Extraction of eco-friendly and biodegradable surfactant for inhibition of copper corrosion during acid pickling

Abstract

A novel, cheap, less toxic, and easier-prepared gelatin surfactant is successfully used as corrosion inhibitor for the corrosion of copper in 0.1 M H₂SO₄ at the temperature range: 25–55°C. The critical micelle concentration of the surfactant was determined from surface tension measurements. The inhibition efficiency was determined from potentiodynamic polarization and electrochemical impedance spectroscopy techniques. For surfactant acted by adsorption at copper/solution interface, an inhibition efficiency up to 68 was obtained at critical micelle concentration (70 ppm) of surfactant at 35°C. The free energy of adsorption was calculated and discussed. The surface parameters of gelatin surfactant were calculated and correlated to the inhibition efficiency. They were also calculated from its surface tension profile including: critical micelle concentration), maximum surface excess (Γ_max), and minimum surface area (A_min). The thermodynamic of micellization, free energies of micellization (ΔG_mic) and entropy of micellization, was calculated and discussed. The formation of compact and adherent monomolecular adsorbed film on copper substrate was confirmed.

Keywords

Gelatin surfactant critical micelle concentration impedance polarization surface activity

Introduction

In view of good corrosion resistance, excellent thermal conductivity, high strength, copper, and copper alloys are used widely as a material of construction for building equipments such as pipelines, storage tanks, heat exchangers, cooling systems, and pumps. Copper and copper alloys suffer from corrosion in aerated electrolytes via the formation of corrosion cells, such as dissimilar metal cells, differential aeration cells, and differential strain cells (Andricacos, 1999; Flott, 1997; Safranek, 1986; Soliman, 2002; Syrett, 1981; Taha, 2014). In electricity production plants, the condenser tubes constructed from brass copper alloy may corrode by the residual of chemicals added during water treatment (Bostwick, 1961) as well as deposit and corrosion products from condensate in copper condensers which act as insulators and reduce the heat transfer process (Fontana, 2005).

Acid pickling is employed to obtain a clean metal surface free of oxides or scales, which can be further treated by processes such as galvanizing, painting, enameling, electroplating, and cold rolling (Ehteshamzade, 2006). The scale deposits on copper condenser tubes are removed by pickling in 0.1 M H₂SO₄ in the presence of corrosion inhibitor (Stupnišek-Lisac, 2002). Recently, due to increasing environmental awareness and the need to develop environmentally eco-friendly processes, attention has been focused on the corrosion control using natural products and eco-friendly corrosion inhibitors (Rani and Basu, 2012). Several corrosion inhibitors for copper (Cu) in various aggressive media were cited in the literature (Antonijevic and Petrovic, 2008; Mihajlović and Antonijević, 2015; Raja et al., 2016; Villamil, 1999); in NaCl and neutral media: 2-carboxymethylthio-4-(p-methoxyphenyl)-6-oxo-1,6-dihy-dropyrimidine-5-carbonitrile and thiadiazoles, 8-aminoquinoline for 5-mercapto-3-phenyl-1,3,4-thiadiazole-2-thione potassium, 2-mercaptobenzimidazole, phytic acid calcium film, bis-(4-amino-5-mercapto-1,2,4-triazol-3-yl) butane for fungicides (myclobutanil and hexaconazole ammonium pyrrolidinedithiocarbamate, olive leaf extract, imidazole, benzimidazole, and methyl and mercapto derivatives (1-methyl-imidazole; 2-mercapto-1-methyl-imidazole and 2-mercaptobenzimidazole polypyrrole (PPy) films for Cu in sodium di-hydrogen phosphate andphytate, cysteine, 1,5-bis(4-dithiocarboxylate-1-dodecyl-5-hydroxy-3-methyl-pyrazolyl) pentane, 3-benzylidene amino 1,2,4-triazole phosphonate, 3-cinnamalidene amino 1,2,4-triazole phosphonate, 3-salicylalidene amino 1,2,4- triazole phosphonate and 3-paranitro benzylidene amino 1,2,4-triazole phosphonate, designed for protection of copper against corrosion; 1,2,3-benzotriazole, 5-mercapto-3-p-nitrophenyl-1-2-4-triazole, and l-cysteine electrodeposit in the presence of benzotriazole, PPY film doped with oxalic ions PPy film containing BTA, polyaniline (PANI) on Cu (Cu/Ni) aniline-containing Na-oxalate solution, copolymer films PANI-co-POA, and poly(aniline-co-o-anisidine); electropolymerized pyrrole; imidazoles (4-Me-1-ph-imidazole, 4-methyl-1-(p-tolyl) imidazole), polyindole film, and poly(o-toluidine) coatings on Cu; N-(5,6-diphenyl-4,5-dihydro-[1,2,4)triazin-3-yl)-guanidine; 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole, benzimidazole, and 2-methylbenzimidazole; trithiocyanuric acid; chitosan biopolymer; 2-thiazoles ingredients: 4-Me-5-vinylthiazole and 5-(2-hydroxyethyl)-4-methylthiazole; imidazole, 1,2,4-triazole, and 1-Me-derivatives; 2-mercapto-4-amino-5-nitroso-6-hydroxy pyrimidine, diniconzole and triadimefon, allylthiourea, tolyl, 4-methyl-1-phenyl imidazole, PPy/polyhedral oligomericsilsesquioxane nanocomposite, electrodeposited thiourea film; hybrid sol–gel (Hy) coating consisting of (3-glycidoxypropyl)methyldiethoxysilane and tetraethyl orthosilicate was immobilized on thiourea-modified copper surface via an epoxy amine cross-linking reaction to form a binary coating. 2-Mercaptobenzoxazole as well as imidazoles; purine, adenine, and 6-benzylaminopurine for Cu/synthetic seawater; purine and adenine for Cu/0.5 M NaNO₃ and Cu/0.5MNa₂SO₄; 5-methoxy-2-(octadecylthio)benzimidazole for Cu/nitric acid; 2,4-hexadienoic acid potassium salt and benzotriazole 1,2,3-benzotriazole for Cu/sulfate solutions; neem leaf extract for Cu/aerobic A.sulfureus neutral medium; cysteine for Cu/HCl; 5-methyl-1H-benzotriazole and benzotrizaole; 1-(2-pyrrole carbonyl)-benzotriazole and 1-(2-thienyl carbonyl)-benzotriazole in combination with TritonX-100 for Cu/goundwater; bentonite/saline groundwater environment; super-hydrophobic film: myristic acid (n-tetradecanoic); 3-vanilidene NH₂-1,2,4-triazole phosphonate and 3-anisalidene amino 1,2,4-triazolephosphonate; 3-amino-5-mercapto-1,2,4-triazole polydopamine coating on Cu surface, Schiff bases, including N,N0-ethylene-bis(salicylidenimine). N,N0-ortho-phenylene-bis (salicylidenimine) on Cu/chloride and acidic solutions, films modified by propane thiol and 1-dodecanethiol, electropolymerized poly(N-methyl aniline) coatings on Cu/0.1M H₂SO₄; several inhibitors exist for copper in neutral media. However, there is a lack in the literature about corrosion inhibitors for copper in the acidic media.

The aim of the present work is to study the corrosion behavior of brass alloy in aerated 0.1 M H₂SO₄ solution in the presence and absence of the newly prepared eco-friendly gelatin surfactant.

Sodium dodecyl sulfate and benzotriazoles were found to inhibit copper corrosion in sulfuric acid solution (Zaferani et al., 2013). The imidizole derivatives showed high inhibition efficiency for copper corrosion in sulfuric acid (Zhang, 2009). The azoles compounds were reported as corrosion inhibition of copper in 0.1 M H₂SO₄ (Stupnišek-Lisac et al., 2002). Recently, addition of surface active agents or surfactants to the sulfuric acid solution to reduce the corrosion rate of copper has been proved to be an effective method (Lalitha, 2005; Ma, 2003).

Gelatin is a colloidal surfactant that was used to reduce the grain size and harden the deposit of copper during electrodeposition (Turner and Johnson, 1962) and was used as an inhibitor during the galvanic corrosion of copper (Tasic et al., 2006). In addition, small concentrations of gelatin (0.1 g/L or less) were used for corrosion protection of copper pipes in alkaline solution (Malik et al., 2011).

Gelatin is extracted via partial hydrolysis of collagen, the main protein of intracellular substance in animal tissue and bone (Bajpai et al., 2015). There are two types of gelatin type A, with an isoionic point of 7–9, is derived via acid pre-treatment of collagen (Bajpai et al., 2015). The actual composition of gelatin depends on the number of factors including the source of collagen, type of hydrolytic treatment, pH, and temperature because the gelatin solution undergoes structural and mechanical (hence physicochemical) transformation under different temperatures (Khanna et al., 2010).

Experimental

Preparation of gelatin surfactant

A sufficient amount of gelatin macromolecule polymer of chemical structure represented in Figure 1 was extracted from the skin, the subcutaneous layer of a medium-old healthy chicken, acidically digested (hydrolyzed) with a mixture of equimolar concentrated nitric acid and sulfuric acid. The structural unit of gelatin contains many glycine amino acid (almost one in three residues, arranged every third residue), proline, and 4-hydroxyproline residues. A typical structure of amino acids in gelatin is: -Ala-Gly-Pro-Arg-Gly-Glu-4Hyp-Gly-Pro- (Te Nijenhuis, 1997).

Figure 1.

Molecular structure of gelatin.

On filtration, the hydrolyzate was taken and processed into consecutive chromatography methods (gel filtration, ion exchange, and HPLC as reported previously (Iwai et al., 2005). The filtrate was neutralized using 0.1 M NaOH before atomic absorption measurement. The free unsaturated fatty palmitic acid (C₁₆H₃₂O₂) was isolated and identified. On dissolving 15 g/L palmitic acid in thionyl chloride as a solvent and a reactant in the same time, giving the acid chloride of palmitic acid, equimolar solution olyelamine (C₁₈H₃₅NH₂) was added to the last mixture. The solution was magnetically stirred at the room temperature, and thin layer chromatography was performed for identification of the reaction completion. The gelatin surfactant, octadecyl cetyl amide surfactant (C₁₈C₁₆CONH₂), was isolated in a good yield.

A stock solution of 2 M H₂SO₄ was prepared from analytical-grade concentrated sulfuric acid solution (98%). The test solution 0.1 M H₂SO₄ was prepared from 2 M H₂SO₄ by an appropriate dilution. A stock solution of 1000 ppm gelatin surface was prepared in 100-mL double-distilled water for the corrosion rate measurements.

Electrochemical techniques

The impedance and polarization measurements were carried out in cleaned electrochemical cell Multiport™ with a jacketed version for temperature control; three necks for introducing working electrode (WE), auxiliary electrode, and reference electrode that is saturated calomel electrode (SCE) were introduced in air bubble-free Luggin–Haber capillary tube filled with the test solution, and its tip was placed very closely at 1–3 mm apart from the surface of WE to minimize Ohmic IR drop across the solution and avoiding error in the subsequent recorded impedance and polarization curves. No liquid junction potential was included in the measured potential (Cummings et al., 1992). Cylindrical brass alloy sample of the chemical composition shown in Table 1 was used to design WE.

Table 1.

The chemical composition of brass alloy.

Element	Ca	Sn	Mo	Fe	Zn	Cu
Wt%	0.9	0.2	0.1	0.8	31.4	59.3

The copper specimen was fixed on a rod of poly tetrafluoroethylene (Teflon) in such a way that surface area of 1.28 cm² was left in contact with the test solution. A thick copper wire was screwed to the specimen for electrical connection introduced in a glass tube holder and insulator. All potentials were recorded using periodically calibrated corning-type SCE: Hg/Hg₂Cl_2(s), KCl_(saturated) of potential: 0.242 V at 25°C (derived from Nernst equation)

E_{cal} = 0. 242 - 0.000 78 (t ° C)

(1)

The SCE acts as a non-polarizable second interface electrode used to measure the potential of WE. The counter electrode (CE) is an inert carbon electrode completing the circuit and allows passage of current across the circuit that being connected to the output of operational amplifier. The cell and its components were carefully cleaned after each experiment to remove any foreign residues, especially metallic products that were formed during polarization. The cell was cleaned with tap water, double-distilled water, and finally with a portion of the test solution.

Each experiment was carried out with a newly cleaned WE surface. The surface of WE was handly wetted, polished using emery papers of grades: 320, 500, 800 and 1000 grades finishing. Starting with a coarse one and proceeding in steps to the fine grade up to a mirror finish, the electrode was degreased by absolute ethanol and washed thoroughly with double-distilled water just before insertion into the test solution. The cell was filled by 100-mL test solution and was thermostated for 20 min at the desired temperature before starting the experiment and connected to the Gamry potentiostat as schematically represented in Figure 2. The rest potential (E_rest) of WE electrode (performed due to build up of the electrical double layer, EDL formed at the electrode/solution interface) was followed for 15 min till steady-state potential was established to ensure reliable impedance and polarization measurements (Abdel-Gaber et al., 2009).

Figure 2.

Schematic representation of the electrical circuit used in electrochemical measurements.

Impedance measurements were achieved by applying10-mV peak-to-peak sine wave alternating current signal with respect to the rest potential (E_rest) of WE in the frequency range: 0.2–1.0 × 10⁵ Hz. Five data points were taken for each decade, or 10 multiplications of frequency and response current were recorded. An impedance analyzer measures the complex impedance, and the lock-in amplifier-integrated system contains all necessary electrical hardwares controlled by personal computer for running reference 600 sequencer software version 6.20 that coordinate execution of the experiments, logging of data, and provide graphical and numerical analysis of EIS spectra. Gamry Echem-analyst version 6.20 software program was used for the analysis of the impedance spectra, as Nyquist plots via simulation with a suitable equivalent with negligible error.

The potentiodynamic polarization curves of copper in 0.1 M H₂SO₄ were recorded via sweeping the potential at 30 mV/min from ±250 mV (E_rest) of WE and measuring the constant corresponding current attained in two minutes relaxation time after the application of the overpotential. The process repeated continuously (Khanna et al., 2010) till the completion of half-cycle potentiodynamic polarization.

Determination of the critical micelle concentration of gelatin surfactant

The surface tension of the surfactant solution in either double-distilled water or in 0.1 M H₂SO₄ was measured at 25, 35, 45, and 55°C using a Du Nouy tensiometer. The temperature (±0.1°C) was reserved constant by circulating the thermostated water through a jacketed vessel containing the solution. The concentration of the solution was varied by adding a known volume aliquot of stock solution in the vessel. The reproducibility in values of CMC was found to be ±1%, calculated from at least three replicated experimental runs.

Results and discussion

Temperature effect on surface parameter of gelatin surfactant

The surface active properties of gelatin surfactant were calculated using surface tension measurements. At critical micelle concentration (CMC), most of the physical and the chemical properties of surfactant solution showed an abrupt variation at CMC (Ali et al., 2014; Domínguez et al., 1997). The plots of measured surface tension as a function of the logarithm of surfactant concentration in double-distilled water or 0.1 M H₂SO₄ at different temperatures indicated that the surfactant is molecularly dispersed at low concentration, leading to a reduction in the surface tension. The reduction in the surface tension increased with increasing surfactant concentration. At higher concentration, however, when a certain concentration is reached, surfactant molecules form micelles, which are in equilibrium molecular dispersed species (Shinoda et al., 2016). The intercept of the two straight lines in each plot designates CMC of gelatin surfactant, where saturation in surface adsorbed layer takes place. Figure 3 represents the plots of surface tension γ (mN m⁻¹) versus logarithm of surfactant concentration (log C) at different temperatures: (a) gelatin in water and (b) gelatin in 0.1 M H₂SO₄, respectively.

Figure 3.

Variation in surface tension with concentration of gelatin surfactant solution: (1) in water and (2) in 0.1 M H₂SO₄ at: (a) 25, (b) 35, (c) 45 and (d) 55°C.

The obtained CMC values show an increasing trend with raising the temperature of surfactant solution (Table 2). This increase in the temperature causes a decrease in the hydration of a hydrophilic group of surfactant molecules enhancing micellization or causes disruption of structured water surrounding hydrophobic group (an effect that disfavors micellization). The relative magnitude of these two opposing effects, therefore, determines whether CMC increases or decreases over a particular temperature range. From data in Table 2, it is clear that CMC increased by raising the temperature, which implies that the magnitude of two factors (increasing temperature and decreasing hydration) is disfavoring micellization (Ali et al., 2014; Malik et al., 2014).

Table 2.

Surface active properties for the Gelatin at different temperatures in water and sulfuric acid.

Gelatin	H₂O					0.1 M H₂SO₄
t°C	25	35	45	55		25	35	45	55
Parameters
CMC (ppm)	500	800	1400	3000	60		70	85	200
CMC (mol L⁻¹)	1.1 × 10⁻⁵	1.7 × 10⁻⁵	2.0 × 10⁻⁵	7.0 ×10⁻⁵	1.0 × 10⁻⁶		1.3 × 10⁻⁶	1.7 ×10⁻⁶	2.9 × 10⁻⁶
γ_CMC (mN m⁻¹)	46.96	47.60	51.98	53.22	56.86		47.34	44.68	42.23
10¹⁰Г_max (mol cm⁻²)	4.06	3.96	2.92	1.56	3.39		3.25	2.87	2.80
Π_CMC. (mN m⁻¹)	27.7	24.9	18.66	15.76	17.69		24.89	25.98	26.76
A_min (A²)	41.32	42.37	57.68	109.28	49.88		51.79	59.40	60.79
−ΔG_mic (kJ mol⁻¹)	27.62	27.54	27.10	25.38	33.69		34.37	34.20	34.10
ΔS_mic (kJ mol⁻¹ K⁻¹)	0.094	0.091	0.087	0.078	0.114		0.113	0.111	0.106
−ΔG_ads (kJ mol⁻¹)	28.28	28.14	27.74	26.36	34.18		35.11	35.82	35.12
ΔS_ads (kJ mol⁻¹ K⁻¹)	0.097	0.093	0.088	0.082	0.117		0.116	0.114	0.109

The surface tension decreased with increasing of the surfactant concentration until Critical micelle concentration (CMC) is reached, above which surface tension becomes constant and not affected further by surfactant concentration. The values of CMC for gelatin surfactant in water and 0.1 M H₂SO₄ are 1.2 × 10⁻⁵, 1.8 × 10⁻⁵, 3.0 × 10⁻⁵, and 8.0 × 10⁻⁵M at 25, 35, 45, and 55°C, respectively. Whereas CMCs for gelatin surfactant in 0.1 M H₂SO₄ were found to be: 1.0 × 10⁻⁶, 1.2 × 10⁻⁶, 1.5 × 10⁻⁶, and 3.0 × 10⁻⁶ M at the temperatures: 25, 35, 45 and 55°C, respectively.

Hence the gelatin surfactant formed micelles at concentration above CMC. The CMC of gelatin surfactant in water is greater than that in 0.1 M H₂SO₄. The CMC is controlled by a number of factors that are dependent on the nature of surfactant molecules and the aqueous environment. The presence of SO₄⁻² and H⁺ ions decreased CMC of surfactant in comparison to that in pure water as these counter ions tightly associated with the surface of the micelle and protected the formed micelle (Rangel-Yagui et al., 2005), by forming of electrical double layer around the micelle surface.

These phenomena are related to the famous Hofmeister series, which is an empirical measure of ion degree of hydration, and arranged the ions with increased salting in potency from left to right as follows (Cacace et al., 1997)

\begin{matrix} {SO}_{4}^{^{- 2}} {,HPO}_{2}^{^{- 4}} {,OH}^{-} {,F}^{-} {,HCOO}^{-} {,CH}_{3} {COO}^{-}, {Cl}^{-} {,Br}^{-} {,NO}_{3}^{-} {,SCN}^{-} {,ClO}_{4}^{-} \end{matrix}

The sulfate ion, SO₄⁻², that is located at the left of the series acts as water structure makers salting out ions when competing with surfactant for water of hydration, reduces amount of water available in micelles for hydration of polar chain, and causes micellization at a lower surfactant concentration.

The surface active properties of gelatin surfactant, effectiveness (π_cmc), maximum surface excess (Г_max), and minimum area per molecule (A_min) were calculated. The surface pressure at CMC, (π_cmc), which is defined as the effectiveness of surfactant in reducing surface tension, was calculated from the following equation (Zhu and Rosen, 1984)

π_{cmc} = γ_{o} - γ_{cmc}

(2)

where γ_o is the surface tension calculated in water at the appropriate temperature and γ_CMC is the surface tension at CMC (mN m⁻¹). The greatest reduction in surface tension at CMC (effectiveness, π_cmc) was achieved by gelatin surfactant at CMC that is corresponding to the highest inhibition efficiency for dissolution of copper in this acid media.

The maximum surface excess (Г_max) is an effective measure of the number of surfactant molecules adsorbed at the solution/interface in excess to that in the bulk solution. The surface excess (Γ_max) was calculated using the Gibbs adsorption (Adamson and Gast, 1967)

Γ_{\max} = \frac{1}{2.303 R T} (\frac{- d γ}{d \log (c)})

(3)

where Г_max is expressed as the concentration of surfactant molecules at the interface per unit area (mol cm⁻²), T is the absolute temperature, R is the molar gas constant (R = 8.314 J/mol K), and C is the molar concentration of surfactant solution. The surfactant lowered the surface energy, present in excess at the solution/air interface, and effectively lowered the surface tension of the solution. From Table 2, it is clear that the Г_max decreased with raising the temperature of the solution.

The minimum area per molecule (A_min) was calculated using Gibbs adsorption equation (Rosen et al., 1982)

A_{\min} = \frac{1016}{Γ_{\max} \cdot N_{A}}

(4)

where N_A is Avogadro number (6.023 × 10²³ molecule/mol). It was assumed that the head group area at air/solution interface was the same as the equilibrium area per surfactant molecule at micelle/solution interface (Jurašin et al., 2010). The increase in values of A_min caused by increasing the temperature is probably due to the increased thermal motion of the molecules (Churaev, 2018). The thermodynamic parameters of micellization expressed by standard free energy

Δ G_{mic}^{o},

and the entropy of micellization

Δ S_{mic}^{o},

are calculated from the equations (Kataoka, 1996)

Δ G_{mic}^{o} = RT ln (CMC)

(5)

Δ S_{mic}^{o} = \frac{- δ Δ G_{mic}^{o}}{δ T}

(6)

The micellization is thermodynamically a spontaneous process ( $Δ G_{mic}^{o},$ < 0) (Rosen et al., 1982). The $Δ G_{micellization}^{o},$ for gelatin surfactant in H₂SO₄ is more negative than in water. This behavior showed that the micelle was formed more easily in 0.1 M H₂SO₄ than in water, which confirmed the obtained lower values of CMC in H₂SO₄. The values of ΔG^o decreased with raising the temperature confirming the increase of CMC at the elevated temperature (Sulthana et al., 1996).

The highly positive values of ΔS^o_mic in 0.1 M H₂SO₄ solution compared to its value in water showed increase in randomness in the system upon the transformation of surfactant molecules from unimeric state into the aggregated micelle (Chen et al., 1998).

The standard free energy of adsorption ( $Δ G_{ads}^{o},$ ) of gelatin surfactant at an air/solution interface was calculated using the equation (Walter, 1991)

Δ G_{ads}^{o} = Δ G_{mic}^{o} - [0.6022] X π_{CMC} A_{\min}

(7)

Δ S_{ads} = (- δ Δ G_{ads} / δ T)

(8)

The negative values of $Δ G_{ads}^{o},$ indicated the spontaneity of the adsorption process. The negative values of $Δ G_{ads}^{o},$ are greater than $Δ G_{micellization}^{o},$ showing that the surfactant favors the adsorption process rather than micellization. Adsorption at an interface is associated with a decrease in free energy of the system (Walter, 1986). These observations agree with the data reported before which indicated that steric factors inhibit the micellization more than they affect adsorption at air/aqueous solution interface (Chen et al., 1998). The values of ΔS_ads are all positive and greater than ΔS_{micellization} values reflecting the greater freedom of motion of hydrocarbon chain at planar air/aqueous solution interface compared to that in relatively cramped interior beneath the convex surface of the micelle.

Impedance and polarization measurements

Figure 4 shows the Nyquist plots for copper in 0.1 M H₂SO₄ at 25°C. The capacitive loops at low the high frequencies region are ended by the diffusion tail at the low frequency region. This locus of impedance Nyquist plots indicated that the corrosion of copper in 0.1 M H₂SO₄ is under diffusion control (Ma et al., 2002; Shaban et al., 2015).

Figure 4.

Nyquist plots for brass alloy in 0.1 M H₂SO₄ at 35°C: (a) 0.1 M H₂SO₄, (b)10, (c) 20, (d) 30, (e) 40 and (f) 70 ppm gelatin surfactant.

Nyquist plots were analyzed via the nonlinear fitting to the equivalent circuit model shown in Figure 5, and the impedance that contains the impedance parameters is collected in Table 3.

Figure 5.

The equivalent circuit model used for simulation of Nyquist plots of brass alloy, where Rs represents the solution resistance, Q_dl represents the capacitance of electrical double layer of copper/solution interface, R_ct the charge transfer resistance across the copper surface, C_f is capacitance associated with surface film formed on the surface of brass alloy, R_f is Ohmic resistance of surface film and n_f is exponent factor associated with the film impedance.

Table 3.

Impedance parameters of brass alloy in 0.1 M H2SO4 in absence and the presence of different concentration of gelatin surfactant.

[Surfactant] (ppm)	R_ct (Ω cm²)	R_s (Ω)	Q_dl (µF/cm²)	R_f	C_f (µF/cm²)	Inhibition efficiency (% P)
0.00	145	0.56	64	4.38	100	0.0
10	253	0.60	60	2.67	97	43
20	290	0.68	58	4.76	86	50
30	350	0.26	56	3.15	81	59
40	402	0.42	53	5.31	80	64
70	460	0.44	52	7.69	76	68

The increase in the values of R_ct and the decrease in the values of Q_dl with increasing concentration of surfactant could be attributed to the adsorption of unimers of gelatin surfactant at copper/solution interface. The values on number n, n_f are smaller than the unity indicating that the copper/H₂SO₄ corrosion system is a heterogeneous system.

The decrease in the values of double-layer capacitance on increasing the concentration of surfactant indicated that the surfactant molecules that were adsorbed on copper electrode, forming double layer with a thickness, increased progressively with increasing the concentration of surfactant.

The inhibition efficiency (%P) was calculated form impedance and polarization data from the equation (Pavan et al., 1999)

% P = \frac{\frac{1}{R_{ct}^{o}} - \frac{1}{R_{ct}}}{\frac{1}{R_{ct}^{o}}} = \frac{i_{o} - i}{i_{o}}

(9)

where

R_{ct}^{o}, i_{o}

are the charge transfer resistance and the current density of the uninhibited acid solution, while

R_{ct}, i

are the charge transfer resistance and the current density in inhibited acid solution.Polarization curves for dissolution of brass alloy in the absence and presence of different concentrations of gelatin surfactant at 35°C are shown in Figure 6.

Figure 6.

Polarization curves of brass alloy in 0.1 M H₂SO₄ at 35°C: (1) 0.1 M H₂SO₄, (2)10, (3) 20, (4) 30, (5) 40 and (6) 70 ppm gelatin surfactant.

In cathodic polarization curves, the current reaches a peak value and then stabilizes at a plateau value that is relatively constant (limiting current i_L) and this shows that the predominant cathodic reaction in the corrosion of copper in 0.1 M H₂SO₄ is the reduction of oxygen gas at the cathodic sites (Shaban et al., 2015) of copper electrode. The polarization curves were analyzed, and the values of polarization parameters, the corrosion potential: E_corr. (anodic and cathodic Tafel slopes: β_a, β_c; corrosion current density (i_corr), are collected in Table 4.

Table 4.

Polarization parameters for brass alloy in 0.1 M H2SO4 at 35°C.

[Surfactant] (ppm)	βa	βc	i_corr (mA cm^–2)	E_corr (mV vs. SCE)	%P
[Surfactant] (ppm)	(mV decade^–1)		i_corr (mA cm^–2)	E_corr (mV vs. SCE)	%P
0.00	47	218	0.018	−21	0.0
10	43	193	0.010	12	45
20	39	195	0.009	17	50
30	42	170	0.007	21	61
40	39	175	0.006	24	67
70	47	200	0.005	24	72

The decrease in the values of limiting current (I_L) values and the percentage of inhibition or protection efficiency (%P) of gelatin surfactant indicate the inhibition of the corrosion rate of brass alloy in 0.1 M H₂SO₄ at 35°C.

Figure 7 shows the adsorption isotherm of gelatin surfactant at copper/solution interface. There is a good agreement with the values of protection efficiency (%P) at low concentration of surfactant. However, the deviation in the values of %P obtained from the two techniques at high surfactant concentration is attributed to the nature of the experimental setup of the two techniques.

Figure 7.

Adsorption isotherm of gelatin surfactant on the surface of brass alloy in 0.1 M H₂SO₄ at 35°C.

The adsorption of gelatin surfactant follows S-shaped adsorption isotherm with the largest value of %P at CMC of gelatin surfactant that correspond to the formation of monomolecular form of adsorbed molecules of gelatin at copper/solution interface.

The dissolution retardation of brass alloy corrosion in 0.1 M H₂SO₄ solutions by gelatin extract can be explained on the basis of molecular adsorption of gelatin surfactant at copper surface. Molecular adsorption of gelatin on copper surface may be considered due to interaction between unshared electron pairs in surfactant molecule and copper surface.

The adsorption on steel surface occurs through ester group (O–C=O), π-electrons of aromatic ring, and lone pair of electrons of oxygen atoms. The high performance of gelatin surfactant was attributed to the presence of many adsorption centers, large molecular sizes, and planarity of surfactant molecule (Churaev, 2018; Jurašin et al., 2010). This colloidal surfactant has a high molecular weight and has the ability to form a monomolecular film on the copper surface and thus retards the acid attack partly by hindering the aggressive attack of sulphate ion. The unimers of gelatin surfactant adsorbed on the copper surface, These adsorbed layers characterized by high electrical resistance formed on copper surface which may be responsible for the reduction in the rates of diffusion of oxygen molecules necessary for the continuation of the dissolution process and subsequently the rate of dissolution process representing in the limiting current, I_L decreased.

Conclusion

The present results not have revealed that the eco-friendly and biodegradable gelatin surfactant inhibited the rate of copper corrosion in 0.1 M H₂SO₄ as an acid pickling solution for storage tanks, pumps, and heat exchangers from brass alloys. The high inhibition efficiency of gelatin surfactant for corrosion of copper in H₂SO₄ also suggests the use of gelatin as inhibitor in acid pickling of copper and copper alloy. The CMC is the key factor in determining the effectiveness of gelatin surfactant as an corrosion inhibitors for copper and copper alloy in the acid media. The inhibition efficiency (%P) of gelatin surfactant is a function of the concentration. The surfactant acts by adsorption on surface of copper substrate in the acid pickling bath.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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