Inhibitive action of Cystine on the corrosion of low alloy steel ASTM A213 grade T22 in sulfamic acid solutions

3.1 Effect of inhibitor concentration and temperature

The inhibition efficiency of Cystine (Cys) for low alloy steel (LAS) in aerated 0.5 M sulfamic acid solution was determined at different temperatures 25, 35, 45 and 55 °C ± 2 by using two different electrochemical techniques as following:

3.1.1

3.1.1 Electrochemical frequency modulation studies

Fig. 1a and b are examples to demonstrate the EFM intermodulation spectra of LAS in aerated stagnant 0.5 M sulfamic acid in the absence of and presence of 0.004 M Cys at 55 °C. Similar results were collected for the other Cys concentrations at different temperatures.

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Figure 1

Intermodulation spectrum for LAS in 0.5 M sulfamic at 55 °C: (a) in absence and (b) in presence of 0.004 M Cys.

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The values of kinetic parameters such as I_corr, β_a, β_c, and both Causality Factors (CF2 and CF3) for Cys are given in Table 2. Inspections of these data infer the following:

1. The addition of Cys to 0.5 M sulfamic acid solution decreases the corrosion current density (I_corr) indicating that Cys acts as corrosion inhibitor.

2. With increasing the inhibitor concentration the corrosion current density (I_corr) decreases as a result of increasing adsorption and surface coverage of Cys on the steel surface.

3. The corrosion current density (I_corr) in the absence and presence of Cys increases with increasing the temperature indicating that the corrosion rate increases with temperature in both inhibited and uninhibited solutions. Moreover, it is seen that the values of (I_corr) are higher in the uninhibited sulfamic acid solution than those obtained in the inhibited solution indicating the inhibitive effect of Cys at all tested temperatures.

Table 2 Electrochemical kinetic parameters obtained from EFM technique for LAS in 0.5 M sulfamic acid with various concentrations of Cys at different temperatures.

Temp (°C)	Cys Conc (M)	βa (mV dec−1)	βc (mV dec−1)	CF2	CF3	Icorr (μA cm−2)	IE%	θ
25	0.00	58.28	181.74	1.905	2.994	549.2	0.00	0.000
	0.0005	57.17	181.09	1.871	2.876	354.5	35.45	0.355
	0.001	57.56	180.82	1.699	2.931	218.6	60.20	0.602
	0.002	56.73	179.45	1.896	2.895	121.6	77.86	0.779
	0.004	53.74	177.81	1.797	3.238	43.39	92.10	0.921
35	0.00	63.79	189.64	2.117	3.372	946.4	0.00	0.000
	0.0005	61.78	186.39	1.981	2.958	659.1	30.36	0.304
	0.001	58.22	187.17	2.215	2.754	454.6	51.97	0.520
	0.002	56.87	189.00	2.088	3.441	287.8	69.59	0.696
	0.004	59.68	187.31	1.955	2.980	139.5	85.26	0.853
45	0.00	72.24	196.87	1.845	3.129	1376	0.00	0.000
	0.0005	71.67	194.11	2.168	3.447	1037	24.64	0.246
	0.001	69.03	195.38	1.871	2.957	792.7	42.39	0.424
	0.002	68.44	193.72	1.642	3.105	520.2	62.19	0.622
	0.004	68.45	192.29	1.749	3.336	286.5	79.18	0.792
55	0.00	86.78	199.45	1.878	3.736	1646	0.00	0.000
	0.0005	85.55	197.00	1.944	3.282	1377	16.34	0.163
	0.001	86.14	198.23	2.162	2.884	1076	34.63	0.346
	0.002	85.68	196.58	2.228	2.957	760.7	53.78	0.538
	0.004	83.32	195.83	2.017	3.238	468.9	71.51	0.715

The inhibition efficiency (IE%) of Cys was calculated using the following equation:

(1)

$$\text{IE}\%=\left[1-\left(\frac{I_{\text{corr}}}{I_{\text{corr}}^{\text{o}}}\right)\right]\times 100$$ where $I_{\text{corr}}^{\text{o}}$ and I_corr are the corrosion current densities for uninhibited and inhibited solutions, respectively.

The calculated values of the inhibition efficiency (IE%) at different concentrations of Cys and at different temperatures (25–55 °C) are also included in Table 2. The values of IE% show that the corrosion process depends on two factors, the inhibitor concentration and the solution temperature.

At a given temperature, the IE% increases with the increasing Cys concentration as a result of increasing adsorption and surface coverage on the electrode surface. However, at a given Cys concentration, the IE% decreases with the increase of solution temperature.

3.1.2

3.1.2 Electrochemical impedance studies

Typical Nyquist plots (representative example) obtained for LAS electrode in aerated stagnant 0.5 M sulfamic acid solution free and containing different concentrations of Cys at 45 °C at OCP are shown in Fig. 2. Similar impedance plots of LAS at different temperatures were obtained (diagrams not given). In all these cases the plots have the same shape where a single depressed capacitive semicircle was obtained. This frequency dispersion can be attributed to inhomogeneities and roughness of the electrode surface (Paskossy, 1994; Growcock and Jasinski, 1989).

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Figure 2

Nyquist plots for LAS in 0.5 M sulfamic without and with various concentrations of Cystine at 45 °C.

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The electrochemical response to the impedance tests for Cys was best simulated with the equivalent circuits shown in Fig. 3. According to Fig. 3, the polarization resistance (R_p), which corresponds to the diameter of Nyquist plot, includes charge transfer resistance (R_ct), diffuse layer resistance (R_d), film resistance (R_f) and all accumulated species at metal/solution interface (R_a). Therefore, in the present study, the difference in real impedance at lower and higher frequencies is considered as the polarisation resistance (R_p = R_ct + R_d + R_f + R_a) (Lorenz and Mansfeld, 1986; Solmaz et al., 2008; Sam et al., 2010).

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Figure 3

Equivalent electrical circuit diagram for EIS results of LAS in inhibited solutions, R_ct, charge transfer resistance; C_dl, double layer capacitance; R_s, solution resistance; R_d, diffuse layer resistance; C_f, film capacitance; R_f, film resistance.

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Table 3 shows the impedance parameters, the double layer capacitance, C_dl and the polarization resistance, R_p. Analysis of the collected data listed in this table indicates the following:

1. The polarization resistance (R_p) increases and the double layer capacitance (C_dl) decreases as the inhibitor concentration is increased. The increase in charge transfer resistance may be the result of decreasing film capacitance due to increase in the surface coverage by the inhibitor molecules.

2. The values of (R_p) decrease and those of (C_dl) increase with increasing temperature.

Table 3 Electrochemical kinetic parameters obtained from EIS technique for LAS in 0.5 M sulfamic acid with various concentrations of Cys at different temperatures.

Temp (°C)	Cys Conc (M)	Cdl (μF cm−2)	Rp (Ω cm2)	IE%	θ
25	0.00	826.7	33.63	0.00	0.000
	0.0005	511.1	53.48	37.12	0.371
	0.001	320.3	91.91	63.41	0.634
	0.002	147.2	178.1	81.12	0.811
	0.004	67.5	427.4	92.13	0.921
35	0.00	874.1	23.01	0.00	0.000
	0.0005	626.9	32.83	29.91	0.299
	0.001	428.7	49.85	53.84	0.538
	0.002	221.5	85.69	73.15	0.731
	0.004	121.6	182.2	87.37	0.874
45	0.00	967.8	17.44	0.00	0.000
	0.0005	718.3	23.12	24.57	0.246
	0.001	569.8	30.90	43.56	0.436
	0.002	345.9	46.77	62.71	0.627
	0.004	210.5	92.45	81.14	0.811
55	0.00	1113.7	13.00	0.00	0.000
	0.0005	900.1	15.82	17.83	0.178
	0.001	709.8	20.51	36.62	0.366
	0.002	566.3	26.42	50.79	0.508
	0.004	278.6	50.22	74.11	0.741

The above results can be explained on the basis that adsorption of the inhibitor species and formation of a physical protective film that retard the charge transfer process and, therefore, increase the value of (R_p) and so the corrosion reactions were inhibited. Moreover, the adsorbed inhibitor species decrease the electrical capacity of the electrical double layer at the electrode/solution interface and, therefore, decrease the values of (C_dl) (Mansfield, 1987).

Since the electrochemical theory assumes that the reciprocal of polarization resistance (1/R_p) is directly proportional to the corrosion rate, the inhibition efficiency (IE%) was calculated from (R_p) values using the following equation:

(2)

$$\text{IE}\%=\left[1-\left(\frac{R_{\text{p}}^o}{R_{\text{p}}}\right)\right]\times 100$$ where $R_{\text{p}}^{\text{o}}$ and R_p are the polarization resistance values in the absence and presence of inhibitor, respectively.

The calculated values of the inhibition efficiency (IE%) for different concentrations of Cys at different temperatures are also included in Table 3. The values of IE% show that the corrosion process depends on both factors, the inhibitor concentration and the solution temperature. The IE% increases with the increase of the inhibitor concentration, but it decreases with the increase of solution temperature.

3.1.3

3.1.3 Apparent activation energy

Further study of the effect of temperature on the inhibition efficiency was carried out. Corrosion activation energies (E_a) in the absence and presence of the inhibitor were calculated using the following Arrhenius equation:

(3)

$$\log (\text{Corr}.\text{Rate})=\frac{-E_{\text{a}}}{2.303\mathit{RT}}+A$$ where E_a is the apparent activation energy, R is the universal gas constant, T is the absolute temperature, and A is the Arrhenius pre-exponential factor.

A plot of logarithm of the corrosion rate of LAS obtained from the three techniques versus 1/T gives straight lines their slopes are the −E_a/2R. Fig. 4 is an example of Arrhenius curves obtained from EIS technique.

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Figure 4

Arrhenius plots for LAS in 0.5 M sulfamic without and with various concentrations of Cys associated with EIS measurements.

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In the literature, the lower activation energy value of corrosion process in the presence of inhibitor rather than the absence of inhibitor is attributed to its chemisorption, while it is found to be opposite in the case with physical adsorption (Mora-Mendoza and Turgoose, 2001; Jovancicevic et al., 1999; Hirozawa, 1995).

The calculated values of E_a extracted from the two techniques (EFM and EIS) were given in Table 4. The analysis of E_a values shows that an increase in corrosion activation energy in the presence of inhibitor compared to its absence with a decrease in inhibition efficiency with the rise in temperature indicates that the type of adsorption of Cys on the steel surface in 0.5 M sulfamic acid solutions belongs to physical adsorption.

Table 4 Activation energies of Cys using the obtained data from EFM and EIS techniques.

Cys conc (M)	Ea (kJ mol−1)
	EFM	EIS
0.00	30.16	25.50
0.0005	37.09	32.64
0.001	43.78	40.63
0.002	49.99	51.56
0.004	64.57	57.92

3.2 Effect of solution stirring on the inhibition efficiency

The corrosion inhibition of Cys for LAS in aerated stirring 0.5 M sulfamic acid solution at stirring speeds (RPM) 200, 400, 800 and 1200 and at 25 °C were carried out using EFM and EIS techniques. The EFM and EIS diagrams for LAS in aerated stirring 0.5 M sulfamic acid in the absence and presence of 0.004 M Cys at different stirring velocities were collected. Fig. 5 is an example that represents the EFM intermodulation spectra of LAS in aerated stirring 0.5 M sulfamic acid containing 0.004 M Cys at 800 RPM.

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Figure 5

Intermodulation spectrum for LAS in 0.5 M sulfamic in presence of 0.004 M Cys at 800 RPM.

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The obtained results from EFM and EIS techniques are presented in Table 5. This table demonstrates the effect of stirring speed on the inhibition efficiency of 0.06 M Cys.

Inspections of the produced results of both techniques reveal the following:

1. The results obtained from the EIS and EFM support that the inhibiting effect of Cys is strongly dependent on the solution movement (hydrodynamic conditions). As the stirring velocity increases, the efficiency of the inhibitor decreases.

2. At 0.06 M of Cys concentration, the corrosion current density (I_corr) increases while (R_p) decreases with increasing the stirring speed as a result of removal of adsorbed inhibitor species from the metal surface which retards the effectiveness of the inhibitor and thus significantly enhances the corrosion rate (Lebrini et al., 2007; Oguzie et al., 2004).

3. The corrosion inhibition efficiency under stagnant conditions is higher than that of the inhibition performance under stirring conditions.

3.3 Adsorption Isotherms

Adsorption isotherm was studied in order to get more information about the interaction between the adsorbed organic inhibitor species and the metal surface and therefore, the mechanism of corrosion inhibition (Hirozawa, 1995; Lebrini et al., 2007).

Two main types of interactions can classify the adsorption of organic compounds: physical adsorption (physisorption) and chemical adsorption (chemisorption). They are influenced by the nature of the charge of the metal surface, the chemical structure of the inhibitor and the type of electrolyte.

Surface coverage degree (θ) is an important parameter for determining the inhibitor adsorption characteristics. The surface coverage degree (θ), as a function of inhibitor concentration was calculated (θ = IE%/100). Surface coverage values for different concentrations of Cys in 0.5 M sulfamic acid solution at different temperatures are given in Tables 2 and 3 using the obtained data from the two techniques.

The surface coverage (θ) results were fitted to different isotherm type models to represent the adsorption behavior of Cys on the steel surface. The best-fitted straight line was obtained for the plot of the surface coverage (θ) versus logarithm of inhibitor concentration (C_inh). This suggests that the adsorption of Cys at the steel surface in sulfamic acid solutions has been found to obey the Temkin adsorption isotherm (Oguzie et al., 2004; Duong, 1980).

Fig. 6 is an example of Temkin adsorption isotherm for various concentrations of Cys at different temperatures using the data obtained from EFM measurements.

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Figure 6

Temkin isotherm plots for LAS in 0.5 M sulfamic in the presence of various concentrations of Cys at different temperatures (data obtained from EFM technique).

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The equilibrium constant (K_ads) was calculated from the intercepts and slopes of the straight lines of Temkin isotherm curves.

The equilibrium constant of adsorption (K_ads) is related to the standard free energy of adsorption, ( $-\mathrm{\Delta }{G}_{\text{ads}}^0$ ) via the following Eq. (4):

The equilibrium constant (K_ads) and the standard free energy of adsorption, ( $-\mathrm{\Delta }{G}_{\text{ads}}^0$ ) values obtained from the data of the three techniques are listed in Table 6.

As shown in Table 6, the values of (K_ads) decrease with increasing the temperature, which confirms the suggestion that Cys is physically adsorbed on the surface of LAS (Keera and Deyab, 2005; Abd El Rehim et al., 2002; El Azhar et al., 2002).

The values of the standard free energy of adsorption, ( $-\mathrm{\Delta }{G}_{\text{ads}}^0$ ) acquire a negative sign emphasis the spontaneity of the adsorption process and stability of the adsorbed layer on the LAS surface which is accompanied with a high efficient adsorption of Cys (El-Etre, 2006).

It was reported earlier that the values of ( $-\mathrm{\Delta }{G}_{\text{ads}}^0<40$  kJ mol⁻¹) as an indicator of physical adsorption (electrostatic interactions between the inhibitor species and the charged metal surface). While values of ( $-\mathrm{\Delta }{G}_{\text{ads}}^0\ge 40$  kJ mol⁻¹) are related to chemical adsorption (sharing or transfer of electrons from organic molecules to the metal surface to form a coordinate bond (Ashaasi-Sorkhabi et al., 2004; Donahue and Nobe, 1965; Khamis et al., 1991; Bouklah et al., 2005; Scendo, 2007; Bouklah et al., 2006).

Therefore, the calculated values of ( $-\mathrm{\Delta }{G}_{\text{ads}}^0$ ) listed in Table 6 indicates that the adsorption mechanism of Cys at LAS surface in 0.5 M sulfamic acid solutions is typical of physisorption.

3.4 Inhibition mechanism

The above results emphasise that the Cys acts as inhibitor for LAS in 0.5 M sulfamic acid solutions. The corrosion inhibition of Cys is due to their physical adsorption and formation of protective film on the steel surface.

The pervious investigation reports that the steel surface in aqueous acid solutions is positively charged (Lagrenée et al., 2002; Solmaz et al., 2008; Wahdan et al., 2002). Moreover, in aqueous acid solutions, the amino acids exist either as neutral molecules or in the form of cations (Amin et al., 2009; Bockris and Yang, 1991; Abdallah and Megahed, 1995).

Therefore, these amino acids may be adsorbed on the positively charged metal surface in the form of neutral molecules involving the displacement of water molecules from the metal surface and sharing electrons between the nitrogen atoms and the metal surface (Bentiss et al., 1999).

Furthermore, adsorption can occur via the already adsorbed sulfamate anions at the positively charged metal surface. The adsorbed sulfamate anions at the metal surface make a negatively charged double layer and consequently it results in an increase in the adsorption capability of the protonated amino acids (Wahdan et al., 2002; Larabi et al., 2004).