^a Corrosion Research Center, PG and Research Department of Chemistry, GTN Arts College, Dindigul 624 005, Tamilnadu, India

^b Department of Chemistry, KLN College of Information Technology, Pottapalayam 630 611, India

^c Department of Chemistry, Yadava Collge, Madurai, India

^d Department of Chemistry, SVN College, Madurai, India

Abstract

The inhibition efficiency of sodium gluconate (SG)-Zn²⁺ system in controlling corrosion of carbon steel in sea water has been evaluated by weight-loss method. The formulation consisting of 250 ppm of SG and 75 ppm of Zn²⁺ has 98% IE. Influence of duration of immersion on the IE of SG-Zn²⁺ has been evaluated. The mechanistic aspects of corrosion inhibition have been investigated by polarization study and AC impedance spectra. The protective film has been analysed by FTIR and luminescence spectra. The surface morphology and the roughness of the metal surface have been analysed by atomic force microscopy. The protective film consists of Fe²⁺-SG complex and Zn(OH)₂. It is found to be UV-fluorescent.

Keywords: Carbon steel, sodium gluconate, synergistic effect, Atomic Force Microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR), corrosion.

Introduction

Several carboxylic acids have been used as corrosion inhibitors to prevent corrosion of metals [1-35]. Their inhibitive action is due to coordination of the oxygen atom of the carboxylate anion to the metal ions to form metal-inhibitor complexes. They have been used to prevent the corrosion of metals such as mild steel [1, 10, 11, 14, 32, 33], aluminium [7, 9] and copper [19, 23].

The environment includes various media like acidic [6, 12, 23, 33], alkaline [6] and neutral [8, 9, 10, 12]. Thermodynamic studies [19], quantum chemical studies [35], and electrochemical studies [4] have been employed to explain the inhibitive actions of carboxylic acids. Phosphono carboxylic acids [29, 31, 32], tartrates [8, 10], amino acids [19] and citrates [1] have been used as corrosion inhibitors.

The present work is undertaken (i) to evaluate the inhibition efficiencies of sodium gluconate (SG)-Zn²⁺ system in controlling corrosion of carbon steel in sea water by weight-loss method, (ii) to investigate the mechanistic aspects of corrosion inhibition by electrochemical studies like polarization study and AC impedance spectra, (iii) to analyse the protective film by FTIR and luminescence spectra, (iv) to analyse the surface morphology by AFM, and (v) to propose a suitable mechanism of corrosion inhibition based on the results from the above study.

Experimental

Preparation of specimen

Carbon steel specimens [0.0267 % S, 0.06 % P, 0.4 % Mn, 0.1 % C and the rest iron] of dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to a mirror finish and degreased with trichloroethylene.

Weight-loss method

Carbon steel specimens in triplicate were immersed in 100 mL of solutions containing various concentrations of the inhibitor in the presence and absence of Zn²⁺ for one day. The weight of the specimens before and after immersion was determined using a Shimadzu balance, model AY62. The corrosion products were cleansed with Clarke's solution [36]. The inhibition efficiency (IE) was then calculated using the equation.

where W₁ = corrosion rate in the absence of the inhibitor, and W₂ = corrosion rate in the presence of the inhibitor.

Biocidal study

Inhibitor - Zn²⁺ formulations that offered the best corrosion inhibition efficiency were selected for biocidal study. The biocidal efficiency of N-cetyl -N, N, N - trimethylammonium bromide (CTAB) in the presence and absence of these formulations and also the effect of CTAB on the corrosion inhibition efficiency of these systems were determined [37-39].

Various concentrations of CTAB 50 ppm, 100 ppm, 150 ppm, 200 ppm, and 250 ppm were added to the formulation consisting of the inhibitor system. The numbers of colony-forming units (CFU/mL) present in the above formulation were determined by step dilution technique. Synergistic formulations were taken into Petri dishes and mixed with 15 mL of sterile nutrient agar. While mixing, the plate was tilted back and forth, so that the medium and sample travels around the plate several times. Then, the Petri dishes were kept in an incubator at 30 °C in an inverted position for 5 days. Distinct colonies of bacteria were developed. Such colonies may be easily isolated. Each organism grew and reproduced itself. To determine the number of colonies, the plate count technique was applied. One organism gives rise to one colony. Hence, a colony count performed on the plate reveals the viable microbial population. Within the range of 30-300, the count can be accurate. Colonies are usually counted from dark field illumination, so that they are easily visible and a large magnifying lens is often used. Number of colonies counted on plate x dilution of sample = number of bacteria per mL.

Surface examination study

The carbon steel specimens were immersed in various test solutions for a period of one day. After one day, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimens was analysed for surface analysis technique by FTIR spectra and fluorescence spectra.

FTIR spectra

The film formed on the metal surface was carefully removed and mixed thoroughly with KBr. The FTIR spectra were recorded in a Perkin Elmer 1600 spectrophotometer.

Fluorescence spectra

These spectra were recorded in a Hitachi F-4500 fluorescence spectrophotometer.

Atomic Force Microscopy

Atomic Force Microscopy (AFM) is an exciting new technique that allows surface to be imaged at higher resolutions and accuracies than ever before [4042]. The microscope used for the present study was PicoSPM I Molecular Imaging, USA. Polished specimens prior to the initiation of all corrosion experiments were examined through an optical microscope to find out any surface defects such as pits or noticeable irregularities like cracks, etc. Only those specimens which had a smooth pit-free surface were subjected for AFM examination. The protective films formed on the carbon steel specimens after immersion in the inhibitor systems for different time durations were examined for a scanned area of 30 × 30 μm² and 15 × 15 μm². The two-dimensional and three-dimensional topographies of surface films gave various roughness parameters of the film.

Polarization study

Polarization studies were carried out in an H & CH electrochemical workstation impedance analyser, model CHI 660A.

A three-electrode cell assembly was used. The working electrode was carbon steel. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode.

AC impedance measurements

The instrument used for polarization study was used for AC impedance measurements too. The cell set up was the same as that used for polarization measurements. The real part (Z') and the imaginary part (Z'') of the cell impedance were measured in Ohms at various frequencies. The values of charge transfer resistance, R_t, and the double layer capacitance, C_dl, were calculated.

where R_s = solution resistance, and C_dl = 1 / 2 Π RT f_max being f_max = maximum frequency.

Results and discussion

Weight-loss method

It is observed that SG alone has good inhibition efficiency. As the concentration of SG increases, the IE also increases.

In the presence of various concentrations of Zn²⁺ (25, 50 and 75 ppm), the IE of SG increases. A synergistic effect exists between SG and Zn²⁺. For example, 250 ppm of SG have only 61% IE; 75 ppm of Zn²⁺ have 39% IE. However, their combination has 98% IE. This suggests a synergistic effect existing between SG and Zn²⁺.

Influence of duration of immersion on the inhibition efficiency of the SG-Zn²⁺ system

It is observed that the IE almost remains constant up to 5 days. Afterwards, the IE decreases. On the seventh day, the IE decreases from 98% to 92%. This is due to the fact that as the immersion period increases, the protective film formed on the metal surface, namely, Fe2⁺-SG complex, is broken by the aggressive chloride ions present in sea water and hence the IE decreases. Further, a competition arises between the formation of Fe2⁺-SG complex and FeCl2/FeCl3. As the immersion period increases, the formation of FeCl2/FeCl3 is favoured to Fe2⁺-SG complex at the anodic sites of the metal and hence the IE decreases [43].

Synergism parameters

Synergism parameter (SI) has been calculated to know the synergistic effect existing between two inhibitors [44-47]. Synergism parameter is calculated using the relation

where θ₁₊₂ = (θ₁ + θ₂) - (θ₁ * θ₂), θ is the surface coverage of inhibitor (=IE%/100), and θ₁ and θ₂ are the surface coverage of inhibitor 1 and 2, respectively:

= combined surface coverage in presence of inhibitors 1 and 2.

This confirms the synergistic effect existing between SG and Zn²⁺. As a model, the Zn²⁺ (75 ppm)-SG (50, 100, 150, 200 and 250 ppm) system has been selected to evaluate the synergism parameters.

Influence of N-cetyl-N, N, N-trimethylammoniam bromide (CTAB) on the corrosion inhibition efficiency of the SG-Zn²⁺ system

This formulation may find application in cooling water systems, if further research is carried out at high temperature under flow condition.

Potentiodynamic polarization study

Polarization study has been used to confirm the formation of a protective film on the metal surface [48-52]. If a protective film is formed on the metal surface, the linear polarization resistance value (LPR) increases and the corrosion current value (I_corr) decreases.

When the inhibitors, namely SG (250 ppm) + Zn²⁺ (75 ppm), are added to sea water, the corrosion potential of carbon steel shifts to the noble side (-676 to -668 mV vs. SCE). This indicates that a film is formed on the anodic sites of the metal surface. This film controls the anodic reaction of metal dissolution (Fe → Fe + 2 e^-) by forming Fe-SG complex on the anodic sites of the metal surface. Formation of Fe³⁺-SG complex to some extent cannot be ruled out.

The formation of a protective film on the metal surface is further supported by the fact that the LPR value increases from 152.8 to 179.5 Ohm cm² and the corrosion current decreases from 2.559 × 10^-4 to 2.090 × 10^-4 A/cm². Thus, polarization study confirms the formation of a protective film on the metal surface.

AC impedance spectra

When the inhibitors [SG (250 ppm) + Zn²⁺(75 ppm)] are added to sea water, the R_t value increases from 6.88 to 8.52 Ohm cm²; the C_dl value decreases from 2.849 × 10^-7 to 2.301 × 10^-7 F/cm²; the impedance value increases from 0.9291 to 1.010.

These results lead to the conclusion that a protective film is formed on the metal surface.

FTIR spectra

The C=O stretching frequency of the carboxyl group appears at 1631 cm^-1.

A peak appears at 690 nm.

Root mean square roughness, average roughness, and peak to valley value

and the cathodic reaction is

-When the system containing 250 ppm of SG and 75 ppm of Zn²⁺ is prepared, there is formation of Zn²⁺-SG complex

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

-On the surface of metal, Zn²⁺-SG complex is converted into Fe²⁺-citrate complex at the local anodic regions. The stability of Fe²⁺-SG complex is higher than the corresponding zinc complex

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

Thus the protective film consists of Fe²⁺-SG complex and Zn(OH)₂.

Conclusions

The present study leads to the following conclusions:

-A synergistic effect exists between SG and Zn²⁺ in controlling corrosion of carbon steel immersed in sea water.

-The formulation consisting of 250 ppm of SG and 75 ppm offers 98% IE.

-Polarization study suggests that the combination of SG and Zn²⁺ functions as an anodic inhibitor.

-AC impedance spectra reveal that a protective film is formed on the metal surface.

-FTIR spectra show that the protective film consists of Fe²⁺-SG complex and Zn(OH)₂.

-The IE of inhibitor formulation depends on the ability of the inhibitor to form complex with Zn²⁺ and ability of Fe²⁺ to react with Zn²⁺ to form iron complex.

-This formulation may find applications in cooling water systems.

-AFM images and luminescent spectra confirm the formation of a protective layer on the metal surface.

-The synergistic formulation with 250 ppm of CTAB has 91% corrosion inhibition efficiency and 100% biocidal efficiency.

References