Corrosion Inhibition of Galvanized Steel in NaCI Solution by Tolytriazole

1 Introduction

As an important material, galvanized steel has been widely employed in construction, power communication, transportation vehicles, and so on, owing to the simplicity of manufacture, low cost and high efficiency [1, 2, 3, 4, 5, 6]. A lot of research has been undertaken to elucidate the corrosion and protection mechanism of galvanized steel [1, 2, 3, 4, 5, 6, 7, 8, 9]. Zinc behaves as a barrier for steel when exposed to corrosive media. Moreover, zinc also provides electrochemical protection for steel as a result of galvanic couple effect. The effect of corrosion protection depends on the thickness and compactness of the zinc coating [10].

Although zinc has protective effect on steel, it is also needed to apply other measures to improve the corrosion resistance of galvanized steel since zinc layer is normally thin. Recently, researchers have attempted to use corrosion inhibitors to protect galvanized steel, which restrains zinc from the formation of white corrosion products in the corrosive media [11]. Literature has reported that some organic molecules with hetero-atoms (such as oxygen, nitrogen, sulphur and so on) can serve as corrosion inhibiting agents, which may be adsorbed on the surface of metals or react with metals to generate undissolved and stable metal complexes [12]. Triazole-type organic compounds, especially benzotriazole, including nitrogen are particularly used as corrosion inhibitors for copper, cast iron, zinc and so on [13, 14, 15, 16, 17, 18]. Benzotriazole, which has low toxicity and is economical, finds use as a good corrosion inhibitor [19, 20, 21, 22].

Benzotriazole has been studied as a corrosion inhibitor for galvanized steel in aerated corrosive solutions [23, 24, 25]. Tolyltriazole (TTA) [26], a mixture of 4- and 5-methyl-1H-benzotriazole, as a derivative of benzotriazole, is similar in chemical structure (Fig. 1). However, the effect and mechanism of TTA on corrosion inhibition of galvanized steel is still not fully understood. The aim of the present work is to study the inhibition effect of TTA on corrosion of galvanized steel in neutral NaCl solution. Additionally, the inhibition efficiency of TTA on galvanized steel was investigated by using Langmuir adsorption isotherm model to obtain better understanding regarding the role of TTA on galvanized steel.

2 Experimental

2.1 Materials

Galvanized steel used in the present work is a commercial one. Figure 2a, b show the SEM images of surface and cross-sectional morphologies of the galvanized steel. The energy-dispersive spectroscopy (EDS) of the cross section of the sample indicates the top layer is 100% Zn, and the bottom layer is 100% Fe. The thickness of the Zn layer is approximately from 6 to 12 μm. The dimension of the samples for the experiments is 10 mm × 10 mm × 2 mm. Methyl-1H-benzotriazole (TTA) was purchased from Sinopharm Chemical Reagent Company, China. It was used as corrosion inhibitor for the galvanized steel, which was added into the aqueous solution of 5 mM NaCl. The appropriate amount of TTA was weighed and mixed with 5 mM NaCl to prepare different concentrations of TTA of in 5 mM NaCl.

2.2 Full Immersion Tests

Samples of the galvanized steel with dimension of 10 mm × 10 mm × 2 mm were used. Before immersion tests, the back side and four cut edges of the samples were sealed by epoxy resin mixed with polyamide hardener (100:32 by weight). After rinsing with distilled water and degreasing with ethanol, the samples were immersed in aerated 5 mM NaCl without TTA or 5 mM NaCl with 0.01 Mol/L TTA for different times (1, 4 and 24 h) at room temperature. After 1, 4 and 24 h, the samples were removed out and taken photos. Before and after the immersion tests, the samples were observed by XL30-type environment scanning electronic microscope (SEM) integrated with energy-dispersive spectroscopy (EDS).

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used for investigation of sample surface after 24 h of immersion in 5 mM NaCl with 0.01 Mol/L TTA. A Spectrum 400 (Perkin Elmer Co., USA) measurement system, fitted with a Universal ATR sampling accessory, was used for infrared spectroscopy.

2.3 Electrochemical Measurements

Galvanized steel samples were used as working electrodes for electrochemical impedance spectroscopy (EIS) and polarization measurements. The back side of the galvanized steel samples was welded by copper wire and then sealed by epoxy resin in the same way as described in Sect. 2.2. The exposed area of the working electrodes is 1 cm².

EIS and polarization measurements were taken using an EG&G 273 Potentiostat/Galvanostat and EG&G 5210 lock-in amplifier integrated with a PC system. The measurements were done in a three-electrode cell system, in which a saturated calomel electrode (SCE) as reference electrode, a platinum foil electrode as counter electrode and a sample as working electrode. The measurements were taken at room temperature in naturally aerated 5 mM NaCl without or with TTA. For EIS measurements, the frequency range is from 100 kHz to 10 mHz at 5 points per hertz decade and an amplitude perturbing signal is 10 mV. EIS data were fitted with ZSimpWin software. For polarization measurements, the scanning rate is 0.33 mV/s. The scanning range was set from - 0.25 V/SCE to + 0.6 V/SCE with reference to open-circuit potential. During the measurements, three parallel samples were used to confirm the repeatability of the results.

2.4 SVET Measurements

The corrosion behaviour of the galvanized steel samples immersed in 5 mM NaCl without and in the presence of 0.01 Mol/L TTA was studied by SVET. A commercial system from Applicable Electronics, controlled by the science wares ASET 2.0 software, was used to perform the SVET measurements. For the tests, the Pt-Ir probes (Microprobe Inc.) were platinized to form a small 30 μm diameter, ball of platinum black at the tip. The frequency of probe vibration in perpendicular direction to the sample surface is 325 Hz. The measurements were taken at open-circuit potential. The time of acquisition for each SVET data point is 1.2 s. The local ionic current densities were mapped on a 30 × 30 grid. The current densities were detected on 150 μm over the sample surface within an area of c.a. 4 mm². The samples were tested after 1, 4 and 24 h of exposure in the 5 mM NaCl without and in the presence of 0.01 Mol/L TTA. The solutions in the cell were added by distilled water to maintain the original level while measuring. The data of current density were visualized by QuikGrid software.

3 Results and Discussion

3.1 Immersion Tests

After immersion in 5 mM NaCl solutions without or with 0.01 M TTA for 24 h, the photographs of the galvanized steel samples are shown in Fig. 3a-f, respectively. From Fig. 3a-c, it can be seen that the galvanized steel sample immersed in 5 mM NaCl was severely corroded, while almost no corrosion was seen on the sample surface immersed in 5 mM NaCl containing TTA (see Fig. 3d-f). Meanwhile, the other two parallel samples immersed in the above solutions were used for SEM observation of surface and cross-sectional morphologies. From the SEM image shown in Fig. 4a, the corrosion products were fully distributed on the surface of the sample after immersion in 5 mM NaCl. Figure 4b shows the surface morphology of the sample after immersion in 5 mM NaCl containing TTA. It is obvious that there is only slight corrosion on the surface, which reflects the effective corrosion inhibition.

ATR-FTIR spectra of TTA, sample surface after 24 h of immersion in 5 mM NaCl with 0.01 Mol/L TTA were recorded in order to examine the presence of TTA on the galvanized steel. As shown in Fig. 5a, the transmission absorption peaks of TTA are shown at 1092 and 1031 cm^-1, which are attributed to N-H in-plane bending and C-H in-plane bending [27, 28]. The peak at 1632 cm^-1 is also attributed to N-H in-plane bending [28]. As shown in Fig. 5b, the presence of peaks at 1632, 1092 and 1031 cm^-1 indicates that TTA was complexed with galvanized steel.

3.2 Polarization Curves

Figure 6 shows the polarization curves of the galvanized steel samples immersed in 5 mM NaCl and 5 mM NaCl solutions containing different concentrations of TTA. Table 1 shows the electrochemical parameters (corrosion potential, E_corr; corrosion current density, I_corr; polarization resistance, R_p) obtained by R_p extrapolation in the vicinity of the open-circuit potential (± 15 mV). The corrosion efficiency IE is formulated as following [29],

$ IE=(1-I'_{corr}/I_{corr})\times 100\% ,$(1)

where I_corr is the corrosion current density in 5 mM NaCl; I′_corr is the corrosion current density in 5 mM NaCl solutions containing different concentrations of TTA. IE is used to evaluate the inhibition effect of TTA acted on the surface of galvanized steel.

From Fig. 6, it can be seen that with the increase of concentration of TTA, the corrosion potential shifts to more anodic direction and the corrosion current density shifts to much lower values in comparison to those of the control sample immersed in 5 mM NaCl, indicating that TTA has good inhibiting effect on corrosion of galvanized steel. Obviously, as the concentration of TTA reaches 0.01 Mol/L, I_corr is the lowest. From Table 1, it can be seen that I_corr shows a decrease of two orders of magnitude for the sample immersed in 5 mM NaCl containing 0.01 M TTA, comparing with I_corr for the sample immersed in 5 mM NaCl. Meanwhile, IE reaches to the maximum value at this concentration. When the concentration of TTA is increased from 0.001 to 0.005 M, R_p shows a sharp increase. Correspondingly, IE increases remarkably from 55.62 to 94.94%.

From Fig. 6, it can be seen that with the increase of concentration of TTA, the corrosion potential shifts to more anodic direction and the corrosion current density shifts to much lower values in comparison to those of the control sample immersed in 5 mM NaCl, indicating that TTA has good inhibiting effect on corrosion of galvanized steel. Obviously, as the concentration of TTA reaches 0.01 Mol/L, I_corr is the lowest. From Table 1, it can be seen that I_corr shows a decrease of two orders of magnitude for the sample immersed in 5 mM NaCl containing 0.01 M TTA, comparing with I_corr for the sample immersed in 5 mM NaCl. Meanwhile, IE reaches to the maximum value at this concentration. When the concentration of TTA is increased from 0.001 to 0.005 M, R_p shows a sharp increase. Correspondingly, IE increases remarkably from 55.62 to 94.94%.

Figure 7 shows the polarization curves of the galvanized steel samples immersed in 5 mM NaCl and 5 mM NaCl solution containing 0.01 M of TTA after different immersion times. Table 2 gives the fitting results of the polarization curves by R_p extrapolation in the vicinity of the open-circuit potential (± 15 mV). From Table 2, it is clear that E_corr shifts to the noble direction when 0.01 M TTA was added to 5 mM NaCl. For I_corr and R_p values, there is an opposite oscillating behaviour, which can be ascribed to the adsorption and desorption of TTA during the immersion period.

3.3 EIS Measurements

EIS measurements were taken, aiming to study the characteristic at the interface of the galvanized steel and electrolyte. Figure 8 shows the EIS plots of galvanized steel samples exposed to 5 mM NaCl and 5 mM NaCl in the presence of 0.01 M TTA at different immersion times. From the EIS spectra, the diameter of capacitance loop increases with the addition of TTA, indicating TTA has a passive effect on the electrode. In comparison to the EIS spectra measured in 5 mM NaCl, there are larger capacitive loops in the low frequency range in the presence of TTA, which is caused by the charge transfer during the procedure of the metal dissolution and adsorption of inhibitor [15, 30, 31, 32]. The diameter of the capacitance loop grows with the immersion time before 72 h. It can be inferred that TTA may be adsorbed at the interface between the metal and the aggressive solution, blocking the available active centre of the galvanized steel. After immersion for 72 h, there is a drop in the diameter of the capacitance loop, which demonstrates that the protecting ability of TTA acting on the surface of the galvanized steel is becoming weaker. The decrease in capacitance loop can be ascribed to corrosion on the surface.

The equivalent electrical circuit, used to interpret the impedance behaviour of the galvanized steel electrode, is represented in Fig. 9. The equivalent electrical circuit is composed of solution resistance R_s, which is connected in series with two time constants Q_f(R_f(Q_dlR_ct)), where Q_f is the capacitance of the absorbed film on the metal/aggressive solution interface, R_f is the film resistance absorbed on the metal surface, Q_dl is the double-layer capacitance at the interface between the metal and the electrolyte, and R_ct is the charge transfer resistance. The constant phase angle elements (CPEs, consisting of Q_f and Q_dl) are more suitable than the capacitor to fit the actual impedance data due to frequency dispersion, which is caused by the roughness and heterogeneity of the metal surface [30]. The impedance of CPE is defined as following formula [15],

$Z_{{CPE}} (w) = Y_{0}^{ - 1} (jw)^{ - n} ,$(2)

where Y₀ is the admittance function; j² = - 1, which is the imaginary part; w is the angular frequency; n is the index of dimensionless (0 < n < 1), which can indicate the degree of the metal surface heterogeneity (for n = 1, the CPE is equivalent to the capacitance; for n = - 1, the CPE is the inductance; for n = 0, the CPE is the resistance).

The EEC, R(Q(R(QR))), was fitted with all the impedance data from 0 to 120 h of the immersion. All the fitted data for the impedance spectra are shown in Table 3. It is clear that the value of the film resistance, R_f increases from 0 to 120 h of immersion due to the chemical adsorption of TTA on galvanized steel, especially after 24 h of immersion. Correspondingly, there shows a decrease of Q_f from 24 to 72 h. It is obvious that the value of R_ct has an oscillating behaviour, indicating the adsorption and desorption process. The increase of Q_dl is possibly due to the intense complexing reactions between TTA and galvanized steel. The active sites on TTA are the positively charged N atoms, which are able to complex with negatively charged Cl^- adsorbed on the metal surface [7].

The EEC, R(Q(R(QR))), was fitted with all the impedance data from 0 to 120 h of the immersion. All the fitted data for the impedance spectra are shown in Table 3. It is clear that the value of the film resistance, R_f increases from 0 to 120 h of immersion due to the chemical adsorption of TTA on galvanized steel, especially after 24 h of immersion. Correspondingly, there shows a decrease of Q_f from 24 to 72 h. It is obvious that the value of R_ct has an oscillating behaviour, indicating the adsorption and desorption process. The increase of Q_dl is possibly due to the intense complexing reactions between TTA and galvanized steel. The active sites on TTA are the positively charged N atoms, which are able to complex with negatively charged Cl^- adsorbed on the metal surface [7].

3.4 SVET Measurements

In the present work, the inhibiting effect of TTA on corrosion of galvanized steel was evaluated in a localized area using SVET. Figure 10a-c shows the current densities on the galvanized steel during immersion in 5 mM NaCl. The maximum anodic current density with a value of c.a. 18 μA/cm² is shown at positions closed the edge and corner of the scanning area after 1 h of immersion. The maximum values of cathodic current density are c.a. - 12 μA/cm² in the centre area. The location of anodic and cathodic current can be ascribed to the crevice corrosion taken placed in the edge and corner. As the immersion time increased to 4 h, the maximum values of anodic current density are c.a. 13 μA/cm² and the maximum values of cathodic current density are c.a. - 12 μA/cm². The values of anodic current density increase to c.a. 63 μA/cm², and the values of cathodic current density maintain c.a. - 13 μA/cm² after 24 h of immersion. In the corrosion process, the anodic and cathodic reactions could be presented in Eqs. (3) and (4).

${\text{Zn}} \to {\text{Zn}}^{2 + } + \, 2{\text{e}},$(3)

${\text{O}}^{2} + \, 2{\text{H}}_{2} {\text{O}} + 4{\text{e}} \to 4{\text{OH}}^{ - } .$(4)

Figure 11a-c shows the SVET current density on the galvanized steel during immersion in 5 mM NaCl containing TTA. The anodic and cathodic current densities show even distribution immediately after immersion (1 h), indicating that TTA effectively blocks crevice corrosion. The maximum values of anodic current density are c.a. 26 μA/cm², and the maximum values of cathodic current density are c.a. - 19 μA/cm². As the immersion time increased to 4 h, the maximum values of anodic current density decrease to c.a. 10 μA/cm² and the maximum values of cathodic current density change to c.a. - 6 μA/cm². After 24 h, the maximum values of anodic current density decrease to c.a. 1 μA/cm² and the maximum values of cathodic current density change to c.a. - 1.5 μA/cm². The shrinking of anodic and cathodic current densities as the elongation of immersion time clearly suggests that TTA is effective in corrosion inhibition of the galvanized steel.

3.5 Study of the Mechanism of Absorption

According to the previous works [12, 33], small organic molecules like triazole compounds protect metal from corrosion by adsorption. The adsorption isotherm equation can be used to account for the adsorption mechanism of TTA on the surface of galvanized steel. Assuming that it is a single molecular adsorption, Langmuir formula of isothermal absorption can be used to analyse the situation, which is defined as following,

${\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 \theta }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\theta $}} + {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {Kc}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${Kc}$}} = 1,$(5)

where K denotes the equilibrium constant between adsorption and desorption; c denotes the concentration of corrosion inhibitor in the solution and θ denotes the degree of surface coverage, which can be replaced by η in that η is directly proportional to the degree of surface coverage (θ) in case of the geometric blocking effect [34]. The equation (5) can be changed into another form as following:

${\raise0.7ex\hbox{$c$} \!\mathord{\left/ {\vphantom {c \eta }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\eta $}} = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {K^{\prime}}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${K^{\prime}}$}} + c.$(6)

According to Table 1, different values of η correspond to different concentrations of c, by which a graph is drawn in Fig. 12, where X-axis denotes c and Y-axis denotes c/η. The linearly dependent coefficient of R is 0.992, which means the data between c/η and c are well linearly dependent (R > 0.99) based on the results of the fitting. Langmuir adsorption isotherm equation can be used to explain the mechanism of the absorption of TTA on the surface of galvanized steel. The equilibrium constant K’ can also be calculated from the fitting curve, which is 3.43 × 10⁵ L/mol.

Depending on previous literature [35, 36, 37, 38], the relation between the equilibrium constant K′ and the free energy of absorption ∆G is defined as following:

$\Delta G = - RT\ln (55.5 \times K^{\prime}),$(7)

where ∆G can be calculated as - 41.55 kJ/mol, which is below 0, indicating that the procedure of absorption is spontaneous in the condition of constant pressure and temperature. Meanwhile, the mechanism of TTA inhibiting the corrosion on the surface of galvanized steel is chemical absorption because the value of ∆G is lower than - 40 kJ/mol, which means that the formation of chemical bonds between the solid and the adsorption needs a larger number of chemical energy than 40 kJ/mol or more and the absorption is single-layer. In contrast, the essence of physical adsorption is van der Waals forces, very small (> - 20 kJ/mol) [39]. From the above analysis, it can be concluded that the adsorption of TTA is chemical adsorption.

4 Conclusions

The corrosion inhibition of the galvanized steel by tolytriazole was studied. Tolytriazole exhibits good inhibiting ability for corrosion of galvanized steel in 5 mM NaCl solution. The efficiency of corrosion inhibition increases with the increase in the concentration of tolytriazole. As the concentration of tolytriazole increased from 1 mM to 5 mM, there is a sharp increase in inhibition efficiency and polarization resistance. The film resistance due to the adsorption of tolytriazole increases with the elongation of immersion time when the galvanized steel was immersed in 5 mM NaCl containing 0.01 M tolytriazole. The adsorption of tolytriazole on the surface of the galvanized steel in NaCl follows Langmuir adsorption isotherm formula and is chemical adsorption.

Acknowledgements The authors acknowledge the financial support from the National Natural Science Fund of China (Grant No. 51571202) and National Basic Research Program of China (No. 2014CB643304).

The authors have declared that no competing interests exist.

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