INTRODUCTION:

Carbon steel is an important material which finds wide applications in manufacturing industry due to its excellent mechanical properties and low cost, e.g. chemical processing and oil production [1, 2]. The corrosion inhibitors reveals that most organic substances employed as corrosion inhibitors can absorb on the metal surface through hetero atoms such as nitrogen, oxygen, sulfur and phosphorus, multiple bonds or aromatic rings and block the active sites decreasing the corrosion rate [3-6].

The higher molecular size of organic inhibitors and high electron density on the adsorption centers may be responsible for high corrosion efficiency. Unfortunately, most of the organic inhibitors used are expensive and environmental hazard. Novel anticorrosion ability of different types of water soluble inhibitors in the industry is good options to solve the problems of corrosion on metallic surfaces to prevent the material and economic loss [7]. Owing to the multiple adsorption sites, polymeric compounds adsorb more strongly on the metal surface compared with their monomer analogues [8]. Therefore, it is expected that the polymers will be better corrosion inhibitors. The use of polymers as corrosion inhibitors have drawn considerable attention recently due to their inherent stability and cost effectiveness. The Literature reveals that a wide range of polymeric compounds have been successfully investigated as potential inhibitors for the corrosion of metals in aqueous media. Polymers such as Poly (1, 8-diaminocarbazole) [9], Sodium Carboxymethyl Cellulose [10], poly anthranilic acid [11], poly(N-methylaniline) [12], polyepoxysuccinate [13], Polyethylene Glycol Methyl Ether [14], poly(o-toluidine) [15]. The aim of the present work is to determine the inhibitive effect of Polyacrylic acid (PAA), TSC and Zn²⁺ ions on the corrosion of mild steel in 60 ppm chloride medium by chemical and electrochemical methods. Surface characterization techniques (FT-IR, SEM, EDS and AFM) are also used to ascertain the nature of the protective film.

EXPERIMENTAL

MATERIALS:

Polyacrylic acid (PAA), Zinc sulphate (ZnSO₄.7H₂O), Trisodium citrate (TSC) and other reagents were analytical grade chemicals. The molecular structures of Polyacrylic acid and Trisodium citrate are shown in Fig. 1 and 2 respectively. All the solutions were prepared by using double distilled water. pH values of the solutions were adjusted by using 0.01 N sodium hydroxide and 0.01 N sulphuric acid solutions. An aqueous solution consisting of 60 ppm of sodium chloride has been used as the control throughout the studies.

Fig.1. The molecular Structure of Polyacrylic acid (PAA)

Fig. 2. The molecular structure of Trisodium citrate (TSC)

Preparation of specimens:

For all the studies, the specimens taken from a single sheet of mild steel of the following composition were chosen: C, 0.1-0.2%; P, 0.03-0.08%; Mn, 0.4-0.5% and the rest iron. For gravimetric measurements and surface analytical techniques, the polished specimens of the dimensions, 4 cm x 1.0 cm x 0.1 cm, were used while for other (electro chemical) studies, the dimensions of the specimens were 1.0 cm x 1.0 cm x 0.1 cm Prior to all measurements, the specimens were polished successively using 1/0 to 6/0 emery papers, decreased with trichloroethylene and washed thoroughly with double distilled water and dried.

Gravimetric studies:

Gravimetric experiments are the easiest way to find the corrosion rate (CR) and inhibition efficiency (IE). In all gravimetric experiments, the polished specimens were weighed and immersed in duplicate, in 100 ml control solution in the absence and presence of inhibitor formulations of different concentrations, for a period of seven days. Then, the specimens were reweighed after washing and drying. The weights of the specimens before and after immersion were determined by the Mettler electronic balance, AE 240 model with a readability of 0.1 mg. Accuracy in weighing up to 0.0001g and its surface area measurement up to 0.1 cm². Corrosion rates of mild steel in the absence and presence of various inhibitor formulations are expressed in mdd. The corrosion rate was calculated according to the following equation.

Loss in weight (mg)

Corrosion rate = ------------------------------------------------(mdd) (1)

Surface area of the specimen (dm²) x Period of immersion (days)

Where,

m - Loss in weight (mg)

d - Surface area of the specimen (dm²)

d - Period of immersion (days)

Inhibition efficiencies (IE) of the inhibitor were calculated by using the formula,

I.E= 100 [1- W₂/W₁] % (2)

Where,

W₁= Corrosion rate in the absence of inhibitor

W₂= Corrosion rate in the presence of inhibitor.

Electrochemical Studies:

Electrochemical impedance spectroscopic (EIS) studies and potentiostatic polarization studies were carried out using an electrochemical workstation CHI model 660A (USA) electrochemical analyzer. The mild steel specimens used as working electrode while platinum and calomel electrodes were used as counter electrode and the reference electrode, respectively. Impedance measurements were carried out at E_corr potential at the range of 100 kHz to 10 mHz at amplitude of 10 mV. The impedance diagrams are given in Nyquist representation. The impedance and polarization parameters such as double layer capacitance (C_dl), charge transfer resistance (R_ct), corrosion current (I_corr), corrosion potential (E_corr), anodic Tafel slope (β_a) and cathodic Tafel slope (β_c) were computed from the polarization curves and Nyquist plots. The IEp values were calculated from potentiodynamic polarization measurements using the equation (3).

(3)

Where, I_corr and I′_corr are the corrosion current densities in case of the absence and presence of the inhibitor respectively. From impedance measurements, the IE_I values were calculated from the following relation,

(4)

Where, Rct_(b) and Rct_(i₎ are the charge transfer resistance values in the absence and presence of the inhibitor respectively.

Surface examination studies:

The mild steel specimens were immersed in various test solutions for a period of seven days. Then, they were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed by Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and Atomic force microscopy (AFM).

Fourier transform infrared spectroscopy:

The mild steel specimens were immersed in various test solutions for a period of seven days. On completion of the seventh day, the specimens were taken out and dried. The protective film formed on the metal specimens was scratched and mixed with KBr and pellets were obtained and the FT-IR spectra were recorded using Spectrum RXI Spectrophotometer over a range of 4000 - 400 cm^-1 with a resolution of 4 cm^-1.

Scanning electron microscopy and EDS:

The surface morphology of the formed layers on the mild steel surface after its immersion in control solutions containing 60 ppm chloride ions in the absence and in the presence of the inhibitor were carried out. After seven days, the specimens were taken out, washed with distilled water and dried. The SEM- EDS photographs of the surfaces of the specimens were investigated using a VEGA3-TESCAN model scanning electron microscope.

Atomic force microscopy:

The atomic force microscope was used for surface morphology studies. The protective films were examined with atomic force microscope (AFM) using A100 model (A.P.E Research, Italy). The topography of the entire samples from a scanned area of 20 μm x 20 μm is evaluated for a set point of 20 nm and a scan speed of 10 mm/s. The three dimensional topography of surface films gave various roughness parameters of the film.

RESULTS AND DISCUSSION

Gravimetric studies:

The gravimetric measurements were carried out to calculate the corrosion rate (CR) and inhibition efficiency (IE) for the mild steel in an aqueous solution containing 60 ppm chloride ions in the absence and presence of various inhibitor formulations consists in various amounts of PAA, Zn²⁺ ions and TSC are given in Table 1. The inhibition efficiency was represented as a function of TSC concentration in Fig. 3.

Table 1: Inhibition efficiency (IE) and Corrosion rate (CR) by gravimetric studies of mild steel in the presence of inhibitor

Zn²⁺ (ppm)	PAA (ppm)	TSC (ppm)	Weight loss (g)	I.E (%)	Corrosion rate (mdd)	Surface Coverage (θ)
60 ppm Cl^-	--	--	0.0136	--	17.66	--
50	200	100	0.0006	95	0.7792	0.9485
50	200	200	0.0012	91	1.5584	0.9117
50	200	300	0.0016	88	2.0779	0.8823
50	200	400	0.0023	83	2.9870	0.8308
50	200	500	0.0029	79	3.7662	0.7867
50	200	600	0.0036	74	4.6753	0.7352

The results of the ternary formulations, that for lower concentration of 50 ppm Zn²⁺ and 200 ppm PAA with 100 ppm TSC, the maximum inhibition efficiency of only 95% is achieved. A further addition to increasing the TSC concentration decreasing order of the inhibition efficiency. For the ternary formulation present investigation, inhibition efficiency decreases from a concentration of 100 ppm due to the weakening inhibitor- metal interaction facilitates the replacement of the adsorbate by aggressive ions and results in a corrosion rate increase. Hence an auto-antagonistic effect of the surfactant with respect to its inhibitive properties.

Fig. 3. Inhibition efficiency as a function of concentration of TSC

Effect of pH:

The influence of pH on corrosion rate of mild steel in the presence of inhibitor system and the maximum inhibition efficiency obtained in the gravimetric measurements were studied. The effect of pH for the formulation consisting of PAA (200 ppm), Zn²⁺ (50 ppm) and TSC (100 ppm) in the pH range of 5-11 is shown Fig. 4. The highest inhibition efficiency could be obtained in the pH range 5-7. But, when the pH is decreased from below 5, 9 and on increasing pH range from 9, the inhibition efficiency is reduced to 30 %. When the pH was increased from 11 inhibition efficiency also increased. The reasons for this decrease in inhibition efficiency in more alkaline and acidic environments are explained under the mechanistic aspects.

Fig. 4. Effect of pH

Electrochemical measurements:

Potentiodynamic polarization studies:

The potentiodynamic polarization studies were carried out to determine the kinetics of the cathodic and anodic reactions. Fig.5. Shows the potentiodynamic polarization curves for mild steel electrodes in control solution at pH 7 in the absence and presence of various inhibitor combinations. Electrochemical kinetic parameters, i.e., the corrosion potential (E_corr), corrosion current density (I_corr), and anodic and cathodic tafel slopes (β_a and β_c), obtained from extrapolation of the polarization curves are listed in Table 2. When mild steel is immersed in 60 ppm Cl^- medium, the corrosion potential (Ecorr) is -598 mV/dec and the corrosion current is 129 µA/cm². When 100 ppm TSC to 200 ppm of PAA and 50 ppm Zn²⁺ are added to 60 ppm Cl^- medium the corrosion potential is found to be -671 mV/dec and corrosion current is 6.11 µA/cm². The corrosion current decreases from 129 µA/cm²to 6.11 µA/cm².

Fig. 5. Potentiodynamic polarization curves of MS in a). 60 ppm Cl^-b). 60 ppm Cl^- + 50 ppm Zn²⁺ + 200 ppm PAA + 100 ppm TSC

This shows that the formulation functions as a cathodic inhibitor controlling both anodic and cathodic processes but more predominantly cathodic process. This suggests, indicate that protective film is formed on the metal surface.

Electrochemical impedance studies:

Nyquist plots for mild steel immersed in 60 ppm of Cl^- solution at pH 7 in the absence and presence of various formulations are shown in Fig.6. The impedance parameters, charge transfer resistance (R_ct), Double layer capacitance (C_dl) from the Nyquist plots and the calculated inhibition efficiency (IE_i) values are shown in Table 3. When mild steel is immersed in 60 ppm Cl^- medium the R_ct value is found to be 112 (Ω cm²). The C_dl value is 25.40 (µF/cm²). When 100 ppm TSC to of 200 ppm PAA and 50 ppm Zn²⁺ are added to 60 ppm Cl^- medium the R_ct value has increased from 112 to 855 (Ω cm²) and the C_dl value has decreased from 25.40 to 0.70 (µF/cm²). The increase in Rct values and decrease in double layer capacitance values obtained from impedance studies justify the good performance of a compound as an inhibitor in 60 ppm Cl^- medium. This behavior means that the film obtained acts as a barrier to the corrosion process that clearly proves the formation of the film.

Fig.6. Electrochemical impedance curves of MS in a). 60 ppm Cl^-b). 60 ppm Cl^- + 50 ppm Zn²⁺ + 200 ppm PAA + 100 ppm TSC

Table 3: Corrosion parameters of mild steel immersed in the absence and presence of inhibitor obtained from AC impedance studies

	Concentration (ppm)			R_ct(Ωcm²)	C_dl (µF/cm²)	q	I.E (%)
Zn²⁺		PAA	TSC	R_ct(Ωcm²)	C_dl (µF/cm²)	q	I.E (%)
	0	0	0	112	25.40	--	--
	50	200	100	855	0.70	0.8690	87

surface studies:

Fourier transform infrared spectroscopy:

The FT-IR spectrum of pure PAA is shown in Fig.7.(a). The C=O stretching vibration at 1719 cm^-1 and the broad absorption peak at 3448 cm^-1 shows OH stretching vibration. The FT-IR spectrum of pure TSC is shown in Fig.7.(b). The C=O stretching frequency appears at 1595 cm^-1 and OH stretching frequency appears at 3454 cm^-1 respectively. The FT-IR spectrum of the film formed on the surface of the mild steel after immersion in the solution consisting of 60 ppm of Cl^-, 50 ppm of Zn²⁺, 200 ppm PAA and 100 ppm of TSC is shown in Fig.7.(c). It is observed that the OH stretching frequency of PAA decreases from 3448 cm^-1 to 3411 cm^-1. This shift is caused by the electron cloud density from the O atom to Fe²⁺. This suggests that the O atom of the polyacrylic acid is coordinate to Fe²⁺ resulting in the formation of Fe²⁺ - PAA, complex on the metal surface.

Fig.7. FT-IR Spectra of (a) pure PAA (b) pure TSC (c) Surface film

The C=O stretching frequency of TSC has decreased from 1595 cm^-1 to 1611 cm^-1. This suggests that TSC has coordinated with Fe²⁺ through an oxygen atom, resulting in the formation of Fe²⁺ - TSC complex at the anodic sites on the metal surface. The peak at 1407 cm^-1 is due to Zn(OH)₂ on the cathodic sites of the metal surface. Thus FT-IR spectral study leads to the conclusion that the protective film consists of Fe²⁺ - PAA complex, Fe²⁺- TSC complex and Zn(OH)₂.

Scanning electron microscopy (SEM):

SEM analysis provides a pictorial representation of the surface. To understand the nature of the surface film in the absence and presence of inhibitors and the extend of corrosion of mild steel. Fig.8.(a) show the polished metal surface of mild steel before corrosion Fig.8.(b) show the metal surface immersion of 60 ppm Cl^- solutions. This shows the roughness of the metal surface by the corrosive environment and there is formation of different forms of corrosion products.

Fig.8.(c) show the metal surface immersion of 600 ppm SPT to of 200 ppm PAA and 50 ppm Ni²⁺ are added to 60 ppm Cl^- medium gives metal surface incorporating into the passive film in order to block the active site present on the mild steel surface. Comparative examination of these images, clearly suggest that the surface of mild steel is smoothened to a very large extent in the presence of the combined inhibitor system. This smoothening might be due to the adsorption of the inhibitor molecules on it and thus the surface is fully covered.

Energy Dispersive X-Ray Spectroscopy (EDS):

The EDS spectra was used to determine the elements present on the surface of mild steel absence and presence of inhibitors in the uninhibited and inhibited 60 ppm Cl^‑. Fig.9.(a) gives characteristics peaks of the elements (Fe, Carbon and Mn) constituting the polished mild steel specimen. Fig. 9.(b) portrays the EDS analysis of mild steel in 60 ppm Cl^- which indicates only the presence of Fe, Na, Cl and O. This confirms that the passive film contained only Fe₂O₃. The EDS of presence of inhibitor Fig. 9(c) in shows the presence of additional lines due to Fe, C, Na, Cl, Zn and O. This may beat tribute to the presence of inhibitor on mild steel which protects the steel surface from corrosion.

Fig.9. EDS images of mild steel in a). polished mild steel b). 60ppm Cl^- solution c). Inhibitor solution

Atomic force micrographs:

AFM is a dynamic tool to examine the surface morphology from nano to micro scale and has become a new choice to study the nature of protective layer formed over the surface of mild steel. The 2D & 3D AFM images of polished mild steel, 60 ppm Cl^-in mild steel and mild steel in 60 ppm Cl^-solution containing 50 ppm of Zn²⁺, 200 ppm of PAA and 100 ppm of TSC, respectively are shown in Fig.10(a-c). As can be seen from the AFM images, the surface is very clear for polished mild steel (Fig.10.a). Whereas in mild steel immersed in 60 ppm Cl^- (Fig.10.b) the surface is severely damaged by the solution. In (Fig.10.c) the surface is protected from attack by the protective layer formed by the inhibitor molecules. From the results, it is clear that, the inhibition of mild steel corrosion in inhibitor is mainly due to the formation of a protective layer by adsorption of inhibitor molecules over the surface of mild steel. AFM data for mild steel is given in Table 4.

Fig. 10. 2D &3D AFM images of mild steel surface (a). Polished mild steel (b). 60 (c). 60ppm Cl^-+ 50 ppm Zn²⁺ + 200 ppm PAA + 100 ppm TSC

Table 4: AFM parameters in different environments

Environment	Average Roughness (nm)	Root-mean- square Roughness (nm)	Maximum peak-to valley height (nm)
a). Polished mild steel	26	36	215
b). 60 ppm Cl^-	218	279	1901
c). 60 ppm Cl^-+ 50 ppm Zn²⁺ + 200 ppm PAA + 100 ppm TSC	44	69	625

Mechanism of Protection:

In order to explain all the experimental results, the following mechanism of corrosion inhibition can be proposed. Mild steel undergoes initial corrosion to form Fe²⁺ ions at anodic sites:

(5)

Fe²⁺ further undergoes oxidation in the presence of oxygen available in the aqueous solution:

(6)

And the cathodic reaction is:

(7)

When the environment containing 60 ppm Cl^- ions/50ppm Zn²⁺/200 ppm PAA/100 ppm TSC was prepared, a [Zn²⁺-PAA-TSC] complex was formed in the solution. Besides this complex, the presence of free PAA, TSC and Zn²⁺ ions. While the metal was immersed in this environment, the [Zn²⁺-PAA-TSC] complex diffused from the bulk of the solution onto the surface of the metal and further complexes with Fe²⁺/Fe³⁺ ions available due to initial corrosion. Free PAA and TSC molecules diffuse from the bulk of the solution to the metal surface and form [Fe²⁺/Fe³⁺-PAA-TSC] complexes. These complexes fill the pores of the film formed on the surface and make it productive.

(8)

Fig. 12 Pictorial representation of mechanism of adsorption of inhibitors on MS surface

Free Zn²⁺ ions diffuse from the bulk of the solution to the metal surface and form Zn (OH) ₂ at the local cathodic sites.

(9)

Thus, PAA, Zn²⁺, and TSC play a very important role in the controlling corrosion through the formation of a protective film on the metal surface.

CONCLUSION:

All the results showed that the PAA has excellent inhibition properties for the corrosion of mild steel in aqueous medium. The gravimetric measurements showed that the formulation containing 50 ppm Zn²⁺ ions and 200 ppm PAA and 100 ppm TSC yield good inhibition efficiency of 95%. The inhibitor system is effective in the pH range of 5-7 and 11. The inhibitor formulation acts as a cathodic in nature. Nyquist plots established that the inhibitors reduced the mild steel corrosion through their effective adsorption of inhibitive layer, which is further evidenced from SEM, EDS, AFM and FT-IR. The protective film may consist of [Fe (III)/Fe (II)/Zn (II)-PAA-TSC] complex, Zn(OH)₂, and hydroxides and oxides of iron.

ACKNOWLEDGEMENT:

The authors are thankful to Indian Institute of Science, Bangalore for providing the support for recording AFM studies. We are also thankful to Department of Chemistry of Thiagarajar Engineering College, Madurai for providing help in carrying out the Electrochemical Studies.

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Received on 13.01.2016 Modified on 27.01.2016

Asian J. Research Chem. 9(1): Jan., 2016; Page 40-46

DOI: 10.5958/0974-4150.2016.00008.0

Concentration (ppm)			E_corr (mV/dec)	I_corr (µA/cm²)	β_a(m0V/dec)	β_c(mV/dec)	q	I.E (%)
Zn²⁺	PAA	TSC	E_corr (mV/dec)	I_corr (µA/cm²)	β_a(m0V/dec)	β_c(mV/dec)	q	I.E (%)
0	0	0	- 598	129.0	192	189	--	--
50	200	100	- 671	06.11	176	185	0.9526	95