Aggressive Anions Research Articles

Studies on the pitting corrosion of lead in carbonate media

Cyclic voltametry and potentiodynamic single sweep techniques are used to study the electrochemical behaviour of lead in Na2CO3 solutions containing various concentrations of ClO4‐ as aggressive anion. The effects of different concentrations, in terms of destruction of passivity and initiation of pitting corrosion, were monitored with reference to the change in integrated anodic charge. It was found that Δqa (taken as a measure of the extent of pitting) varies linearly with log CClO4‐. The pitting corrosion potential, Epitting, varies with log CClO4‐ according to sigmoidal curves. These curves are explained on the basis of formation of passive, active and continuously propagating pits. Additions of aliphatic amines shift the pitting corrosion potential, Epitting, into the noble (positive) direction, indicating the inhibition action of the added amines on the pitting attack. Epitting varies with the logarithm of the inhibitor concentration according to: Epitting = a + b log Cinh. The inhibition of pitting corrosion by the aliphatic amines is assumed to be due to either competitive adsorption between the CO32– with ClO4‐ anions, and/or the chemisorption of the amine on the metal with the formation of a metal‐nitrogen coordination bond. The efficiency of these compounds as pitting corrosion inhibitors increases with the increase in the chain length of the alkyl group.

Examination of the metastable and stable pitting corrosion of aluminum modified with carbon by ion beam techniques

It is well known that aluminum and aluminum alloys are sensitive to pitting corrosion when exposed to aqueous solutions containing aggressive anions like halides. The destructive nature of pitting is due to its high local dissolution rates at electrode potentials above the so-called pitting potential U p. Recently, it has been realized that also at potentials below U p, in the passive and cathodic regions and around the free corrosion potential, anodic current transients appear which have been attributed to metastable pitting events. For the purpose of full characterization of the pitting behavior, a program routine has been developed where the occurrence frequency, lifetime and rate of metastable pitting events are extracted from potentiostatic current/time-measurements depending on the electrode potential. The routine has been applied to measurements of carbon modified pure aluminum. Carbon modifications were done with carbon evaporation and carbon sputtering under concurrent argon ion bombardment. The results are discussed in terms of the applied modification technique, their parameters and their effects on the corrosion protection ability of aluminum modified by carbon.

Pitting corrosion of aluminium coated with amorphous carbon films by argon ion beam assisted deposition at low process temperature

It is well known that aluminium alloys are prone to pitting corrosion when they are exposed to a wet environment in the presence of aggressive anions such as chloride. When a dense inert coating is applied pitting may be inhibited. Amorphous carbon films (a-C) are particularly suitable as they are chemically resistant and show a low microporosity. However, owing to the temperature sensitivity of aluminium alloys, the films have to be deposited at low process temperature. Low process temperature often leads to coatings which are porous and not well adherend. The situation may be improved when the films are irradiated with energetic ions. Amorphous carbon (a-C) films were deposited by electron beam evaporation from graphite under concurrent bombardment with 10 keV Ar + ions at process temperature below 150°C. Both mirror-polished and ground aluminium samples were used as substrates. Films of different thickness up to 0.8 μm were deposited. For evaluating the corrosion behaviour, polarisation measurements were carried out in neutral aqueous solution of sodium chloride. The results show that for polished surfaces the films reduce the pitting corrosion susceptibility of aluminium considerably when they are thick enough, while for ground samples with a roughness beyond the film thickness no corrosion protection could be achieved.

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

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The Effect of Chloride Ions on the Ambient Electrochemistry of Pyrite Oxidation in Acid Media

The influence of chloride ions on the electrochemical oxidation of pyrite in sulfuric acid solutions has been studied using ring (glassy carbon)‐disk (pyrite) voltammetry and in situ Raman spectroelectrochemistry at ambient temperatures (25°C). The results indicate that at disk (pyrite) potentials greater than 0.7 V vs. saturated calomel electrode, where ring collection is evident, chloride ions inhibit the deposition of elemental sulfur on the pyrite electrode surface by promoting the oxidation of an adsorbed intermediate, believed to be the thiosulfate ion, to soluble tetrathionate ions. In the absence of chloride ions, the thiosulfate intermediate undergoes acid decomposition on the pyrite surface to yield elemental sulfur. During pyrite oxidation, chloride ions are proposed to act as aggressive anions, removing passive layers containing ferric ions by adsorption on the electrode surface. This mechanism is consistent with the increase in the collection of ferric ions at the ring electrode in the presence of chloride ions. The adsorption phenomenon is also proposed to account for the inhibition of pyrite oxidation in the presence of chloride ions. © 2000 The Electrochemical Society. All rights reserved.

Influence of microstructure on the microbial corrosión behaviour of stainless steels

Several stainless steels (Types UNS S30300, S30400, S30403, S31600, S31603 and S42000) with different microstructural characteristics have been used to study the influence of heat treatments on microbiologically influenced corrosion (MIC). Biocorrosion and accelerated electrochemical testing was performed in various microbiological media. Two species of sulphate-reducing bacteria (SRB) have been used in order to ascertain the influence of microstructure. The morphology of corrosion pits produced in both chloride and chloride plus sulphide -SRB metabolites- was inspected by optical and scanning electron microscopy (SEM) complemented with energy-dispersive X-ray (EDX) analysis. Results have shown different behaviours regarding corrosion resistance in each case studied. Sensitized austenitic stainless steels were more affected by the presence of aggressive anions and pitting potential (Ep) values were more cathodic than those of as-received state. A corrosion enhancement is produced by the synergistic action of biogenic sulphides and chloride anions. Pitting corrosion in martensitic stainless Steel Type UNS S42000 was found in a- biocorrosion test. The pitting morphology is correlated to the chemical composition, the microstructure and the electrolyte.

Effect of Cl − anions on zinc passivity in borate solution

The anodic behaviour of zinc in Na 2B 4O 7 solutions in the absence and presence of Cl − anion as an aggressive anion has been investigated by the galvanostatic polarization technique. In the absence of Cl − anion, the polarization curves are characterized by one distinct arrest corresponding to Zn(OH) 2 or ZnO, after which the potential increases linearly with time before reaching the oxygen evolution region. Addition of low concentrations of Cl − anions, has no effect on the passive film formed on the metal surface. The potential starts to oscillate within the oxygen evolution region with an increase in the concentration of the Cl − anion. This suggests interference of Cl − anion with oxygen evolution. Further increases in the concentration of Cl − anions are associated with breakdown of Zn passivity, this denoting the destruction of the passivating film and initiation of pits. The susceptibility of zinc to pitting corrosion is enhanced with increasing Cl − anion concentration but decreases with an increase of both the Na 2B 4O 7 concentration and current density. Addition of increasing concentrations of tungstate, phosphate or molybdate anions causes a shift of the breakdown potential in the noble direction, indicating the inhibitive effect of the added anion on the pitting attack. In contrast, addition of the sulphate anion enhances the pitting attack.

Random Instability Induced by Active Dissolution

With the nonequilibrium fluctuation theory, pitting process in the active state was examined: in the active state, anodic dissolution yields random irregular surface inside a pit. This type of morphology is quite different from that of the polishing-state pit, which is composed of hemispherical round hollows. Consequently, it was concluded that pit diameter is, as well as the polishing-state pit, determined by the asymmetrical fluctuations which result from strong specific adsorption of anions. On the other hand, the irregularity inside the pit is created by the symmetrical fluctuations arising from the variation of the amount of adsorbed ions. That is, the aggressive anions form a complex with metal, which accelerates irregular dissolution. In the polishing-state pit, passive film hinders the formation of the complex, so that the symmetrical fluctuations do not occur. As a result, only hemispherical pits are yielded by the asymmetrical fluctuations.

A simplified method for estimating corrosion cavity growth rates

A simplified method is proposed for calculating corrosion cavity propagation rates. This method is based on an assumption that if the rate of an electrode reaction depends (in an explicit form) only on the potential, the pit growth rate depends only on the concentration of those species that determine the potential distribution near the metal within the cavity. The advantage of this method is that it permits one to predict the rates of cavity propagation without knowing various parameters, such as the equilibrium constants of some chemical reactions and diffusion coefficients of species that are present at relatively low concentrations near the electrode surface. The analytical expressions for calculating propagation rates of cylindrical and hemispherical pits are compared with available experimental data. The influence of aggressive anions on the pit propagation rate has been investigated. It is shown that transport processes in the internal environment do, in the general case, influence the kinetics of metal corrosion.

Inhibition of chloride pitting corrosion of tin in alkaline and near neutral medium by some inorganic anions

The effect of dichromate, chromate, molybdate, nitrite and nitrate anions on the chloride pitting corrosion of tin in each of the following solutions: Na 2CO 3 ( pH = 10.9), Na 2B 4O 7 ( pH = 8.35) and Na 2SO 4 ( pH = 6.8) (each solution containing 0.1 M NaCl) has been studied. The potentiodynamic technique was used, complemented by X-ray and SEM. Addition of increasing concentrations Cr 2O 7 2−, CrO 4 2− or MoO 4 2− caused a shift of the pitting potential ( E pit) in the positive direction indicating the inhibitive effect of the added anions on the pitting corrosion. The absorption characteristics of these anions on the metal surface plays a significant role in inhibition. The NO 2 − anion has a little inhibitive effect of pitting corrosion but the NO 3 − anion is ineffective as an inhibitor, and actually increases pitting corrosion. The pitting corrosion of chloride ions and other aggressive anions in the solutions is completely suppressed at concentration $ ̆ /0.01 M sodium dichromate. The MoO 4 − species limits the growth of the passive film, restricts Cl − anions and inhibits the pitting corrosion.

Localized corrosion of 1024 mild steel in slightly alkaline bicarbonate solution with Cl− ions

The passive film breakdown of 1024 mild steel induced by the presence of 0.05m chloride ions had been investigated in 0.075–0.75m bicarbonate solutions at pH8.9–9.7. A rotating disc electrode with a Kel-F holder was used in conjunction with a rotating ring-disc electrode. The resistance to localized attack is closely linked to the preanodization potential (Eox) applied in the absence of Cl- ions. For Eox below about 0.2–0.3V vs SCE, the resistance to localized attack provided by the passive film is independent of Eox; above the breakdown potential, the localized attack is manifested by the formation of pits at the mild steel surface. The breakdown potential increases linearly with NaHCO3 concentration and pH. Passive film breakdown for Eox below about 0.2–0.3V vs SCE most likely begins with a surface film dissolution prior to the penetration of the aggressive anions through the film. For Eox above about 0.3V vs SCE under the same conditions, no pitting is noticed and the potential associated with localized attack shifts considerably in the anodic direction due to interstitial (formation of crevices) corrosion at the mild steel/Kel-F interface.

A simplified method for estimating corrosion cavity growth rates

A simplified method is proposed for calculating corrosion cavity propagation rates. This method is based on an assumption that if the rate of an electrode reaction depends (in an explicit form) only on the potential, the pit growth rate depends only on the concentration of those species that determine the potential distribution near the metal within the cavity. The advantage of this method is that it permits one to predict the rates of cavity propagation without knowing various parameters, such as the equilibrium constants of some chemical reactions and diffusion coefficients of species that are present at relatively low concentrations near the electrode surface. The analytical expressions for calculating propagation rates of cylindrical and hemispherical pits are compared with available experimental data. The influence of aggressive anions on the pit propagation rate has been investigated. It is shown that transport processes in the internal environment do, in the general case, influence the kinetics of metal corrosion.

Mechanism of corrosion inhibition by sodium molybdate

Sodium molybdate, Na 2MoO 4, is a non-toxic, environment-friendly corrosion inhibitor for the protection of soft-water cooling systems. The agent is classified as anodic inhibitor, but its use requires the presence of oxygen or other oxidizing agents in solution. Also, the lack of inherent oxidizing characteristics allows its mixing with a large number of organic compounds, promoting inhibition synergism. Little and contradicting information is known about the mechanism of action of MoO 4 2− as corrosion inhibitor. We have examined in some detail the inhibition of corrosion of mild-steel by Na 2MoO 4. Work was carried out in distilled and tap waters as well as in NaCl and Na 2SO 4 solutions of increasing concentrations. The nature of the MoO 4 2−/steel surface interaction was established from the measurement of the variation with time of the open circuit potential (OCP) of steel coupons in the test solutions under a wide variety of conditions. The type of potential/time curve obtained depended primarily on whether or not dissolved oxygen was present in the solution. In air-free distilled water, the OCP of the steel changed towards more negative values and a well defined arrest was recorded. The arrest represented the destruction of the air-formed oxide film on the metal surface. The length of the arrest increased with the MoO 4 2− content in the solution. The additive fortified to some extent the oxide film. The OCP ultimately tended to a constant final steady potential, E st , corresponding to the free corrosion potential under immune conditions. On the other hand, in aerated water the potential tended directly towards positive values, denoting the ready passivation of the steel. In aerated tap water, a different behavior was recorded. Dilute MoO 4 2− solutions promoted corrosion and led to final negative potentials. Concentrated solutions, on the other hand, induced passivation. The active/passive transition followed an S-shaped curve when the final steady-state potentials were plotted as function of the logarithm of additive concentration. The point of inflexion of the curve was the threshold concentration of the inhibitor. The same behavior as in tap water was noted in solutions of NaCl and Na 2SO 4. Two factors influenced the value of the threshold concentration. The first was the concentration of foreign ions in the solution. The threshold concentration, C inh , moved towards higher values when the concentration of aggressive anions (e.g., Cl − or SO 4 2−), C agg , was increased. The two variables were related as log C inh = a + b log C agg , where “a” and “b” were constants. The constant “b” was the ratio of electric charges carried by the two counter-acting anions. The double logarithmic relation signified the occurrence of competitive adsorption. A rise in solution temperature also augmented C inh . The plot of log C inh as function of the reciprocal of absolute temperature (Arrhenius plot) was linear. The activation energy of passivation was calculated to be 17.33 kcal/mole. The high value suggested chemical bond formation. The factors influencing the stability of the passive state were identified. These were: the time of treatment with the inhibitor, temperature, type and concentration of foreign ion in solution, continuous supply of O 2 and eventual reduction in inhibitor concentration. X-ray mapping of the distribution of Mo on the surface suggested the role of MoO 4 2− and O 2 to be the healing and repair of defects in the air-formed oxide. Prior to bond formation, the two species were assumed to be absorbed on the surface.

High Nitrogen Steels. The Role of Nitrogen in the Corrosion of Iron and Steels.

The beneficial effect of nitrogen in steels in resistance against localized corrosion, pitting and crevice corrosion is well proven, but not yet fully understood. The results on dissolution of nitrogen steels in acids are controversial, but the dissolution of Fe-N alloys is enhanced compared to pure iron. Many surface analytical studies have been conducted in the recent years to find out about the state of nitrogen, and mainly the presence of Nδ– in the metal surface and NH3 or NH4+ in the passive layer are well established, and moreover, NH4+ is the species transferred into the electrolyte in a wide range of potentials. Formation of NH4+ will somewhat buffer the acidification in pits. The possible role of N in the steps of pitting, initiation, growth and possible repassivation are discussed, and most important appears the favourable effect on passivation and repassivation, which is effective also for Fe-N alloys. Many authors assume formation of stable Cr- and Mo-nitrides in the steel surface upon dissolution, and retardation of pit growth by these nitrides, however, this mechanism would not apply for Fe-M alloys. Desorption of the aggressive anions induced by the segregated Nδ– just after the local failure of the passive layer is proposed to be the mechanism by which nitrogen favours the rapid repassivation of pits.

An investigation into the mechanisms of protection afforded by a cathodic current and the implications for advances in the field of cathodic protection

The influence of a cathodic current on the corrosion behaviour of mild steel exposed to both dilute sodium nitrate solutions and chloride contaminated concrete has been examined. In both cases the formation of passive films was promoted by stimulating a cathodic reaction on the metal surface, the cathodic kinetics being under activation control in the near passive region. The sodium nitrate solution was not capable of supporting a passive film on its own and it is suggested that, in this environment, film formation is promoted by the production of inhibitors (hydroxyl ions, free radical intermediates and metal oxide intermediates) resulting from the cathodic reduction reaction. In chloride-contaminated concrete an additional effect is the removal of aggressive anions via the flow of negative ionic current away from the metal surface. These passivating effects of the cathodic current provide persistent protection and may be harnessed to develop intermittent cathodic protection for use in areas such as the tidal zone where an electrolyte is not always present. The improvement in the local environment at the steel surface may facilitate a reduction in the protection current density, a factor which may be used to reduce the risk of overprotection when cathodic protection is applied to prestressing steel. The use of potential depolarisation and current density related criteria may be more appropriate than the achievement of a predetermined absolute potential in cases where the purpose of the cathodic protection system is to induce passivation.

Corrosion of iron in anhydrous methanol

Cyclic voltammograms and anodic polarization curves of an iron electrode prepared by abrasion and cleaning upon exposure to air were measured in anhydrous methanol plus 0.1 M electrolyte solutions (water content < 450 ppm) under a nitrogen atmosphere. Results of these measurements and surface analyses by X-ray photo-electron spectroscopy showed that an oxide film formed on the surface during the preparation protects iron against corrosion in methanol solutions. Breakdown of the film occurs with aggressive anions. The pitting potentials of the oxide-covered iron electrode in methanol with the anions decreased in the order, I − > Br − > Cl −. The corrosion rate of an iron electrode prepared under an oxygen-free and dry argon atmosphere was measured in an anhydrous methanol solution of 0.1 M LiClO 4 (water content < 150 ppm) by impedance measurements. X-Ray photo-electron spectra of the electrode surface suggested the formation of ferrous methoxide which is sparingly soluble in methanol. The corrosion rate decreased with an increase in time by accumulation of the methoxide on the surface.

Breakdown of Passive Film on Copper in Bicarbonate Solutions Containing Sulfate Ions

The formation, breakdown, and composition of the surface films formed on a copper rotating disk electrode at 25°C were studied by potentiodynamic and potentiostatic pulse measurements. The present results explain the nature of the pit initiation process, especially some characteristic values of pitting phenomena such as the critical breakdown potential and the induction time for pit nucleation on passive copper in contact with bicarbonate solution, , containing aggressive sulfate ions. It was found that the critical breakdown potential, , and the inhibition potential, depend on the concentration of the aggressive anion A and inhibitor anion I according to the equations: and . The critical potentials are explained by the adsorption of sulfate anions on the metal surface and their competition with the inhibitor. On the basis of the experimentally obtained dependencies for the critical breakdown potential and induction time on halide concentration and potential, respectively, the various theories for pit initiation on a passive metal in contact with a solution containing aggressive ions were tested. The experimental data are analyzed in terms of the point defect model, the 2‐D nucleation and growth model, and the sulfate nuclei model for passivity breakdown. The validity of the particular models is discussed. Numerical analysis enabled the evaluation of several unknown parameters which are characteristic for the models proposed.

Effect of Inorganic and Biogenic Sulfide on Localized Corrosion of Heat-Treated Type 304 Stainless Steel

Abstract Type 304 (UNS S30400) stainless steel (SS) specimens with different microstructural characteristics are used to study the influence of heat treatment on microbiologically induced corrosion. The effects of chloride and inorganic and biogenic sulfide are analyzed. Sensitized metal probes were affected by the presence of aggressive anions and pitting potential values were more cathodic than those in the as-received state. Corrosion is enhanced by the synergistic action of biogenic sulfides and chloride anions.

Influence of Microstructure on the Electrochemical Behavior of Type 410 Stainless Steel in Chloride Media with Inorganic and Biogenic Sulfide

Abstract The effect of aggressive anions chloride and sulfide and their mixtures on the localized corrosion resistance of type 410 (UNS S41000) stainless steel (SS) has been studied. A synergistic effect of these anions on pitting attack was found. The quenched condition offers the highest resistance followed by the as-received condition. Tempered states show worse corrosion resistance than the other states. A comparison between the effect of biogenic and inorganic sulfides and the study of bacterial adhesion on type 410 SS surfaces were made. The adhesion of Desulfovibrio vulgaris on the metal probes facilitated localized attack in chloride media.

Diffuse Reflectance Infrared (Drifts) Studies of Corrosion Products

Abstract The diffuse reflectance IR technique has permitted the study of corrosion products to be carried out in a very simple manner. The products formed in the presence of aggressive anions such as chlorides and sulfates were studied. The advantage of this technique was the fact that the products could be studied while intact on the specimen and thus not requiring sample preparation and avoiding interferences that can occur during sample preparations. The technique is also relatively easier and less costly compared to other techniques such as ESCA.