Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad-09, Gujarat, India

Abstract

The inhibition performance of two Schiff bases, namely (A) Benzenamine, 2-chloro-N[( 4-methoxyphenyl)methylene] and (B) Benzenamine, 3-chloro-N-[(4methoxyphenyl) methy-lene] as corrosion inhibitors for Al-Pure in 1.0 M HCl has been investigated by galvanostatic polarization measurement, electrochemical impedance spectroscopy (EIS) and weight loss method. Galvanostatic polarization study showed that two inhibitors are mixed type with predominant action on the cathode. The adsorption of inhibitors on Al-Pure surface was found to follow Langmuir adsorption isotherm and the thermodynamic parameters (E_a, ΔG_ads and Q_ads) were determined. Two Schiff bases have shown remarkable inhibition on the corrosion of Al-Pure in 1.0 M HCl solution. The high inhibition efficiency was attributed to the blocking of active sites by adsorption of inhibitor molecules on the metal surface.

Keywords: Al-Pure, Schiff bases, Inhibition efficiency, EIS.

Introduction

Corrosion is defined as the deterioration of the material, usually a metal, because of reaction with its environment and which requires the presence of an anode, a cathode, an electrolyte and an electrical circuit [1]. Acid solutions are generally used for the pickling, industrial acid cleaning, acid descaling, oil well acidizing, etc. [2-5]. Al-Pure and its alloys are important materials due to their high technological value and wide range of industrial applications, especially in aerospace, household industries, and commonly used in marine applications as well. In addition, they are justified by low price, high electrical capacity and high energy density [6].

Because of widely used in many industrial processes, aluminium and its alloys could be corroded during these acidic applications particularly with the use of hydrochloric acid and sulphuric acid. Corrosion prevention of metals has always been an important subject to be dealt with. The prevention of corrosion is vital not only for increasing the life-time of equipments, but also for decreasing the dissolution of toxic metals from the components into the environment. The use of organic molecules as corrosion inhibitors is one of the most practical methods for protecting metals against the corrosion and it is becoming increasingly popular. Inhibitors are used to prevent metal dissolution and minimize acid consumption. Most of the efficient acid inhibitors are organic compounds that contain mainly nitrogen, sulphur or oxygen atoms in their structure. Despite the large number of organic compounds, there is always a need for developing new organic corrosion inhibitors [7-9].

Recently, Schiff base compounds have been of interest in order to obtain efficient corrosion inhibitors since they provide much greater inhibition by adsorption on metal surface compared with corresponding amines and aldehydes [10-17]. Some Schiff bases have recently reported as effective corrosion inhibitors for steel [18, 19], aluminium [20], aluminium alloys [21, 22] and copper [23] in acidic media. The presence of -C=N- group in Schiff base molecules enhances their adsorption ability and corrosion inhibition efficiency [24, 25]. The action of such inhibitors depends on the specific interaction between the functional groups and the metal surface. So it is very important to clarify the interactions between the inhibitor molecules and metal surfaces in order to search new and efficient corrosion inhibitors [26].

In earlier work the inhibition of corrosion of zinc in sulphuric acid and aluminium in hydrochloric acid by Schiff bases of ethylenediamine [27], orthosubstituted aniline-N-salicylidenes [28], ortho-, meta-, and para-aminophenol-Nsalicylidenes [29], meta-substituted aniline-N-salicylidenes [30], salicylidineN,N'-dimorpholine [31] and aniline-N-benzylidene [32] has been reported. In the present work, the inhibitive effectiveness of (A) and (B) Schiff bases have been studied in retarding corrosion of Al-Pure in 1.0 M HCl. Galvanostatic polarization, electrochemical impedance spectroscopy (EIS) and weight loss method were used to investigate the inhibiting influence of (A) and (B). Both inhibitors are substitutants of chloroaniline-N-(p-methoxy benzylidene). So comparatively both inhibitors give very similar results with negligible differences.

Experimental

Synthesis of Schiff bases

Electrodes and electrolytes

99.9% pure aluminium was used in this study. Rectangular specimens of Al-Pure of size 6 cm × 3 cm and thickness 0.050 cm with a small hole of about 2 mm diameter just near the upper end of the specimen were used for the determination of weight losses. The specimens were polished with '0', '00', '000' and '0000' grade emery paper. The specimens degreased by A. R. carbon tetrachloride (sulphur free).

For polarization and impedance measurements, metal coupons of circular design, diameter 2.802 cm with a handle 3 cm long and 0.5 cm wide and thickness 0.050 cm with a small hole of about 2 mm diameter just near the upper end of the specimen were used. The handle and the back of the coupon and of the auxiliary platinum electrode were coated with Perspex leaving only the circular portion of the specimen of apparent surface area 6.156 cm² exposed to the solution.

The corrosive solution (1.0 M HCl) was prepared by dilution of analytical grade 37% HCl (NICE) with double distilled water. The concentration range of employed inhibitors was 0.001 to 0.5% in 1.0 M HCl. The concentration range of employed inhibitor was 0.001 to 0.5% in 1.0 M HCl.

Measurements

Three methods, namely polarization study, impedance spectroscopy and weight loss method, were used to determine the corrosion inhibition efficiencies of Schiff bases.

Electrochemical measurements

Electrochemical experiments were carried out using a standard electrochemical three-electrode cell. Al-Pure was used as working electrode, platinum as counter electrode and saturated calomel electrode (SCE) as reference electrode. The test solution was contained in a H-type (80 mL in each limb) Pyrex glass cell with Luggin capillary as near to the electrode surface as possible, and a porous partition to separate the two compartments. The potential was measured against a saturated calomel electrode (SCE), in polarization study. Corrosion parameters such as corrosion potential (E_corr), corrosion current density (I_corr) and Tafel plots were measured in polarization method. In this study, the current density was varied in the range of 2 × 10^-4 to 3.25 × 10^-2 A cm^-2. The inhibition efficiencies at different inhibitor concentrations are calculated using the following equation:

where, I⁰_corr and I_corr are the corrosion current densities in the absence and presence of the inhibitors, respectively.

Electrochemical impedance measurements were carried out in the frequency range of 20 kHz - 0.1 Hz at the open circuit potential with applying amplitude of 5 mV ac signal (AUTOLAB). Double layer capacitance (C_dl) and charge transfer resistance (R_ct) values were calculated from Nyquist plots as described by Hosseini [34]. EIS data were analyzed using frequency response analyzer (FRA) electrochemical setup. The inhibition efficiencies at different inhibitor concentrations were calculated using the following equation:

where, R_ct and R⁰_ct are the charge transfer resistance of the electrode with and without the inhibitors, respectively.

Weight loss (chemical) method

In weight loss method, the specimens were exposed to 1.0 M HCl solutions containing controlled additions of two Schiff bases in the range 0.001% to 0.5% inhibitor concentration. One specimen only was suspended by a glass hook in each beaker containing 230 mL of the test solution which was open to the air at 35 ± 0.5 °C, to the same depth of about 1.5 cm below the surface of the test solution. The experiments were repeated at different temperatures, ranging from 35 ± 0.5 °C to 65 ± 0.5 °C in the absence and presence of 0.5% inhibitor's concentration after 60 minutes.

The percentage inhibition efficiency (%IE) and surface coverage (Θ) were calculated using the following equations:

where, Wu = weight loss of Al-Pure in uninhibited acid and Wi = weight loss of Al-Pure in inhibited acid, respectively.

Results and discussion

Polarization study

The inhibition efficiencies obtained by extrapolation of the cathodic Tafel line to the corrosion potential agree well with those calculated from weight loss data. This suggests that in presence of the inhibitors, both the cathode and the anode are polarized, but the cathode is polarized to a greater extent at higher inhibitor concentration. The inhibitors thus appear to be mixed type inhibitors with predominant action on the cathode.

Electrochemical impedance spectroscopy

From the figures, it is clear that the impedance diagrams obtain yield a semi-circle shape. This indicates that the corrosion process is mainly controlled by charge transfer.

The results show that there are increasing R_ct values with the addition of the inhibitors when compared with those obtained without inhibitors. It should be noted that while R_ct values increase with the addition of inhibitors, the C_dl values decrease indicating the formation of a surface film. Thus, increase in R_ct value and decrease in C_dl values by the inhibitors are related to the increased degree of protection of Al-Pure in 1.0 M HCl. The electrochemical theory shows that C_dl is proportional to the corrosion rate.

Weight loss measurements

Effect of inhibitor concentration:

Increasing the concentration of (A) and (B) inhibitors, increases the inhibition efficiency (%IE) upto a maximum value of > 99% at 0.5% inhibitor concentration. The inhibition was found to depend on inhibitor's concentration. The inhibitory action of (A) and (B) against Al-Pure corrosion can be attributed to the adsorption of their molecules on Al-Pure surface, which limits the dissolution of the latter by blocking its corrosion sites and hence decreasing the weight loss, with increase in efficiency as the concentration increases.

Adsorption isotherm

Effect of temperature

The decrease in inhibition efficiencies with increase in temperature can be explained by desorption of the inhibitor molecules due to a higher rate of hydrogen evolution at elevated temperatures which may disturb or detach the adsorbed film from the metal surface.

Thermodynamic parameters

The activation parameters for the corrosion process can be evaluated from Arrhenius-type plot according to the following equation [38]:

The values of the free energy of adsorption (ΔG_ads) were calculated from the following equation [37]:

were

Further, these inhibitors, which are in a distinct ionic form, also may get attached to the metal surface of opposite polarity through electrostatic attraction. The adsorbed layer then will block the dissolution of the metal. The size, orientation and shape of the molecule, and the electronic charge on it, will determine the degree of adsorption and hence the effectiveness of the inhibitor.

It is also possible that the compounds may form onium ions in acidic medium and move to the cathodic regions and then the adsorption will take place through the iminic nitrogen and also through the -OCH3 group (+R, -I) which supplies electron density to the aromatic benzaldehydic ring and thus the delocalized π-electrons of this ring help in the adsorption of the compound with stronger bonds.

It appears that the inhibitor molecules lie flat on the metal surface and cover greater areas when present in sufficient amount.

Conclusions

The inhibition of Al-Pure in 1.0 M HCl with and without different concentrations of (A) and (B) has been investigated by polarization method, EIS and weight loss method, and the main conclusions are:

(1) Two Schiff bases (A) and (B) are good inhibitors in 1.0 M HCl solution and a inhibition efficiency upto 99% was obtained. The inhibition efficiencies of (A) and (B) increased with the inhibitor concentration increase.

(2) Adsorption of (A) and (B) in 1.0 M HCl solution on Al-Pure surface was found to obey the Langmuir adsorption isotherm.

(3) (A) and (B) act as mixed type inhibitors with predominant action on the cathode.

(4) The adsorption takes place due to the iminic group and -OCH3 group, therefore the inhibitive efficiency of both (A) and (B) should be almost the same.

(5) Obtained results about inhibition efficiencies from polarization study, EIS and weight loss method are in good agreement with each other.

References