Corrosion inhibition of copper in HNO3 solution using thiophene and its derivatives

A.S. Fouda; H.A. Abdul Wahed

doi:10.1016/j.arabjc.2011.02.014

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Original article

9 (

1_suppl

); S91-S99

doi:

10.1016/j.arabjc.2011.02.014

Corrosion inhibition of copper in HNO₃ solution using thiophene and its derivatives

A.S. Fouda^⁎, H.A. Abdul Wahed

Department of Chemistry, Faculty of Science, El-Mansoura University, El-Mansoura 35516, Egypt

⁎Corresponding author. asfouda@mans.edu.eg (A.S. Fouda)

Received: 2010-12-16, Accepted: 2011-2-14, Published: 2011-09-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The corrosion behavior of copper in the presence of thiophene (T) and some of its derivatives [2-thiophene carboxylic acid (TC) and 2-thienyl ethanol (TE)] has been investigated in 2 M HNO₃ solution using electrochemical frequency modulation (EFM), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and weight loss techniques. Polarization studies showed that these compounds act as mixed type inhibitors. EFM technique provides a new tool for electrochemical corrosion monitoring and was used as an effective method for corrosion rate determination. In EFM measurements corrosion current density was determined without prior knowledge of Tafel slopes. Inhibition efficiency of these compounds has been found to vary with concentrations of the compounds and temperature. The adsorption of these compounds on the copper surface from the acid solution has been found to obey Langmuir adsorption isotherm. The thermodynamic activation parameters of copper corrosion in 2 M HNO₃ were determined and discussed. The results obtained from EFM, EIS, Tafel and weight loss measurements were in good agreement.

Keywords

Copper corrosion

Thiophene and thiophene derivatives

HNO3

Electrochemical frequency modulation

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1 Introduction

Considerable attention has been drawn during the past few decades to inhibit corrosion of copper, as it has wide application in industry. Copper is used in microelectronics, fabrication of heat exchanger tubes and cooling water systems, due to its high thermal and electrical conductivities, low cost and malleability (Kane Jennings and Laibins, 1996; Ma et al., 2003, Sayed and Sherif, 2006 ). Corrosion inhibition of copper can be achieved through the modification of its interface by forming self assembled ordered ultrathin layers of organic inhibitors. Commonly used inhibitors for copper corrosion are toxic compounds that should be replaced with the new eco-friendly inhibitors. Most of the inhibitors are organic compounds and their derivatives such as azoles (Sayed and Sherif, 2006, Zucchi et al., 1996; Szocs et al., 2005, Subramanian and Lakshminarayanan, 2002, Ramesh and Rajeswari, 2005, El-Naggar, 2002; Zhong et al., 2004, Baartly et al., 2003, Frignani et al., 1999; Bastidas et al., 2003; Otieno-Alego et al., 1999; Huynh et al., 2002 and El-Morsi and Hassanein, 1999 ), amines (Stupnisek-Lisac et al., 2000), amino acids (Moretti and Guidi, 2002), thiophene and its derivatives (Szklarska-Smialowska and Kaminski, 1973; Talai and Gandhi, 1983; Galal et al., 2005; Fouda, 1986 ) and many others. It is noticed that the presence of heteroatom such as N-, S-, and P- in organic compound molecule improves its action as copper corrosion inhibitor. This is explained by the presence of vacant d orbitals in copper atom that form coordinative bonds with atoms able to donate electrons. Interaction with rings containing conjugated bonds, π electrons, is also present.

In this paper the EFM was used as a new non-linear distortion technique, evaluated as an instantaneous corrosion monitoring technique and is used here for online monitoring of corrosion rate of copper in the absence and presence of investigated compounds. The purpose of this paper is to compare the corrosion inhibition data derived from EFM with that obtained from Tafel extrapolation, EIS and weight loss techniques.

2 Experimental detail

2.1

2.1 Chemicals

The electrolyte was prepared using analytical grade HNO₃ (67.5% LR) and bidistilled water. Copper sheet and wire (99.9%) were used throughout the experiments. Thiophene (T) and some of its derivatives [2-thiophene carboxylic acid (TC) and 2-thienyl ethanol (TE)] were purchased from Aldrich chemicals (Darmstadt Germany) (see Fig. 1).

Names, molecular structure and molecular weight of investigated thiophene and its derivatives.

2.2

2.2 Procedures used for corrosion measurements

2.2.1

2.2.1 Weight loss tests

For weight loss measurements, rectangular specimens of size 2 × 2 × 0.3 cm were used. The specimens were abraded with SiC papers grit sizes (800 and 1200), degreased with ethanol, rinsed several times by bidistilled water, and finally dried between two filter papers. The specimens were then immediately immersed in the test solution without or with desired concentration of the investigated compounds. Triplicate specimens were exposed for each condition and the mean weight losses were reported.

The percentage of inhibition efficiency (IE_wt %) over exposure time is calculated according to the following equation:

(1)

({IE}_{wt} %) = [1 - (R_{corr}^{o} / R_{corr})] \times 100

where

R_{corr}^{o}

and R_corr are the corrosion rates in the absence and presence of the inhibitor, respectively.

2.2.2

2.2.2 Potentiodynamic polarization measurements

Polarization experiments were carried out in a conventional three-electrode cell with a platinum counter electrode and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. The working electrode was in the form of a square cut from copper sheet embedded in epoxy resin of polytetrafluoroethylene so that the flat surface area was 1.0 × 1.0 cm. The working electrode was abraded as in weight loss. Before measurement, the electrode was immersed in solution at natural potential for 2 h until a steady state was reached. All polarization curves were recorded by PGZ100 Volta Lab Radiometer analytical controlled by Volta Master 4 corrosion software at 30 °C. The potential was started from −600 to +400 mV vs. open circuit potential (E_ocp). The working electrode was abraded with SiC papers grit sizes (800 and 1200) as before. All experiments were carried out in freshly prepared solutions and results were always repeated at least three times to check the reproducibility. The inhibition efficiencies are defined as:

(2)

{IE}_{p} % = [(j_{corr}^{o} - j_{corr}) / j_{corr}^{o}] \times 100

where

j_{corr}^{o}

and j_corr are the uninhibited and inhibited corrosion current densities, respectively.

2.2.3

2.2.3 Electrochemical impedance spectroscopy measurements

All EIS measurements were performed at open circuit potential E_ocp at 30 °C over a wide frequency range of (2 × 10⁴–8 × 10⁻² Hz). The sinusoidal potential perturbation was 10 mV in amplitude peak to peak.

The inhibition efficiency is calculated from the charge-transfer resistance data as follows (Ma et al., 2002):

(3)

% IE = [1 - (R_{ct} - R_{ct}^{o})]

(4)

Θ = [1 - (R_{ct} - R_{ct}^{o})]

R_{ct}^{o}

and R_ct are the charge-transfer resistance values without and with inhibitor, respectively.

2.2.4

2.2.4 Electrochemical frequency modulation technique

EFM experiments were performed with applying potential perturbation signal with amplitude 10 mV with two sine waves of 2 and 5 Hz. The choice for the frequencies of 2 and 5 Hz was based on three arguments (Bosch et al., 2001). The larger peaks were used to calculate the corrosion current density (j_corr), the Tafel slopes (β_c and β_a) and the causality factors CF₂ and CF₃ (Abdel–Rehim et al., 2006; Trabanelli et al., 1987).

EIS and EFM experiments were carried out using Gamry PCI300/4 potentiostat/galvanostat/Zra analyzer, EIS300 Electrochemical Impedance software, EFM140 Electrochemical Frequency Modulation software and Echem 5.21 for results plotting, graphing, data fitting and calculating.

The inhibition efficiencies IE_EFM% was calculated as follows:

(5)

{IE}_{EFM} % = [(1 - j_{corr} / j_{corr}^{o})] \times 100

3 Results and discussion

3.1

3.1 Weight loss measurements

The weight loss of copper in 2.0 M HNO₃ is nearly varied linearly with immersion period in the absence and presence of different concentrations of compound (TE) at 30 °C as shown in Fig. 2. Similar curves were obtained for the other inhibitors (not shown). The linearity of the weight loss with time from the beginning was interpreted by El-Hosary et al. (1972) as the breakdown of the oxide film at the start of the attack.

Weight loss – time curves of copper corrosion in 2 M HNO3 in the presence and absence of compound (TE) at 30 °C.

The corrosion rates (R_corr), degree of surface coverage (θ) and the inhibition efficiency (IE_wt%) for all investigated compounds are listed in Table 1. The results obtained show that the addition of investigated inhibitors limits the dissolution of copper by blocking its corrosion sites and hence decreasing the weight loss and the corrosion rate and hence increasing the inhibition efficiency. The inhibitor action depends on the sulfur atom of heterocyclic ring and the nature of substituent.

Table 1 Weight loss (W), corrosion rates (R_w·corr) and inhibition efficiencies (IE_w) in 2 M HNO₃ solutions after 2 h immersion at 30 °C, calculated from the weight loss method.

Blank	Wt. loss (mg cm⁻²)	5.18
Blank	R_corr (mm y⁻¹)	26.04
Concentration/M		1 × 10⁻⁶	1 × 10⁻⁵	1 × 10⁻⁴	1 × 10⁻³

[TE]	Wt. loss (mg cm⁻²)	1.71	1.54	1.34	1.29
	R_corr (mm y⁻¹)	8.34	7.5	6.59	6.17
	θ	0.671	0.712	0.747	0.763
	%IE	67.1	71.2	74.7	76.3

[TC]	Wt. loss (mg cm⁻²)	4.01	3.41	2.47	1.35
	R_corr (mm y⁻¹)	19.61	16.69	12.06	6.71
	θ	0.247	0.359	0.537	0.742
	%IE	24.7	35.9	53.7	74.2

[T]	Wt. loss (mg cm⁻²)	1.79	1.56	1.06	0.96
	R_corr (mm y⁻¹)	8.72	7.62	5.16	4.53
	θ	0.665	0.707	0.802	0.826
	%IE	66.5	70.7	80.2	82.6

3.2

3.2 Adsorption isotherm

Basic information on the interaction between the inhibitor and the metal surface can be provided by the adsorption isotherm, and the type of the inhibitors on metal is influenced by (i) the nature and charge of the metal (ii) chemical structure of the inhibitor and (iii) the type of electrolyte. The degree of surface coverage (θ) of copper electrode by adsorption of investigated compounds was calculated using the following equation:

(6)

Θ = [1 - (R_{corr} / R_{corr}^{o})]

where R_corr and

R_{corr}^{o}

were defined before. The values of (θ) for different concentrations of the studied compounds at 30 °C have been used to explain the best isotherm to determine the adsorption process. The adsorption of the thiophene and its derivatives, on the surface of Cu electrode is regarded as a substitution adsorption process between organic compounds in aqueous phase (org_aq) and the water molecules adsorbed on Cu surface (H₂O_ads) (Moretti et al., 1994):

(7)

{Org}_{(sol)} + x {(H_{2} O)}_{ads} = {Org}_{(ads)} + x H_{2} O_{(sol)}

Where x is the number of water molecules replaced by one organic molecule. Attempts were made to fit (θ) values to various isotherms, including Langmuir, Frumkin, Temkin and Freundlich isotherms. In this study, Langmuir adsorption isotherm was found to be suitable for the experimental findings. The isotherm is described by Eq. (8):

(8)

C / Θ = (1 / K) + C

With

(9)

Δ G_{ads}^{o} = - RT \ln (K_{ads} \times 55.5)

where C is the inhibitor concentration, K_ads is the adsorption equilibrium constant,

Δ G_{ads}^{o}

is the standard free energy of adsorption, 55.5 is the concentration of water in the solution in mol dm⁻³, R is the universal gas constant and T is the absolute temperature in Kelvin. The plot of C/θ vs. C gives straight lines (Fig. 3) (correlation > 0.99), the deviation of the slopes from unity (for ideal Langmuir isotherm) can be attributed to the molecular interaction among the adsorbed inhibitor species (Fiala et al., 2007). The increasing negative value of

Δ G_{ads}^{o}

(Table 2) reflects the increasing adsorption capability and spontaneous adsorption of these inhibitors on copper and the higher values of K_ads indicate strong adsorption of these compounds on Cu surface (Morales-Gil et al., 2004; Abdallah, 2002) and the increasing value of K_ads (K_ads(TE) > K_ads(T) > K_ads(TC)) reflects the increasing adsorption capability, due to structural formation on the metal surface (Da Costa and Agostinho, 1989).

Langmuir adsorption isotherms for the investigated compounds on copper surface in 2 M HNO3.

Table 2 Adsorption equilibrium constant, k_ads, regression coefficient, R²_, and standard free energy of adsorption, ΔG^o_ads of all additives at 1 × 10⁻³ M on copper in HNO₃ solutions at 30 °C.

Name	Slope	K_ads × 10⁻⁴ M⁻¹	R²	−ΔG˚_ads kJ mol⁻¹
[T]	1.33	12.87	1.00000	41.9
[TC]	1.30	4.02	0.99891	36.8
[TE]	1.22	29.8	0.99979	45.6

3.3

3.3 Effect of temperature

Generally the corrosion rate increases with the rise of temperature. It was found that the inhibition efficiency decreases with increasing temperature. This can be attributed to the shift of the adsorption–desorption equilibrium towards desorption. Such behavior suggests that investigated compounds were physically adsorbed on Cu surface. The activation energy ( $E_{a}^{*}$ ) of the corrosion process was calculated using Arrhenius equation:

(10)

R_{corr} = A \exp (- E_{a}^{*} / RT)

where A is Arrhenius constant, R is the gas constant and T is the absolute temperature.

Fig. 4 shows the Arrhenius plot (log R_corr vs. 1/T) in the presence and absence of investigated compounds. The values of activation energies $E_{a}^{*}$ can be obtained from the slope of the straight lines and are given in Table 3, it is noted that the values of activation energy is higher in the presence of inhibitors than in their absence. This is due to the presence of a film formation on Cu surface. The activation energy for the corrosion of Cu in 2.0 M HNO₃ is equal to 78.2 kJ mol⁻¹ which is in good agreement with the work carried out by Fouda et al. (2006) and others (Asaf et al., 2002). An alternative formulation of the Arrhenius equation is the transition state equation (Arab and Noor, 1993):

(11)

R_{corr} = RT / Nh \exp (Δ S^{*} / R) \exp (- Δ H^{*} / RT)

where h is Planck’s constant, N is Avogadro’s number, ΔS^∗ is the entropy of activation and ΔH^∗ is the enthalpy of activation. Fig. 5 shows a plot of log (k/T) vs. (1/T). Straight lines are obtained with a slope of (ΔH^∗/2.303R) and an intercept of (log R/Nh + ΔS^∗/2.303R) from which the values of ΔH^∗ and ΔS^∗ are calculated and listed in Table 3. The negative values of ΔH^∗ reflect that the process of adsorption of the inhibitors on Cu surface is an exothermic one. The negative values of ΔS^∗implies that the activation complex is the rate determining step that represents an association rather than dissociation step (Abd El-Rehim et al., 1999). This means that the activated molecules are in higher order state than that the initial state.

Arrhenius plots of copper in 2 M HNO3 without and with 10−3 M of the investigated inhibitors.

Table 3 Activation parameters for the dissolution reaction of copper in 2 M HNO₃ with and without thiophene and its derivatives at 1 × 10⁻³ M at 120 min.

Name	$- E_{a}^{*}$ kJ mol⁻¹	−ΔH^∗ kJ mol⁻¹	−ΔS^∗ J mol⁻¹ K⁻¹
Blank	78.2	75.6	−23.3
[T]	95.1	92.4	22.6
[TC]	94.3	91.6	21.9
[TE]	98.4	95.4	30.0

Transition state plots of copper in 2 M HNO3 without and with 10−3 M of the investigated inhibitors.

3.4

3.4 Potentiodynamic polarization measurements

Both anodic and cathodic polarization curves for Cu in 2.0 M HNO₃ at various concentrations of compound (TE) is shown in Fig. 6. Similar curves for other compounds were obtained (not shown). It is clear that the presence of inhibitors cause a marked decrease in the corrosion rate. The inhibitors have a significant effect on the rate of the hydrogen evolution and anodic dissolution reactions i.e. these investigated inhibitors act as mixed-type inhibitors.

Potentiodynamic polarization curves of copper in 2 M HNO3 in the absence and presence of various concentrations of compound (TE).

The values of corrosion current densities (j_corr), corrosion potential (E_corr), the cathodic Tafel slope (β_c), anodic Tafel slope (β_a) and inhibition efficiency (IE%) are given in Table 4. Results in Table 4 revealed that the corrosion current decreases obviously after inhibitors are added in 2.0 M HNO₃, and IE% increases with increasing the inhibitor concentration. The presence of inhibitors shifts the corrosion potential (E_corr) with no definite trend, indicating that these compounds act as mixed-type inhibitors in 2.0 M HNO₃ (Fig. 6). Polarization resistance values (R_p) obtained from the LPR method (Table 4) are increased in inhibited system compared to the free system and consequently the corrosion rate is decreased at different inhibitor concentrations indicating that the increase in the inhibition efficiency.

Table 4 Electrochemical kinetic parameters obtained by Tafel extrapolation method for Cu in 2 M HNO₃ without and with various concentrations of investigated compounds.

Comp.	Conc. M	−E_corr mV vs. SCE	j_corr mA cm⁻²	β_c mV dec⁻¹	β_a mV dec⁻¹	R_P Ω cm²	k_corr mm y⁻¹	θ	%IE
Blank	0	116	3.38	278	61	7.79	39.58	0	0

[T]	1 × 10⁻⁶	104	2.27	301	56	8.70	26.58	0.328	32.8
	1 × 10⁻⁵	106	1.85	203	53	9.69	21.58	0.453	45.3
	1 × 10⁻⁴	125	1.46	173	39	9.11	16.90	0.568	56.8
	1 × 10⁻³	115	0.97	111	39	9.93	14.90	0.713	71.3

[TC]	1 × 10⁻⁶	103	2.31	251	58	10.40	26.96	0.318	31.8
	1 × 10⁻⁵	106	2.06	223	52	10.53	24.12	0.389	38.9
	1 × 10⁻⁴	108	1.73	219	54	10.93	20.27	0.488	48.8
	1 × 10⁻³	112	1.67	241	47	12.24	19.53	0.506	50.6

[TE]	1 × 10⁻⁶	86	1.74	238	56	9.83	22.95	0.485	48.5
	1 × 10⁻⁵	88	1.53	225	54	8.78	17.16	0.547	54.7
	1 × 10⁻⁴	125	1.25	138	34	8.61	14.57	0.630	63
	1 × 10⁻³	89	0.83	112	34	8.81	9.74	0.754	75.4

IE_p% of these compounds follows the sequence: TE > T > TC. This behavior may be attributed to the free electron pair in sulfur atom, π electrons on the aromatic nuclei and the substituent in the molecular structure of the inhibitor.

3.5

3.5 Electrochemical impedance spectroscopy (EIS)

EIS is a well-established and powerful technique in the study of corrosion. Surface properties, electrode kinetics and mechanistic information can be obtained from impedance diagrams (Lorenz and Mansfeld, 1981). Fig. 7 shows the Nyquist plot obtained at open-circuit potential both in the absence and presence of increasing concentrations of investigated compound (TE). Similar curves were obtained for other compounds (not shown). Nyquist plots are analyzed in terms of the equivalent circuit shown in Fig. 8. It is concluded that the curves were approximated by single capacitive semicircles, showing that the corrosion process was mainly charge-transfer controlled (Trabanelli et al., 2005). The general shape of the curves is very similar for all samples (in the presence or absence of inhibitors) indicating no change in the corrosion mechanism (Reis et al., 2006). From the impedance data (Table 5), we conclude that the value of R_ct increases with an increase in concentration of the inhibitors and this indicates an increase in IE%.

Nyquist plots for copper in 2 M HNO3 without and with different concentrations of compound (TE) at 30 °C.

Equivalent circuit model for the studied inhibitors.

Table 5 Impedance data of Cu in 2 M HNO₃ without and with different concentrations of investigated compounds.

Name	Conc. M	R_ct Ω cm²	C_dl μF cm⁻²	θ	%IE
Blank	0.0	39.96	93.63	–	–

[T]	1 × 10⁻⁶	80.66	90.67	0.505	50.5
	1 × 10⁻⁵	84.34	112.10	0.526	52.6
	1 × 10⁻⁴	172.9	68.56	0.769	76.9
	1 × 10⁻³	205.6	63.22	0.806	80.6

[TC]	1 × 10⁻⁶	43.33	87.32	0.078	7.8
	1 × 10⁻⁵	49.95	72.70	0.200	20.0
	1 × 10⁻⁴	79.21	55.64	0.496	49.6
	1 × 10⁻³	86.45	52.72	0.538	53.8

[TE]	1 × 10⁻⁶	60.0	84.83	0.320	32.0
	1 × 10⁻⁵	62.0	73.55	0.360	36.0
	1 × 10⁻⁴	73.0	67.45	0.452	45.2
	1 × 10⁻³	245.0	54.29	0.836	83.6

Values of double layer capacitance are also brought down to the maximum extent in the presence of inhibitor and the decrease in the values of C_dl follows the order similar to that obtained for j_corr in this study. The decrease in C_dl is due to the adsorption of these compounds on Cu surface leading to the formation of a film from the acidic solution (Caigman et al., 2000).

IE_EIS% of these compounds follows the sequence: TE > T > TC and is the same trend as in weight loss and electrochemical methods.

3.6

3.6 Electrochemical frequency modulation technique (EFM)

EFM is a nondestructive corrosion measurement technique that can directly give values of the corrosion current without a prior knowledge of Tafel constants. The great strength of the EFM is the causality factors which serve as an internal check on the validity of EFM measurement. The causality factors CF-2 and CF-3 are calculated from the frequency spectrum of the current responses. Fig. 9 shows the frequency spectrum of the current response of pure copper in nitric acid solution (real corroding system). The EFM intermediation spectra of Cu in nitric acid solution containing (1 × 10⁻³ M) of the studied inhibitors are shown in Figs. 10–12 . Similar results were recorded for the other concentrations of the investigated compounds (not shown). The harmonic and intermediation peaks are clearly visible and are much larger than the background noise. The two large peaks, with amplitude of about 200 μA, are the response to the 40 and 100 mHz (2 and 5 Hz) excitation frequencies. It is important to note that between the peaks there is nearly no current response (<100 nA). The experimental EFM-data were treated using two different models: complete diffusion control of the cathodic reaction and the “activation” model. For the latter, a set of three non-linear equations had been solved, assuming that the corrosion potential does not change due to the polarization of the working electrode (Sherif and Park, 2006). The electrochemical parameters of corrosion are calculated and presented in Table 6. The causality factors obtained under different experimental conditions are approximately equal to the theoretical values (2 and 3) indicating that the measured data are verified and of good quality (Abdel-Rehim et al., 2006). The data presented in Table 6 obviously show that the addition of any one of the tested compounds at a given concentration to the acidic solution decreases the corrosion current density, indicating that these compounds inhibit the corrosion of Cu in 2.0 M HNO₃ through adsorption.

Intermediation spectrums for copper in 2 M HNO3.

Intermediation spectrums for copper in 2 M HNO3 in presence of 1 × 10−3 M compound (T).

Intermediation spectrums for copper in 2 M HNO3 in presence of 1 × 10−3 M compound (TC).

Intermediation spectrums for copper in 2 M HNO3 in presence of 1 × 10−3 M compound (TE).

Table 6 Electrochemical kinetic parameters obtained by EFM technique for Cu in 2 M HNO₃ solutions containing various concentrations of investigated compounds at 30 °C.

Name	Conc. M	j_corr μA cm⁻²	β_a mV dec⁻¹	β_c mV dec⁻¹	Causality factor (2)	Causality factor (3)	%IE
Blank	0.0	311.8	62	199	1.92	3.36	0.0

[T]	1 × 10⁻⁶	117.2	101	141	2.26	2.62	62.4
	1 × 10⁻⁵	102.4	60	115	1.87	3.45	67.2
	1 × 10⁻⁴	89.19	83	135	1.70	3.05	71.4
	1 × 10⁻³	64.04	53	190	1.90	2.82	79.5

[TC]	1 × 10⁻⁶	113.6	89	158	1.95	2.63	63.6
	1 × 10⁻⁵	104.9	53	102	2.21	2.98	66.4
	1 × 10⁻⁴	102.4	89	102	1.89	3.51	67.2
	1 × 10⁻³	86.36	53	112	2.11	3.38	72.3

[TE]	1 × 10⁻⁶	106.6	51	84	2.23	3.23	65.8
	1 × 10⁻⁵	98.40	111	123	2.04	2.71	68.4
	1 × 10⁻⁴	84.45	72	129	1.94	3.58	72.9
	1 × 10⁻³	61.31	68	104	1.82	3.67	80.3

3.7

3.7 Comparison between inhibition efficiencies obtained by the techniques used

Inhibition efficiency obtained from weight loss, electrochemical, impedance and electrochemical frequency modulation is presented in Table 7. It is evident from Table 7 that IE% of inhibitors (TE), (T), and (TC) tested increases with increasing of inhibitor concentration and attain a maximum value of 84% for compound (TC) at 1 × 10⁻³ M. We note that the values of inhibition efficiency obtained by the methods used are in quite reasonable agreement.

Table 7 Comparison between inhibition efficiencies obtained by the techniques used.

Compound	Conc./M	IE%
Compound	Conc./M	Wt. loss	Polarization	EIS	EFM
[T]	1 × 10⁻⁶	67.1	32.8	50.5	62.4
	1 × 10⁻⁵	71.2	45.3	52.6	67.2
	1 × 10⁻⁴	74.7	56.8	76.9	71.4
	1 × 10⁻³	76.3	71.3	80.6	79.5

[TC]	1 × 10⁻⁶	24.7	31.8	7.8	63.6
	1 × 10⁻⁵	35.9	38.9	20.0	66.4
	1 × 10⁻⁴	53.7	48.8	49.6	67.2
	1 × 10⁻³	74.2	50.6	53.8	72.3

[TE]	1 × 10⁻⁶	65.5	48.5	32.0	65.8
	1 × 10⁻⁵	69.9	54.7	36.0	68.4
	1 × 10⁻⁴	79.6	63.0	45.2	72.9
	1 × 10⁻³	81.5	75.4	83.6	80.3

3.8

3.8 Effect of chemical structure on the inhibition efficiency

Many of the organic corrosion inhibitors are compounds with at least one polar unit having atoms of N-, S-, O- and in some cases P- and Se. Polar organic compounds acting as corrosion inhibitors are adsorbed on the metal surface forming a charge transfer complex bond between their polar atoms and the metal. The size, orientation, shape, and electric charge on the molecule determine the degree of adsorption and hence the effectiveness of the inhibitor. The inhibitor may affect both the cathodic and anodic reactions by blocking the active sites on the metal surface (Trabanelli, 1987). The extent of corrosion inhibition of the investigated compounds at all concentrations followed the following order: (TE) > (T) > (TC).

The above mentioned trend can be explained on the basis of adsorption. These compounds can be adsorbed on the metal surface through the lone pair of electrons of the sulfur atom and delocalized π-electrons of the hetero atom. The difference in inhibition can be explained on the basis of the type of the substituent in position 2 in the hetero ring. Compound (TE) is more efficient, this improved behavior may be due its has two donating groups –CH₃ and –OH which increase the electron density on the molecule, increase its adsorption on Cu surface and hence increase its IE%. The O atom of OH group may be considered as adsorption center and results in improved adsorption of this compound on Cu surface. Compound (T) has –H atom in position 2 which does not contribute any electron density to the molecule, so it comes after compound (TE) in the sequence of IE%. Compound (TC) is the least efficient one and comes after compound (T) in the sequence of IE%. This is due to the presence of electron withdrawing group (–COOH) in position 2, which decreases the electron density on the molecule, and hence decreases its adsorption and IE%.

4 Conclusions

All the investigated compounds are good corrosion inhibitors for Cu in 2.0 M HNO₃ solution. The effectiveness of these inhibitors depends on their structures. The variation in inhibitive efficiency depends on the type and the nature of the substituent present in the inhibitor molecule. (–OH > –H > –COOH).
EFM can be used as a rapid and nondestructive technique for corrosion measurements without prior knowledge of Tafel slopes.
The results of EIS revealed that an increase in the charge transfer resistance and a decrease in double layer capacitances when the inhibitor is added and hence an increase in IE%. This is attributed to an increase of the thickness of the electrical double layer.
Results obtained from potentiodynamic polarization indicated that the thiophene and its derivatives are mixed-type inhibitors.
The results indicate that the inhibitors are adsorbed on Cu surface following the Langmuir adsorption isotherm.
The results obtained from chemical and electrochemical measurements were in good agreement with the results obtained from EFM. The IE% of these investigated compounds is in the following order: compound (TE) > compound (T) > compound (TC).

References

Abd El-Rehim S.S., Ibrahim M.A.M., Khaled K.F., . J. Appl. Electrochem.. 1999;29:593.
Abdallah M., . Corros. Sci.. 2002;44:717.
Abdel–Rehim S.S., Khaled K.F., Abd–Elshafi N.S., . Electrochim. Acta. 2006;51:3269.
Arab S.T., Noor E.M., . Corrosion. 1993;49:122.
Asaf F.H., Abou-Krisha M., Khodari M., EL-Cheihk F., Hussien A.A., . Mater. Chem. Phys.. 2002;93:1.
Baartly J., Huynh N.E., Bottle S., Flitt H., Notoya T., Schweinsberg D.P., . Corros. Sci.. 2003;45:81.
Bastidas M.J., Pinilla P., Cano E., Polo J.L., Miguel S., . Corros. Sci.. 2003;45:427.
Bosch R.W., Hubrecht J., Bogaerts W.F., Syrett B.C., . Corrosion. 2001;57:60.
Caigman G.A., Metcalf S.K., Holt E.M., . J. Chem. Cryst.. 2000;30:415.
Da Costa A.S.L.F., Agostinho S.M.L., . Corrosion. 1989;45:472.
El-Hosary A.A., Saleh R.M., Shams El Din A.M., . Corros. Sci.. 1972;12:897.
El-Morsi M.A., Hassanein A.M., . Corros. Sci.. 1999;41:2337.
El-Naggar M.M., . Corros. Sci.. 2002;42:773.
Fiala A., Chibani A., Darchen A., Boulkamh A., Djebbar K., . Appl. Surf. Sci.. 2007;253:9347.
Fouda A.S., . Monat. Chem.. 1986;117:159.
Fouda A.S., Abd E., Aal A., Kandil A.B., . J. Dasalination. 2006;201:216.
Frignani A., Tommesani L., Brunoro G., Monticelli C., Fogagnolo M., . Corros. Sci.. 1999;41:1205.
Galal A., Atta N.F., Hassan M.H., . Mater. Chem. Phys.. 2005;89:28.
Huynh N., Bottle S.E., Notoya T., Schweinsberg D.P., . Corros. Sci.. 2002;44:83.
Kane Jennings G., Laibins P.E., . Colloids and Surfaces A: Physicochem& Eng Asp.. 1996;116:105.
Lorenz W.J., Mansfeld F., . Corros. Sci.. 1981;21:47.
Ma H., Chen S., Niu L., Zhao S., Li S., Li D., . J. Appl. Electrochem.. 2002;32:65.
Ma H.Y., Yang C., Yin B.S., Li G.Y., Chen S.H., Luo J.L., . Appl. Surf. Sci.. 2003;218:143.
Morales-Gil P., Negron-Silva G., Romero-Roma M., Angeles-Chavez C., Palomar-Pardave M., . Electrochim. Acta. 2004;49:4733.
Moretti G., Quartorone G., Tassan A., Zingales A., . Werkst. Korros.. 1994;45:641.
Moretti G., Guidi F., . Corros. Sci.. 2002;44:1995.
Otieno-Alego V., Huynh N., Notoya T., Bottle S.E., Schweinsberg D.P., . Corros. Sci.. 1999;41:685.
Ramesh S., Rajeswari S., . Corros. Sci.. 2005;47:151.
Reis F.M., de Melo H.G., Costa I., . J. Electrochem. Acta. 2006;51:17.
Sayed E.I., Sherif M., . Appl. Surf. Sci.. 2006;252:8615.
Sherif E.M., Park Su-Moon, . Corros. Sci.. 2006;48:4065.
Stupnisek-Lisac E., Brnada A., Maance A.D., . Corros. Sci.. 2000;42:243.
Subramanian R., Lakshminarayanan V., . Corros. Sci.. 2002;44:535.
Szklarska-Smialowska Z., Kaminski M., . Corros. Sci.. 1973;13:1.
Szocs E., Vastag Gy., Shaban A., Kalman E., . Corros. Sci.. 2005;47:893.
Talai J.D., Gandhi D.K., . Corros. Sci.. 1983;23:1315.
Trabanelli G., . Mansfeld F., ed. Corrosion Mechanisms. New York: Marcel Dekker; 1987. p. :122.
Trabanelli G., . Mansfeld F., ed. Corrosion Mechanisms. New York: Marcel Dekker; 1987. p. :119.
Trabanelli G., Montecelli C., Grassi V., Frignami A., . J. Cem. Concr. Res.. 2005;35:1804.
Zhong Da-quan, Gao Li-xin, Zhou Guo-ding, . Corros. Sci.. 2004;46:3031.
Zucchi F., Trabanelli G., Fonsati M., . Corros. Sci.. 1996;38:2019.

Show Sections

[1] Abd El-Rehim S.S., Ibrahim M.A.M., Khaled K.F., . J. Appl. Electrochem.. 1999;29:593.

[2] Abdallah M., . Corros. Sci.. 2002;44:717.

[3] Abdel–Rehim S.S., Khaled K.F., Abd–Elshafi N.S., . Electrochim. Acta. 2006;51:3269.

[4] Arab S.T., Noor E.M., . Corrosion. 1993;49:122.

[5] Asaf F.H., Abou-Krisha M., Khodari M., EL-Cheihk F., Hussien A.A., . Mater. Chem. Phys.. 2002;93:1.

[6] Baartly J., Huynh N.E., Bottle S., Flitt H., Notoya T., Schweinsberg D.P., . Corros. Sci.. 2003;45:81.

[7] Bastidas M.J., Pinilla P., Cano E., Polo J.L., Miguel S., . Corros. Sci.. 2003;45:427.

[8] Bosch R.W., Hubrecht J., Bogaerts W.F., Syrett B.C., . Corrosion. 2001;57:60.

[9] Caigman G.A., Metcalf S.K., Holt E.M., . J. Chem. Cryst.. 2000;30:415.

[10] Da Costa A.S.L.F., Agostinho S.M.L., . Corrosion. 1989;45:472.

[11] El-Hosary A.A., Saleh R.M., Shams El Din A.M., . Corros. Sci.. 1972;12:897.

[12] El-Morsi M.A., Hassanein A.M., . Corros. Sci.. 1999;41:2337.

[13] El-Naggar M.M., . Corros. Sci.. 2002;42:773.

[14] Fiala A., Chibani A., Darchen A., Boulkamh A., Djebbar K., . Appl. Surf. Sci.. 2007;253:9347.

[15] Fouda A.S., . Monat. Chem.. 1986;117:159.

[16] Fouda A.S., Abd E., Aal A., Kandil A.B., . J. Dasalination. 2006;201:216.

[17] Frignani A., Tommesani L., Brunoro G., Monticelli C., Fogagnolo M., . Corros. Sci.. 1999;41:1205.

[18] Galal A., Atta N.F., Hassan M.H., . Mater. Chem. Phys.. 2005;89:28.

[19] Huynh N., Bottle S.E., Notoya T., Schweinsberg D.P., . Corros. Sci.. 2002;44:83.

[20] Kane Jennings G., Laibins P.E., . Colloids and Surfaces A: Physicochem& Eng Asp.. 1996;116:105.

[21] Lorenz W.J., Mansfeld F., . Corros. Sci.. 1981;21:47.

[22] Ma H., Chen S., Niu L., Zhao S., Li S., Li D., . J. Appl. Electrochem.. 2002;32:65.

[23] Ma H.Y., Yang C., Yin B.S., Li G.Y., Chen S.H., Luo J.L., . Appl. Surf. Sci.. 2003;218:143.

[24] Morales-Gil P., Negron-Silva G., Romero-Roma M., Angeles-Chavez C., Palomar-Pardave M., . Electrochim. Acta. 2004;49:4733.

[25] Moretti G., Quartorone G., Tassan A., Zingales A., . Werkst. Korros.. 1994;45:641.

[26] Moretti G., Guidi F., . Corros. Sci.. 2002;44:1995.

[27] Otieno-Alego V., Huynh N., Notoya T., Bottle S.E., Schweinsberg D.P., . Corros. Sci.. 1999;41:685.

[28] Ramesh S., Rajeswari S., . Corros. Sci.. 2005;47:151.

[29] Reis F.M., de Melo H.G., Costa I., . J. Electrochem. Acta. 2006;51:17.

[30] Sayed E.I., Sherif M., . Appl. Surf. Sci.. 2006;252:8615.

[31] Sherif E.M., Park Su-Moon, . Corros. Sci.. 2006;48:4065.

[32] Stupnisek-Lisac E., Brnada A., Maance A.D., . Corros. Sci.. 2000;42:243.

[33] Subramanian R., Lakshminarayanan V., . Corros. Sci.. 2002;44:535.

[34] Szklarska-Smialowska Z., Kaminski M., . Corros. Sci.. 1973;13:1.

[35] Szocs E., Vastag Gy., Shaban A., Kalman E., . Corros. Sci.. 2005;47:893.

[36] Talai J.D., Gandhi D.K., . Corros. Sci.. 1983;23:1315.

[37] Trabanelli G., . Mansfeld F., ed. Corrosion Mechanisms. New York: Marcel Dekker; 1987. p. :122.

[38] Trabanelli G., . Mansfeld F., ed. Corrosion Mechanisms. New York: Marcel Dekker; 1987. p. :119.

[39] Trabanelli G., Montecelli C., Grassi V., Frignami A., . J. Cem. Concr. Res.. 2005;35:1804.

[40] Zhong Da-quan, Gao Li-xin, Zhou Guo-ding, . Corros. Sci.. 2004;46:3031.

[41] Zucchi F., Trabanelli G., Fonsati M., . Corros. Sci.. 1996;38:2019.