¹ PG and Research Department of Chemistry, Arulmigu Palaniandavar College of Arts & Culture, Palani-624601, Tamilnadu, India

² Arulmigu Palaniandavar College for Women, Palani-624601, Tamilnadu, India.

³ Sri Subramaniya College of Engineering & Technology, Palani-624601, Tamilnadu, India.

⁴ Christian College of Engineering, Oddanchathiram, Tamilnadu, India.

Abstract

The inhibition efficiency (IE) of metronidazole (MZ)-Zn(II) system in controlling corrosion of mild steel in aqueous solution containing 60 ppm of Cl^- ion has been evaluated by weight loss method. Weight loss study reveals that the formulation consisting of 140 ppm of MZ and 50 ppm of Zn(II) has 84% inhibition efficiency in controlling corrosion of mild steel immersed in aqueous solution containing 60 ppm of Cl^- ion. Polarization study reveals that this system as a mixed type of inhibitor controlling the cathodic and anodic reaction to an equal extent. AC impedance reveals that a protective film is formed on the metal surface. The FTIR spectra revealed that the protective film consists of Fe(II)-MZ complex.

Keywords: metronidazole drug, mild steel, corrosion inhibition, weight loss, polarization and AC impedance.

Introduction

Corrosion inhibition of mild steel is a matter of theoretical as well as practical importance [1]. It has been widely used in industries such as pickling, cleaning, descaling, etc., and because of their aggressiveness, inhibitors are used to reduce the dissolution of metals. Many organic hetero cyclic compounds containing N, S, O, & P have been reported as inhibitors [2-6].

A large number of organic compounds have been used as corrosion inhibitors for mild steel and most of them are highly toxic to both human beings and environment. Due to the increasing environmental awareness and the negative effects of some chemicals, research activities in recent times are geared towards developing the less toxic and environmentally safe corrosion inhibitors [7-12].

It has been investigated [13] that some drugs like tryptamine, succinic acid, L- ascorbic acid, sulfamethaxazole, cefratrexyl, cefradroxil and alpendazole were effective and eco-friendly inhibitors for acid environments. The inhibitive effect in acidic media through complex formation on metal surfaces by anti-bacterial drugs, namely ampicillin, cloxacillin, flucloxacillin and amoxicillin on controlling corrosion of Al was investigated [14].

Metronidazole has been widely used in the treatment of trichomoniasis, amoebiasis, giardiasis, vincent infections and eradications of cysts. Many literature reviews revealed the corrosion inhibitive effect on mild steel in acidic media. Even though many drugs have been used as corrosion inhibitors, the mechanistic aspects of corrosion inhibition in aqueous media have not been studied in detail. The present work is undertaken a) to evaluate the inhibition efficiency (IE) of metronidazole in controlling the corrosion of mild steel in aqueous solution containing 60 ppm of Cl^- ion in the absence and presence of Zn²⁺ ion, b) to examine the influence of pH and duration of immersion on the IE of MZ drug, c) to analyze the protective film formed on the mild steel by FTIR spectra, d) to understand the mechanistic aspects of corrosion inhibition by potentiodynamic polarization and AC impedance analysis.

Experimental methods

Inhibitor

Stock solution of metronidazole in ppm level was made in DD water and used as corrosion inhibitor in the present study.

Preparation of the specimens

Mild steel specimens (0.026% S, 0.06% P, 0.4% Mn, 0.1% C and rest iron) of the dimensions 1.0×4.0×0.2 cm were polished to a mirror finish, degreased with trichloroethylene, and used for the weight-loss method and surface examination studies.

Weight-loss method

Mild steel specimens were immersed in 100 mL of a solution containing 60 ppm of Cl^- ion and various concentrations of the inhibitor in the presence and absence of Zn²⁺ ion for a period of one day. The weights of the specimens before and after immersion were determined using a balance, Schimadzu AY62 model. The inhibition efficiency (IE) was then calculated using the formula,

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

Potentiodynamic polarization study

This study was carried out using a CHI 660A electro chemical impedance analyzer model. A three-electrode cell assembly was used. The working electrode used was mild steel with 1 cm 2 exposed area. A saturated calomel electrode (SCE) was used as reference electrode. A rectangular platinum foil was used as the counter electrode. Polarization curves were recorded after doing iR compensation. The parameters such as Tafel slopes, ICorr and Ecorr were calculated.

AC impedance measurements

A CHI 660A electro chemical impedance analyzer was used to record AC impedance measurements. The cell set up was the same as that used for polarization measurements. The real part (Z') and imaginary part (Z'') of the cell impedance were measured in Ohms for various frequencies. The Rt (charge transfer resistance) and C_dl (double layer capacitance) were calculated.

Surface examination study

The mild 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 were analysed by surface analysis techniques such as FTIR spectra. The FTIR spectra were recorded with a Perkin-Elmer 1600 Spectrophotometer. The FTIR spectrum of the protective film was recorded by carefully removing the film mixing it with KBr and making the pellet.

Results and discussion

Analysis of the results of weight-loss study

It is observed that as the duration of immersion increases, the inhibition efficiency decreases. This is due to the fact that as the time of immersion increases, the protective film formed on the metal surface is not able to withstand the attack of Cl^-. The inhibitive film is broken and hence the IE decreases [16,17].

Influence of pH on IE of metrinidazole -Zn²⁺ system

When pH is lowered to 5 by addition of dilute sulphuric acid, the IE decreased to 75%. This is due to the fact that when the acid is added the protective film is broken by the aggressive H+ ions present in the acid. When the pH is increased to 9.2 by addition of diluted sodium hydroxide solution, the IE increased to 80% when compared to an acidic medium. However, this 80% IE in basic medium (pH 9.2) is lower than the IE of 84% in neutral medium. This is due to the fact that when NaOH is added, a portion of Zn²⁺ is precipitated as Zn(OH)₂ in the bulk of the solution and hence not available for the complex formation.

Analysis of potentiodynamic polarization study

When mild steel is immersed in aqueous solution containing 60 ppm of Cl ion, the corrosion potential is -515 mV vs. SCE. The formulation consisting of 140 ppm of MZ and 50 ppm of Zn²⁺ shifts the corrosion potential to -605mV vs. SCE suggesting that the reaction is predominantly catholically controlled. The Rp value increases from 1511 ohm cm² to 7073 ohm cm². This suggests that a protective film is formed on the metal surface. Further, the corrosion current decreases from 3.60 × 10^-5 A/cm² to 9.9 × 10^-6 A/cm². The IE calculated from corrosion current data is 73%. This value is lower than the IE obtained by weight loss method (84%). The discrepancy may be explained by the fact that in electro chemical process, the instantaneous corrosion current is measured. However, in the case of weight loss method, IE is calculated after a long time. The protective film formed is strengthened as the duration of immersion increases. In the presence of inhibitors, the cathodic and anodic Tafel slopes are more or less equal. This indicates that the formulation consisting of MZ-Zn²⁺ system as a mixed type of inhibitor controlled by both the cathodic and anodic reaction to an equal extent. In several investigations, acidic media of some drugs such as Albendazole and Ketorol [27, 28] has been reported as a mixed type but predominantly cathodic inhibitor.

Analysis of AC impedance study

The appearance of a single semi-circle shows the single charge transfer process during dissolution which is unaffected by the presence of the inhibitor molecule. Deviation from perfect circular shape is often referred to the frequency dispersion of interfacial impedance.

In this study, when mild steel is immersed in aqueous solution containing 60 ppm of Cl^- ion, the Rt value is 678 ohm cm² and the C_dl value is 9.53 × 10^-5 F/cm². When MZ drug and Zn²⁺ are added, the Rt value increases from 678 ohm cm² to 1192 ohm cm² and C_dl decreases from 9.53 × 10^-5 F/cm² to 1.71 × 10^-5 F/cm². This suggests that a protective film is formed on the surface of the metal. This accounts for the good IE of MZ-Zn²⁺ system.

Analysis of FTIR spectra

The active principle in metronidazole (MZ) is given as follows.

The saturated carbon of C-H stretching frequency appears at 2845.13 cm^-1. The alkene mono substituted C-H stretching frequency appears at 3098.78 cm^-1. The band due to intra molecular hydrogen bonding of O-H stretching appears at 3218.37 cm^-1. The peak at 1533.47 cm^-1 is due to the C-NO2. The C = N stretching frequency appears at 1654.25 cm^-1. Thus the structure of metronidazole is confirmed by FTIR spectrum.