Corrosion protection of steel by bi-layered polypyrrole doped with molybdophosphate and naphthalenedisulfonate anions

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPy coating consisted of the inner layer doped with PMo₁₂ (PMo₁₂) and ions and the outer layer doped with naphthalenedisulfonate (NDS) ions. PMo₁₂ ions doped in the inner layer make the passive oxide film on steel stabilized and the outer layer doped with large organic ions of NDS restricts decomposition and release of PMo₁₂ ions in the inner layer. Corrosion tests were made in 3.5% NaCl aqueous solution for the steels covered by the single PPy-PMo₁₂, single PPy-NDS, and bi-layered PPy-PMo₁₂/PPy-NDS coatings. The single PPy-PMo₁₂ and PPy-NDS coatings maintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating kept the steel in passive state and prevented corrosion of the steels for the longer time period.     

Electrochemical behavior of tin in sodium borate solutions and the effect of halide ions and some inorganic inhibitors

The corrosion of tin electrode in sodium borate (Na₂B₄O₇) solutions was investigated using cyclic voltammetry and potentiostatic current transient techniques. In absence of halide ions, the E/j response exhibits active/passive transition. The active region involves one anodic peak corresponding to the formation of Sn(OH)₂ and/or SnO. Addition of Cl⁻, Br⁻ or I⁻ (C ? 0.01 M) ions inhibits the active dissolution of tin, but higher concentrations enhance the active dissolution and tend to breakdown the passive film and induce pitting attack. The effect of , , and as inorganic inhibitors on the pitting corrosion of tin in (0.1 M Na₂B₄O₇ + 0.1 M NaCl) solution has also been studied. The presence of these anions (except ) inhibits pitting corrosion. Chronoamperometry measurements showed that nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation () increases with increasing halide ions concentration and applied potentials, but decreases with increasing the concentration of the inorganic inhibitors (except ). The inhibition efficiency of these inhibitors decreases in the order:

     

The effect of ultrasonic irradiation during electropolymerization of polypyrrole on corrosion prevention of the coated steel

Ultrasonic irradiation was imposed during electropolymerization of polypyrrole (PPy) in acid phosphate solution containing molybdophosphate () ions and pyrrole monomer. Corrosion of the steel coated by the PPy film prepared under ultrasonic irradiation was tested in 3.5 wt.% NaCl solution and compared with corrosion of the steel coated by the PPy film without ultrasonic irradiation. The PPy film prepared under ultrasonic irradiation kept the steel in the passive state one and a half times as long as that prepared without ultrasonic irradiation. Imposition of ultrasound enhanced the doping of and decreased the doping of . The surface morphology of the PPy film was changed with imposition of ultrasound in electropolymerization process. Under imposition of ultrasound, a dense and compact PPy layer was formed. The structure of the PPy film obtained under ultrasonic irradiation was assumed to result from change in nucleation-growth mechanism.     

The application of electrochemical noise resistance to evaluate the corrosion resistance of AISI type 304 SS in nitric acid

The paper presents the application of noise resistance to evaluate the corrosion behaviour of sensitized AISI type 304 SS in nitric acid of varying concentration (4 N, 12 N, 16 N) and temperature (298 K, 323 K, 348 K). Electrochemical noise data was acquired from a three identical electrode configuration in the required conditions at open circuit potential. The noise resistance was evaluated as the ratio of the standard deviation of the potential to that of the current noise after removing the DC component. The inverse relationship between noise resistance and corrosion rate was exploited to qualitatively assess the corrosion behaviour of AISI type 304 SS in nitric acid. Noise resistance decreased with increase in concentration implying an increase in corrosion rate with increase in nitric acid concentration. An increase in temperature from 298 K to 323 K and 348 K decreased the noise resistance in 4 N and 12 N nitric acid implying higher corrosion rates at higher temperatures. The corrosion rates were similar at 323 K and 348 K in these concentrations. The simultaneous measurement of current and potential noise facilitated the evaluation of the frequency dependence of the noise data to determine the spectral noise resistance (R_sn) and the DC limit of the spectral noise resistance . The results from R_sn and also indicated higher corrosion rates at higher concentration and temperature. Also R_n and correlated well in 4 N and 12 N nitric acid at 323 K and 348 K while disparity was observed at room temperature in 4 N and 12 N nitric acid.     

Effects of temperature and aggressive anions on corrosion of carbon steel in potable water

Experiments on the corrosion of carbon steel in three kinds of artificial potable waters containing different concentrations of aggressive anions, Cl⁻ and , open to air in the temperature range from 5 to 60 °C were conducted using weight loss and electrochemical methods. The corrosion rate increased in proportion to the concentration of aggressive anions and with increasing temperature. At lower concentrations of aggressive anions and temperatures, the corrosion rate was lower and the corrosion potential was higher, at intermediate concentrations and temperatures, the corrosion rate increased logarithmically with decreasing potential, and at higher concentrations and temperatures, the corrosion rate was higher and the potential was lower. The corrosion behavior in potable water can be determined by the balance between inhibitive action of oxygen (passive film formation) and aggressive action of Cl⁻ and ions.     

Bis-isoxazolidines: A new class of corrosion inhibitors of mild steel in acidic media

Two new bis-isoxazolidines were synthesized in excellent yields via cycloaddition reaction of 1-pyrroline 1-oxide with 2,7-di(10-undecenyloxy)naphthalene and 1,4-di(10-undecenyloxy)benzene. One of the bis-isoxazolidines, on reaction with two equivalents of 1-bromododecane, afforded a bis-quaternary ammonium salt. All three inhibitor molecules in the presence of 400 ppm at 60 °C achieved inhibition efficiencies (IE) in the ranges 97-98% and 92-96% as determined by gravimetric method for corrosion of mild steel in 1 M HCl and 0.5 M H₂SO₄, respectively. The results obtained by the electrochemical methods using Tafel plots and electrochemical impedance spectroscopy corroborated the findings of the weightloss method. Both bis-isoxazolidines demonstrated remarkable efficiency in the lower range of inhibitor concentrations - in the presence of a meager 1.5 ppm of the inhibitor molecules, IEs of ∼70% in 1 M HCl were achieved. Thermodynamic parameters (, , ) for the adsorption process in the presence of the bis-isoxazolidines were determined. The values of around -90 kJ/mol in 1 M HCl and −50 kJ/mol in 0.5 M H₂SO₄, pointed towards the chemisorption of the inhibitor molecules, especially in HCl media. While the corrosion inhibition by these molecules was predominantly under cathodic control in 1 M HCl, the inhibition in 0.5 M H₂SO₄ was found to be under anodic control. The bis-isoxazolidines were found to provide a suitable inhibition mechanism for the corrosion inhibition in HCl as well as in H₂SO₄ media.     

Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C

New Pourbaix diagrams were calculated at 25, 75 and 95 °C for the Nb-H₂O system. The species and were considered. Potentiodynamic polarization and mass loss experiments (14 days) were conducted in concentrated H₂SO₄ (20, 40 and 80 wt%) and HCl (20 and 38 wt%) solutions at 75 and 95 °C. Nb forms a metastable pentoxide (Nb₂O₅) in H₂SO₄ and HCl solutions which dissolves as . Corrosion rates decrease between the 40% and the 80% H₂SO₄ solutions. SEM micrographs show generalized pitting in the 20% and 40% H₂SO₄ solutions. Mass loss corrosion rates did not exceed 306 μm/yr. Corrosion rates estimated by Tafel extrapolation were within two orders of magnitude of those measured by mass loss and it is shown that this finding is consistent with the thickening of the oxide.     

Voltammetric current oscillations due to general and pitting corrosion of tantalum: Implications for electrochemical-mechanical planarization

Anodic corrosion of Ta is examined for potential applications in electrochemical-mechanical planarization (ECMP) of diffusion barriers. This strategy involves electro-oxidation of Ta in the presence of (or Br⁻) to form mechanically weak surface-oxide films, followed by mechanical removal of the latter. The voltammetric currents exhibit oscillatory behaviour with frequencies that are signature attributes of localised pitting by Br⁻ or general surface corrosion by . SEM, voltammetry, and impedance spectroscopy are used to probe these corrosion mechanisms. Apart from their relevance for ECMP, the results also address certain fundamental aspects of pitting and general corrosion of valve metals.     

Effects of inorganic additions on the performance of manganese-based conversion treatments

The electrochemical behaviour of permanganate-based no-rinse conversion coatings on AA3003-H14 aluminium alloy shows that they have promise as an alternative to toxic chromate-based coatings. A selection of inorganic additions has been made here to a simple 0.1 M KMnO₄ formulation investigated previously. The performance of the different coatings has been evaluated from potential-time and polarisation measurements in 0.5 wt % (0.085 M) NaCl solution for periods up to 3 days. The addition of fluoride ions produced no benefit, whereas Al(NO₃)₃ shows an improvement when added in small amounts. Tests with the individual ions reveal that it is Al³⁺ cations, rather than anions, which produce the greatest benefit.     

Inhibition of iron corrosion in acid solutions by Cefatrexyl: Behaviour near and at the corrosion potential

The corrosion inhibition of iron in HCl, HClO₄, H₂SO₄ and H₃PO₄ solutions (1M for each) by cefatrexyl has been studied by polarization resistance (R_p) and electrochemical impedance spectroscopy (EIS) at the corrosion potential. The results obtained at 30 °C revealed that cefatrexyl acts as a weak inhibitor in HCl solution while it shows excellent inhibition performance in the remaining acids. Adsorption of cefatrexyl in HCl solution obeys Langmuir’s isotherm with a very low value of the free energy of adsorption (physisorption) while its adsorption in the other acids follows Temkin’s isotherm with very high negative values of (chemisorption). Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. The calculated values of the apparent activation energy (E_a) and the pre-exponential factor (λ) indicate that cefatrexyl blocks nearly the whole active centers of iron surface in H₃PO₄ solution even at elevated temperatures. The inhibition mechanism of cefatrexyl was discussed.     

Inhibitive effect of diethylcarbamazine on the corrosion of mild steel in hydrochloric acid

Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurements and atomic force microscopy techniques were used to investigate the inhibitory effect of diethylcarbamazine (DECM) on corrosion of mild steel in HCl solution. The inhibitor showed >90% inhibition efficiency at 5.01 × 10⁻⁴ M. Results obtained revealed that inhibition occurs through adsorption of inhibitor molecules on metal surface without modifying the mechanism of corrosion process. Potentiodynamic polarization studies suggested that it is a mixed type inhibitor, predominantly controls cathodic reaction. Activation parameters (E_a, ΔH and ΔS) and thermodynamic parameters (, and ) were calculated to investigate mechanism of inhibition.     

Reaction model for kinetic of cobalt dissolution in carbonate/bicarbonate media

Cobalt dissolution in a carbonate/bicarbonate media was studied in order to elucidate the kinetic of the metal corrosion processes. Steady-state polarization and electrochemical impedance measurements were carried out during the active metal dissolution at different electrode rotation speeds. A reaction mechanism is presented in order to explain the metal dissolution: cobalt electrodissolution starts with the formation of intermediate (CoHCO₃)_ads on the electrode surface, followed by the simultaneous formation of a complex and the formation of a CoO film. Mass transport is characterised by diffusion of the complex away from the electrode. The reaction model was validated by an accurate simulation of the experimental steady-state polarisation and impedance measurements.     

Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions

The pitting corrosion current of reinforcing steel is measured under natural corrosion conditions in Ca(OH)₂ solutions in presence of Cl⁻ as aggressive ions and as inhibiting anions. The corrosion current starts to flow after an induction period which depends on solution composition (concentration, pH and presence or absence of the aggressive and the inhibiting anions). The limiting corrosion currents increase with increasing the Cl⁻ ion concentration and decrease with increasing the pH and inhibiting ions concentration. The inhibition efficiency of the studied inhibiting ions increases in the following order: , and depends on the way by which the inhibitor is added to the solution. Injection of the inhibiting anions in solution causes repassivation of the pre-formed pits through competition with Cl⁻ ions for adsorption sites on metal oxide surface. The adsorbability constant and the free energy of repassivation of the inhibiting anions are calculated.     

Photoelectron spectroscopy study of the inhibition of mild steel corrosion by molybdate and nitrite anions

Passive films formed on mild steel in aqueous 8.6 mM NaCl solutions (pH 8), containing either or , have been studied with X-ray photoelectron spectroscopy. For either anion these films are ∼5 nm deep, and the primary chemical state of iron is Fe³⁺. Following exposure to , the film consists of a sub-layer (∼4.1 nm) composed largely of ferric oxide/hydroxide, overlaid by Fe₂(MoO₄)₃ (∼0.6 nm). As regards , spectra are consistent with the film being closely related to γ-Fe₂O₃. Furthermore, a reduction product of , potentially N₂, is present, displaying a depth profile comparable to that of molybdate.     

Pitting and pitting control of Al in gluconic acid solutions - Polarization, chronoamperometry and morphological studies

The anodic behaviour of Al in gluconic acid (HG) solutions was studied. Al was found to pit in such solutions. Surface and cross-sectional views of the SEM images recorded beyond the breakdown potential (E_b) revealed the occurrence of intense pitting attack with the formation of large hemispherical pits. The effect of adding some environmentally acceptable inorganic inhibitors (tungstates, molybdates or silicates) on the pitting corrosion behaviour of Al in HG solutions was also studied. Measurements were carried out under the influence of various experimental variables based on polarization and chronoamperometric techniques. These measurements were complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) examinations of the electrode surface. The presence of these compounds in HG solutions decreased the passive current density (j_pass) and increased E_b. In HG solutions, chronoamperometric measurements showed that the anodic current density first decreased, due to growth of a passive oxide film, then increased with time after a pit incubation time, t_i and finally attained a steady-state value. Value of t_i was shortened and simultaneously the steady-state current was elevated, corresponding to an increase in the rate of pit initiation and growth, with increasing applied anodic potential and HG concentration. The rate of pit nucleation () was found to decrease to an extent depending on the type and concentration of the introduced inhibitor. The inhibitory effect of these compounds decreased in the order:  >  > .     

Experimental and theoretical investigation of organic compounds as inhibitors for mild steel corrosion in sulfuric acid medium

Three synthesized organic compounds were tested as corrosion inhibitors for mild steel in sulfuric acid medium by potentiostatic polarization, FTIR spectroscopy and SEM techniques. Quantum chemical parameters were also calculated to characterize adsorption mechanism. Acceptable correlations were obtained between inhibition efficiency and the calculated quantum chemical parameters. It was found that the investigated compounds exhibit a good inhibition effect especially at 8-10 ppm range concentration, which makes them commercially important. The adsorption of inhibitors on the surface obeys Langmuir adsorption isotherm. The values of activation energy and the thermodynamic parameters, such as K_ads, , and were calculated.     

Effect of 4-(N,N-diethylamino)benzaldehyde thiosemicarbazone on the corrosion of aged 18 Ni 250 grade maraging steel in phosphoric acid solution

4-(N,N-diethylamino)benzaldehyde thiosemicarbazone (DEABT) was studied for its corrosion inhibition property on the corrosion of aged 18 Ni 250 grade maraging steel in 0.67 M phosphoric acid at 30–50 °C by potentiodynamic polarization, EIS and weight loss techniques. Inhibition efficiency of DEABT was found to increase with the increase in DEABT concentration and decrease with the increase in temperature. The activation energy E_a and other thermodynamic parameters (Δ, Δ, Δ) have been evaluated and discussed. The adsorption of DEABT on aged maraging steel surface obeys the Langmuir adsorption isotherm model and the inhibitor showed mixed type inhibition behavior.     

Corrosion behaviour of vacuum induction-melted Ni-based alloy in sulphuric acid

A vacuum induction-melted (VIM) Ni-based alloy was immersed in 60% H₂SO₄ solution to investigate its corrosion behaviour and resistance. The results indicate that the microstructure contains a γ-Ni solid solution + Ni₃Si particles, dendrite Ni₃Si, Ni₃B, Cr₇C₃, and CrB. The corrosion started at the zones of the γ-Ni solid solution + Ni₃Si particles and dendrite Ni₃Si. These zones transformed to oxide films and protected the alloy from significant attack. However, the pitting corrosion created paths for acid solution and/or to further attack. Therefore, the corrosion rate decreased and then stabilised at a high value as the immersion time increased.     

Influence of 1,2-diaminoethane on the mechanism of aluminium corrosion in sulphuric acid solutions

The influence of 1,2-diaminoethane (DAE) on aluminium corrosion in H₂SO₄ solutions (pH 3) was investigated. In pure H₂SO₄, rapid uniform corrosion is followed by inhibition due to the formation of stable Al-sulphate binuclear bidentate metal bound surface complexes via a ligand exchange mechanism with two neighbouring sites. Metastable pitting is also observed. DAE acts as a strong corrosion inhibitor for both uniform and localised corrosion, due to the formation of Al-DAE monodentate hydrogen-bond surface complexes either by direct adsorption of the protonated molecule on Al-OH sites or via a ligand exchange mechanism with the proton of an site.     

Influence of lactate ions on the formation of rust

The formation of rust can be simulated by oxidation of aqueous suspensions of Fe(OH)₂ obtained by mixing solutions of NaOH and a Fe(II) salt. The aim of this study was to investigate the influence of organic species associated with microbially influenced corrosion. The lactate anion, often used as a carbon and electrons source for the development of microorganisms, was chosen as an example. Then, in the first part of the study, Fe(OH)₂ was precipitated using iron(II) lactate and NaOH. Its oxidation process involved two stages, as usually observed. The first stage led to a Fe(II-III) intermediate compound, the lactate green rust, . This compound has never been reported yet. Its existence demonstrates that the GR structure is able to incorporate a very wide range of anions, whatever the size and geometry. The second stage corresponded to the oxidation of . It led to ferrihydrite, the most poorly ordered form of iron(III) oxides and oxyhydroxides. In the second part of the study, the formation of rust in seawater was simulated by oxidation of Fe(OH)₂ in an aqueous media containing both Cl⁻ and anions. The first stage led to the sulphate green rust, , the second stage to lepidocrocite γ-FeOOH. Small amounts of iron(II) lactate were added to the reactants. Lactate ions did not modify the first stage but drastically perturbed the second stage, as ferrihydrite was obtained instead of γ-FeOOH.