¹ Chemistry Department Faculty of Science, Al-Azhar University, Assiut , Egypt

² Chemistry Department Faculty of Science, Al-Azhar University, Nasr City, Egypt

³ Laboratory for Environment Quality Monitoring, National Water Research Center, Egypt

Abstract

The effect of some mono-,bis -and tris-azo dyes compounds on the dissolution of mild steel in 1.0 M HCl solution was studied using weight loss and potentiodynamic polarization measurements. The inhibition efficiency was found to increase with increasing concentration of the inhibitors and with decreasing temperature. The adsorption of different azo dyes on the mild steel surface obeys the Langmuir adsorption isotherm. Polarization studies indicate that the compounds used are mixed type inhibitors. The thermodynamic functions of adsorption processes were calculated from weight loss at different temperatures data and were used to analyze the inhibitor mechanism. The surface morphology of the mild steel specimens was evaluated using SEM and EDAX analysis. To provide an evidence for the formation of the complex, UV-VIS spectra of solutions were investigated. From previous results (I.E.%, ΔG_ads, SEM, EDAX and UV-VIS spectra) are likely to get into the possiblity of configure the complex on the mild steel surface.

Keywords: azo dyes, mild steel, inhibitors, surface morphology.

Introduction

Mild steel is an alloy of iron, which undergoes corrosion easily in acidic medium. Acidic solutions are extensively used in chemical laboratories and in several industrial processes such as acid pickling, acid cleaning, acid descaling and oil wet cleaning, etc. Also mild steel is used under different conditions in chemical and allied industries for handling alkaline, acid and salt solutions.

Inhibitors are often added in industrial processes to secure metal dissolution from acid solutions. Authors show that organic compounds containing O, S and/or N atoms act as effective inhibitors [1-3]. These organic compounds adsorb on metallic surface and then decrease the corrosion rate [4-7]. It has been observed that adsorption depends mainly on certain physico-chemical properties of the inhibitor group, like functional groups, electron density at the donor atom, π-orbital character, and the electronic structure of the molecule [8-10]. Azo compounds are the most widely used class of dyes due to their versatile application in various fields, such as the dyeing of textiles, and fibbers, to coloring of different materials, and high-technology areas, such as eletro-optical devices and ink-jet printers. Some studies have shown that the inhibition of the corrosion by organic dyes is mainly attributed to the formation of complex compounds between the metal-ions and the nitrogen of azo binding at the electrode surface [11-38].

The objective of this study is to investigate the corrosion behavior of mild steel in 1.0 M HCl at different temperatures in the presence of some mono-,bis -and tris-azo dyes compounds by using weight loss, polarization resistance and Tafel polarization. The effects of temperature were also studied.

Experimental

Materials

Before immersion the obvious electrodes in the test solution each of them is prepared by polishing with emery paper from 250 to 1200 grade to obtain a smooth surface, washed with distilled water and then degreased with acetone about 5 minutes, washed again with distilled water, then dried using filter papers.

Selection and assessment of dyes used

Adopted techniques

Weight loss measurements

Weight loss measurements were carried out by weighing the mild steel specimens before and after immersion in 500 cm³ acid solutions for different time intervals in the presence and absence of various concentrations of dyes. Experiments were also performed at temperature range (298-328 K) in 1.0 M HCl solutions. Duplicate experiments were performed in each case and the mean value of the weight loss was determined.

Open circuit potential measurements (OCP)

The potential of the mild steel electrode (working electrode) was measured against a saturated calomel electrode (SCE) (reference electrode) in 1.0 M HCl solution in absence and presence of different concentrations of the inhibitor.

Potentiodynamic polarization measurements

The electrochemical cell used in potentiodynamic polarization consists of three electrodes: working electrode (mild steel), reference electrode (saturated calomel electrode (SCE)) and platinum wire used as counter electrode. The exposed area of the working electrode to solution was (1 cm2). For the anodic and cathodic potentiodynamic polarization (Tafel plots) the entire potential scan was programmed to take place within ±250 mV of the corrosion potential. The measurements were conducted at scanning rate of 0.2 mV/s.

Surface examinations techniques:

I-Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is a type of electron microscope giving images of the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. SEM images should confirm the electrode surface. The specimens have been rinsed before and after immersion in different solutions.

II-Energy Dispersive X-ray Analysis (EDAX)

EDAX is an analytical technique used for the elemental analysis or chemical characterization of a sample. The identification of the elements present in the surface of the specimens before and after immersion in different solutions will be performed using an energy dispersion X-ray analysis. This technique is used in conjunction with SEM.

Results and discussion

Weight loss measurements

Weight loss of mild steel electrode was determined at various time intervals in absence and presence of different concentrations of mono-,bis-and tris azo-dyes. It is obvious that the weight-loss of mild steel electrode in the presence of different azo-dyes (D-1--D-5) varies linearly with time, being much lower than that obtained in blank solution. The linearity obtained indicates the absence of insoluble surface film during corrosion and the inhibitors were fast adsorbed onto the metal surface and, therefore, impeding the corrosion process [39]. The corrosion rate (C.R.), the inhibition efficiency (IE %) and the surface coverage (θ), that represent the weight of metal surface covered by inhibitor molecules, were calculated using the following equations:

At first, when there is not enough Fe (Inh)_ads. to cover the metal surface, because the inhibitors concentration is low or because the adsorption rate is slow, metal dissolution takes place in sites on the mild steel surface free of Fe (Inh)_ads. With high inhibitor concentration, a compact and coherent inhibitor over filmed is formed on the mild steel which reduces chemical attacks on the metal.

The results show that the slopes and R² values are very close to unity indicating strong adherence of the adsorbed inhibitors to the assumptions of Langmuir [44].

where R is the universal gas constant , ΔG_ads. is the free energy of adsorption and 55.5 is the concentration of water in solution (mol L^-1) [45].

where ORG is the organic compound adsorbed and deadsorbed at the same speed on the metal surface. The adsorption phenomenon occurs in an aqueous solution containing the organic compound [ORG _(aq)]. This involves the displacement of a certain number of water molecules (χ) by the organic compound which is adsorbed on the electrode surface [H2O _(ads)].

Ateya et al. [48] described this situation as the adsorption of the organic compound which is accompanied by the desorption of water molecules from the surface. Thus, while the adsorption process is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. Therefore the gain in entropy that accompanies the substantial adsorption process is attributed to the increase in solvent entropy. The values of ΔH_ads provide further information about the mechanism of corrosion inhibition. The endothermic adsorption process is ascribed unambiguously to chemisorption and an exothermic process may involve either physisorption or chemisorption or a combination of both [49]. In the present study, the obtained negative values of ΔH_ads indicate a combination of both physisorption and chemisorption processes.

The values of ΔH_ads and ΔG_ads can also be calculated by using the following equation [37]:

where (C.R.) is the corrosion rate, E_a is the apparent activation energy, R is the molar gas constant, T is the absolute temperature and A is the frequency factor.

It is clear from this table that E_a values in presence of the azo-dyes are higher than in their absence. Inspection of these data reveals that the apparent activation energy Ea in HCl in the absence of azo dye compounds was 13.47 kJ /moL^-1.

Low activation energy means fast reaction, and high activation energy means slow reaction. High activation energy corresponds to a reaction rate that is very sensitive to temperature. Conversely small activation energy indicates a reaction rate that varies only slightly with the temperature. If reaction has zero activation energy, its rate is independent of temperature. In some cases, activation energy (E_a) is found to be negative, which indicates that the rate decreases when temperature is raised and such behaviour is a signal that the reaction has a complex mechanism [30,51].

Enthalpy and entropy of activation ΔH^* and ΔS^* were obtained by applying the transition state equation:

In almost all azo-dye compounds, there is always a general tendency for the OCP to drift with time towards more stable values at which it tends to be stabilized after 30 minutes.

Accordingly, the behavior can be classified into two different behaviors depending upon the mode of the variation of the steady state potentials with time and concentration of the test solutions.

In the first behavior which is represented by the potential time curves of mild steel electrode in 1.0 M HCl containing different concentrations of D-1, D-2, D-3 and D-5, there is always a general tendency for the immersion potential (E_im) to shift from negative to the positive direction. The values of steady state potential (E_ss) in this case are higher than the value in free acid.

The second behavior, on the other hand, belongs to the azo-dye D-4 (which contains tris azo-group) where the values of steady state potential are lower than the value in free HCl solution. This behavior indicates that the first adsorbed molecules of the inhibitors are attached to the most active anodic sites. In contrast to some azo-dyes, there is some inhibition of the cathodic reaction probably due to steric hindrance of the combination of the adsorbed hydrogen atoms to form molecules.

Potentiodynamic polarization measurements

This difference can be attributed to the fact that the first method gives instantaneous corrosion rate, whereas the second method gives average corrosion rates.

This difference may also be expected to arise because of the difference in the time required to form an adsorbed layer of inhibitor on metal surface that can inhibit corrosion [54].

Surface morphology of the metal electrodes

The scanning electron microscope images and energy dispersive X-ray analysis further supported the formation of a surface film by the inhibitors and their interaction with surface atoms of mild steel.

Scanning Electron Microscope (SEM) analysis

The presence of the peaks of carbon, nitrogen, chloride, silicon and sulphur is explained by the adsorption of the inhibitors [D-3, D-5 and beet root extract] on the products of corrosion of the mild steel.

Mechanism of inhibition

The inhibition action of mono-,bis-and tris-azodyes towards the corrosion of mild steel in 1.0 M HCl could be attributed to several factors including the structure, the number and types of adsorption sites, the nature of the molecule, the metal surface, and the ability to form complexes.

The results obtained in this study show the order of increase of I.E% of the compounds in solution and the extent of their ability to adsorb, hence protecting mild steel, as follows:

Bis-azodye D-3(82.6%) and D-5 (80.41%) > Tris-azodye D-4(76.12%) > Mono-azodye D-1(71.08%) and D-2 (60.25%)

The sequence may be attributed to the electronic structure of these compounds. The lone pairs of electrons on the S and N atoms are delocalized which stabilizes the compounds.

The inhibition mechanism of the azo compounds under investigation is believed to be as a result of the complex formation between Fe²⁺ and the azo compounds. The formed complex is adsorbed on the metal surface and thereby isolating the metal from further corroding attack.

To provide an evidence for the formation of the complex, UV-VIS spectra of solutions in presence of 1.0 M HCl and different under test azo dyes in absence (blank) and in presence of mild steel sample were investigated.

Additionally, there are two significant peaks at lower wavelengthes after addition of dyes to mild steel electrode, which may indicate a new structure unit formed and the degradation of the original structure under test dyes [55].

Conclusions

1 - The corrosion of mild steel in 1.0 M HCl is inhibited by the addition of some mono-, bis-, and tris-azo dyes compounds.

2 - The inhibition efficiency increases with increase in the concentration of these compounds, but decreases with an increase in temperature.

3 - The adsorption of the azo dye compounds on the mild steel surface in acid solution obeyed Langmuir adsorption isotherm.

4 - The azo dye compounds influence both the cathodic and the anodic reactions in the HCl solution. This indicates that the additives act as mixed-type inhibitors.

5 - Previous results (I.E.%, ΔG_ads, SEM, EDAX and UV-VIS spectra) are likely to get into the possiblity of configure the complex on the mild steel surface.

References