Electrochemical and Quantum Chemical Studies of 1, 2, 3-Benzotriazole as Inhibitor for Copper and Steel in Simulated Tap Water

Abstract

The corrosion inhibition effect of benzotriazole on copper and steel in simulated tap water was investigated by using potentiodynamic polarization, electrochemical impedance spectroscopy and a quantum chemistry analysis. The corrosion inhibition efficiency for copper and steel was accordingly increased with an increasing of the benzotriazole concentrations. However, the electrochemical test showed a significant difference between the inhibition efficiency of copper and steel. From molecular theory and the adsorption isotherm, the difference between the d-orbital structure of copper and steel affects the adsorption stability of benzotriazole on the metal surface. This postulation is well correlated with the results of the quantum chemical analysis.

1. Introduction

Water that is supplied from underground and surface water is subject to a number of chemical and physical treatments namely water treatment, for water quality and protection of facility. The treated water is distributed to various places for applications such as landscape irrigation, sanitary services, residential customers, industry plants, and districted heating systems^1,2). The prevention of corrosion is an important consideration for water treatment because the corrosion of the water distribution system problems such as decline of water quality and failure of distribution system parts in various fields.

One of the typical issues due to the corrosion of the water distribution system is the colored water phenomena. The corrosion products change the water color, leading to undesirable health and aesthetic aspects, as follows^3,4):

• (1)   Green or blue water: Usually caused by the corrosion of copper plumbing. If corrosion is occurring, dripping water will leave a bluish-green stain on porcelain fixtures. Certain metals that can get into drinking water from corrosion such as copper or lead, may pose a health concern.
• (2)   Brown, red, orange or yellow water: Usually caused by iron in the galvanized iron, steel, or cast iron pipes of a home or business, or the water main can cause rusty water. While it is unpleasant and potentially damaging to clothes and fixtures, iron in drinking water is not a human health concern. However, excessive iron ions in water can lead to the propagation of bacteria, so such excessive levels need to be prevented.

Also, the failure of a distribution system can be caused by corrosion. The hydraulic pressure from flowing water applies stress to parts of the distribution system and the weak point caused by corrosion such as pitting, roughness and notches can be an initiation point of cracking⁵⁾. The corrosion problems of the water distribution system are illustrated in Fig. 1. Especially, these corrosion problems have occurred mainly in pipelines that consist of metallic materials such as copper and steel.

Fig. 1

Schematic of water distribution system and corrosion problems in pipeline.

In industry, various methods have been used to prevent corrosion problems. The use of an inhibitor in water treatment is a general method of corrosion prevention for the water pipeline system. Among the corrosion inhibitors, the 1, 2, 3-benzotriazole (BTAH, C₆H₅N₃) shown in Fig. 2 has been reported as an effective corrosion inhibitor for steel and copper^6–9) in various environments. BTAH is absorbed on the metal surface and forms the BTA-metal complex as a protective barrier on the metal surface in various environments. The donor-acceptor reaction between the π-electrons of an inhibitor and the d-orbital of metal has a close relation with the inhibition mechanism of BTAH because the metal-N bond in the BTA-metal complex is explained by overlapping of the metal sp-hybrid atomic orbital with the N-hybridized lone-pair atomic orbital^10,11). The understanding of the electron or the orbital structure of the inhibitor and the materials is therefore important for the interpretation of the inhibition efficiency. Recently, computational quantum chemical research has reported the interpretation of the electron interactions at the molecular level^12–14). Among the electronic parameters, the popular parameters are the highest-occupied (ε_HOMO) and the lowest-unoccupied (ε_LUMO) molecular orbitals, dipole moment (μ), HOMO-LUMO gap, electronegativity (χ) and chemical hardness (η).

Fig. 2

Molecular structure of BTAH.

The corrosion inhibitor efficiency of BTAH has been studied with respect to various materials and environments. Although BTAH is known as a corrosion inhibitor for copper, it can cause the negative effect on the other metals in specific environments¹⁵⁾. Therefore, a comparison between the inhibition efficiency of BTAH regarding the different metals that can be used in the same environment is necessary, as most of the corrosion researches have considered the inhibition of one metal beside the different metal in same environment. The aim of this study is the comparison between the corrosion inhibitions of BTAH between the copper and steel in simulated tap water. Also, the mechanisms of the corrosion inhibition of BTAH for copper and steel are evaluated using the correlation between the adsorption isotherm and the quantum chemical analysis.

2. Experimental

2.1 Specimens and solution preparation

All of the electrochemical measurements and immersion tests were performed on steel and copper panels, the chemical compositions of which are given in Table 1. The BTAH used in this work was obtained from Sigma-Aldrich. The specimens were abraded with silicon carbide (SiC) paper from 220 to 600 grit size, rinsed with ethanol, distilled water and finally dried with pure nitrogen gas. The specimens are in the shape of a square (1 cm × 1 cm) embedded in the epoxy resin of polytetrafluoroethylene (PTFE). Table 2 gives the chemical composition of the simulated tap water. To adapt the chemical composition of the tested simulated tap water, NaCl, Mg(OH)₂, CaCO₃ and H₂SO₄ were used and the pH was controlled by 0.1 M of NaOH solution. The test solution is made based on average contents of several tap water in Korea. The pH level is satisfied the Environmental Protection Agency (EPA) recommendation (pH 6.5～8.5). The prepared specimens were exposed to samples of the simulated tap water containing 0 ppm, 400 ppm and 1000 ppm (weight/volume) BTAH at 25℃ without deaeration.

Table 1 Chemical composition of steel and copper specimens (mass%).

Specimen	C	Mn	S	P	Fe	Cu
Steel	0.15	0.60	0.050	0.050	Bal.	-
Copper	-	-	-	0.02	-	Bal.

Table 2 Chemical composition of simulated tap water (mg/L = ppm).

pH	Cl−	Mg2+	Ca2+	SO42−	BTAH
8.5	15.8	12.9	51.7	13.2	0, 400, 1000

2.2 Electrochemical measurements

To evaluate the corrosion behavior and properties according to the BTAH concentration, electrochemical measurements were carried out. Potentiodynamic polarization tests were conducted in a three-electrode cell with a specimen working electrode, two pure graphite counter electrodes and a saturated calomel electrode (SCE) coupled with Luggin capillary. Before the electrochemical measurements, the working electrode was immersed in the test solution for 6 h until the stable open-circuit potential (OCP) was attained, which was taken as the corrosion potential (E_corr).

For the polarization experiments, potentials from −0.25 V vs. E_corr to 1 V were applied and sweep rate was fixed to 0.166 mV/s. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range from 100 kHz to 1 mHz at OCP with sinusoidal amplitude of 10 mV. All of the electrochemical measurements were accomplished with the Princeton Applied Research PARSTAT 2263. The impedance plots were interpreted on the basis of an equivalent circuit according to a suitable fitting procedure using the ZsimpWin software.

2.3 Immersion test

To clarify the influence of the BTAH on the corrosion behavior of steel and copper, the immersion test was carried out in the simulated tap water with and without the BTAH. Measurements of the specimen weight were performed before the immersion test. After the immersion test, the weight of the specimens which remove the corrosion products was measured. The corrosion rate (CR, μm/yr) was calculated as follows:   

\[CR = \frac{87600W}{At\rho}\](1)

where W is the weight loss, A is the original surface area exposed to the corrosive media, t is the exposure time and ρ is the density⁵⁾. After the immersion test, the surface of the specimens was investigated by optical microscope.

2.4 Quantum chemical analysis

The electronic properties of BTAH were evaluated for the interpretation of the adsorption mechanism on the metal surface in simulated tap water. The B3LYP¹⁶⁾ nonlocal exchange correlation functional and the local Gaussian-type-orbital set were applied for calculations. Depending on the convergency examination, at least triple-zata basis set disputed with the polarization and the diffusion function should be used to obtain converged results. Thus, the 6-311++G(d, p) basis set was used. All of the calculations were performed using the GAUSSIAN-09 program and the molecular graphics were made using the Gauss-View 5.0 graphical package.

3. Results and Discussion

3.1 Potentiodynamic polarization measurements

Figure 3 shows the potentiodynamic polarization curves of (a) steel, (b) steel in magnifying anodic curve region and (c) copper specimens in simulated tap water as a function of the BTAH concentration at 25℃. The corrosion potential shifted toward more positive values in all of the polarization curves with the increase of BTAH concentrations. It may be due to the adsorption of BTAH on the metal surface, which made the surface into a more noble state. To clarify the tendency of anodic polarization curve in the case of steel according to the concentration of the BTAH, the potential region of polarization curves in the case of steel is magnified in the potential range from −0.8 V_SCE to −0.2 V_SCE as shown in Fig. 3(b). In Fig. 3(b), the anodic curve is shifted to the direction of the lower current density according to the increase of the BTAH concentration, meaning that the BTAH decrease the anodic reaction of the steel. Consequently, it is indicated the BTAH on the steel acts as an anodic-type inhibitor. On the other hand, in Fig. 3(c), the BTAH showed a mixed-type inhibitor on copper since the anodic and cathodic reactions were affected in the simulated tap water containing BTAH¹⁷⁾. These results indicate that the inhibition reaction of BTAH can be different depending on the type of metals.

Fig. 3

Potentiodynamic polarization curves in simulated tap water as a function of BTAH concentration: (a) steel, (b) steel in magnifying anodic curve region and (c) copper.

The corrosion behavior of copper and steel in simulated tap water containing BTAH were different. The steel showed a uniform corrosion behavior under all conditions. There is no breakdown potential (E_b), and the protective film generated by the adsorption of BTAH on the steel surface did not sufficiently obstruct the electron transfer reactions. In the case of copper, however, the E_b which indicated the passive property and breakdown of passive films⁵⁾ was appeared and increased with the increasing of the BTAH concentration. In previous studies on the mechanism of the BTAH inhibition of the copper^18,19), the layer formed by the adsorption of BTAH or complex formation acts as a physical barrier to the inward diffusion of aggressive ions. Due to this barrier, the passive behavior is indicated in the polarization curves. If the potential across this barrier exceeds a certain value, the barrier breaks down and corrosion activated. This potential is called breakdown potential. Thus, the stability of physical barrier on the surface can be evaluated by positive value of E_b. The current density of polarization curves is decreased and E_b is increased according to the increase of the BTAH concentration in the case of copper, indicating that the protective property of the barrier formed by the BTAH is increased with the increase of the BTAH concentration. This difference between the corrosion behaviors of copper and steel from the addition of BTAH in simulated tap water indicates that the bonding strength or interaction energy among copper, steel and BTAH are dissimilar.

Table 3 shows the electrochemical parameters that were obtained from the polarization curves for the copper and steel electrodes as a function of BTAH concentrations in simulated tap water. The i_corr values can be calculated as the inhibition efficiency (IE%) using the following equation:   

\[IE\% = 100 \times \left[ \frac{\left( i_{corr}^0 - i_{corr} \right)}{i_{corr}^0} \right]\](2)

where $i_{corr}^0$ and i_corr are the corrosion current densities in the absence and the presence of the inhibitor in the solution, respectively. The increase of the IE value was shown under all of the conditions, indicating that the BTAH depressed corrosion of copper and steel in simulated tap water. However, the degree of the IE at the same concentration of BTAH showed a huge difference between the copper and steel specimens. It inferred that the adsorption strength or the stability of BTAH to copper and steel are different.

Table 3 Electrochemical parameters from polarization measurements on the copper and steel specimens in simulated tap water at 25℃ as a function of BTAH concentration.

Electrode	CBTAH (ppm)	Ecorr (mVSCE)	icorr (μA/cm2)	Eb (mVSCE)	IE%
Carbon Steel	0	−613	26.7	-	-
	200	−516	22.3	-	16.4
	400	−491	19.1	-	28.4
	600	−469	14.6	-	45.3
	1000	−427	10.5	-	60.6
Copper	0	−42	0.379	-	-
	200	−22	0.047	304	87.5
	400	−8	0.018	525	95.2
	600	54	0.004	672	98.9
`	1000	104	0.001	779	99.7

3.2 Electrochemical impedance spectroscopy (EIS)

Figure 4 shows the Nyquist plots that were obtained for (a) steel and (b) copper in the simulated tap water with different BTAH concentrations. The roughness and nonhomogeneity of the specimen surface can affect the variance of the pure capacitive loops^20–22). Figure 4 shows that the increase of the BTAH concentration increases the radius of the capacitive loops as well as the inhibition efficiencies in all cases. However, the amount of radius increase is larger in the copper case than steel. BTAH is therefore more effective for copper than carbon steel in simulated tap water.

Fig. 4

Nyquist plots in simulated tap water as a function of BTAH concentrations: (a) steel, (b) copper.

The equivalent circuits in Figs. 5(a) and (b) fit well with these EIS results. In the case of steel under none and lower BTAH concentrations (0, 200 and 400 ppm), the Nyquist plots shows the two-time constant behavior. It may be due to the insufficient inhibition of the BTAH on the steel surface. The insufficient inhibition formed the defect such as pore in the physical barrier, so the metal substrate can be exposed to the corrosion²³⁾. It was therefore applied two-time equivalent circuit as shown in Fig. 5(a). On the other hand, the other cases shows the one time constant behavior, which indicated that the physical barrier formed by the BTAH prevents the aggressive ion from the metal substrate^20,24–27). Thus, the one-time circuit was applied in the other cases. The equivalent circuits consist of the following elements: R_s is the solution resistance, R_barrier is the barrier resistance from the formation of an ionic conduction path through defect or pore in the physical barrier, C_barrier (CPE1) is the barrier capacitance, R_ct is the charge transfer resistance, C_dl (CPE2) is the double-layer capacitance generated by the electrical double layer capacitance at the water/substrate interface. The constant phase element (CPE) is applied in this equivalent circuit instead of perfect capacitor to achieve a more accurate fit²⁸⁾. The impedance of the CPE is expressed as:   

\[Z_{CPE} = \frac{1}{Y_0(j\omega)^n}\](3)

where Y₀ is the magnitude of the CPE, j is the imaginary number ($j^2=-1$), α is the phase angle of the CPE and $n=\alpha/(\pi/2)$. The parameter n usually lies between 0.50 and 1.0 and the CPE expresses the ideal capacitor when $n=1$. The parameter n is often referred to as frequency dispersion due to the surface states like nonhomogeneity^28–31) and roughness^32,33).

Fig. 5

Equivalent circuit for fitting the EIS result for steel and copper as a function of BTAH concentration in simulated tap water at 25℃: (a) two-time circuit for steel in simulated tap water contained 0, 200 and 400 ppm BTAH, (b) one-time circuit for the others.

Some parameters extracted from the EIS data are listed in Table 4. The inhibition efficiency and double layer capacitance are calculated using the following equations^22,34):   

\[IE\% = 100 \times \left( \frac{R_{total} - R_{total}^0}{R_{total}} \right)\](4)

  

\[C = Y_0(2\pi f_{max})^{n-1}\](5)

In eq. (4), R_total and $R_{total}^0$ are the total resistance (R_total = R_barrier + R_ct or R_barrier) in the presence and absence of the inhibitor, respectively. In eq. (5), f_max is the frequency at which the imaginary component of the impedance reaches the maximum value.

Table 4 Electrochemical parameters from EIS measurements on the steel and copper electrodes in simulated tap water at 25℃ as a function of BTAH concentration under open circuit condition.

Electrode	CBTAH (ppm)	Rs (Ω-cm2)	Rbarrier (Ω-cm2)	CPE1		Rct (Ω-cm2)	CPE2		IE%
				Cbarrier (μF/cm2)	n		Cdl (μF/cm2)	n
Carbon Steel	0	79.4	1.41 × 103	9.68 × 10−4	0.781	2.09 × 102	3.25 × 10−3	0.731	-
	200	86.1	1.69 × 103	8.12 × 10−4	0.753	2.36 × 102	2.71 × 10−3	0.711	15.7
	400	70.3	1.82 × 103	6.86 × 10−4	0.762	5.52 × 102	2.05 × 10−3	0.689	31.5
	600	98.8	3.16 × 103	4.23 × 10−4	0.721	-	-	-	48.6
	1000	96.2	5.76 × 103	1.73 × 10−4	0.750	-	-	-	71.8
Copper	0	95.7	8.46 × 104	8.89 × 10−5	0.591	-	-	-	-
	200	111.5	9.93 × 105	3.88 × 10−6	0.909	-	-	-	91.4
	400	119.3	2.62 × 106	2.16 × 10−6	0.946	-	-	-	96.8
	600	138.2	8.24 × 107	9.69 × 10−7	0.977	-	-	-	99.8
	1000	155.1	1.17 × 108	2.80 × 10−7	0.993	-	-	-	99.9

Figure 6 shows the change of EIS parameters (R_total and C_barrier) as a function of the BTAH concentration. Figure 6(a) shows that the R_total obtained from the copper and steel electrodes is increased with the increase of the BTAH concentration. Due to the increase of the R_total, the IE% of the two electrodes was also increased with the increase of the BTAH concentration. However, the increase of the $\underline{R}_{\rm total}$ and IE% was significantly different according to the kind of electrodes. The results are in a good agreement with the potentiodynamic polarization results. On the contrary to the R_total, the value of C_barrier decreased with the increase of BTAH concentration. The C can be expressed in accordance with Helmholtz model as the following equation^35–37):   

\[C = \frac{\varepsilon \varepsilon_0 A}{d}\](6)

where $\varepsilon$ is the dielectric constant of protective layer, $\varepsilon_0$ the vacuum permittivity, d the thickness of the physical barrier and A is the effective surface area of electrode. The $\varepsilon_0$ and A are not variable value so that the increase of C_barrier is related to the increase of physical barrier thickness and decrease of dielectric constant. In Fig. 6(b), the decrease of C_barrier is indicated according to the increase of the BTAH concentration, meaning the increase of physical barrier thickness is increased. Also, the physical barrier that is caused by adsorption and complex formation of the BTAH on the copper and steel surfaces could change the dielectric constant because the adsorption energy and the complex formation (BTA-Fe and BTA-Cu) are different.

Fig. 6

The change of total resistance and barrier capacitance of the steel and copper as a function of the BTAH concentration.

Another parameter extracted from the EIS data is the n implying various surface conditions such as heterogeneity, roughness, inhibitor adsorption and porous layer³⁸⁾. In the case of steel, the n is not changed according to the increase of the BTAH concentration. It means that the stability of the physical barrier on the steel surface, which is caused by the BTAH, was not significantly influenced the BTAH concentration. On the other hand, in the case of the copper electrode, the n is increased with the increase of the BTAH concentration. It indicated that the physical barrier by the BTAH on the copper electrode is more stable according to the increase of the BTAH concentration and forms the homogeneous barrier unlike the steel electrode.

3.3 Immersion test

Corrosion rates of the copper and steel calculated from the immersion test are shown in Fig. 7. The corrosion rate of the steel is decreased with the increase of the BTAH concentration. In the case of copper, the corrosion rate is also decreased with the increase of the BTAH concentration. However, the corrosion rate of the copper is significantly decreased in the small concentration of the BTAH (200 ppm), whereas that of the steel is not. It is the similar tendency with the results of the electrochemical tests, i.e., that the BTAH is more effective to the inhibition of copper than steel.

Fig. 7

Corrosion rate of the steel and copper calculated from the immersion test.

After the immersion test, the steel and copper surfaces in the simulated tap water as a function of the BTAH concentration were observed using the optical microscope as shown in Fig. 8. In the case of the solution contained the 0 ppm BTAH, the steel and copper were uniformly corroded. On the other hand, the corrosion behavior was different in solution contained the BTAH. The localized corrosion was observed on the steel surface regardless of the BTAH concentration, whereas the copper was practically not corroded. The result suggests that the BTAH effectively prevents the corrosion of copper, but causes the localized corrosion on the steel surface. The localized corrosion on the steel surface is caused by the unstable physical barrier on the surface, which can generate the localized corrosion^15,39). Also, the generation of the localized corrosion on the steel surface does not depend on the concentration of the BTAH. It means that an insufficient reaction between the BTAH and the steel surface is occurred irrespective of the concentration despite the decrease of the totally corrosion reaction.

Fig. 8

Optical microscope images of the copper and steel surface after immersion test.

3.4 Adsorption mechanism

The adsorption of BTAH on the copper and steel surfaces was analyzed using the calculation of the adsorption isotherm. The values of the surface coverage (θ) for the copper and steel electrodes in the simulated tap water containing different BTAH concentrations were calculated according to the following equations⁴⁰⁾:   

\[\theta = \left( \frac{i_{corr}^0 - i_{corr}}{i_{corr}^0} \right)\](7)

The correlation between θ and the inhibitor concentration (C, mol L⁻¹) in the solution can be represented by the Langmuir adsorption isotherm, as follows^40,41):   

\[\theta = \frac{KC}{1 + KC}\](8)

Equation (8) is rearranged as follows:   

\[\frac{C}{\theta} = \frac{1}{K} + C\](9)

where K is the constant of adsorption. The plots between the C/θ and C values of steel and copper are shown in Fig. 9.

Fig. 9

C/θ vs. C plots of (a) steel and (b) copper in simulated tap water as a function of BTAH concentration.

The plot of C/θ vs. the BTAH concentration showed a linear correlation of the slope close to unity. This tendency indicates that the adsorption of BTAH on the steel and copper surfaces follows the Langmuir adsorption isotherm. The obtained values of K are as follows: K_Fe = 123.62 and K_Cu = 3294.9 L mol⁻¹.

The K value is related to the standard free energy of adsorption, $\Delta G_{ads}^0$ according to the following equation^41,42):   

\[K = \frac{1}{55.5} \exp \left( \frac{-\Delta G_{ads}^0}{RT} \right)\](10)

where 55.5 is the concentration of water in the mol concentration (M) of the solution, R is the gas constant and T is the absolute temperature.

The $\Delta G_{ads}^0$ values on steel and copper were calculated as follows: $\Delta G_{ads}^0$ of steel = −21.89 kJ mol⁻¹ and $\Delta G_{ads}^0$ of copper = −30.02 kJ mol⁻¹. The higher value of K and negative value of $\Delta G_{ads}^0$ indicate the spontaneous adsorption of the inhibitor and are characteristics of a strong interaction with the metal surface. Thus, the values of K and $\Delta G_{ads}^0$ between the BTAH and copper indicate that the BTAH is more powerfully absorbed on the copper than the steel.

A chemisorption is a strong interaction in which the electronic states of the inhibitor molecules hybridize with the metal electron state^43–49). The interaction between BTAH and metal surface is associated with the π-electron of BTAH and the d-orbital of metal surface, which could be regarded as the chemisorptions. Thus, the d-orbital difference between steel and copper would affect the inhibition efficiency in this study.

The state of BTAH on the copper and steel surfaces can be the adsorption and the complex formation according to the following eqs. (11) and (12), respectively^50,51):   

\[ \begin{split} & n({\rm BTAH})_{\rm ads} + n{\rm Fe}\ {\rm or}\ n{\rm Cu} \\ &\quad \leftrightarrow ({\rm Fe} \mbox{-} {\rm BTA}\ {\rm or}\ {\rm Cu} \mbox{-} {\rm BTA})_n + n{\rm H}^+ + n{\rm e}^- \end{split} \](11)

  

\[ \begin{split} & 4({\rm BTAH})_{\rm ads} + 4{\rm Fe}\ {\rm or}\ 4{\rm Cu} + {\rm O}_2 \\ &\quad \leftrightarrow 4({\rm Fe} \mbox{-} {\rm BTA}\ {\rm or}\ {\rm Cu} \mbox{-} {\rm BTA}) + 2{\rm H}_2{\rm O} \end{split} \](12)

According to the above equations, the BTAH can exist as the adsorption layer and the complex formation on the surface. Also, the BTAH is dominantly present at the BTA⁻ ion in the simulated synthetic tap water due to the pH of 8.5⁵²⁾ so that the complex formation can be generated directly. The protective layer formed by BTAH was mainly consisted of the complex formation. The formation of the metal complex is referred to as the orbital interaction^53–56). When BTAH is close to the metal surface, the π-orbital of BTAH can be hybridized with the d-orbital of metal forming the π-d bonding. The strength of the π-d bond is related to the quantity of electrons that are sharing in the complex formation such as the single, double and triple bonding in molecules. The numbers of the electron filled d-orbital in copper and iron are different, i.e., copper and iron are five and one, respectively. Therefore, the copper electrode, which showed fully filled electrons in the d-orbital, is capable of a greater bonding strength in the complex formation than the steel electrode.

3.5 Quantum analysis

To investigate the difference between the inhibition efficiency and behaviors of BTAH for copper and steel, the following electronic parameters that are listed in Tables 5 and 6 were considered. The electronic parameters include the highest occupied (ε_HOMO) and lowest unoccupied (ε_LUMO) molecular orbital chemical hardness (η), electronegativity (χ), an estimate of the number of electrons transferred from the molecule to metal (ΔN) and an approximate calculation of the associated molecule-metal interaction energy (ΔE). The inhibitor effectiveness is often referred according to these parameters⁵⁷⁾. In addition, the protective barrier is generated by two processes, as follows: the complex formation from the adsorption of BTAH and the direct reaction with the BTA⁻ and metals. Although the BTA⁻ mainly exists, the BTAH can affect the formation of the protective barrier. Therefore, the molecular state is divided into BTAH and BTA⁻ in the quantum analysis. The geometry models of BTAH and BTA⁻ that were used in this study are shown in Fig. 10.

Table 5 Electronic parameters of BTAH: orbital energies of HOMO (ε_HOMO) and LUMO (ε_LUMO), chemical hardness (η), electronegativity (χ).

	εHOMO (eV)	εLUMO (eV)	η (eV)	χ (eV)
BTAH	−7.201	−1.759	2.738	4.48
BTA−	−1.436	2.439	1.937	−0.50

Table 6 Parameters of work-function (Φ), the number of electron transferred from molecule to metal (ΔN) and the molecule-metal interaction energy (ΔE) in copper and steel.

Electrode	Φ (eV)	Molecular state	ΔN (eV)	ΔE (eV)
Copper	4.7	BTAH	0.040	−4.419 × 10−3
		BTA−	1.343	3.4919
Carbon steel	4.5	BTAH	0.004	−3.652 × 10−5
		BTA−	1.291	3.229

Fig. 10

Geometry models of (a) BTAH and (b) BTA⁻ applied in quantum analysis.

In Table 5, the orbital chemical hardness and electronegativity were obtained using the following equations⁵⁸⁾:   

\[\eta \approx \frac{1}{2} (\varepsilon_{LUMO} - \varepsilon_{HOMO}) = \frac{1}{2} \Delta \varepsilon\](13)

  

\[\chi \approx -\frac{1}{2} (\varepsilon_{HOMO} + \varepsilon_{LUMO})\](14)

where Δε is the HOMO-LUMO gap. ΔN and ΔE are listed in Table 6 and were obtained using the following equations^58,59):   

\[\Delta N = \frac{\chi_{metal} - \chi_{mol}}{2(\eta_{metal} + \eta_{mol})} = \frac{\varPhi - \chi_{mol}}{2\eta_{mol}}\](15)

  

\[\Delta E = \frac{(\chi_{metal} - \chi_{mol})^2}{4(\eta_{metal} + \eta_{mol})} = -\frac{(\varPhi - \chi_{mol})^2}{4\eta_{mol}}\](16)

In the above two equations, the electronegativity of the metal surface (χ_metal) is described according to its work-function (Φ), while its chemical hardness is neglected because the chemical hardness of a bulk metal is inversely proportional to its density of states at the Fermi level⁶⁰⁾ which is an exceedingly small value. The work-function of copper and steel were used in Ref. 61).

The difference of the electronegativity between the metal surface and the BTAH molecule and BTA⁻ ion determines the flow direction of the electrons from (to) the molecule to (from) metal. As listed in Tables 5 and 6, the electronegativity of BTAH and BTA⁻ is smaller than the copper and steel work-functions, which implies that the donations of BTAH and BTA⁻ charge to metal surface, namely with positive ΔN values. Especially, the difference between the electronegativity and ΔN of BTA⁻ is remarkably higher than the difference between those of BTAH, which is caused by the negatively charged state, and this indicates an easier interaction with metal. The ΔN of copper in both cases (BTAH and BTA⁻) is larger than that of steel, meaning that the electron transfer from the molecule to metal is more active in the adsorption of the BTAH and BTA-Cu cases. In addition, the larger ΔN value is related to a more exothermic prediction of the molecule-surface interaction energy, ΔE¹⁰⁾. As listed in Table 6, the interaction energy between the adsorption of BTAH and the complex formation for copper is much higher than that for steel. It indicates that the interaction of the BTAH adsorption and the complex formation for copper is more stable than that of the steel case because a high exothermic energy means more stable state in thermodynamics⁶²⁾. In addition, it is confirmed that the complex formation is more stable than the adsorption state, and this result is correlated with the MO interpretation in the isothermal parts.

4. Conclusions

In this study, electrochemical measurements were accomplished to evaluate the inhibition efficiency of 1, 2, 3-benzotriazole (BTAH) regarding copper and steel in simulated tap water. The BTAH showed the significant inhibition efficiency for copper, changing the corrosion behaviour from uniform to passive. It means that a stable adsorption layer was formed on the surface. However, in the case of steel, the inhibition efficiency is small and the corrosion behaviour did not change. The difference of the inhibition efficiency is related to the d-orbital structure of the metal surface because the adsorption of BTAH and the metal is considered as the π-d bonding. Therefore, the distinction of the d-orbital is related to the stability and strength of the BTAH adsorption on the metal surface. From the quantum analysis, the ability of electron transfer and the interaction energy between the BTAH adsorption and the complex formation on metal surface were calculated, and the results show a high correlation with the electrochemical and orbital interpretation results.

Acknowledgments

This research was supported by Global Ph.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015H1A2A1033362).

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