Passivation Of Iron Research Articles

The influence of the polarization time on the passivation of iron in sulphuric acid

The aim of the investigations was to define the time necessary to completely cover an iron surface with a passivating solid layer of FeSO 4.7H 2O resp. with FeSO 4.1H 2O. This was determined by anodic passivation of Armco iron in a wide range of concentrations of sulphuric acid and also by varying the time of the polarization. A log/log relationship between the passivation time ( t) and acid concentration ( N) was found. This indicated that the film was formed from the nuclei of FeSO 4 which precede the crystallization of the appropriate form of hydrated FeSO 4.

The Effects of Bicarbonate on the Corrosion and Passivation of Iron

Abstract The corrosion of iron in bicarbonate solutions has been investigated potentiostatically and by Auger electron spectroscopy. The dissolution of iron in the active and prepassive regions is greatly accelerated by the presence of bicarbonate, owing to the formation of the stable soluble complex anion Fe(CO3)22−. A mechanism is proposed for the dissolution process. Pitting of the metal surface is observed under conditions where dissolution as the complex occurs. Raising the potential into the passive region where Fe(III) forms in the film prevents complexing and so anodically protects the metal from pitting. Possible implications of complex formation in the stress corrosion cracking of mild steel in bicarbonate electrolytes are discussed.

On the Mechanism of breakdown of passivity of iron for instationary conditions

AbstractThe formation of Fe2+‐ions is studied in 0.5 M H2SO4 and 1 M HClO4 during changes of the potential of passivated iron specimen with the rotating‐ring‐disk technique. An Fe2+‐pulse is found during the decrease of the electrode potential serving as an indicator for the short exposure of small areas of bare metal to the electrolyte. In the presence of aggressive anions the repassivation of these defects is prevented leading to the formation of pits. The increase of the pit nucleation after potential changes coincides qualitatively with the occurrence of the Fe2+ transients. The adsorption mechanism may be excluded for these conditions because it postulates the action of aggressive anions for the very beginning of the breakdown process.

The effect of water and oxyanions on the passivation of iron in mixed dimethylformamide-water solvents

The passive film on iron has been produced anodically in mixed dimethylformamide-water solvents with and without oxyanion, and reduced cathodically in phosphate-borate buffer solution of pH 8.5. The thickness of the passive film was estimated from the chronopotentiogram. The passivation was expedited by the addition of water, molybdate and chromate. These species were directly involved in the passivation process to form the passive film, but the extent of the involvement relied on the nature of oxyanion. The addition of greater quantities of water led to no formation of the passive film. The film growth process was dependent evidently on the stability of the ferric hydroxide and/or (FeL) ads (L : oxyanion) in the DMF-water media.

The Effect of Peripheral Velocity on the Corrosion and Passivation of Iron in Chloride Containing Media

Abstract The anodic dissolution kinetics of iron in chloride containing solutions (LiCI) has been studied as a function of peripheral velocity by using a rotating cylinder electrode. The results ob...

ChemInform Abstract: STIMULATION OF THE PASSIVATION OF IRON BY TUNGSTATE, MOLYBDATE AND CHROMATE IONS

AbstractCrO4(2‐), MoO4(2‐) und WO4(2‐) stimulieren die Passivierung von Fe in schwach sauren wäßrigen Lösungen besonders bei niedrigeren pH‐Werten.

AES Investigations of Passive Films on Iron and Iron Base Alloys

Abstract Auger Electron Spectroscopy (AES) combined with the ion sputtering technique has recently proved to be a powerful method of investigation of passive films. The recent findings in the field of passivation of iron and its alloys, which have been obtained by using AES, are summarized. The experimental data concerning the composition profiles of the films are discussed, and the effects of both the solution and the matrix components on the composition of the films are emphasized. The problems arising from the effects of ion sputtering in the interpretation of data, as well as quantitative aspects are considered separately.

ChemInform Abstract: FUNCTION OF THE OXIDIZING AGENT IN THE CHEMICAL PASSIVATION OF IRON

Chemischer InformationsdienstVolume 10, Issue 20 Physical Inorganic Chemistry ChemInform Abstract: FUNCTION OF THE OXIDIZING AGENT IN THE CHEMICAL PASSIVATION OF IRON M. FISCHER, M. FISCHERSearch for more papers by this author M. FISCHER, M. FISCHERSearch for more papers by this author First published: May 15, 1979 https://doi.org/10.1002/chin.197920025Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue20May 15, 1979 RelatedInformation

Stimulation of the passivation of iron by tungstate, molybdate and chromate ions

The effects of tungstate, molybdate and chromate ions on the passivation of iron were investigated, and these oxyanions stimulated the passivation especially at lower pH. The results were rationalized by the dissolution-precipitation mechanism of iron passivation. In order to bring about the passivity of iron, it was required that iron first dissolves into solution, and then a ferric hydroxide or a ferric complex with oxyanion should be produced on the metal surface, which was followed by the dyhydration or the condensation reaction of these compounds to form ferric oxide.

Electrochemical and eclipsometric study of iron corrosion inhibition in sodium sulphate solutions containing aliphatic acids

The inhibition effect of normal aliphatic acids with 6–10 carbon atoms in the molecule has been investigated in aerated solutions of 0·05m sodium sulphate at 6–12 pH. It was found that the depolarization of oxygen is not the only factor responsible for the passivation of iron in aerated solution containing a critical concentration of the inhibitor. The inhibiting ability of aliphatic acids in Na2 SO4 solutions is brought about by a synergistic action of dissolved oxygen and of molecules (or anions) of the aliphatic acid.

Mechanism of the action of weak acids and their salts on the passivation of iron by oxygen

Oxygen containing solutions of salts of weak acids passivate iron only when the pH of the solution exceeds a critical value. The dependence of the critical pH on the nature (pK a) and concentration of the salt ( c s) as well as dissolved oxygen ( c 0 2 ) can often be represented by an equation of the following form: pH c = log c B − log c 0 2 + pK a − const . A quantitative explanation for this relationship is derived in terms of the pH shift nearest the iron surface in course of the diffusion controlled corrosion process.

Processes of the activation of passive iron by thiocyanate ion

The activation of passive iron has been examined in neutral or weak alkaline solutions containing thiocyanate ions. The passivation of iron was carried out potentiodynamically, and the specimen potential was allowed to decay under open-circuit conditions. The residual passive film was then subjected to the cathodic reduction in order to estimate its thickness. The spontaneous potential decay was very much influenced by the addition of thiocyanate ions. The duration of the potential arrest was shortened with increasing concentration of thiocyanate ions. The potential-determining reaction and the role of the thiocyanate ion are discussed.

Mechanism of the action of weak acids and their salts on the passivation of iron by oxygen

Oxygen containing solutions of salts of weak acids passivate iron only when the pH of the solution exceeds a critical value. The dependence of the critical pH on the nature (pK a) and concentration of the salt ( c 8) as well as dissolved oxygen ( c 0 2 ) can often be represented by an equation of the following form: pH c=log c s-log c o2+pK a-const. A quantitative explanation for this relationship is derived in terms of the pH shift nearest the iron surface in course of the diffusion controlled corrosion process.

Processes of the activation of passive iron by thiocyanate ion

The activation of passive iron has been examined in neutral or weak alkaline solutions containing thiocyanate ions. The passivation of iron was carried out potentiodynamically, and the specimen potential was allowed to decay under open-circuit conditions. The residual passive film was then subjected to the cathodic reduction in order to estimate its thickness. The spontaneous potential decay was very much influenced by the addition of thiocyanate ions. The duration of the potential arrest was shortened with increasing concentration of thiocyanate ions. The potential-determining reaction and the role of the thiocyanate ion are discussed.

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential ( E c ) was noted in each solution, the value of E c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E c , indicating that the kinetics of pit initiation are independent of mass-transfer effects. During anodic dissolution at potentials more regative than E c , an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH + (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH − to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential ( E c ) was noted in each solution, the value of E c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E c , indicating that the kinetics of pit initiation are independent of mass-transfer effects. During anodic dissolution at potentials more negative than E c , an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH + (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH − to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.

The dissolution and passivation of pure iron and the effect of phosphate and chromate corrosion inhibitors

The dissolution characteristics of pure iron undergoing an active-passive transition have been studied using impedance techniques, in carbonate-bicarbonate solutions at 70°C. The impedance spectra have been shown to consist of three overlapping semicircles, from which two time-constants for incremental passivation can be identified. The potential-dependence of these time-constants is related to stress corrosion cracking phenomena. The electrochemical behaviour of two stress corrosion cracking inhibitors have also been examined.

The breakdown of passivity of iron and nickel by fluoride

The breakdown of passivity of iron and nickel by fluoride is investigated in acid solutions. Localized corrosion is observed for iron at pH ⩾ 5 whereas nickel shows general corrosion. In strong acids only general corrosion for both metals is obtained. The breakdown of passivity of iron in acids occurs in two stages. Fluoride leads first to an increased passive current density ( 0.5 mA cm −2) where Fe 3+ ions are formed. After a non-reproducible induction period of minutes to hours a steep increase of the dissolution current density is obtained going along with Fe 3+ ion production. Apparently, at this stage the passive layer is removed leading to a similar situation as on the surface of pits during localized corrosion caused by other halides.

The breakdown of passivity of iron and nickel by fluoride

The breakdown of passivity of iron and nickel by fluoride is investigated in acid solutions. Localized corrosion is observed for iron at pH ⩾ 5 whereas nickel shows general corrosion. In strong acids only general corrosion for both metals is obtained. The breakdown of passivity of iron in acids occurs in two stages. Fluoride leads first to an increased passive current density (≈ 0.5 mA cm −2) where Fe 3+ ions are formed. After a non-reproducible induction period of minutes to hours a steep increase of the dissolution current density is obtained going along with Fe 3+ ion production. Apparently, at this stage the passive layer is removed leading to a similar situation as on the surface of pits during localized corrosion caused by other halides.

The effects of added impurities upon the electrochemical behaviour of iron in liquid ammonia solutions

Polarization data are presented for an iron electrode in liquid ammonia solutions containing ammonium salts and small amounts of the impurities: water, oxygen, nitrogen, ammonium carbamate, urea and boron trifluoride at — 40°C. Additions of water up to a limiting concentration increase the iron dissolution currents in the active region. Additions of oxygen have a similar effect. The presence of either oxygen or water assists the onset of passivation and gives the phenomenon a more irreversible character. In ammonia, one mole of oxygen with two moles of Fe(II) forms a complex which can be cathodically reduced on vitreous carbon (1.0F per mol of Fe(II)) or on iron (2.0F per mol of Fe(II)). On an iron cathode molecular oxygen is reduced at a rate determined by the mass transfer of oxygen, except at values of electrode potential more positive than the iron passivation potential. Nitrogen and urea do not affect the polarization behaviour of iron. Ammonium carbamate increases the iron dissolution currents obtained with nitrate or fluoroborate electrolytes but considerably inhibits the process in chloride electrolytes. Boron trifluoride profoundly increases the activity of iron towards dissolution.