DOI:https://doi-0004.org/6812/17934263262751
1Chemistry Research Unit Environmental Molecular CHEMS, Mentouri Brothers Constantine 1 University, Constantine, Algeria
*Corresponding author (e-mail: ibrahim-adouani.hayet@umc.edu.dz)
Received : 12/06/2026 ; Accepted : 27/09/2026 ; Published : 31/10/2026
ABSTRACT
The corrosion resistance of a stainless steel in a hydrochloric acid medium, in the absence and in the presence of two organic compounds, namely ethyl 2-cyano-2-(1,3-dithietan-2-ylidene) acetate (A) and ethyl 2-cyano-2-(1,3-dithian-2-ylidene) acetate (B), was investigated using gravimetry (measurement of mass loss) and electrochemical methods (open-circuit potential, polarisation curves and electrochemical impedance spectroscopy). The results obtained show that both compounds act as corrosion inhibitors for stainless steel in 1.0 M HCl, and that the inhibitory efficacy increases with the concentration of the compounds. The electrochemical impedance spectroscopy data indicate that the charge transfer resistance increases with the concentration of the inhibitor, whilst the capacitance of the double layer decreases.
Keywords: Inhibition, corrosion, stainless steel, 1,3-dithietan-2-yl derivative, HCl.
Received: September 2026; Accepted: December 2026
1. INTRODUCTION
Stainless steels are steel-based metal alloys (alloy: iron-carbon) which must contain at least 10.5 per cent chromium (according to European standard EN 10088-1) and often other alloying elements such as nickel (or its substitute, manganese) and, to a lesser extent, molybdenum [1]. Electropolishing is a surface treatment process in which the workpiece, placed in the anodic position, is polished in the presence of an acidic electrolyte; it produces surfaces of excellent quality and is widely used for the treatment of stainless steel [2]. Due to the aggressiveness of acidic solutions, the use of organic inhibitors has become one of the most cost-effective methods for limiting the loss of metallic materials. Organic compounds containing unsaturated bonds and/or polar atoms such as O, N and S are often effective inhibitors of iron corrosion in hydrochloric acid [3]–[6]. In addition to investigating the corrosion of steel in 1 M HCl, we felt it would be of interest to study the inhibitory effect of two ketene dithioacetal derivatives on the corrosion of steel in this medium. It is hoped that these derivatives, containing these heteroatoms (O, N and S), will significantly reduce the rate of the corrosion process.
2. EXPERIMENTAL METHOD
2.1. Materials and chemicals
The material tested in this study is a stainless steel with the following chemical composition: 0.03% C, 17.2% Cr, 10.58% Ni, 2.31% Mo, 1.02% Si, 2.04% Mn, 0.04% P, 0.08% S, with iron making up the balance. The samples were machined into discs measuring 1.2 cm in diameter and 0.5 cm in thickness. The compounds tested were synthesised in the laboratory according to the procedure described by Thuillier and Vialle [7]. The molecular structure of the products (Figure 1) was identified by measuring the melting point and by IR spectroscopy, ¹H NMR and ¹³C NMR.
FIGURE1 . Molecular structure of the organic compounds
2.2. Gravimetric measurements:
Gravimetric measurements, or mass loss tests, were carried out in a 100 ml beaker. A water bath manufactured by Kottermann Labortechnik was used to maintain the electrolyte at the desired temperature. The volume of electrolyte was 50 mL. The steel samples were immersed in 1 M HCl, both with and without the addition of various concentrations of inhibitors. In order to obtain reproducible results, prior to each test the samples were polished using sandpaper of progressively finer grit sizes (i.e. #800, #1000, #1200, # 2000), followed by rinsing with distilled water, then degreasing in ethanol and finally air-drying. Each value from the gravimetric tests is the average of at least two tests carried out under the same conditions for each concentration; a Sartorius analytical balance accurate to four decimal places was used to weigh the samples.
2.3. Electrochemical measurements
The electrochemical experiments are carried out in a double-walled, thermostatically controlled Pyrex glass cell with three electrodes (Tacussel type CEC/TH), with a platinum counter-electrode and a saturated calomel electrode (SCE) (Tacussel XR 100) as the reference electrode. The steel sample, in the form of a disc, is inserted into a polytetrafluoroethylene sample holder and acts as the working electrode; the working surface area is 0.636 cm². The potential sweep is carried out using a Radiometer potentiostat/galvanostat (PGP201), connected to a PC (HP Vectra 5/75) for data acquisition. This data is processed using the VoltaMaser1 software. The anodic and cathodic potentiodynamic polarisation curves are recorded at a scan rate of 20 mV min⁻¹, following 30 minutes’ immersion of the working electrode in the solution. Temperature control is achieved using a thermostatic bath (Bioblock Scientific Polystat). Electrochemical impedance spectroscopy plots were generated using a Solartron 1287 potentiostat and a Solartron 1255B frequency response analyser; measurements were carried out at open-circuit potential in the frequency range from 100 kHz to 0.01 Hz using a sinusoidal wave of 10 mV amplitude. To ensure reproducible results, the working electrode (WE ) undergoes pre-treatment prior to each test, consisting of polishing the electrode surface with sandpaper of progressively finer grit sizes (Sic #800, #1000, #1200, # 2000), followed by rinsing with distilled water, then degreasing in ethanol and finally air-drying.
3.RESULTS AND DISCUSSION
3.1. Gravimetric study
Figure 2 shows the results of mass loss as a function of immersion time for stainless steel at 25 °C, both in the absence and in the presence of 1 mM of the two products under investigation. It can be seen that the mass loss is considerable over time in the solution containing HCl alone, whilst it is low in the solutions containing both inhibitors. These results confirm the inhibitory effect of the compounds studied.
Figure 2.Mass loss of steel in 1M HCl in the absence and in the presence of 10-3 M of A and B
3.2. Electrochemical study
Open-circuit potential:
Figure 3 shows the evolution of the potential as a function of immersion time for solutions of HCl alone and with the addition of 1 mM of each inhibitor, A and B. It can be seen that the presence of both products shifts the potential towards less negative values.
Polarisation curves:
The cathodic and anodic polarisation curves of steel in a 1 M HCl solution, in the absence and presence of 10⁻³ M of inhibitors A and B, are shown in Figure 4. These were obtained after 30 minutes of immersion at Ecorr and at a temperature of 25°C. The Tafel plot is plotted by plotting the decimal logarithm of the absolute value of the current density against the potential. It can be seen that the addition of the compounds under investigation reduces the anodic and cathodic current densities and shifts the corrosion potential towards anodic values, indicating that the tested compounds can be considered as anodic inhibitors.
Figure 3. Change in the open-circuit potential of steel in 1 M HCl alone and with the addition of 10⁻³M of A and B
Figure 4. Polarisation curves of steel in 1 M HCl alone and with the addition of 10⁻³ M of the inhibitors.
Figure 5 shows the polarisation curves of steel in 1 M HCl at 25 °C in the absence and in the presence of different concentrations of A and B, respectively. The variation of the Log|i|=f(E) curves as a function of concentration reveals a marked reduction in cathodic and anodic currents, which gradually decreases as the concentration of inhibitors decreases.
In order to confirm the hypothesis that the inhibitor’s action is based on a mechanism involving simple adsorption onto the metal surface, thereby blocking the active sites and consequently reducing the currents measured during the recording of the polarisation curves, we attempted to correlate the experimental results with the adsorption isotherm. The selection of the appropriate isotherm, which appears graphically as a straight line, was determined using the correlation coefficient (R). Various isotherms, including Langmuir, Temkin, Frumkin and Freundlich, were tested in order to identify the appropriate adsorption isotherm.
Figure 5. Polarisation curves of steel in 1 M HCl at 25 °C in the absence and presence of different concentrations of A and B
Table 1. Parameters for steel corrosion in 1 M HCl at 25 °C in the absence and presence of different concentrations of A and B, obtained from the polarisation curves.
| inhibitor | Concentration (× 10⁻³M) | Ecorr (mV) | icorr (mA/cm²) | Bc (mV) | ϴ | E% |
| A | 1 M HCl | -411.5 | 0.3162 | -150.0 | / | / |
| 1.00 | -321.3 | 0.0058 | -140.3 | 0.981 | 98.1 | |
| 0.50 | -346.7 | 0.0084 | -125.0 | 0.973 | 97.3 | |
| 0.10 | -373.3 | 0.0199 | -110.0 | 0.937 | 93.7 | |
| 0.05 | -388.3 | 0.0224 | -107.5 | 0.929 | 92.9 | |
| 0.01 | -410.0 | 0.0596 | -115.0 | 0.811 | 81.1 | |
| B | 1.00 | -311.2 | 0.0049 | -162.5 | 0.985 | 98.5 |
| 0.50 | -332.0 | 0.0097 | -145.0 | 0.969 | 96.9 | |
| 0.10 | -361.5 | 0.0128 | -125.0 | 0.960 | 96.0 | |
| 0.05 | -376.1 | 0.0154 | -117.5 | 0.951 | 95.1 | |
| 0.01 | -400.4 | 0.0461 | -111.7 | 0.854 | 85.4 |
Figure 6 shows the Langmuir adsorption isotherm model with the best correlation coefficient, which is 0.99999. This model assumes that there is a fixed number of energetically identical sites on the surface. Each site can adsorb only a single particle. Furthermore, the model assumes that interactions between adsorbed particles are negligible and, consequently, the adsorption energy is considered constant[8] . Under these assumptions, and for a given temperature, the relationship between the adsorbed quantity of a species and its concentration in the liquid phase in contact with the surface is given by the following equation:
The coverage ratio θi can be determined using the formula:
(2)
Where and
are, respectively, the corrosion current in the absence and in the presence of inhibitor
Figure 6. Langmuir adsorption isotherm for compounds A and B on steel in 1 M HCl at 25 °C.
The adsorption equilibrium constant is related to the standard free energy of adsorption ∆Gads0 by the following equation:
The values of the standard free enthalpies of adsorption (∆G°ads) are –16.29 kJ/mol for compound A and –16.21 kJ/mol; these negative values indicate spontaneous adsorption of the inhibitors onto steel.
Electrochemical impedance spectroscopy:
Any liquid or solid can conduct current when a voltage is applied; when the voltage is constant, the ratio of the voltage to the current (V/I) is known as the material’s electrical resistance, but if the applied voltage is variable, the ratio (V/I) is known as the material’s impedance. Impedance spectroscopy is a powerful analytical technique; it is widely used in various fields of research, such as the study of the properties of chemical reactions, corrosion and the characterisation of dielectrics, and it can aid in the interpretation of electrochemical and electronic processes[9].
Impedance spectra are represented as Nyquist plots (Figure 7). As can be seen, the impedance plots feature semicircles, indicating that the corrosion process is essentially controlled by charge transfer [6]. The sizes of the capacitive loops, which are out of phase with the real axis, increase as the inhibitor concentration rises. The intersection of the Nyquist plots at high frequency with the real axis represents the electrolyte resistance (Rs), whilst the intersection at low frequency represents Rs + Rp (Rp), the polarisation resistance [10].
Figure 7. Nyquist impedance diagrams obtained for steel in 1 M HCl in the absence and presence of various concentrations of A and B.
The impedance of ideally polarisable liquid electrodes (e.g. mercury, indium-gallium amalgams) can be modelled by an RC circuit (resistance, capacitance). However, most impedance studies are now carried out using solid electrodes. The capacitance of the double layer of these electrodes is not purely capacitive and often exhibits some frequency dispersion. Such behaviour cannot be modelled by a simple circuit consisting of R, L and C components (electrical resistance, inductor and capacitance). To explain this behaviour, a constant-phase element (CPE) is generally used [11]. The electrochemical parameters relating to impedance spectroscopy are simulated by the equivalent circuit model shown in Figure 8.
The inhibitory efficacy obtained using the impedance diagrams was calculated using the following equation:
(4)
Where are the polarisation resistances in the presence and absence of inhibitors, respectively.
In Table 2, the double-layer capacitance was calculated using the following equation[12] :
(5)
Where Q and n are the CPE parameters.
The results shown in Table 2 regarding inhibitory efficacy are in good agreement with those obtained from the polarisation curves; these data confirm that these compounds exhibit effective inhibitory performance against steel corrosion in hydrochloric acid. It is noted that the inhibitory efficacy increases with the concentration of the compounds tested. The increase in Rpcan be explained by the adsorption of the inhibitors onto the metal surface. Furthermore, the decrease in Cdlcan be explained by an increase in the electric double layer, suggesting that both products act via adsorption onto the metal surface.
Table 2. Parameters of steel corrosion in 1 M HCl at 25 °C in the absence and presence of different concentrations of A and B, obtained by electrochemical impedance spectroscopy
| inhibitor | Concentration mmol/L | CPE | Cdl (μF/cm²) | Rs (Ω cm²) | Rp (Ω cm²) ) | E % | |
| n | Q (sn /Ωcm²) | ||||||
| White | 0.86605 | 4.48E-04 | 294.86 | 1.736 | 145.9 | / | |
| A | 1 | 0.85654 | 7.64E-05 | 54.13 | 1.361 | 1669.0 | 91.34 |
| 0.5 | 0.88124 | 9.10E-05 | 66.89 | 1.307 | 1115.0 | 87.03 | |
| 0.1 | 0.89782 | 1.23E-04 | 92.79 | 1.320 | 698.2 | 79.29 | |
| 0.05 | 0.85676 | 1.47E-04 | 92.84 | 1.351 | 448.2 | 67.56 | |
| 0.025 | 0.88152 | 1.64E-04 | 111.72 | 1.310 | 348.0 | 58.45 | |
| B | 1 | 0.86442 | 1.06E-04 | 74.20 | 1.236 | 990.8 | 85.41 |
| 0.5 | 0.86492 | 1.26E-04 | 89.03 | 1.426 | 874.6 | 83.47 | |
| 0.1 | 0.87222 | 1.23E-04 | 87.69 | 1.427 | 826.2 | 82.50 | |
| 0.05 | 0.84623 | 1.58E-04 | 103.68 | 1.403 | 619.0 | 76.64 | |
| 0.025 | 0.83239 | 1.84E-04 | 113.71 | 1,266 | 501.0 | 71.14 | |
Figure 8. Equivalent circuit for impedance spectra, where: Rs is the electrolyte resistance, Rp is the polarisation resistance and CPE is the constant-phase element
CONCLUSION
The addition of the compound under study reduces the anodic and cathodic current densities and shifts the corrosion potential towards more noble values; the anodic and cathodic current densities decrease as the inhibitor concentration increases and increase with rising temperature in the range from 20 to 60 °C.
Analysis by electrochemical impedance spectroscopy indicates the formation of a stable, insoluble deposit which limits the electrolyte’s access to the metal surface.
The negative value of ∆Gads0indicates spontaneous adsorption of the inhibitor molecules onto the metal surface, following a Langmuir isotherm.
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