1. INTRODUCTION:

Carbon steel is an important construction material extensively used in all kinds of industries and corrosion of carbon steel known to occur especially in neutral environment such as cleaning and to control the carbon steel corrosion by both corrosion scientist and material technologist[1-6]. Corrosion is the destruction of metals and alloys by chemical and electrochemical reactions with its environment. It is a natural phenomenon which cannot be avoided, but it can be controlled using appropriate techniques like metallic coating, anodic protection, cathodic protection and using inhibitors, etc. Inhibitors impart very good role in the process of corrosion inhibition [7,8].

Carbon steel is used under different conditions in chemical and allied industries for handling alkaline acid and salt solutions. Chloride, sulphate and nitrate ions in aqueous media are particularly aggressive and accelerate corrosion. Corrosion products are formed when a metal give its electrons to the oxidising substances. This can be delayed by painting the metal, or other way of protecting these metal from corrosion is to use corrosion inhibitors[9,10]. The RO product water in pipelines, when comes into contact with pumps, valves or other metallic components, has the capability to corrode these components. A survey of the literature indicates that although numerous references are available on corrosion resistant materials but only limited references are available on carbon steel related to RO product water[11-13].

The present work were taken to (i) study the IE of TSC and Zn²⁺ on carbon steel used as RO water storage system (ii) confirm the formation of inhibitive film on carbon steel using surface analysis techniques like potentiodynamic polarization study, SEM, EDX and FT-IR.

2. EXPERIMENTAL DETAILS:

2.1. Preparation of the Specimen:

Carbon steel specimens (0.026% S, 0.6% P, 0.4% Mn, 0.1% C, and rest iron) of the dimensions1.0cm × 4.0cm × 0.2cm were polished to a mirror finish and degreased with trichloroethylene and used for the weight loss method and surface examination techniques.

2.2. Weight - loss method

Carbon steel specimens were immersed in 100ml of the RO water, containing various concentration of the inhibitor in the absence and presence of Zn²⁺for 7 days. The weights of the specimens before and after immersion were determined using a digital balance model AUY 220 SHIMADZU. The corrosion products were cleaned with Clarke’s solution. The IE was then calculated using the equation.

IE = 100[1-(W₂/W₁] %

Where W₁ is the corrosion rate in the absence of inhibitor and W₂ is the corrosion rate in the presence of inhibitor. Corrosion rate was calculated using the formula

Corrosion rate[CR] = 87.6 W/ DAT mmy^-1 [14]

W= Weight loss in milligrams

D= density of specimen, g/cm³

A= area of specimen, cm²

T= exposure in hours=168hrs

2.3. Potentiodynamic Polarization Technique:

Polarization studies were carried out in a CHI-electrochemical work station with impedance model 660A. It was provided with IR facility. A three electrode cell assemble was used and the working electrode was carbon steel. A SCE was the reference electrode. Platinum was the counter electrode. From polarization study, corrosion parameters such as corrosion potential (E_corr), corrosion current (I_corr) (Tafel slopes anodic = β_a and cathodic=β_c) were calculated and a linear polarization study was done. The scan rate (V/S) was 0.01. Hold time at E_fcs was zero and quit time (s) was two.

2.4. Surface Examination Techniques:

The carbon steel specimens were immersed in blank as well as inhibitor solutions, for a period of 7 days. After the immersion period is over, the specimens were taken out and dried. The nature of the thin film formed on the surface of the metal specimen analyzed by various surface analysis techniques. The morphology of carbon steel specimen surface (corroded and inhibited) was studied by recording SEM images of the corresponding samples using OXFORD instruments analytical scanning electron microscope.

2.5. Energy Dispersive Analysis of x – ray (EDX):

EDX (Model: OXFORD Instrument) system attached with Scanning Electron Microscope was used for elemental analysis or chemical characterization of the film formed on the carbon steel surface. As a type of spectroscopy, it relies on the investigation of sample through interaction between electromagnetic radiation and the matter, so that a detector was used to convert X-ray energy into voltage signals. This information is sent to pulse processor, which measures the signals and passed them to analyzer for data display on the analysis.

2.6. Fourier Transform Infrared Spectra:

These spectra were recorded in a JASCO 460 plus FT-IR spectrometer using KBr pellet. The FT-IR spectrum of the protective film was corded by carefully scratching the film, mixing it with KBr, and making the pellet.

2.7. The physico- chemical parameters of RO water used to prepare solutions

S. NO	Parameters	Level of content
1.	Appearance	Colorless and clear
2.	Odour	None
3.	TDS	200ppm
4.	pH	7.5
5.	Total hardness as CaCO₃	200ppm
6.	Chloride as Cl	200ppm
7.	Fluoride as F	Nil
8.	Sulphate as SO₄	Nil
9.	Phosphate as PO₄	Nil

3. RESULTS AND DISCUSSION:

3.1. Weight-loss Method:

The corrosion conjunction behavior of carbon steel in RO water in the absence and presence of different concentrations of TSC alone and in conjunction with Zn²⁺was studied using a weight – loss method and data obtained after seven days of immersion is shown in Table 1 to 3. Table 2 shows IE increases with increasing TSC concentrations showing an optimum IE of 20.1% at 150ppm. The increased IE% with increasing inhibitor concentrations revealed more TSC molecules adsorbed onto carbon steel surface, leading to greater surface coverage and hence the formation of a protective film[15]. A relatively low IE% at lower concentrations of TSC could be attributed to the modest surface coverage owing to its small molecular area and desorption of protective film from the metal surface into the bulk of the solution. A further increase in TSC concentration causes a slight lowering in IE%. This phenomenon is attributed to the dissolution of adsorbed inhibitive film [16]. To obser ve the effect of Zn²⁺ on the corrosion inhibition behavior of TSC, the corrosion of carbon steel in RO water in the absence and presence of different concentrations of TSC, in conjunction with various concentrations of Zn²⁺ was separately studied. The results are shown in Table 1. The corrosion rates of carbon steel in the presence of TSC in combination with 10ppm Zn²⁺ are further studied and the results are shown in table 3. When 150ppm TSC used alone, the IE is 20.1 % and when 10ppm Zn²⁺ used alone, the IE is 8.6% and when both these two used, the IE is 72.5%. It reveals the existing of synergism (S_I) [17-19].

Table 1. Corrosion Rates (CR) of carbon steel in RO water in the absence and presence of Zn²⁺ ions

Conc. of Zn²⁺ ppm	Corrosion Rate (mmy^-1)	Inhibition efficiency %
Blank	171.5	-
10	156.7	8.6
25	155.0	9.6
50	140.6	18.0
75	131.9	23.1
100	125.7	26.6
125	122.4	28.6
150	114.0	33.5

Table 2. Corrosion Rates (CR) of carbon steel in RO water in the

absence and presence of TSC

Conc. of TSC ppm	Corrosion Rate (mmy^-1)	Inhibition efficiency %
Blank	171.5	-
10	148.2	13.6
50	148.0	13.7
75	145.4	15.2
100	138.7	19.1
125	137.5	19.8
150	137.0	20.1
175	137.3	19.9

Table 3. Corrosion Rates (CR) of carbon steel in RO water in the absence and presence of TSC and 10ppm Zn²⁺ ions

Conc. of Zn²⁺ ppm	Conc. of TSC ppm	Corrosion Rate (mmy^-1)	Inhibition efficiency %
Blank	-	171.5	-
10	10	74.8	56.4
10	50	73.5	57.1
10	75	70.3	59.0
10	100	53.8	68.6
10	125	47.7	72.3
10	150	47.1	72.5
10	175	48.8	71.5

3.2. Analyses of Polarization Curves

Polarization study has been used to confirm the type of inhibition during corrosion inhibition processes [20-23]. The potentiodynamic polarization curves of carbon steel immersed in RO water in the absence and presence of inhibitors are shown in Fig1. The corrosion parameters are given in Table 4. When carbon steel immersed in RO water, the corrosion potential is -606.623 mV Vs SCE. The corrosion current is 40.225µA. When the addition of (150ppm TSC and 10ppm Zn²⁺) to blank the corrosion potential shift to anodic side (84 mV). This result suggests that this formulation act as a mixed type inhibitor. In the presence of this inhibitors system, the corrosion current decreases from 40.225µA to 2.698µA .This result are consistent with the results of the weight-loss measurements. According to Ferreira and W.H. Li, if the displacement in corrosion potential is more than 85mv with respect to the corrosion potential of the blank, the inhibitor can be seen as cathodic or anodic type [24-25]. Study reveals the inhibitor formulation is anodic.

Figure 1. Tafel plots for carbon steel immersed in the absence and presence of inhibitor formulation obtained from polarization measurements

3.3. Analysis of SEM:

SEM images provide a support of the adsorbed film onto the carbon steel surface. To know the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of carbon steel, the SEM images of the surface are examined[26-27]. The SEM images of carbon steel surface immersed in RO water in the presence and absence of inhibitors are shown in Figures 2a & 2b. Figure 2a shows the highly damaged surface, fault of the metallic properties and roughness of the metal surface which indicates the corrosion of carbon steel in RO water. Figure 2b shows the smoothness of the metal surface which reveals the confirmation of protective film.

3.4. Analyses of energy dispersive X-Ray (EDX)

The protective film formed on carbon steel surface was analyzed using EDX is shown in Figure 3. Figure 3a shows highly weakened peaks and corrosion products present onto the metal surface. Figure 3b shows a formation of strong protective film of the inhibitor molecules on the surface of the carbon steel [28].

3.5. Analyses of FT-IR spectra:

FTIR spectra have been used to analyze the protective film formed on metal surface [29-31]. The FT-IR spectrum of pure TSC is shown in Figure 4a. The C = O stretching frequency of the carboxyl group appears at 1597cm^-1. The OH stretching frequency appears at 3454 cm^-1. The FT-IR spectrum of film (KBr) formed on the surface of carbon steel after immersion in the test solution containing 150 ppm TSC and 10 ppm Zn²⁺ is shown in the Figure 4b. The C = O stretching frequency has shifted from 1597 cm^-1 to 1626 cm^-1. The OH stretching frequency shifted from 3454 cm^-1 to 3427 cm^-1. This indicates that oxygen atom of carboxyl group and OH have coordinated with Fe²⁺ resulting in the formation of Fe²⁺ - TSC complex formed on the anodic sites of the metal surface. The peak at 1392 cm^-1 due to the formation of Zn(OH)₂ on the metal surface [32].

3.6 Mechanism of Corrosion Inhibition:

Analyses of the results of weight- loss method reveals the inhibitor formulation consisting of 150ppm of TSC and 10ppm of Zn²⁺ offers an IE of 72.5%. Results of polarization study suggest that the formulation functions as an anodic inhibitor.

From the above observations, the following mechanism of corrosion inhibition is proposed. When carbon steel specimen is immersed in RO water, the anodic reaction is

Fe → Fe²⁺ + 2e^-

And the cathodic reaction is

2H₂O + O₂ + 4e ^- → 4OH ^–

When the inhibitor system consisting of 150 TSC and 10ppm of Zn²⁺ is prepared, there is a formation of Zn²⁺ - TSC complex in solution.

Zn²⁺ + TSC → Zn²⁺ - TSC

When the carbon steel is immersed in the solution, the Zn²⁺ - TSC complex diffuses from the bulk of the solution to the metal surface.

On the surface of the metal, Zn²⁺ - TSC complex is converted into Fe²⁺ - TSC complex at the local anodic regions.

Zn²⁺ - TSC + Fe²⁺ → Fe²⁺ - TSC + Zn²⁺

The released Zn²⁺ ions combine with OH^- ions to form Zn(OH)₂ on the cathodic sites.

Zn²⁺ + 2OH^- → Zn (OH)₂ ↓

Thus the protective film consists of Fe²⁺ - TSC complex, Zn(OH)₂and oxides of Fe.

4. CONCLUSION:

The present study leads to the following conclusions

1. The Zn²⁺ - TSC system shows a synergistic effect in controlling the corrosion of carbon steel immersed in RO water.

2. The formulation consisting of 150ppm of TSC and 10ppm of Zn²⁺has 72.5% IE.

3. Polarization study reveals that the formulations Zn²⁺- TSC functions as an anodic inhibitor.

4. SEM and EDX study corroborated that the inhibitor molecules form a good protective film onto the metal surface.

5. FT-IR spectra reveal that the protective film consist of Fe²⁺ - TSC complex, Zn(OH)₂, and oxides of Fe.

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Received on 31.08.2015 Modified on 17.09.2015

Asian J. Research Chem. 8(10): October 2015; Page 613-617

DOI: 10.5958/0974-4150.2015.00097.8

Concentration ppm		E_corr mV VS SCE	βa mV/dec	βc mV/dec	I_corr µA	IE_p%
Zn²⁺	TSC	E_corr mV VS SCE	βa mV/dec	βc mV/dec	I_corr µA	IE_p%
Blank	-	-606.623	391.2	269.0	40.225	-
10	150	-522.836	240.3	425.3	11.698	70.91