1 Introduction

C-steel, the most widely used engineering material, accounts for approximately 85% of the annual steel production world wide. Despite its relatively limited corrosion resistance, C-steel is used in large tonnages in marine applications, chemical processing, petroleum production and refining, construction and metal-processing equipment. The use of chemical inhibitors to decrease the rate of corrosion processes of carbon steels is quite varied [1–5]. A variety of organic compounds containing heteroatoms such as O, N, S and multiple bonds in their molecule are of particular interest as they give better inhibition efficiency than those containing N or S alone [6, 7]. Sulfur and/or nitrogen containing heterocyclic compounds with various substituents are considered to be effective corrosion inhibitors. Thiadiazole derivatives offer special affinity to inhibit corrosion of metals in acid solutions [8–11].

The present work was designed to study the corrosion inhibition of C-steel in H2SO4 solutions by new thiadiazole derivatives as corrosion inhibitors using four different techniques: weight loss, potentiodynamic polarization, linear polarization and electrochemical impedance spectroscopy (EIS). The synergistic effect brought about by combination of the inhibitors with KSCN, KI and KBr was examined and explained. The effect of temperature on the corrosion behavior also was investigated.

2 Experimental techniques

1 M H2SO4 solution was prepared from an analytical reagent grade of H2SO4 98% and bi-distilled water and was used as corrosive media in this study. All the experiments were performed at 298 ± 1 K with C-steel samples containing (weight %): 0.200% C, 0.350% Mn, 0.024% P, 0.003% Si and the remainder iron. The inhibitors were synthesized in the laboratory according to a previously described experimental procedure [12] purified and characterized by NMR and IR spectroscopies and element analysis before use. The structure formula of the inhibitors examined is given in Fig. 1.

Fig. 1
figure 1

The Chemical structure of the investigated thiadiazole derivatives

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2.1 Weight loss method

C-steel specimens of dimensions 2 × 2 × 0.2 cm, in triplicate, were mechanically polished successively with different grades emery papers, up to 1,200 grade, degreased in acetone rinsed with bi-distilled water and finally dried then immersed in 100 mL of 1 M H2SO4 solutions containing various concentrations of the inhibitors. The weight of the specimens before and after immersion was determined. The weight loss was used to calculate the percentage inhibition efficiency (% IE) using the equation:

$$ \% \,{\text{IE}} = \frac{W - W^{\prime} }{W} \times 100 $$
(1)

where W and W′ are the corrosion rates (mg cm−2 min−1) of carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of inhibitors, respectively.

2.2 Potentiodynamic polarization method

Electrochemical polarization experiments were carried out in a glass cell with a capacity of 250 mL. A platinum electrode and a saturated calomel electrode (SCE) were used as a counter electrode and a reference electrode, respectively. The working electrode was in the form of a disc with an exposed area of 0.12 cm2 that was cut from C-steel under investigation. A time interval of about 30 min was given for the system to attain a steady state and the open circuit potential (OCP) was noted. Both cathodic and anodic potentiodynamic polarization curves were recorded at 300 mV ± OCP and scan rate of 2 mV s−1 using potentiostat-galvanostat Wenking model-PGS95 and a LINSEIS model Ly 1600-IIXYt recorder.

% IE was calculated using the equation:

$$ \% \,{\text{IE}} = \frac{{i_{\text{corr}} - i^{\prime}_{\text{corr}} }}{{i_{\text{corr}} }} \times 100 $$
(2)

where i corr and i′corr are the corrosion current densities in the absence and presence of the inhibitors, respectively.

2.3 Linear polarization resistance method

Polarization was done in the range of 25 mV ± OCP at a scan rate of 2 mV s−1 and the slope of the linear segment was obtained as polarization resistance, R p.

The % IE is evaluated as follow:

$$ \% \,{\text{IE}} = \frac{{R^{\prime}_{\text{p}} - R_{\text{p}} }}{{R^{\prime}_{\text{p}} }} \times 100 $$
(3)

where R p and R′p are the polarization resistance values in the absence and presence of the inhibitors, respectively.

2.4 Electrochemical impedance spectroscopy (EIS)

EIS experiments were conducted at 298 ± 1 K at the OCP over a frequency range of 1 kHz to 1 Hz, with a signal amplitude perturbation of 10 mV using IM6e system (Zahner Elektrik, Germany) and personal computer. Nyquist plots were obtained from the results of these experiments. Values of the charge transfer resistance (R ct) were obtained from these plots by determining the difference in the values of impedance at low and high frequencies, as suggested by Tsuru et al. [13].Values of the double layer capacitance (C dl) were calculated from the frequency at which the impedance imaginary component (−Z′′) was maximum, using the following equation:

$$ f\left( { - Z^{\prime \prime}_{ \max } } \right) = \frac{1}{{2\pi C_{\text{d1}} R_{\text{ct}} }} $$
(4)

% IE was calculated using the equation:

$$ \% \,{\text{IE}} = \frac{{\left( {1/R^{\prime}_{\text{ct}} } \right) - \left( {1/R_{\text{ct}} } \right)}}{{\left( {1/R^{\prime}_{\text{ct}} } \right)}} \times 100 $$
(5)

where R ′ ct and R ct are the charge transfer resistance values in the absence and presence of the inhibitors, respectively.

3 Results

3.1 Weight loss

Weight loss of C-steel, in mg cm−2 of the surface area, was determined at various time intervals in the absence and presence of different concentrations (1 × 10−6–20 × 10−6 M) of the thiadiazole derivatives (I–VI). The curves obtained in the presence of different concentrations of inhibitors fall significantly below that of free acid. Similar behaviors were obtained for the other inhibitors. Values of % IE are tabulated in Table 1. In all cases, the increase in the inhibitor concentration was accompanied by a decrease in the weight loss and an increase in % IE. These results lead to the conclusion that, these compounds under investigation are fairly efficient as inhibitors for C-steel dissolution in H2SO4 solution. Careful inspection of these results showed that, at the same inhibitor concentration, the ranking of the inhibitors according to % IE is as follow: VI > V > IV > III > II > I.

Table 1 % IE of C-steel dissolution at 180 min immersion in 1 M H2SO4 in the presence of different concentrations of compounds (I–VI) at 298 K
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3.1.1 Synergistic effect

The effect of addition 1 × 10−2 M KSCN, KI and KBr on the corrosion rate of C-steel in the absence and presence of different concentrations of inhibitors (I–VI) in 1 M H2SO4 solutions was investigated using weight loss method. Results of % IE obtained are summarized in Table 2. It was observed from these results, these additives improved the % IE significantly, due to the stabilization of adsorbed halide ions by means of electrostatic interactions with the inhibitor, which leads to greater surface coverage [14]. The interactions of these additives with the inhibitor molecules can be described by introduction of the synergistic parameter (S θ ) which is defined as [15]:

$$ S_{\theta } = \frac{{1 - \theta_{1 + 2} }}{{1 - \theta^{\prime }_{1 + 2} }} $$
(6)

where θ1+2 = (θ1 + θ2) − (θ1θ2); θ1 = the degree of surface coverage by the anions; θ2 = the degree of surface coverage by the cations; θ′1+2 = measured surface coverage by both anions and cations.

Table 2 % IE at different concentrations of inhibitors (I–VI) for C-steel at 180 min immersion in 1 M H2SO4 containing 10−2 M KSCN, KI and KBr at 298 K
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S θ approaches 1 when no interaction between the inhibitor compounds exists, while S θ  > 1 points to a synergistic effect. In the case of S θ  < 1, the antagonistic interaction prevails.

Values of S θ summarized in Table 3 are more than unity, suggesting that the phenomenon of synergism exists between the inhibitors and these additives used. Aramaki [16] has proposed two kinds of joint adsorption (competitive and cooperative) to explain the synergistic action observed between an anion and a cation [17]. For competitive adsorption or interference adsorption, the anion (X−) and cation (inhibitor molecules in acid medium) are adsorbed at different sites on the electrode surface and for cooperative adsorption, the anion (X−) is chemisorbed on the surface and the cation (inhibitor molecules) is adsorbed on the layer of the anions. The two types of adsorption are represented schematically in Fig. 2 and can be characterized by a synergistic factor (S θ ) calculated as above.

Table 3 Synergistic parameters (S θ ) of 10−2 M KSCN, KI and KBr at different concentrations of compounds (I–VI) for C-steel in 1 M H2SO4 at 298 K
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Fig. 2
figure 2

Schematic representations of a competitive and b cooperative adsorption of the anions (X−) and cations (A+) on carbon steel surface in acid solutions

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The synergistic inhibitive effect brought about by combination of the inhibitors with KSCN, KI and KBr for the corrosion of C-steel in 1 M H2SO4 can be explained as follows: The strong chemisorption of X− ions (SCN−, I− or Br−) on the metal surface is responsible for the synergistic effect of these anions, in attraction with protonated inhibitor [18]. X− ions are adsorbed on the anodic sites on the metal surface. Surface charge is changed to negative by the specific adsorption of these ions resulting in the joint adsorption of anions with the inhibitor cations. The inhibitors are believed to be adsorbable, not only on the cathodic sites by electrostatic attraction using the charge of the protonated molecule, but also on the anodic sites by virtue of donation of the electron-pair on the nitrogen atom of the unprotonated molecule [19] therefore, interference adsorption can take place at the anodic sites.

3.1.2 Adsorption isotherm

Attempts were made to fit the degree of surface coverage determined by weight loss (θ) to various isotherms. The best fit was obtained with the Langmuir’s isotherm which is in good agreement with Eq. 7 [20]. The variations of (θ/1 − θ) with the inhibitor concentrations (C) of compounds (I–VI) are represented in Fig. 3. According to the following equation:

$$ \frac{\theta }{1 - \theta } = K_{\text{ads}} \times C $$
(7)

where K ads is the equilibrium constant for the adsorption process, a linear relationship is obtained with slope equal to K ads which is related to the standard free energy of adsorption (ΔG°ads) by the equation:

$$ K_{\text{ads}} = \frac{1}{55.5}{ \exp }\frac{{ - \Updelta G^{\circ}_{\text{ads}} }}{\text{RT}} $$
(8)

where 55.5 is the molar concentration of water in the solution, R is the universal gas constant and T is the absolute temperature.

Fig. 3
figure 3

Langmuir adsorption isotherm for C-steel in 1 M H2SO4 in the presence of different concentrations of inhibitors (I–VI) at 298 K

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The thermodynamic parameters for the adsorption process were obtained from these figures are shown in Table 4. The values of \(\Updelta G_{\rm ads}^{\circ}\) are negative and increased as the % IE increased which indicate that these investigated compounds are strongly adsorbed on the C-steel surface and show the spontaneity of the adsorption process and stability of the adsorbed layer on the C-steel surface. Generally, values of \(\Updelta G_{\rm ads}^{\circ}\) up to −20 kJ mol−1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than −40 kJ mol−1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [21]. The values of \(\Updelta G_{\rm ads}^{\circ}\) obtained were approximately equal to −39 ± 1 kJ mol−1, indicating that the adsorption mechanism of the thiadiazole derivatives on C-steel in 1 M H2SO4 solution involves both electrostatic adsorption and chemisorption [22]. The K ads follows the same trend in the sense that large values of K ads imply better more efficient adsorption and hence better inhibition efficiency [23].

Table 4 Thermodynamic parameters of the adsorption of compounds (I–VI) on C-steel surface in 1 M H2SO4 at 298 K
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3.1.3 Effect of temperature

Corrosion reactions are usually regarded as Arrhenius processes and the rate (k) can be expressed by the relation:

$$ { \log }\,k = A - \frac{{E_{\text{a}} }}{{2.303\,{\text{RT}}}} $$
(9)

where E a is the activation energy of the corrosion process and A is a Arrhenius pre-exponential constant depends on the metal type and electrolyte. Arrhenius plots of log k vs. 1/T for carbon steel in 1 M H2SO4 in the absence and presence of 11 × 10−6 M of inhibitors (I–VI) are shown graphically in Fig. 4. The variation of log k vs. 1/T is a linear one and the values of E a obtained are summarized in Table 5. These results show that the values of E a are similar and ranging from 61.34 to 68.40 kJ/mol suggest that the inhibitors are similar in the mechanism of action. The increase in E a with the addition of 11 × 10−6 M of inhibitors (I–VI) indicating that the energy barrier for the corrosion reaction increases. It is also indicated that the whole process is controlled by surface reaction, since the activation energy of the corrosion process is over 20 kJ/mol [24].

Fig. 4
figure 4

Log k − 1/T plots for carbon steel dissolution in 1 M H2SO4 in the absence and presence of 11 × 10−6 M of inhibitors (I–VI)

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Table 5 Activation parameters of carbon steel dissolution in 1 M H2SO4 in the absence and presence of 11 × 10−6 M of compounds (I–VI)
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Enthalpy and entropy of activation (ΔΗ*, ΔS*) are calculated from transition state theory using the following equation [25]:

$$ k = \frac{RT}{Nh}{ \exp }\left( {\frac{{\Updelta S^{*} }}{R}} \right){ \exp }\left( {\frac{{ - \Updelta H^{*} }}{RT}} \right) $$
(10)

where h is Planck’s constant, N is Avogadro’s number. A plot of log k/T vs. 1/T also gave straight lines as shown in Fig. 5 for carbon steel dissolution in 1 M H2SO4 in the absence and presence of 11 × 10−6 M of inhibitors (I–VI). The slopes of these lines equal −(ΔΗ*/2.303R and the intercept equal log RT/Nh + (ΔS*/2.303R) from which the value of ΔΗ* and ΔS* were calculated and tabulated in Table 5. From these results, it is clear that the presence of the tested compounds increased the activation energy values and consequently decreased the corrosion rate of the carbon steel. These results indicate that these tested compounds acted as inhibitors through increasing activation energy of carbon steel dissolution by making a barrier to mass and charge transfer by their adsorption on carbon steel surface. The values of ΔΗ* reflect the strong adsorption of these compounds on carbon steel surface. The values of ΔS* in absence and presence of the tested compounds are large and negative; this indicates that the activated complex in the rate-determining step represents an association rather than dissociation step, meaning that a decreases in disordering takes place on going from reactants to the activated complex and the activated molecules were in higher order state than that at the initial state [26, 27].

Fig. 5
figure 5

Log (k/T)–(1/T) plots for carbon steel dissolution in 1 M H2SO4 in the absence and presence of 11 × 10−6 M of inhibitors (I–VI)

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3.2 Potentiodynamic polarization

Anodic and cathodic polarization were carried out potentiodynamically in unstirred 1 M H2SO4 solution in the absence and presence of various concentrations of the inhibitors (I–VI) at 298 K over potential range 300 mV ± OCP. The results are represented in Fig. 6 for compound (VI), similar behaviors were obtained for other compounds. The obtained potentiodynamic polarization parameters are given in Table 6. These results indicate that the cathodic and anodic curves obtained exhibit Tafel-type behavior. Additionally, the form of these curves is very similar either in the cathodic or in the anodic side, which indicates that the mechanisms of carbon steel dissolution and hydrogen reduction apparently remain unaltered in the presence of these additives. Addition of thiadiazole derivatives decreased both the cathodic and anodic current densities and caused mainly parallel displacement to the more negative and positive values, respectively, i.e. the presence of thiadiazole derivatives in solution inhibits both the hydrogen evolution and the anodic dissolution processes with overall shift of E corr to more negative values with respect to the OCP.

Fig. 6
figure 6

Potentiodynamic polarization curves for carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of inhibitor (VI) at 298 K

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Table 6 Potentiodynamic polarization and LPR parameters for corrosion of carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of inhibitors (I–VI) at 298 K
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The results also show that the anodic and cathodic Tafel slopes (βa and βc) slightly changed on increasing the concentration of the tested compounds. This indicates that there is no change of the mechanism of inhibition in presence and absence of inhibitors. This could be interpreted as an action of mixed inhibitor control over the electrochemical semi-reactions. This means that the thiadiazole derivatives are mixed type inhibitors, but the cathode is more preferentially polarized than the anode. The higher values of Tafel slope can be attributed to surface kinetic process rather the diffusion-controlled process [28]. The constancy and the parallel of cathodic slope obtained from the electrochemical measurements indicate that the hydrogen evolution reaction was activation controlled [29] and the addition of these derivatives did not modify the mechanism of this process. This result suggests that the inhibition mode of the thiadiazole derivatives used was by simple blockage of the surface by adsorption.

3.3 Linear polarization resistance (LPR)

LPR values were obtained from current/potential plots. The obtained linear polarization plots for C-steel in 1 M H2SO4 in the absence and presence of different concentrations of compound (VI) is shown in Fig. 7, similar behaviors were obtained for other compounds. LPR parameters of different thiadiazole derivatives (I–VI) are given in Table 6. The slope of these curves (R p) showed an increase values from 39.8 Ω cm2 for the inhibitor free solution to 153.4 Ω cm2 in the presence of 11 × 10−6 M of compound (VI). Also, the values of R p, and consequently the % IE, increase with increasing the concentrations of the thiadiazole derivatives which indicates that these compounds act as inhibitors.

Fig. 7
figure 7

Linear polarization plots for carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of inhibitor (VI) at 298 K

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3.4 Electrochemical impedance spectroscopy

Impedance diagrams (Nyquist and Bode) at frequencies ranging from 1 Hz to 1 kHz with 10 mV amplitude signal at OCP for carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of compounds (I–VI) are obtained. The equivalent circuit that describes our metal/electrolyte interface is shown in Fig. 8 where R s, R ct and CPE refer to solution resistance, charge transfer resistance and constant phase element, respectively. EIS parameters and % IE were calculated and tabulated in Table 7. In order to correlate impedance and polarization methods, i corr values were obtained from polarization curves and Nyquist plots in the absence and presence of different concentrations of compounds (I–VI) using the Stern-Geary equation:

$$ i_{\text{corr}} = \frac{{\beta_{\text{a}} \beta_{\text{c}} }}{{2.303\left( {\beta_{\text{a}} + \beta_{\text{c}} } \right)R_{\text{ct}} }} $$
(11)
Fig. 8
figure 8

Equivalent circuit of constant phase element (CPE)

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Table 7 EIS Data for carbon steel in 1 M H2SO4 in the absence and presence of different concentrations of inhibitors (I–VI) at 298 K
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The obtained Nyquist plot for compound (VI) is shown in Fig. 9. Each spectrum is characterized by a single full semicircle. The fact that impedance diagrams have an approximately semicircular appearance shows that the corrosion of carbon steel is controlled by a charge transfer process. Small distortion was observed in some diagrams, this distortion has been attributed to frequency dispersion [30]. The diameters of the capacitive loop obtained increases in the presence of thiadiazole derivatives, and were indicative of the degree of inhibition of the corrosion process. The obtained Bode plot for compounds (VI) is shown in Fig. 10. The high frequency limits corresponds to R s (Ω), while the lower frequency limits corresponds to (R ct + R s). The low frequency contribution shows the kinetic response of the charge transfer reaction [31].

Fig. 9
figure 9

The Nyquist plots for carbon steel dissolution in 1 M H2SO4 in the absence and presence of different concentrations of inhibitor (VI) at 298 K a: 0.0 M, b: 1 × 10−6 M, c: 3 × 10−6 M, d: 7 × 10−6 M and e: 11 × 10−6 M

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Fig. 10
figure 10

The Bode plots for carbon steel dissolution in 1 M H2SO4 in the absence and presence of different concentrations of inhibitor (VI) at 298 K. a: 0.0 M, b: 1 × 10−6 M, c: 3 × 10−6 M, d: 7 × 10−6 M and e: 11 × 10−6 M

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It was observed from the obtained EIS data that R ct increases and C dl decreases with the increasing of inhibitor concentrations. The increase in R ct values, and consequently of inhibition efficiency, may be due to the gradual replacement of water molecules by the adsorption of the inhibitor molecules on the metal surface to form an adherent film on the metal surface and this suggests that the coverage of the metal surface by the film decreases the double layer thickness. Also, this decrease of C dl at the metal/solution interface with increasing the inhibitor concentration can result from a decrease in local dielectric constant which indicates that the inhibitors were adsorbed on the surface at both anodic and cathodic sites [32].

The impedance data confirm the inhibition behavior of the inhibitors obtained with other techniques. From the impedance data (Table 7), we conclude that the value of R ct increases with increase in concentration of the inhibitors and this indicates an increase in the corrosion inhibition efficiency, which is in concord with the weight loss and electrochemical results obtained. In acidic solution, the impedance diagrams show perfect semi-circles (Fig. 9) whose size increases with the concentration of the inhibitor indicating a charge-transfer process mainly controlling the corrosion of steel. In fact, the presence of inhibitors (I–VI) enhances the value of the transfer resistance in acidic solution. Values of double layer capacitance are also brought down to the maximum extent in the presence of inhibitors and decrease in the values of C dl follows the order similar to that obtained for i corr in this study. The decrease in C dl is due to the adsorption of this compound on the metal surface leading to the formation of a film from the acidic solution [33].

3.5 Chemical-quantum calculations

Table 8 shows the energy (in eV) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for inhibitors. It has been reported that the higher the HOMO energy (E HOMO) level of the inhibitor, the greater is the ease of offering electrons to unoccupied d orbital of metallic iron and the greater the inhibition efficiency [34]. The present results show that E HOMO and E LUMO decrease in the order: VI > V > IV > III > II > I which is parallel to the order of inhibition efficiency.

Table 8 Some quantum chemical calculations for the studied inhibitors
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4 Corrosion inhibition mechanism

From the previous results of various experimental techniques used, it was concluded that thiadiazole derivatives (I–VI) inhibit the corrosion of C-steel in 1 M H2SO4 by adsorption at the metal/solution interface. A skeletal representation of the proposed mode of adsorption of studied compounds is shown in Fig. 11 and clearly indicates the active adsorption centers.

Fig. 11
figure 11

Skeletal representation of the proposed adsorption mode of the thiadiazole derivatives on C-steel surface

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The adsorption of the thiadiazole on the metal surface occurs either directly, on the basis of donor acceptor interactions between the π-electrons of the hetero atoms and the vacant d-orbital of iron surface atoms, or interaction of thiadiazole derivatives with already adsorbed SO4 − ions [35]. In aqueous acidic solutions thiadiazole derivatives exists either as neutral molecules or in the form of cations. The thiadiazole derivatives may be adsorbed on the metal surface in the form of neutral molecules, involving the displacement of water molecules from the metal surface and sharing of electrons between the nitrogen atoms and metal surface [36]. Heterocyclic nitrogen compounds may also adsorb through electrostatic interactions between the positively charged nitrogen atom and the negatively charged metal surface [37]. It is well known that SO4 2− ions are strongly adsorbed [38] on the metal surface leaving less space available for organic molecules to adsorb. Molecular structure of organic compound is important in synergistic inhibition [39]. The order of the inhibition efficiency of these investigated compounds is as follows: VI > V > VI > III > II > I.

figure a

Compound (VI) comes on top of the investigated compounds because of the presence of highly electron releasing para two methyl groups (with Hammett constant σ = −0.17) [40] which increase the electron density on the active centers and this leads to greater surface overage, thereby giving higher inhibition efficiency. Compound (V), which has the same structure as compound (VI) with the replacement of the two methyl groups by H atom and Br atom [with σBr = 0.23 and σH = 0.0] comes next to compound (VI) in the order of % IE. This is due to the operation of a mesomeric effect (+M) involving electron pairs on (Br) which act in opposite direction to the inductive effect (−I), thus the overall effect of Br atom is to release electrons on the molecule but less than compound (VI). Compound (VI) comes after compound (V) in % IE due to its lesser molecular weight than compound (V). In addition to that, the values of E ΗΟΜΟ decreases in an order runs parallel to increase of % IE obtained which support the order : IV > V > VI.

Compound III is more efficient than compound II due to its higher molecular size. It follows that the ethoxy group is more efficient than the methyl one. From structural organic point of view, both methyl and ethoxy groups have +R effect but the inductive effect is +I For methyl group and −I for ethoxy group. Although the methyl group has +R, but its effect is very little as a result of hyperconjugation. In the case of ethoxy group both the effects of +R and −I are larger. Moreover, ethoxy group may add an additional active center to compound (III) and this increases the inhibition efficiency of compound (III) than compound (II). Compound (I) is the least efficient inhibitors in spite of its larger molecular size and it has the same number of adsorption active centers. This can be explained on the basis of the absence of the conjugation between the carbonyl group (–C=O) with azomethine (–N=C–) group in the thiadiazole ring of compounds (I). This leads to lesser surface overage by compound (I) thereby giving lower inhibition efficiency than other tested compounds. In addition to that, the values of E ΗΟΜΟ decreases in an order runs parallel to increase of % IE obtained which support the order: III > II > I.

5 Conclusion

  1. (1)

    The investigated thiadiazole derivatives exhibit inhibiting properties for carbon steel in 1 M H2SO4.

  2. (2)

    The % IE increased with the increase in inhibitor concentration. At all concentrations, % IE followed the ranking order: VI > V > IV > III > II > I.

  3. (3)

    The compounds were inhibitors of the mixed type but the cathode is more polarized than the anode, i.e. they affect both anodic dissolution of carbon steel and hydrogen evolution reactions.

  4. (4)

    The adsorption of the investigated compounds was found to follow the Langmuir’s adsorption isotherm indicating that the inhibition process occurs via adsorption.

  5. (5)

    The negative value of \(\Updelta G_{\rm ads}^{\circ}\) obtained from this study indicates that these compounds are strongly spontaneously adsorbed on the carbon steel surface.

  6. (6)

    A synergistic effect on % IE of a combination of the inhibitors and KSCN, KI and KBr was observed.

  7. (7)

    The % IE obtained from weight loss, polarization curves, electrochemical impedance spectroscopy measurements and the Stern-Geary equation are in good agreement.