Passivity of iron

Eig. 2. The thermodynamic regions of corrosion, immunity, and passivation of iron in an iron—water system assuming passivation by a film of Ee202 (H)-... 

The possible passivation of iron and steel surfaces this can be a problem prior to chemical conversion coatings. 

Figure 60. Christian Friedrich Schonbein (1799-1868). (Courtesy E. Berl.) Discovered guncotton, 1846. Discovered ozone, worked on hydrogen peroxide, auto-oxidation, the passivity of iron, hydrosulfites, catalysts, and prussic acid. Professor of Chemistry at Basel from 1829 until the time of his death. He published more than 300 papers on chemical subjects. Reproduced from original in Kekule s portrait album.

H. Beyers and M. Darrin have studied the passivity of iron in phosphoric acid ... 

Much of our knowledge of passivity is concerned with the passivity of iron. The reason of course is that this is the most important layer technologically. However, there is an increasing amount of work being done on other passive layers (for Al, see Section 12.3) and on Cr and Ti. For details of the structure on Cr and T, see the further reading sections. 

Fig. 12.8. Passivation of iron by chromate and nitrite. Raw X-ray absorption data of (a) Fe and Fe304 and (b) Fe films after treatment (c) EXAFS data after Fourier transform (adapted from Ref. 31 with permission).

Enhancing -> passivation of iron and steel and dissolving refractory substances such as titanium slag, copper-yttrium oxide, and metal fluorides. 

Passivity of Iron.—In 1790 Keir 8 drew attention to the fact that a piece of iron, when placed in contact with nitric acid of density 1 45, is rendered inactive or passive. It does not appear to dissolve in the acid9 when placed in a dilute solution of copper sulphate it does not effect the deposition of copper and when immersed in ordinary water it exhibits remarkable resistance to corrosion. And this, in many cases, to quote the words of Keir 44 without the least diminution of metallic splendour or change of colour (see p. 57). 

Bergmann (Opuscula Chimica et Physica, 1779-88, 3, 140. See also de Benneville, J. Iron Steel hist, 1897, II., 40. Senderens (Bull Soc. chim., 1896, 15, 691 1897, 17, 279) had already observed, prior to the discovery by K.eir of the passivity of iron, that this metal does not precipitate silver from concentrated aqueous solutions of silver mtrate. 

Fig. 17M Schematic representation of the corrosion and passivation of iron in sulfuric acid. The primary passivation potential and the corresponding critical current density for corrosion i are shown. Breakdown of the passive film occurs at potentials more positive than E. ...

High temperature and high pressure processing of materials often involves the use of supercritical fluids. Corrosion studies are quite essential for evaluation of the equipment in supercritical fluid operations. Previous electrochemical measurements for alloys in supercritical fluids are rare (1-1). The reported measurements (3) show that passivation of iron alloys is different at supercritical conditions compared to ambient conditions. The study of the electrochemistry of iron alloys can lead to control of corrosion of equipment utilizing the alloys. Thermodynamic analysis provides the information about stable species, i.e. corrosion products under given temperatures and pressures. 

Passivation of iron under critical conditions is predicted. Hematite (Fe203) may still be the main corrosion product in the neutral water pH (pH 7.2) region, but the passivation potential range is narrower and shifts to negative potentials, compared with regions on the diagram for ambient conditions. For chromium, no solid chromium oxide stable species is predicted within the stable region of neutral water. This indicates chromium oxidation without any passivation oxide film formation. 

W. Hittorf concluded that the passivity of chromium is not due to the formation of an oxide film, but rather to the metal assuming a different electrical. state the metal in the passive state is in a strained or coerced condition— Zwangzustand—so that instead of dissolving as a bivalent element it dissolves as a sexivalent element. The film hypothesis, discussed in connection with the passivity of iron, best fits the facts. C. W. Bennett and W. 8. Burnham stated that the film is best regarded as a film of oxide which is rendered stable by adsorption into the metal. The oxide is usually unstable, but becomes stable when adsorbed by the... 

Other Non-Electrochemical Methods. B. Cahan(56) is examining the passivation of iron and ferrous alloys by a... 

Passivation of iron granulates in permeable barriers used for in situ groundwater remediation may result in a shorter life time and in contaminant breakthrough earlier than expected. Therefore, mineral reactions or generally the effect of other groundwater constituents on the long term reactivity of iron is of major interest for the application of this technology in environmental clean up. For interpretation of column experiments it is also important to estimate the effect of flow velocity on the extent of passivation due to mineral reactions. 

Even if the mechanisms or the extent of passivation of iron by mineral reactions are still unknown in detail the inorganic reactions most probably effect the degradation behaviour of CHC. A simplified model is capable of simulating the effects of these reactions on CHC reduction at different flow velocities on the assumption that mineral precipitation as well as continuous anaerobic corrosion decrease the reactive iron surface. PHREEQC (Parkhurst and Apello, 1999) can be used for qualitative simulation after modification of the data set integrating ethene and chlorinated ethene species as well as some iron minerals. Figure 13.6 shows the results of two simulations at five times different pore velocities and the model assumptions in Table 13.1. 

Calcite, Siderite, Magnetite - Equilibrium precipitation and dissolution (if present) - Precipitation causes passivation of iron... 

The content of iron was used in place of an iron surface in the simplified model and linear relationships were presumed between mineral precipitation and passivation of iron. Only a small iron content was used in each cell to achieve results in acceptable simulation times. The model results in PCE concentration profiles which are typically observed in column experiments (Fig. 13.6), even if the assumptions cannot be verified as the real mechanisms of passivation. Nevertheless, the simulation shows a migration... 

This reaction occurs, for example, with passive iron. The inverse reaction is also well known as an additional mechanism of passivation of iron in solutions containing Fe +-ions [50]. If the solubility product is not reached, chemical dissolution takes place... 

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