNPs was confirmed by the negative zeta potential (-8.95 mV) as shown in Figure 4.

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Figure 4: Zeta potential of SNPs. (SNPs: Silver nanoparticles)

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Zeta (ζ) potential at the slipping plane of colloidal SNPs expressed the magnitude of electrostatic repulsion between SNPs. This highly negative zeta potential indicated that SNPs carry a net negative surface charge, causing repulsion between the SNPs and preventing aggregation.

Characterization of the microemulsion

The loading of SNPs on the co-polymer matrix was confirmed as follows: Particle size distribution (PSD) showed the frequency of particle sizes within the SNPs-polymer sample [Figure 5].

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Figure 5: Particle size distribution curve of SNPs-COP microemulsion. (SNP-COP: Silver nanoparticles capped with a co-polymer)

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The monodispersed particle size peaked at 140 nm, confirming good mechanical properties (tensile strength, elasticity), large surface area, reactivity, and thermal stability. The small PDS (∼100 nm) indicated the microemulsion is a colloidal system where quantum effects and high surface-to-volume ratios dominate. This narrow PSD (∼100 nm): confirmed uniform dispersed non-aggregates and strong interfacial bonding of SNPs within the co-polymer matrix. The surfactant and the co-surfactant controlled the particle size.

The amorphous structure of the microemulsion was confirmed from the XRD pattern [Figure 6a and b].

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Figure 6: XRD patterns of (a) SNPs and (b) microemulsion. (SNP: Silver nanoparticles; XRD: X-ray diffraction)

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The amorphous structure of the microemulsions provided superior corrosion inhibition through uniform surface coverage, enhanced solubilization, improved adsorption, and long-term stability. These properties enabled good protection of C-steel against corrosion. The single, narrow particle size distribution peak at ∼100 nm, and the XRD pattern confirmed the successful formation of a uniform microemulsion with SNPs incorporated and stabilized within the polystyrene-DVB co-polymer matrix.

The glass transition of the microemulsion differed from that of polystyrene. The thermal stability of the microemulsion is recorded in the DSC thermogram in Figure 7, with narrow endothermic peaks indicating that the thermal decomposition requires heat input. The high glass temperature at 203.49^oC confirmed thermal stability.^[15]

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Figure 7: DSC curves: The area under the melting peak at T_melting equals the enthalpy of melting. (DSC: Differential scanning calorimetry)

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Polystyrene (PS) has a glass transition temperature (T_g) (100-105)°C, and decomposition temperature (200-250)°C.^[19] Enthalpy of decomposition at glassy state (-12 Jg^-1) is much lower than enthalpy of decomposition (-650 Jg^-1). The weak adhesion between the PS and DVB in the polymer matrix caused the interface to be softened at T_g.^[20] For an amorphous co-polymer like polystyrene-DVB, the key thermal transition is T_g. The loaded SNPs reinforced the adhesion and created an interphase with a higher T_g 203.49^oC due to the strong non-covalent interaction between SNPs and the polymer matrix, such as Van der Waals forces.^[21] The higher T_g of the Co-polymer than pure polystyrene indicates restricted chain mobility due to cross-linking (DVB) and the interaction of SNPs. The high decomposition temperature (256^oC) confirmed the thermal stability of the microemulsion. The amorphous polystyrene co-polymer has no true melting point and exhibits no melting enthalpy but rather T_g.^[22] High ΔH_{decomposition} confirmed loading SNPs into the polymer chain.^[18]

Heat capacity (Cp) at constant pressure was calculated as.^[23]

The lower value 1.23 Jg^-1K^-1 than that of polystyrene (PS) (∼1.3-1.5 J g^-1 K^-1 at the room temperature) confirmed that cross-linking by the crosslinker DVB restricted the chain mobility, which was further decreased by the loaded SNPs.^[24]

The antimicrobial activity of the microemulsion

The microemulsion showed large inhibition zone and potent activity for the tested bacterial cells, as shown in Table 2 and Figure 8 (a-d).

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Figure 8: The inhibition zone for the antibacterial activity of the microemulsion - (a) S. aureus, (b) B. subtilis, (c) S. typhi, (d) E. coli.

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The microemulsion showed superior activity for both Gram-negative bacteria, Escherichia coli & Salmonella typhimurium, and Gram-positive bacteria, Staphylococcus aureus & Bacillus subtilis.

The potent antibacterial activity of microemulsion suggested inhibition and mitigation of MIC of steel in the cooling water system. In this aqueous solution, especially natural water and seawater, various bacterial species cause MIC: Pseudomonas species S9, Serratia marcescens, as well as sulphate-reducing bacteria (SRB), which reduce sulphate (SO₄^-2) ions in seawater into sulphide (S^-2), produce the corrosive gas H₂S_(gas) that forms black ferrous sulphide (FeS) as corrosion product. The biological activity of microemulsion was attributed to the well-dispersed SNPs on the polymer matrix. The penetration of SNPs into the biofilm disrupted its structure and decreased the localized corrosion sites. Microemulsion is favored in comparison to the conventional corrosion inhibitors, which exhibit poor biofilm penetration and uneven surface coverage.^[16]

The evaluation of the microemulsion as a corrosion inhibitor for C-steel in the cooling water system at 25^oC

From weight loss results, Figure 9 (a-g) showed the glass Jar filled with cooling water, and the visual inspection of the corrosion coupons immersed up to 3 months in the test solutions contained 30 ppm concentration of the microemulsion.

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Figure 9: (a) Glass jar filled with cooling water(microemulsion, blue arrow), and C-steel(red arrow); (b-g) Visual inspection of the weight loss results of C-steel in the cooling water system at 25⁰C.

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The morphology of the steel coupon after the weight loss test at different immersion time intervals showed massive brown rust in cooling water and a clean appearance in the presence of 30 ppm microemulsion.

Table 3 shows the weight loss and the CR for the C-steel corrosion coupons in the absence and the presence of different concentrations of the microemulsion after 1 month of immersion.

The CR decreased with increasing concentration of the microemulsion ^[18]. The %IE of 90 ppm microemulsion C-steel was 86.42% at 90 ppm.

The %IE was confirmed from the potentiodynamic polarization curves for C-steel in cooling water at 25^oC containing different ppm concentrations of microemulsion [Figure 10].

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Figure 10: Potentiodynamic polarization curves of carbon steel in cooling water at 25^oC. (SNPs: Silver nanoparticles)

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At high irreversible applied overvoltage (η) or applied potential (E, mV) of steel, the polarization curves showed Tafel behavior and obeyed Tafel equation 2:

$\text{Tafel equation Overvoltage}\eta =\text{a}+\text{b}\text{log }\left(\text{current density},\text{ i}\right):$

Where the constants a and b are characteristic for each electrode process, a = -(2.303RT/αF) log i_o, and b = (2.303RT/αF).

The polarization curves showed Tafel behavior, indicating that corrosion of steel in cooling waste was under activation control.

The polarization parameters: Corrosion potential (E_corr.), corrosion current density (i_corr.), anodic-, and cathodic Tafel slopes β_a, β_c, respectively, collected in Table 4 were calculated using the Tafel extrapolation method at ±50 mV around E_corr..^[17,18]

The values of the linear polarization resistance (LPR) were calculated using equation 6:

By increasing the concentration of the microemulsion, both cathodic and anodic polarization curves were shifted toward higher overvoltages (large values of Tafel slopes β_a and β_c) and the slight shift in (E_corr.)_, indicating the microemulsion was a mixed-type inhibitor for the corrosion of C-steel in cooling water (retarded both redox reactions). The decrease in the corrosion current density and the increase in the values of the linear polarization resistance confirmed the high corrosion resistance of the steel.^[17,18]