1. INTRODUCTION

The damage caused by corrosion within drinking water distribution systems is one of the largest problems for the water utility industry. The effects on the safety and health throughout a community, as well as the associated costs resulting from corrosion, are indeed a major concern for numerous water quality managers. All waters are corrosive to some degree. This corrosivity of the water affects the infrastructure of a distribution system by attacking the piping, concrete and pumps which are often very difficult to budget for a municipality. Factors that affect the corrosive nature of water are: pH, oxygen content, total hardness, total dissolved solids, temperature and alkalinity. The selection of the proper corrosion inhibitor is critical to resolve the corrosion tendency of a water system.

Corrosion of iron pipes in a distribution system can cause three distinct but related problems. First, pipe mass is lost through oxidization to soluble iron species or iron-bearing scale. Second, the scale can accumulate as large tubercles that increase head loss and decrease water capacity. Finally, the release of soluble or particulate iron corrosion-byproducts to the water decreases its aesthetic quality and often leads to consumer complaints of “red water” at the tap. The water industry must be concerned with all three of these aspects of corrosion.

Steel is a very important alloy that finds wide applications in industry, metallurgy and construction fields. Mild steel is a very important electrode material for a lot of applications. However, high corrosion rates in most cases, especially at low pH values limit its applications. The use of water as thermal fluid in cooling water system usually leads to three problems namely: corrosion, scale and biological fouling processes. These phenomena are caused of the concentration of salts and suspended matters. [1-6].

If these problems cannot be solved timely, it will cause the production equipments of long period, full load and influence the safety and stability of equipments [4-7]. In order to limit the damage, many formulations have been developed to protect circuits, piping and materials structures against this scourge .Currently, agent used for simulated cooling water treatment are mainly phosphorus-containing formulas and they are easily to produce eutrophication and red tide phenomenon, so promoting a green chemistry and developing a phosphate-free water treatment agent have became the urgent matter. [8-12] In recent years ,phosphate-free corrosion and scale inhibitors are mainly molybdate salts, chromic acid salts, natural polymer and synthetic polymer , when they are used alone, the dosage is large and the cost is high , so they are not widely used.

The study of corrosion behaviour of steel in different environments is very important to achieve the most suitable passivators and inhibitors to increase the life period of steel equipments. While organic compounds have been successfully applied in the inhibition of general corrosion of materials [13–28], Gluconate and the gluconic acid are known to be effective non-toxic inhibitors for iron and mild steel in cooling water systems [29-31].

The present work was designed to study the corrosion inhibition of mild steel used in distribution system in potable water by sodium gluconate and zinc ions as corrosion inhibitors using three different techniques: electrochemical studies, scanning electron microscopy and energy dispersive x-ray analysis, hoping to get some general ideas to guide the composing of inhibitor in reality.

2. EXPERIMENTAL:

2.1 Materials

The composition of mild steel used for corrosion inhibition studies was (Wt %): 0.026% S, 0.06% P, 0.4% Mn, 0.1% C and balance being Fe. The specimens of size 1.0cm×4.0cm×0.2cm were press cut from the mild steel sheet, were machined and abraded with a series of emery papers. This was followed by rinsing in acetone and bidistilled water and finally dried in air. Before any experiment, the substrates were treated as described and freshly used with no further storage. The inhibitors SG, molecular mass 218.14g mol^-1, Zn²⁺ ions were used as received.

A stock solution of 1000ppm of SG was prepared in bidistilled water and the desired concentration was obtained by appropriate dilution. All solutions were using potable water (Perambalur, Tamil Nadu, India). The study was carried out at room temperature. The physic-chemical parameters shown in Table 1 and molecular structure of SG is given in Fig 1.

Table 1 Physico-chemical parameter of potable water

Parameters	Values
pH	7.84
TDS	251ppm
Chloride	30ppm
Alkalinity	113ppm
Total Hardness	102ppm
Conductivity	358µmhos/cm

Figure 1: Molecular structure of SG

2.2 Electrochemical studies

Both the potentiodynamic polarization studies and electrochemical impedance spectroscopic (EIS) studies were carried out using the electrochemical workstation model CHI- 760d and the experimental data were analysed by using the electrochemical software (Version: 12.22.0.0). The measurements were conducted in a conventional three electrode cylindrical glass cell with platinum electrode as auxiliary electrode and saturated calomel electrode as reference electrode.

The working electrode was mild steel embedded in epoxy resin of polytetrafluoroethylene so that the flat surface of 1cm² was the only surface exposed to the electrolyte. The three electrodes set up was immersed in control solution of volume 100ml both in the absence and presence of the inhibitors formulations and allowed to attain a stable open circuit potential (OCP). The pH values of the solution were adjusted to 7.0 and the solutions were unstirred during the experiments.

Polarization curves were recorded in the potential range of -750 to -150 mV with a resolution of 2mV. The curves were recorded in the dynamic scan mode with a scan rate of 2mVS^-1 in the current range of -20mA to +20mA. The Ohmic drop compensation has been made during the studies. The corrosion potential (E_corr), corrosion current (I_corr), anodic Tafel slope (β_a) and cathodic Tafel slope (β_c) were obtained by extrapolation of anodic and cathodic regions of the Tafel plots. The inhibition efficiency (IE_p) values were calculated from the I_corr values using the equation.

Where I_corr and I^’_corr are the corrosion current densities in case of control and inhibited solutions respectively.

Electrochemical impedance spectra in the form of Nyquist plots were recorded at OCP in the frequency range from 60 KHz to 10MHz with 4 to 10 steps per decade. A sine wave, with 10mV amplitude, was used to perturb the system. The impedance parameters viz., charge transfer resistance (R_ct), double layer capacitance (C_dl) were obtained from the Nyquist plots. The inhibition efficiencies (IE_i) were calculated using the equation,

Where R_ct and R^’_ct are the charge transfer resistance values in the absence and presence of the inhibitor respectively.

2.3 Scanning Electron Microscopy

The surface morphology of the corroded steel sample surface in the presence and absence of the inhibitors was studied using SEM (Model: TESCAN vega3 USA). To study the surface morphology of mild steel, polished specimens prior to initiation of any corrosion reaction, were examined in optical microscope to find out any surface defect, such as prior noticeable irregularities like cracks etc.

Only those specimens, who had a smooth pit-free surface, were subjected to immersion. The specimens were immersed for 24h at 30⁰C. After completion of the tests specimens were thoroughly washed with bidistilled water and dried and then subjected to SEM examination.

2.4 Energy Dispersive Analysis of X-ray (EDAX)

EDAX (Model: BRUKER Nano Germany) system attached with Scanning Electron Microscope was used for elemental analysis or chemical characterization of the film formed on the mild 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 a pulse processor, which measures the signals and passed them into an analyzer for data display on the analysis.

3. RESULT AND DISCUSSION:

3.1. Electrochemical Impedance Spectroscopy

The Nyquist plots of mild steel immersed in potable water in the absence and presence of inhibitors are shown in Fig 2.

Fig 2. Nyquist plots for mild steel in the absence and presence of inhibitor formulation obtained by electrochemical impedance

As the Nyquist plots were not semicircles, these plots were not used for calculating the impedance parameters were obtained by using the semicircle fitting method [32]. This can be achieved by selecting the best fitting circuit (Figure 3) for the semicircle on the Nyquist plot.

Fig 3. Nyquist curves equivalent electrical circuit

Table 2. Nyquist plots for mild steel immersed in the absence and presence of various test solution obtained by AC impedance analysis

Concentration (ppm)		Charge transfer Resistance R_ct(Ω)	Double layer capacitance C_dl CPE µF/cm²	Inhibition efficiency (%)
SG	Zn²⁺	Charge transfer Resistance R_ct(Ω)	Double layer capacitance C_dl CPE µF/cm²	Inhibition efficiency (%)
Blank	-	350.09	2.543	-
100	10	1650.59	0.117	78.75

As seen from the impedance results the increase in charge transfer resistance in the presence of inhibitors compared to blank solution is related to the corrosion control effect of the inhibitors. The value of C_dl decreases in the presence of inhibitors, indicating that the surface oxide layer thickness decrease and changes the influence of the oxide layer on the kinetics of the electrode process.

The EIS parameters were calculated and presented in Table 2 mild steel in the presence of inhibitor formulation exhibited higher charge transfer resistance and impedance but lower double layer capacitance than for the blank.

3.2 Potentiodynamic Polarization study

Tafel curves of mild steel in potable water in the absence and presence of inhibitor formulations (10ppm Zn²⁺ + 100ppm SG) were carried out. The optimum concentrations of inhibitors were evaluated based on inhibition efficiency.

Potentiostatic polarization parameters of mild steel immersed in potable water is given in Table 3. Corresponding polarization curves are also shown in Fig 4. The corrosion current density was reduced considerably in the presence of inhibitor formulation. In the presence of inhibitor formulation the corrosion potential slightly shifted to the anodic region compared to the result obtained in the absence of inhibitor formulation (blank).

Fig 4. Tafel curves for mild steel in the absence and presence of inhibitor formulation obtained by polarization study

The results suggesting for this formulation could acts as a mixed type inhibitor. For this formulation control both anodic and cathodic metal dissolution, corrosion rate.

3.3. Scanning electron microscope (SEM)

Scanning electron microscopy (SEM) was employed in order to get additional information on the inhibition mechanisms. The SEM micrographs of mild steel specimens exposed to potable water over a period of 7 days are given in Fig. 5a. It reveals an uneven surface with highly damaged with corrosion products on metal surface. Fig. 5b shows the SEM micrographs of mild steel in potable water containing (10ppm Zn²⁺ + 100ppm SG). Absence of any corrosion pit and other form of corrosion products in the micrograph suggest that the specimen is covered with an inhibitor layer.

3.4. Energy dispersive X-ray analysis

Energy dispersive X-ray analysis (EDAX) technique was employed in order to get additional information on the inhibition mechanisms. The results obtained from these techniques showed that the corrosion inhibition process was related to the development of inhibitor film over the mild steel surface.

The cross section analyses of the corrosion layers were performed by EDXA. It is important to take into consideration the percentage of the elements present on the surface of the mild steel. The EDAX spectrum of polished mild steel sample shows good surface properties (Fig 6a). In Fig (6b), the presence of the peaks of O and Fe indicates the formation of iron oxides and other corrosion products formed onto the metal surface. In Fig. (6c), the peak appeared at C, O, Na and Zn are present in the inhibitor formulation due to confirm the protective film on the metal surface and decreased metal corrosion rate.

This observation clearly proves that the inhibition is due to the formation of an insoluble stable film through the process of complexation of the organic molecules on the metal surface. Studies also reveal the formation of a thin and very adherent organic film on the metal surface, which is actually responsible for the inhibition of corrosion of the mild steel in potable water distribution system.

CONCLUSION:

The present study leads to the following conclusion in corrosion control of mild steel as distribution system in potable water

Polarization data showed that the investigated inhibition formulation acts as a mixed type inhibitor.

AC impedance spectra revealed that a protective film is formed onto mild steel surface.

The inhibition efficiencies calculated from ac measurement show good agreement observed from dc polarisation results.

SEM and EDAX clearly proved that the inhibition is due to the formation of an insoluble stable protective film through the process of complexation of the organic molecules on the mild steel surface.

This new inhibitor formulation free from hazardous.

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Received on 03.12.2014 Modified on 11.12.2014

Asian J. Research Chem 8(1): January 2015; Page 16-20

DOI: 10.5958/0974-4150.2015.00004.8

Concentration (ppm)		-E_corr mV vs SCE	β_c mV/dec	β_amV/dec	I_corr ×10^-5 A/cm²	Inhibition efficiency (%)
SG	Zn²⁺	-E_corr mV vs SCE	β_c mV/dec	β_amV/dec	I_corr ×10^-5 A/cm²	Inhibition efficiency (%)
Blank	-	602	218.4	186.0	1.831	-
100	10	556	179.7	215.3	0.387	78.86