The influence of different aggressive anions on the electrochemical behaviour of aluminium in sodium nitrate aqueous solutions

The effects of different experimental parameters influencing the determination of critical pitting and protection potentials of aluminium and its alloys have been studied by potentiostatic and potentiodynamic methods. The resistance of aluminium against corrosion in aqueous media can be attributed to a rapidly formed surface oxide film. The addition of the aggressive anions like: chloride, thiocyanide, hydroxyl, sulphide, formate, and acetate (Cl⁻, SCN⁻, OH⁻, S²⁻, HCOO⁻ and CH₃‐ COO⁻) lead to extensive localized attack in all of the cases. The breakdown of the passive film takes into account the migration of aggressive anions through the film. Breakdown occurs when aggressive anions reach the metal‐film interface. E_π is the critical pitting potential, E_p ist the protection potential and the pitting can be formed only in the E_π–E_p polarization range as it was proved in many experiments [1–3]. The most likely action mechanism of aggressive anions is not a complete dissolution of the film, nor penetration of aggressive anions through solid oxide as suggested for nickel and iron [4]. It is more likely to be somewhere in between the two i.e. action of aggressive anions is that of complexing aluminium ions and pulling in water to hydrate the layer in a way similar to that occurring at cathodic hydrogen evolution, where such dramatic increase of hydrogen evolution rate is observed after a certain cathodic potential is reached. Localized corrosion can be prevented by the action of adsorptive inhibitors which prevent the adsorption of the aggressive anions or by the formation of a more resistant oxide film on the metal surface. The corrosion mechanism is not modified by the addition of ammonium rhodanide but only slowed down.     

The influence of light on corrosion phenomena: the behaviour of mild steel in citric acid solution

Experiments using photopotential, galvanic current, a.c. impedance and harmonic analysis techniques were carried out to observe the effect of visible light on mild steel in a citric acid solution. The photopotential and galvanic current techniques show that the steady state potential and current after illumination of one of the electrodes is negative with respect to the electrode in darkness. Both the impedance and harmonic analysis techniques indicate an increase in the corrosion rate of the illuminated electrode. On coating the mild steel with a vinyl lacquer, the effect of illumination is eliminated.     

Electrochemical behaviour of chromium-implanted magnesium in hydroxide, chloride and sulphate solutions

Despite the development and subsequent improvement of new Mg-based alloys, their vulnerability to oxidation and corrosion continues to pose a major obstacle to their more generalized use. The possibility of blocking high diffusivity paths, such as grain boundaries, by ion implantation may help to improve their oxidation and corrosion resistance, because mass transport through these short circuiting paths is reduced.Electrochemical techniques were used to investigate the effect of Cr ion implantation in the electrochemical behaviour of Mg in aqueous solutions. In particular ion implanted fluences of 5 × 10¹⁶ and 5 × 10¹⁷ at./cm² have been evaluated in solutions of NaCl, NaOH and Na₂SO₄.The corroded surfaces and products were analysed by ion beam analysis, scanning electron microscopy and X-ray diffraction. A model of the corrosion mechanism is proposed to explain the obtained results.     

The effect of chloride and sulfate anions on the iron dissolution rate in neutral and nearly neutral media

Analysis of results of steady-state potentiostatic measurements on iron in the neutral and nearly neutral borate solutions is complicated by the fact that the metal is in the state of active-passive transition near the free-corrosion potential. This difficulty is overcome using the method of neural network simulation. The study of the behavior of neural-network model shows that chloride and sulfate ions can promote or inhibit iron dissolution depending on their concentration and the pH value of solution. The ambiguous effect of anions is explained by the fact that, when they are adsorbed at the metal, they prevent its cations from passing to the solution, and, entering into the metal-water adsorption complexes, they assist the decomposition of complexes thus promoting the dissolution. Sulfate ions, compared with chloride ions, are, probably more adsorption-active with respect both to iron and to water adsorbed on it. Original Russian Text ? A.Yu. Aleksanyan, I.I. Reformatskaya, A.N. Podobaev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 135–138.     

The electrochemical behaviour of mild steel in phosphate-borate-sulphide solutions

The localized corrosion of mild steel in neutral buffered solutions containing sodium sulphide is studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy and energy dispersive X-ray analysis. The breakdown potential depends linearly with the logarithm of the sodium sulphide concentration. At potentials close to ? 0.70 V (SCE), the nucleation and growth of a ferrous sulphide layer replacing the initial ferrous hydroxide film occurs prior to pit growth. In the ? 0.70 to ? 0.60 V range the logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential. At potentials more positive than ? 0.60 V the oxide formation hinders pit growth leading to a partial passivation. The results suggest that two-dimensional salt islands are formed before pitting as it has been postulated for iron in halide-containing solutions.     

The influence of sodium chloride on the atmospheric corrosion of steel

The influence of sodium chloride on the atmospheric corrosion of mild steel has been studied. Samples covered with sodium chloride crystals (8 m?g NaCl · cm^?2) have been exposed to an SO₂-free atmosphere at different relative humidities. The combined effect of sodium chloride crystals on a steel surface and sulphur dioxide in the atmosphere has been investigated at an SO₂-supply of 1 m?g SO₂ · cm^–2 · h^–1 (1 ppm SO₂). The corrosion attack was followed by periodic weighings. The corrosion products formed have been studied using the SEM-technique. On exposure of samples with sodium chloride in an SO₂-free atmosphere the extent of corrosion increased with increasing relative humidity from 58% to 90%, interrupted by a sharp minimum at about 87% relative humidity. The SEM-studies showed that tower shaped corrosion products were formed at a high relative humidity while filiform corrosion appeared when the relative humidity was lowered. At 90% relative humidity more corrosion was observed with clean steel samples and an SO₂-supply of 1 m?g ·cm^–2 ·h^–1 than with sodium chloride crystals on the surfaces (8 m?g NaCl ·cm^–2) in the absence of SO₂. In the combined influence of sodium chloride on the steel surfaces and sulphur dioxide in the atmosphere a synergic effect was noticed at 90% relative humidity. At 70% relative humidity no influence of an SO₂-supply of 1 m?g SO₂ ·cm^–2 ·h^–1 on the corrosion of steel samples with sodium chloride crystals on their surfaces could be observed.     

The pitting of mild steel in phosphate-borate solutions in the presence of sodium sulphate

The localized corrosion of mild steel in neutral buffered solutions containing Na₂SO₁ was studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy. The breakdown potential changes linearly with the logarithm of Na₂SO₄ concentration. The logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential, until the inhibition potential region is reached. Open circuit potential decay measured shows that the apparent thickness of the passive oxide film decreases at potential values more negative than the breakdown potential. The pitting current density is close to the diffusion limiting current of iron dissolution through a FeSO₄ film. Results suggest that two-dimensional FeSO₄ salt islands are formed before pitting, as it has been postulated for iron in halide-containing solutions.     

The influence of zinc cations on steel oxidation in nitrate solutions

The influence of zinc nitrate additives on the protective properties of the magnetite coating (MC) obtained by the oxidation of low-carbon steel in the nitrate solution at temperatures of 96–98°C has been studied by physical, electrochemical, and corrosion methods. It has been shown that, under these conditions, iron zinc spinel ZnFeO₄, which is introduced into an MC, can be formed. Low concentrations ($ C_{Zn(NO_3 )_2 } $ C_{Zn(NO_3 )_2 } = 25–150 mg/l) of it increase the corrosion stability of steel with MC in water containing chlorides and sulfates and also at the periodic moisture condensation of the sample surface. At $ C_{Zn(NO_3 )_2 } $ C_{Zn(NO_3 )_2 } > 500 mg/l, MC formation stops.     

Corrosion behaviour of mild steel in chloride solutions with Na2HPO4 additive and the development of antifreeze

ABSTRACT

Chloride salts can prevent roads from freezing in cold regions, together with severe corrosion on steel constructions. To develop an ecological and low-corrosive antifreeze, di-sodium hydrogenphosphate (Na₂HPO₄) was chosen as the additive into chloride salts. The addition of Na₂HPO₄ into either of the antifreezes of natural salt (N/S), NaCl, CaCl₂ or MgCl₂ can suppress the corrosion reaction of mild steel in the 3.0% antifreeze solution. Moreover, the addition of CaCl₂ or MgCl₂ into the antifreeze containing NaCl and 2.0% Na₂HPO₄ significantly decreased the corrosion rate. According to polarisation and XPS analyses, it is evident that the obtained low corrosion rates on mild steel are related to the suppressed cathodic and anodic reactions and the formation of a protective film containing Fe, P, O, Ca or Mg.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

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.     

The influence of chloride ions on the stress corrosion cracking of Al-brass in acidic sulphate solutions

Chloride ions can play a stimulating or an inhibitive role on the occurrence of the stress corrosion cracking (SCC) of Al-brass in acidic sulphate solutions depending upon their concentration, the pH and the Cu²⁺ content of the electrolyte. The stimulating effect is observed either in environmental conditions in which competition between the precipitation of Cu₂O and CuCl can occur or in conditions in which Cu₂O is thermodynamically stable but the presence of a critical amount of Cl⁻ induces an alteration of its defect structure. The inhibiting effect is observed at high Cl⁻ concentrations where CuCl is the prevailing stable phase.     

Effect of glycerine concentration on dissolution and oxidation of mild steel during anodic cementation

The possibility of improving roughness of mild steel after its anodic cementation at reduced content of ammonium chloride and glycerine in working electrolyte is shown. The method of parts flow-around with distributed longitudinal streams is used, allowing to reduce thickness dispersion of case-hardened layer vertically. Examples of practical application of anodic cementation are given.     

The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl

The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl⁻ were investigated by electrochemical measurements. MoO₄²⁻ ion suppressed both pit nucleation and propagation. Cr₂O₇²⁻ ion suppressed pit nucleation, but stimulated pit growth. The different effects of the two anions on pit propagation were explained by the opposite effects on pH value within pits. The pH value in molybdate-containing solution increased as a result of polymerization of MoO₄²⁻, while in dichromate-containing solution, pH value decreased due to hydrolysis of Fe³⁺ and Cr³⁺ which are the products of oxido-reducing reactions between Cr₂O₇²⁻ and Fe²⁺ ions.     

Effect of cyclic deformation on Barkhausen noise in a mild steel

The rms value of the Barkhausen noise and residual stresses were followed as a function of loading cycles in alternating bending fatigue of mild steel specimens. The measurements were carried out in several directions so that the changes in the angular anisotropy of the Barkhausen noise and reorientation of the macroscopic easy axis of magnetisation could be detected. In the loading direction the Barkhausen noise amplitude decreased due to build-up of compressive residual stresses. This was followed by either an increase or a decrease in the Barkhausen noise amplitude depending on the direction of the initial magnetic easy axis with respect to the loading direction. After the commencement of macroscopic plastic deformation the angular anisotropy of Barkhausen noise augmented substantially. In the transverse direction to the loading axis Barkhausen noise decreased in the beginning of loading, reached thereafter a minimum followed by an increase in Barkhausen noise. When cycling under larger strain amplitudes the loading direction and the direction transverse to the loading axis exhibited least changes as a function of loading cycles compared to other directions. The results indicated that following Barkhausen noise only in one direction, usually in the loading direction, as a function of loading cycles, may lead to misevaluation of the fatigue damage development. Monitoring of several directions gives much more reliable results and therefore it is highly recommended.     

The crevice corrosion behaviour of stainless steel in sodium chloride solution

The crevice corrosion behaviour of 13Cr stainless steel in NaCl solution was investigated mainly by electrochemical noise measurements, considering the influences of the crevice opening dimension (a) and the area ratio of the electrode outside the crevice to the one inside the crevice (r). Results show that the increase of r value prolongs the incubation period of crevice corrosion, but crevice corrosion develops rapidly once the crevice corrosion occurs. The crevice corrosion develops preferentially at the crevice bottom and then spreads to the whole electrode surface. Proton could reduce on the uncorroded area and hydrogen bubbles form inside the crevice.     

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.     

Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate

The effects of dilute sulphate on metastable and stable pitting of 304 stainless steel in chloride solution have been studied. The presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, implying that pit nucleation is inhibited. Pit propagation, in both the metastable and stable states, is also inhibited by the sulphate ion. The reduced pit propagation current densities are described quantitatively with respect to the effect of sulphate on the solubility of the metal cation in the pit anolyte. The results are consistent with the observation that metastable and stable pits grow under diffusion control, at a rate which is independent of electrode potential. Pit nucleation and propagation in stainless steel are two distinct processes, of which only the former is directly affected by the potential.     

Inhibiting the anodic dissolution of a steel with polydiallyldimethylammonium chloride and acrylamide copolymers with trimethylammoniumethylacrylate chloride and sodium acrylate

Polydiallyldimethylammonium chloride, as well as acrylamide copolymers with trimethylammoniumethylacrylate, which are industrial flocculants, are tested as corrosion inhibitors on St45 steel. The inhibitor properties are manifested by polydiallyldimethylammonium chloride and acrylamide copolymer with trimethylammoniumethylacrylate chloride. These are determined by the adsorption of polymer molecules on the metal surface. The protective effect of inhibitors is estimated. A mechanism of the adsorption of the surface-active substances on an iron electrode surface is proposed. Original Russian Text ? M.V. Boiko, E.E, Akimova, S.B. Bulgarevich, V.A. Fevraleva, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 669–672.     

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 influence of movement and temperature on the corrosion of mild steel. III. Chloride solutions

Abstract

The corrosion of mild-steel tubes, through which chloride solutions are flowing at velocities up to 400 ft/min., has been studied at temperatures up to 55° and test periods from 3 days to 10 weeks. Corrosion was assessed from the change in weight of tube specimens and the products characterised by X-ray examinations and bulk density determinations.

Where the corrosivity of the solution is relatively low the weight loss due to corrosion w, is a rectilinear function of period of test, t, i.e., constant corrosion rate, k₁. In more corrosive solutions the corrosion rate decreases with time owing to the restraining influence of the corrosion product and also by restriction in flow which under certain conditions leads to complete blockage. Both forms of corrosion/time curve may be represented by an equation of the form found previously.

w = K₁ t/1 + K ₂ t

where k₁ is the initial corrosion rate and k₂ is a ‘blocking factor’ describing the retardation in corrosion rate.

A relation is developed for the initial rate, k₁ (which varies from about 20 to over 2000 mg/dm²/day according to the chloride concentration, c), involving temperature, t, and flow velocity, ν. This equation gives values for k₁ Which agree with the experimental values to within ± 30%. Optimum values of k₂ are found in dilute chloride solutions at medium and high flow velocities.

At slow flow velocities, attack is in the form of longitudinal bands of etched metal while in more rapidly moving corrosive solutions a coarse deposit with nodules up to 6 mm high may be formed. The corrosion product consists of a mixture of magnetite and hydrated ferric oxide, the latter being the major constituent in the outer layers. The hydrated oxide is mainly lepidocrocite, γ-FeOOH, but goethite, α-FeOOH, and haematite, α-Fe₂0₃, are often present in small amounts. From bulk density determinations the product is shown to be spongy in character With voids occupying up to 50% of the total volume.