The influence of pH on the inhibition of corrosion of iron and mild steel by sodium silicate

The inhibition of iron and mild steel in solutions of sodium silicate of different concentration and pH has been studied by means of potentiostatic and potentiokinetic polarization measurements. The mechanism of formation of protective layers formed in the presence of inhibitor has been discussed on the basis of experimental measurements and literature data. The difference in the protective action due to the presence of sodium silicate as distinguished from the effect of the alkaline media has been demonstrated. The influence of the metal corrosion products in the formation of the protective layers has also been investigated.     

The inhibition activity of ascorbic acid towards corrosion of steel in alkaline media containing chloride ions

The activity of ascorbic acid towards steel corrosion in saturated Ca(OH)₂ solution containing chloride ions was investigated in this study. Concentration and time dependence of the protective properties of the passive film were acquired by electrochemical impedance spectroscopy. The best inhibitive performance, i.e. the longest pitting initiation time was obtained in the presence of 10⁻³ M ascorbic acid, while both lower and higher concentrations showed shortening of the pitting-free period. The overall behaviour of ascorbic acid was attributed to its ability to form chelates of various solubility having various metal/ligand ratios and oxidation states of the chelated iron. The assumption of ascorbic acid assisted reductive dissolution of the passive layer at higher inhibitor concentrations was confirmed by cyclic voltammetry and ATR FTIR spectroscopy. It is proposed that the overall inhibitive effect at lower concentrations is due to the formation of insoluble surface chelates and the effective blocking of the Cl⁻ adsorption at the surface of passive film. A pronounced inhibitive effect observed after the pitting had initiated was ascribed to the formation of a resistive film at the pitted area.     

The inhibition of mild steel corrosion in phosphoric acid solutions by some N-heterocyclic compounds in the salt form

The effect of some quaternary N-heterocyclic compounds on the corrosion of mild steel in solutions of phosphoric acid (H₃PO₄) has been investigated in relation to the concentration of the inhibitor and acid as well as temperature by various monitoring corrosion techniques. Surface morphology was studied by scanning electron microscopy (SEM). Results obtained revealed that these compounds are good mixed-type inhibitors without changing the mechanism of the corrosion process. In general, at constant acid concentration, inhibitor efficiency increased with concentration of the inhibitor. On the other hand, at constant inhibitor concentration, inhibitor efficiency decreased with concentration of the acid up to a critical concentration above which it started to increase. The studied compounds appeared to function through general adsorption following the thermodynamic-kinetic adsorption isotherm. The thermodynamic parameters were determined and discussed. A quantitative correlation between inhibitor structure and inhibition efficiency was discussed.     

The corrosion of mild steel in simulated SO2 containing atmospheres

Electrochemical tests were carried out on steel/steel multi-laminar cells under thin layers of sulphate solution simulating atmospheres with contents of 20, 50 and 75 μg m⁻³ of SO₂ and a relative humidity of 85%. Measurements of the current flowing between the two electrodes of the cell vs drying time and impedance measurements for different times were taken. Potentiodynamic cathodic polarization plots were also obtained for one concentration of contaminant at various drying times using three electrode multi-laminar cells. The results showed that the multi-laminar cells allowed the drying process of the thin electrolyte films to be followed. The use of electrochemical techniques gave information on the kinetics and mechanism of the corrosion process occurring during the drying period. Initially the process is controlled by oxygen diffusion through the liquid film, but for longer periods the control changes to the anodic reaction.     

A comparative pitting corrosion study of mild steel in different alkaline solutions containing salts with sulphur-containing anions

The localized corrosion of mild steel in alkaline solutions containing a salt with a sulphurcontaining anion (sodium sulphate, sodium sulphite, sodium thiosulphate, potassium thiocyanate and sodium sulphide) is studied by using potentiostatic and potentiodynamic techniques complemented with scanning electron microscopy.Alkaline solutions containing one of those salts produce pitting of mild steel at potential values more positive than those of the active-passive transition of iron in base. In the presence of either thiocyanate or thiosulphate anion iron pitting takes place through sulphide formation which reacts at the electrode surface yielding poorly protective ferrous sulphide. At potential more positive than the breakdown potential the kinetic behaviour fits a competitive surface reaction mechanism involving the formation of the passive film and the nucleation and growth of a ferrous sulphide salt layer. The proposed reaction model reproduces the corresponding experimental current-transients.     

Electrochemical characterisation of the ability of dicarboxylic acid salts to the corrosion inhibition of mild steel in aqueous solutions

Electrochemical measurements (electrochemical impedance spectroscopy (EIS), polarisation and open circuit potential (OCP) measurements) were used to study the corrosion protection of mild steel in aerated neutral solution of selected dicarboxylates. It was shown that all investigated carboxylates are effective in the presence of air. Most of the carboxylates act predominantly at local defects in the primary oxide layer by forming of weakly soluble Fe(III) compounds, in agreement with the pore plugging concept. As known, carboxylates, which can also be adsorbed on the oxide covered mild steel surface, should own an enhanced efficiency. It was found that the adsorption ability of dicarboxylates their selves are weak, but the corrosion protective effect could be improved considerably, if mixtures of dicarboxylates with monocarboxylates are used. The additional inhibition effect was ascribed to the ability of monocarboxylates to adsorb stronger on the oxide covered mild steel surface than dicarboxylates. The sebacate/caprinate mixture provides the best protection comparable with the synergistic effect observed with carboxylate in the presence of benzotriazole, too.     

Inhibition of steel and galvanised steel corrosion by zinc and calcium ions in the presence of phosphate

Abstract

Mixtures of zinc, calcium and phosphate ions have been found to be effective (>90%) in inhibiting the corrosion of steel and zinc during immersion in acid rain solution and when used as pigment additions to organic coatings. The presence of calcium ions, in addition to zinc ions, appears to be fundamental in the inhibition mechanism since, in the absence of calcium, the inhibition efficiency is 50% or less. Although some anodic inhibition occurs, the predominant mechanism appears to be classical cathodic inhibition caused by the precipitation of a sparingly soluble, thin, but persistent, solid film. Surface analysis, by XPS, confirms that phosphorus is present in the form of phosphate species containing zinc and calcium. Also, the atomic ratios of zinc:calcium:phosphorus present in the inhibitive film strongly suggest that it consists of a mixture of CaZn₂(PO₄)₂ (Scholzite) and Zn₃(PO₄)₂ (Hopeite) in a 1 : 1 ratio.     

Anodic dissolution of mild steel in HCl solutions containing thio-ureas

The effect of thio-urea and its derivatives on the anodic dissolution of mild steel in HCl solutions has been studied. Thio-urea and N-substituted derivatives show maximum anodic polarisation at low concentrations. The anodic Tafel slope shows an abrupt change in the intermediate concentration range, i.e. 4–40 mM. This has been explained in terms of protonated species taking part in metal dissolution. S-substituted derivatives exert increasing anodic polarisation with increasing concentration.     

Synergism and antagonism in mild steel corrosion inhibition by sodium dodecylbenzenesulphonate and hexamethylenetetramine

The inhibition effects of sodium dodecylbenzenesulphonate (SDBS) and hexamethylenetetramine (HA) on the corrosion of mild steel in sulphuric acid solution have been studied using weight loss, electrochemical impedance and Tafel polarisation measurements. For HA, a monotonous increase in inhibition efficiency is observed as a function of concentration. For SDBS, however, an optimum in the inhibition efficiency is observed for a concentration close to 250 ppm, which is ascribed to the formation of hemi-micellar aggregates that provoke inhibitor desorption from the metal/solution interface at higher concentrations. Upon mixing HA and SDBS, concentration regions showing synergistic and antagonistic inhibition behaviour are identified, and it is concluded that electrostatic interactions between adsorbate ions are likely responsible for both phenomena. Langmuir and Frumkin isotherms were tested for relevance in describing the adsorption behaviour of both HA and SDBS.     

Corrosion inhibition of mild steel in aqueous solutions using nonionic surfactants

Protection of Metals and Physical Chemistry of Surfaces - The inhibiting effect of nonionic surfactant of Tween-20 and 60 on the corrosion of mild steel in 0.5 M HCl was studied by weight loss,...     

The inhibition of mild steel corrosion in acidic medium by 1-methyl-3-pyridin-2-yl-thiourea

The effect of 1-methyl-3-pyridin-2-yl-thiourea on the corrosion resistance of mild steel in H₂SO₄ solution was investigated by different techniques. The results show that the inhibition efficiency increases with the increase of inhibitor concentration. This compound affects both the anodic dissolution of steel and the hydrogen evolution reaction in 0.5 M H₂SO₄. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25-45 ± 1 °C E_a, ΔH_a and possible mechanism have been proposed.     

The effect of stress on the corrosion behaviour of mild steel in alkaline solutions

The influence of stress, type of aggressive anion and anodic current density on the corrosion behaviour of mild steel in lime water has been studied. Under open circuit condition the passivity of steel in lime water containing low concentrations of C1?, SO₄^2? or S^2? ions was impaired by the application of stress. In the presence of high concentrations of these ions corrosion was enhanced by the application of stress. Sulphide ions were found to be more dangerous than C1- or SO₄^2? ions at the free corrosion potential. With anodic polarization, S^2? ions are inhibitive both in the absence and in presence of stress. In early stages at low current densities, the anodic process was stimulated by the application of stress.     

Corrosion behaviour of steel in solutions containing mixed inhibitive and aggressive ions

The action of sodium benzoate, sodium chromate and sodium nitrite in relation to the presence of sodium chloride, sodium nitrate, sodium sulphate and sodium sulphide has been studied by means of visual observation as well as potential measurements. It has been found that the critical concentration of the inhibitor for protection in distilled water could tolerate a range of concentration of the aggressive anion. The linear double logarithmic relationship between the concentration of an inhibitive anion and the maximum tolerated concentration of an aggressive anion is only beyond a certain critical concentration of the aggressive anion. The results are interpreted in terms of competitive adsorption between the inhibitor and aggressive ions on the bare metallic parts.     

A comparative study of the corrosion inhibition of mild steel in sulphuric acid by 4,4-dimethyloxazolidine-2-thione

The corrosion protection of mild steel in a 2.5 M H₂SO₄ solution by 4,4-dimethyloxazolidine-2-thione (DMT) was studied at different temperatures by measuring changes in open circuit potential (OCP), potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). Corrosion current densities calculated from EIS data were comparable to those obtained from polarisation measurements. Results showed that DMT inhibited mild steel corrosion in a 2.5 M H₂SO₄ solution and indicated that the inhibition efficiencies increased with the concentration of inhibitor, but decreased proportionally with temperature. Polarisation curves showed that DMT is a mixed-type inhibitor. Changes in impedance parameters suggested the adsorption of DMT on the mild steel surface, leading to the formation of protective films. The DMT adsorption on the mild steel surface followed the Langmuir adsorption isotherm. The kinetic and thermodynamic parameters for dissolution and adsorption were investigated. Comprehensive adsorption (physisorption and chemisorption) of the inhibitor molecules on the mild steel surface was suggested based on the thermodynamic adsorption parameters.     

Effect of calcium ions on pitting corrosion and inhibition performance in CO2 corrosion of N80 steel

The electrochemical impedance spectroscopy (EIS) and polarization resistance technique were used to investigate the susceptibility of N80 steel to pitting corrosion under stagnant condition and the corrosion behavior under flowing condition with/without precorrosion in 3% NaCl solution, in 3% NaCl + 1.5% CaCl₂ solution and in 4.6% NaCl solution (equivalent Cl⁻ concentration to 3% NaCl + 1.5% CaCl₂ solution) saturated by CO₂ at 57 °C. The results showed that under stagnant condition the initiation period of pitting corrosion decreased with increasing Cl⁻ concentration, but at the same Cl⁻ concentration, Ca²⁺ could prolong the initiation period. Under flowing condition the corrosion rate without stagnant precorrosion was bigger than that with precorrosion, and the corrosion rate with precorrosion in 3% NaCl solution was bigger than that in 3% NaCl + 1.5% CaCl₂ solution. Inhibition of quaternary alkynoxymethyl amine (IMC-80-Q) under stagnant condition showed that the optimum inhibitor concentration significantly increased with the increase of Cl⁻ concentration from 3% NaCl solution to 4.6% NaCl solution, and for the solutions with the same Cl⁻ concentration, adding Ca²⁺ did not change the optimum inhibitor concentration.     

Temperature effect on pitting corrosion of mild steel in de-aerated sodium bicarbonate-chloride solutions

The pitting potential of mild steel in bicarbonate-chloride solutions has been studied as a function of temperature. Raising the temperature up to 90°C restricts or completely prevents the breakdown of the passive film at chloride concentrations up to about 130 ppm. At concentrations above this level the temperature effect is reversed and raising the temperature facilitates film breakdown. The two phenomena are explained in terms of two competing processes concurrently occurring at the passive film surface: reduced adsorption of chloride ions and reduced stability and/or increased solubility of an Fe-Cl complex at enhanced temperatures.     

A study on the localized corrosion of cobalt in bicarbonate solutions containing halide ions

The localized attack of cobalt in bicarbonate aqueous solutions containing halide ions was investigated using electrochemical techniques, scanning electron microscopy, UV-visible and Raman spectroscopies. Rotating disc and rotating ring-disc electrodes were used to determine the effect of bicarbonate concentration, solution pH, nature and concentration of the halide ions, convection and potential sweep rate on the corrosion processes. These parameters were found to play a key role on the localized attack induced by halide ions by influencing the production of a Co(HCO₃)₂ precipitate on the pit surface. Potentiostatically generated cobalt oxide films (CoO and Co₃O₄) were found to be efficient to reduce pitting corrosion of cobalt.     

Role of oxide films in stress corrosion cracking of mild steel in nitrate solutions

Time to failure of iron with varying carbon content was determined during stress corrosion at 75°C in various nitrate solutions. Ability of these solutions to passivate iron was estimated. It has been suggested that the high rate of the intergranular stress corrosion cracking of mild steel is connected with the formation of the passive γ-Fe₂O₃ film and its sensitivity to the depassivating action of steel constituents such as carbon, segregated at grain boundaries. It is thought that the Fe₃O₄ film also protects the grain boundaries.     

Environmental factors affecting the corrosion behavior of reinforcing steel II. Role of some anions in the initiation and inhibition of pitting corrosion of steel in Ca(OH)2 solutions

Potential-time curves are constructed for the steel electrode in naturally aerated Ca(OH)₂ solutions simulating the corrosion behavior in concrete. Cl⁻ and SO₄²⁻ ions cause the destruction of passivity and initiation of pitting corrosion. The rate of oxide film growth by Ca(OH)₂ and oxide film destruction by Cl⁻ and SO₄²⁻ ions follows a direct logarithmic law as evident from the linear relationships between the open-circuit potential and the logarithm of immersion time. Chromate, phosphate, nitrite, tungstate and molybdate ions inhibit the pitting corrosion of steel. The rate of oxide film healing and thickening increases with their concentrations. In presence of constant inhibitor concentration, the efficiency of pitting inhibition increases in the order: (weak) CrO₄²⁻ < HPO₄²⁻ < NO₂⁻ < WO₄²⁻ < MoO₄²⁻ (strong).     

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.