Iron passivity

Under the effect of oxidizing agents, a metal may become passivated even when not anodically polarized by an external power source. In this case, passivation is evident from the drastic decrease in the rate of spontaneous dissolution of the metal in the solution. The best known example is that of iron passivation in concentrated nitric acid, which had been described by M. V. Lomonosov as early as 1750. Passivation of the metal comes about under the effect of the oxidizing agent s positive redox potentiaf. 

Interpretation of the AC data for iron passivation in 1 M H2S04 can be carried out with the basic model shown in Scheme 3.8. 

Figure 3.67 Typical disc potential and ring current behaviour during galvanoslatic reduction of the passive him on pure iron (area 0.5 cm2) at a rotation speed of 25 Hz. The Pt ring potential is maintained at 0.2 V to oxidise all Fc(II) to Fellll), and the collection efficiency is 0.28. Note that the residual current detected on the ring beyond 70s corresponds to re-oxidation of hydrogen generated galvanostatically on the disc. Reprinted from Corrosion Science, 28, P. Southworth, A. Hamnett. A.M. Riley and J.M. Sykes, An Ellipsometric and RRDE Study of iron Passivation and Depassivation in Carbonate Buffer , pp. 1139-1161 (1988), with kind permission from Pergamon Press Ltd.. Headington Hill Hall. Oxford 0X3 OBW. UK.

Hugot-LeGoff, A. Pallotta, C. (1985) In situ Raman spectroscopy for the study of iron passivity in relation to solution composition. 

A useful illustrative example is shown in Fig. 12.8 of iron passivated by chromate and nitrite. Fourier transform of the EXAFS data to give distance-dependent signals shows the similarities and differences between the two passivation methods. 

Cold, concentrated nitric acid renders iron passive in this state it does not react with dilute nitric acid nor does it displace copper from an aqueous solution of a copper salt. 1 +1 Nitric acid, or hot, concentrated nitric acid dissolves iron with the formation of nitrogen oxide gas and iron(III) ions ... 

Testing for Passivity. — According to Heathcote,6 iron may be regarded as passive when no chemical action can be detected by the unaided eye after immersing, shaking, and finally holding motionless a piece of the metal in nitric acid of density 1-20, at the room temperature (15° to 17° C.). This is a preferable method to that of Schonbem,7 who employed nitric acid of density 1 35 in a similar manner, because this latter concentration of acid is sufficient to render active iron passive,8 whereas acid of density 1 20 does not do so, at the room temperature. 

It is important to note this temperature restriction, however, for whereas nitric acid of density 1 250 does not render iron passive at 0° C., yet if the temperature is raised to 10° C. or above, the metal is readily passivified 9 by it. 

Chemical properties of iron. Passivity. Ferrous compounds ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, ferrous hydroxide, ferrous sulfide, ferrous carbonate. Ferric compounds ferric nitrate, ferric, sulfate, iron alum, ferric chloride, ferric hydroxide, ferric oxide (rouge, Venetian red). Potassium ferro-cyanide, potassium ferricyanide, Prussian blue. 

The role of chemisorption in the mechanism of passivity is borne out by the typical patterns of data, having the same shape as adsorption isotherms, which describe concentration of radioactive chromium on the surface of iron passivated by chromates (11), or by potential changes induced by surface concentration of chromates (12), both as a function of chromate concentration in solution (Figure 1). It is also illustrated by the initially rapid rate, followed by a measurably slow rate, with which metals achieve passivity as followed by potential change with time for iron immersed in chromates or by 18-8 immersed in aerated water (IS) (Figure 2), and by... 

Figure 11.58. Corrosion current density/potential for iron passivated with polyaniline with different coat thickness [106].

Solid Electrolyte-Ionic Electronic Transport Iron Passivation Valve Metals Dielectric Layers... 

Corrosion Phenomena Novel Energy Conversion Processes Iron Passivation... 

Similar curves can be given to lllust aspects of Iron passivation ... 

If compared with other models of Iron passivation the present model, showing clearly that an elllpsometrlcally detectable phase is formed negative to the peak potential with an effective thickness of one, perhaps two, monolayers at V l not ln agreement with models favoring the adsorption 8 0, OH, etc. It Is In agreement with a more general picture... 

Active iron (Total unconsumed iron) - (Passivated iron)... 

The effect of acid concentration on polarization of active-passive metals is shown in Fig. 4.8. Higher hydrogen ion concentration increases the critical anodic current density and decreases the passive potential range. Severe corrosion conditions present at higher acidity also increase current densities and corrosion rates at all potentials. Figure 4.9 presents the data for iron passivation in phosphoric acid/phosphate buffer solutions of... 

H. Deng, H. Nanjo, P. Qian, A. Santosa, I. Ishikawa, Y. Kurata, Potential dependence of surface crystal strucmre of iron passive films in borate buffer solution, Electrochim. Acta 52 (2007) 4272-4277. 

M. Buchler, P. Schmuki, H. Bohni, Iron passivity in borate buffer formation of a deposit layer and its influence on the semiconducting properties, J. Electrochem. Soc. 145 (1998) 609-614. 

Three main mechanisms for passive film breakdown and pit initiation have been suggested in the literature through penetration, adsorption, or film breaking [20—22]. These mechanisms apply to pure metal systems because they do not consider second-phase particles in the passive film matrix, which very often initiates pitting. For example, as already discussed, dissolution of MnS inclusion at the MnS/matrix is the initial pit formation step in steel [15]. In the absence of chloride ions, the protective hydrated iron passive film slowly converts into dissolved ferric ions ... 

According to Weissenrieder et al. [4], in the absence of either NO2 or O3, iron passivates in 200 ppb SO2. In the presence of oxidants, such as NO2 or O3 in humidified air, locahzed corrosion was detected by the authors and was described as sulfate nests [4]. The observed corrosion is autocatalytic in nature with a rapid dissolution of iron and production of Fe and Fe cations. Localized corrosion promotes the catalytic conversion of SO2 to sulfate anions, which enhance the sulfate-induced corrosion of iron, thus creating more dissolved iron cations. Next, the sulfate nests distribute through increased deposition of SO2 at high pH cathodic sites and locally create new anodic sites by decreasing the pH. 

Concrete is an alkaline environment where iron passivates and the amphoteric materials, aluminum and zinc, react with fresh concrete by evolving hydrogen. In the case of zinc, the reaction can be quenched by addition of chromates to the canent or prepassivating the zinc used as a protective layer for reinforcing iron bars in concrete. Lead in concrete, in conditions of high humidity, corrodes, and the concrete prevents the formation of the protective layers of basic lead carbonate, which would be formed in its absence. 

The standard Flade potential of iron passivated by chromates = 0.54 V) is less noble than for iron passivated by HNO3 (( )f = 0.63V). One explanation [11] is that chromates adsorb on the passive film more stron y than do nitrates, thereby reducing the overall free energy of the system and increasing the stability of the passive film. Other passivators presumably adsorb similarly, but with differing energies of adsorption. 

Iron passivity and Iranspassivily were investigated in aqueous solutions of either sodium sulfate (Na2S04) or potassium nitrate (KNO3) after different pretreatments. In all cases, ti first treatmmit consisted of an alkaline rinsing in order to remove grease fi om the sur ce. Then, we conqiared the effect of treatm t in various aqi us n dia ... 

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