Corrosion inhibition of mild steel by alkylimidazolium ionic liquids in hydrochloric acid

The acid corrosion inhibition process of mild steel in 1 M HCl by 1-butyl-3-methylimidazolium chlorides (BMIC) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) has been investigated using electrochemical impedance, potentiodynamic polarization and weight loss measurements. Potentiodynamic polarization studies indicate the studied inhibitors are mixed type inhibitors. For both inhibitors, the inhibition efficiency increased with increase in the concentration of the inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO₄ > BMIC. The adsorption of the inhibitors on mild steel surface obeyed the Langmuir's adsorption isotherm. The effect of temperature on the corrosion behavior in the presence of 5 × 10⁻³ M of inhibitors was studied in the temperature range of 303-333 K. The associated activation energy of corrosion and other thermodynamic parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), adsorption equilibrium constant (K_ads) and standard free energy of adsorption (ΔG_ads) were calculated to elaborate the mechanism of corrosion inhibition.     

Adsorption and inhibition effect of vanillin on cold rolled steel in 3.0 M H3PO4

The adsorption and inhibition effect of vanillin (4-hydroxy-3-methoxy-benzaldehyde) on cold rolled steel (CRS) in 3.0 M H₃PO₄ at 30–60 °C was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The results show that inhibition efficiency increases with the inhibitor concentration, while decreases with temperature. The adsorption of vanillin obeys Temkin adsorption isotherm. The thermodynamic parameters (adsorption enthalpy ΔH_ads, adsorption free energy ΔG_ads and adsorption entropy ΔS_ads) have been calculated and discussed in detail. Polarization curves show that vanillin acts as a mixed-type inhibitor. EIS shows that charge transfer resistance increases while the capacitance of double layer decreases with the inhibitor concentration, confirming the adsorption process mechanism. The adsorbed film on CRS surface containing vanillin was examined by atomic force microscope (AFM). Quantum chemical calculation was applied to elucidate the adsorption mode of the inhibitor molecule onto steel surface. Depending on the results, the inhibitive mechanism is proposed from the viewpoint of adsorption theory.     

Electrochemical study of 2-mercaptoimidazole as a novel corrosion inhibitor for steels

From electrochemical methods, polarization resistance and electrochemical impedance spectroscopy, the corrosion susceptibility of pipeline steel samples immersed in 1 M H₂SO₄ was determined using nil and different concentrations of the molecule 2-mercaptoimidazole (2MI). It was found that a corrosion inhibiting efficiency, IE, value of 98.5% was reached when the 2MI concentration in the system increased to 25 ppm. Moreover, at this concentration, the 2MI IE was measured as a function of time finding that the IE kinetics follows the relationship: %IE = 98.5 − 0.03t after 800 h of evaluation. During the first 200 h 2MI IE was higher that 90% then, it decreased to 70% and it remained constant up to 1200 h. It is shown that this compound can affect both the anodic and cathodic processes, thus it can be classified as a mixed-type inhibitor however, from variation of both corrosion potential and polarization resistance with [2MI] it was possible to state that the anodic reaction rate, of the corrosion process, decreases at a greater proportion than the cathodic one. 2MI follows an adsorption mechanism, which can be adequately described by the Langmuir isotherm with an adsorption standard free energy difference (ΔG°_ads) of −26.8 kJ mol⁻¹. In order to analyze the influence of substituting groups, both electron-donating and electron-attracting and the number of π-electrons on the corrosion inhibiting properties of organic molecules, an electrochemical study was carried out on four different molecules having similar chemical framework structure: 2-mercaptoimidazole (2MI), 2-mercaptobenzoimidazole (2MBI) 2-mercapto-5-methylbenzimidazole (2M5MBI) and 2-mercapto-5-nitrobenzimidazole (2M5NBI). It was found that the IE order followed by the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. Thus 2MI turned out to be the best inhibitor, even superior to the 2MBI. This fact strongly suggests that, contrary to a hitherto generally suggested notion, an efficient corrosion inhibiting molecule does not require to be a large one, also bearing an extensive number of π-electrons.     

Adsorption and corrosion inhibition behaviour of N-(phenylcarbamothioyl)benzamide on mild steel in acidic medium

Corrosion inhibition property of N-(phenylcarbamothioyl)benzamide (PCB) on mild steel in 1.0 M HCl solution has been investigated using chemical (weight loss method) and electrochemical techniques (potentiodynamic polarization and AC impedance spectroscopy). The inhibition efficiencies obtained from all the methods are in good agreement. The thiourea derivative is found to inhibit both anodic and cathodic corrosion as evaluated by electrochemical studies. The inhibitor is adsorbed on the mild steel surface according to Langmuir adsorption isotherm. The adsorption mechanism of inhibition was supported by spectroscopic (UV-visible, FT-IR, XPS), and surface analysis (SEM-EDS) and adsorption isotherms. The thermodynamic parameter values of free energy of adsorption (ΔG_ads) reveals that inhibitor was adsorbed on the mild steel surface via both physisorption and chemisorption mechanism.     

Electrochemical impedance spectroscopy study of the passive films of alloy 22 in low pH nitrate and chloride environments

Utilizing electrochemical impedance spectroscopy (EIS), we characterize the passive film properties of alloy 22 during immersion in low pH nitrate and chloride solutions. In pure HCl, the passive film grows thinner with increasing acid concentration. In contrast, in HNO₃, the passive film corrosion protection properties are enhanced, which leads to low corrosion rates, even at pH < −0.5. The combined influence of both HCl and HNO₃ in contact simultaneously with the alloy 22 surface shows multiple phases in the passive film properties depending on the pH. EIS results show that the passive film changes either thickness and/or composition as the system is driven chemically through different corrosion states, including: active, passive, active/passive and transpassive.     

A dissymmetric bis-quaternary ammonium salt gemini surfactant as effective inhibitor for Q235 steel in hydrochloric acid

The inhibition effect and adsorption behavior of a novel dissymmetric bisquaternary ammonium salt (DBAL) for Q235 steel in 1 M hydrochloric acid medium were investigated using weight loss method, polarization and electrochemical impedance spectroscopy (EIS). The result of weight loss method indicates that the inhibition efficiency increased with DBAL concentration and temperature in the studied range. The inhibition efficiency is above 90% at a concentration of 3.28 × 10⁻⁴ M in the temperature range of 298–328 K. The polarization measurements reveal that DBAL is a mix-type inhibitor which mainly inhibits cathodic process. EIS results show that changes in the impedance parameters (R_ct and C_d) were due to the formation of a protective layer on the Q235 steel surface by the adsorption of inhibitor molecules. Adsorption follows the Langmuir isotherm via chemical adsorption on the Q235 steel surface. Thermodynamic and kinetic parameters were evaluated from the effect of temperature on corrosion and inhibition processes to discuss the adsorption mechanism.     

Bis-Schiff bases of isatin as new and environmentally benign corrosion inhibitor for mild steel

Inhibition effect of three bis-Schiff bases of isatin namely (2-methoxybenzylidene) hydrazono) indolin-2-one (HZ-1), (2-hydroxybenzylidene) hydrazono) indolin-2-one (HZ-2) and (4-nitrobenzylidene) hydrazono) indolin-2-one (HZ-3) was studied on mild steel corrosion in 1.0 M HCl by gravimetric measurements, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and quantum chemical study. The values of activation energy (E_a), equilibrium constant (K_ads), free energy of adsorption ΔG°_ads, activation enthalpy ΔH^* and activation entropy ΔS^*were discussed. The adsorption of inhibitors on metal followed Langmuir's adsorption isotherm. SEM and EDX observations confirmed the existence of protective inhibitor film on metal surface. Quantum chemical study supports the comparative inhibition effect of HZs.     

Corrosion inhibition of pipeline steel grade API 5L X52 immersed in a 1 M H2SO4 aqueous solution using heterocyclic organic molecules

The effect of the concentration of four types of inhibiting heterocyclic molecules on the corrosion susceptibility of steel pipeline samples grade API 5L X52 in 1 M H₂SO₄ was studied by electrochemical testing and SEM analysis. The compounds used were: 2-mercaptobenzoimidazole (MBI), 5-mercapto-1-tetrazoleacetic sodium salt (MTAc), 1-hydroxybenzotriazole (HBT) and benzimidazole (BIA). The results showed that there was an optimum inhibitor concentration at which the maximum inhibiting efficiency, IE, was reached. Further, the MBI displayed the best inhibiting characteristics for this system, with a maximum IE of approximately 99% having added only 25 ppm. It is shown that this compound can affect both the anodic and cathodic processes, thus it can be classified as a mixed-type inhibitor for API 5L X52 steel corrosion in sulphuric acid. Moreover, this compound follows an adsorption mechanism, which can be adequately described by the Langmuir isotherm with an adsorption standard free energy difference (ΔG°) of −28.5 kJ mol⁻¹.     

Investigation of corrosion behaviors of Mg-6Gd-3Y-0.4Zr alloy in NaCl aqueous solutions

The corrosion behaviors of Mg-6 wt.%Gd-3 wt.%Y-0.4 wt.%Zr (GW63K) alloy in 5 wt.% NaCl aqueous solutions have been investigated in both as-cast and T6 heat treatment conditions by immersion test and electrochemical techniques. The corrosion products and their morphologies formed in the solution after immersing various time have been analyzed by FE-SEM and XRD. It was found that the morphologies of the film of corrosion products varied with immersion time and changed from honeycomb into porous structure composed of tiny erect flaks after immersing 60 min. The phenomena of undermining and falling away of the noble second phase particles observed in the test can be used as an evidence to support the Mechanism II explaining the mechanism of negative difference effect (NDE) about magnesium and its alloys. It was indicated that the E_CORR of GW63K alloy in T6 condition (−1.71 V) is little nobler than that in as-cast (−1.73 V). It was shown by pitting scans that GW63K alloy has tendency to pit whether in as-cast or in T6 condition and the tendency in T6 condition is more serious than that in as-cast. An equivalent circuit was proposed based on the EIS experimental data and the honeycomb or porous characteristics of the corrosion products formed on the surface of GW63K alloy.     

Inhibitory effect of non-ionic surfactants of the TRITON-X series on the corrosion of carbon steel in sulphuric acid

The corrosion inhibition characteristics of non-ionic surfactants of the TRITON-X series, known as TRITON-X-100 and TRITON-X-405, on stainless steel (SS) type X4Cr13 in sulphuric acid were investigated by potentiodynamic polarisation measurements. It was found that these surfactants act as good inhibitors of the corrosion of stainless steel in 2 mol L⁻¹ H₂SO₄ solution, but the inhibition efficiency strongly depends on the electrode potential. The polarisation data showed that the non-ionic surfactants used in this study acted as mixed-type inhibitors and adsorb on the stainless steel surface, in agreement with the Flory-Huggins adsorption isotherm. Calculated ΔG_ads values are −57.79 kJ mol⁻¹ for TRITON-X-100, and −87.5 kJ mol⁻¹ for TRITON-X-405. From the molecular structure it can be supposed that these surfactants adsorb on the metal surface through two lone pairs of electrons on the oxygen atoms of the hydrophilic head group, suggesting a chemisorption mechanism.     

Acetyl thiourea chitosan as an eco-friendly inhibitor for mild steel in sulphuric acid medium

Acetyl thiourea chitosan polymer (ATUCS) has been synthesized and evaluated as corrosion inhibitor. The electrochemical behavior of mild steel in naturally aerated 0.5 M H₂SO₄ acid containing different concentrations of ATUCS has been studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) measurements and surface examination via scanning electron microscope (SEM) technique. The results of EIS showed that the resistance (R_t) increases slightly with increasing immersion time indicating a slight decrease in corrosion rate of the steel with time. Also, the corrosion rate increases with either increasing temperature or decreasing the polymer concentration as observed by polarization technique. Electrochemical impedance spectroscopy measurements under open-circuit conditions confirmed well polarization results. ATUCS has shown very good inhibition efficiency (IE) in 0.5 M sulphuric acid solution reaches to 94.5% for 0.76 mM concentration. IE of this compound has been found to vary with the concentration of the polymer solution, immersion time and temperature.     

Influence of silica nanoparticles added to an organosilane film on carbon steel electrochemical and tribological behaviour

The electrochemical behaviour and tribological properties of carbon steel coated with bis-[trimethoxysilylpropyl]amine (BTSPA) filled with SiO₂ were evaluated. The silane film filled with SiO₂ was prepared by adding different SiO₂ concentrations. The electrochemical behaviour of the coated steel was mainly evaluated by means of open-circuit potential (E_OC), electrochemical impedance spectroscopy (EIS) and polarization curves, in 0.1 mol L⁻¹ NaCl solution. Structural and morphological characterizations were made by optical, electron and atomic force microscopy (AFM). E_OC and EIS data showed that sample filled with 300 ppm SiO₂ presented the highest E_OC and total impedance value. AFM measurements showed a homogeneous particle distribution of SiO₂ particles. Nanohardness measurements showed SiO₂ promoted an increase of the hardness mean value (1.70 ± 0.11 GPa to non-filled BTSPA and 2.21 ± 0.05 GPa for sample filled with 300 ppm SiO₂). Silane films when filled with SiO₂ particles improved the corrosion resistance of the steel substrate. The optimum SiO₂ particles concentration in silane solution is 300 ppm SiO₂. Incorporation of an extra amount of silica into BTSPA film led to degradation of the corrosion protection of the film to the substrate.     

Corrosion inhibition of mild steel in 1.0 M hydrochloric acid medium by new photo-cross-linkable polymers

Photo-cross-linkable polymers namely, poly((E)-(1-(5-(4-(3-(4-chlorophenyl)-3-oxoprop-1-enyl)phenoxy)pentyl)-1H-1,2,3-triazol-4-yl)methyl acrylate) (Cl-5) and poly((E)-(1-(5-(4-(3-(4-chlorophenyl)-3-oxoprop-1-enyl)phenoxy)decyl)-1H-1,2,3-triazol-4-yl)methyl acrylate) (Cl-10) were synthesized by click-chemistry. The polymers were characterized by using various spectroscopic techniques and the rate of cross-linking was evaluated by absorption spectroscopy. The inhibitory action of the photo-cross-linkable polymers was evaluated for mild steel (MS) corrosion in 1.0 M hydrochloric acid solution (HCl) by means of electrochemical impedance spectroscopy, potentiodynamic polarization measurements, adsorption isotherms and surface analysis. To the best of our knowledge, these are the most efficient inhibitors (Cl-5 and C-10) for the corrosion of MS in HCl reported so far. Tafel polarization measurements showed that the polymers act as mixed type inhibitors and the adsorption of the inhibitors onto the MS surface followed the Langmuir adsorption isotherm. The values of the Gibbs free energy of adsorption (ΔG_ads) strongly supported spontaneous physicochemical adsorption of inhibitor molecules on the MS surface. The SEM-EDX results confirmed that the cross-linked polymers inhibited the corrosion to a greater extent than the intact polymer.     

Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc

A trivalent chromium based conversion coating (CCC), based on chromium nitrate solution with Co(II) ions, was developed on Zn substrate. The corrosion resistance of the trivalent CCC, measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution using anodic polarization and electrochemical impedance spectroscopy (EIS), was very sensitive to both immersion time and bath pH. Micro-cracks were found on the surface of the CCC. Besides, the density of micro-crack and the coating thickness also depended on immersion time and bath pH. With increasing the coating thickness its pitting potential increased and passive current density decreased. The trivalent CCC formed on Zn for 40 s in pH 1.7 bath showed the best corrosion resistance, and the pitting potential increased significantly from −355 mV_SCE for Zn to 975 mV_SCE for the trivalent CCC on Zn. To explain the corrosion behavior of the trivalent CCC using EIS analysis, a modified equivalent circuit, which considered the micro-cracks in the coating and chromium corrosion product (CCP) deposited in the micro-cracks, was designed and the variation of each electrical parameter was examined. Especially, its corrosion behavior was well described by the variation of the resistance of CCP (R_ccp).     

Electrochemical characterization of the protective film formed by the unsymmetrical Schiff's base on the mild steel surface in acid media

The corrosion inhibition efficiency of a newly synthesized Schiff's base for the corrosion of mild steel was studied in 1.0 M HCl and 0.5 M H₂SO₄ solutions. The results of weight loss measurements, electrochemical impedance and potentiodynamic polarization measurements consistently demonstrated that the Schiff's base synthesized is a good corrosion inhibitor with an inhibitory efficiency of approximately 92% at an optimum inhibitor concentration of 600 mg/L. The inhibition in both of the corrosive media was observed to be a mixed type. The potential of zero charge (PZC) at the metal–solution interface was determined for both the inhibited and uninhibited solutions to provide the mechanism of inhibition. The inhibitor formed a film on the metal surface through chloride or sulfate bridges depending upon the medium. The temperature dependence of the corrosion rate was also studied in the temperature range from 27 to 50 °C. The value of the activation energy (E_a) calculated showed that the inhibition film formation on the metal surface occurred through chemisorption. The thermodynamic parameters such as the adsorption equilibrium constant (K_ads) and the free energy of adsorption (ΔG_ads) were calculated and discussed. Several adsorption isotherms were tested and the experimental data fit well with the Langmuir adsorption isotherm.     

A study of adsorption kinetics and thermodynamics of ω-mercaptoalkylhydroquinone self-assembled monolayer on a gold electrode

The adsorption kinetics and thermodynamics for the formation of redox active self-assembled monolayer (SAM) of 2-(n-mercaptoalkyl)hydroquinone (abbreviated as H₂Q(CH₂)_nSH, where n = 4, 6, 8, 10, and 12) on gold electrode has been investigated by cyclic voltammetry to study the effects of concentration and alkyl chain length. The time dependence of surface coverage, differential capacitance, and formal potential of electroactive hydroquinone(H₂Q) moiety supports that the adsorption of H₂Q(CH₂)_nSH molecules typically processes with a two-step adsorption consisted of a fast initial adsorption and a slowly following reorganization. The adsorption processes can be satisfactorily described by simple Langmuir adsorption kinetics, irrespective of concentration and alkyl chain length of adsorbate molecule. Based on Langmuir kinetics, the adsorption rate constant was determined at the initial step for the formation of all H₂Q(CH₂)_nSH-SAMs studied in this work. The rate constant value was found to be decreased with increasing alkyl chain length and decreasing bulk solution concentration (≤10 μM). The dependence of a surface coverage (Γ_e) at adsorption equilibrium on the bulk concentration is accurately described by the Langmuir isotherm at several concentrations ranging from 8 × 10⁻⁶ to 1 × 10⁻⁵ M for all H₂Q(CH₂)_nSH molecules. Parameters characterizing the adsorption thermodynamics, such as Γ_s, adsorption coefficient (β), and adsorption free energy (ΔG_ads) were determined from this isotherm.     

Corrosion inhibition of stainless steel by polyaniline,poly(2-chloroaniline), and poly(aniline-co-2-chloroaniline) in HCl

Polyaniline (PANi), poly(2-chloroaniline) (PClANi), and poly(aniline-co-2-chloroaniline) (co-PClANi) films were synthesized by electrochemical deposition on 304-stainless steel (SS) from an acetonitrile solution. The structural properties of these polymer films were characterized by spectroscopic (FTIR and UV–vis) and electrochemical (cyclic voltammetry) methods. Open circuit potential–time (E_ocp–time) curves, potentiodynamic polarization, and electrochemical impedance (EIS) measurements showed that these films have significant protective performance against corrosion of SS in 0.5 M HCl solution. It was found that co-PClANi film has acted as a passivator as well as barrier for cathodic reduction reaction in a similar manner as PANi film. However, PClANi film has behaved only as barrier for corrosion protection of SS in 0.5 M HCl.     

Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions

The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E_oc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. E_oc shifts positively with time until attaining a steady (E_st) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F⁻ > I⁻ > Br⁻ > Cl⁻. This sequence is consistent with that for surface film resistance (R_T) and its relative thickness (1/C_T). Nevertheless, in F⁻ medium each of the above parameters increases with [F⁻] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br⁻ and I⁻ solutions, as well as the lower Cl⁻ concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E_corr) to the knee point (E_pt) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl⁻ concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where E_pt lies negative to E_corr. Addition of F⁻ to the Cl⁻ solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection.     

Electrochemical behaviour of titanium in H2SO4-MnSO4 electrolytes

The corrosion of titanium in H₂SO₄ electrolytes (0.001-1.0 M) at temperatures from ambient to 98 °C has been investigated using steady-state polarization measurements. Four distinct regions of behaviour were identified, namely active corrosion, the active-passive transition, passive region and the dielectric breakdown region. The active corrosion and active-passive transition were characterized by anodic peak current (i_m) and voltage (E_m), which in turn were found to vary with the experimental conditions, i.e., d(log?(im))/dpH=−0.8±0.1 and dE_m/dpH which was −71 mV at 98 °C, −58 mV at 80 °C and −28 mV at 60 °C. The activation energy for titanium corrosion, determined from temperature studies, was found to be 67.7 kJ mol⁻¹ in 0.1 M H₂SO₄ and 56.7 kJ mol⁻¹ in 1.0 M H₂SO₄. The dielectric breakdown voltage (E_d) of the passive TiO₂ film was found to vary depending on how much TiO₂ was present. The inclusion of Mn²⁺ into the H₂SO₄ electrolyte, as is done during the commercial electrodeposition of manganese dioxide, resulted in a decrease in titanium corrosion current, possibly due to Mn²⁺ adsorption limiting electrolyte access to the substrate.