Potential chromium-iron alloys

As the chromium content in the alloy increases from 8% to 13%, the corrosion rate of iron decreases from 0.08 mm/year to very low values [9]. The Flade potentials of chromium-iron alloys in 4% NaCl solutions increases from —0.57 V (vs. SHE) in the absence of chromium to +0.17 V (vs. SHE) for the ahoy with 12% chromium [7,10]. The critical current density for the passivation of Cr-Fe aUoys at pH = 7 reaches a... 

Figure 6.4. Standard Flade potentials for chromium-iron alloys and chromium [7-9].

Figure 6.10. Potentials of chromium-iron alloys in 4% NaCi. [Reprinted with permission from H. Uhlig, N. Carr, and P. Schneider, Trans. Electrochem. Soc. 79, 111 (1941). Copyright 1941, The Electrochemical Society.]...

In this figure, for zero pH (pH = 0 rev,oxide = the passivation potentials of chromium and iron do not match the standard potentials of the oxides Cr203 and Fc203 listed in Table 6.8. One explanation is that kinetic limitations lead to a higher passivation potential than predicted by thermodynamics. Another reason could be that the listed standard potentials were measured on bulk samples. The extreme thinness of passive films could influence their thermodynamic properties. In addition, their composition does not always correspond to a simple stoichiometry. For example, chromium-iron alloys form passive films containing both iron and chromium cations and their passivation (Figure 6.9) lie between those of iron and chromium. Then again, they exhibit the same pH dependence as the pure metals. 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

Polarization curves for iron, chromium, and alloys with 1, 6, 10, and 14 weight percent (wt%) chromium in iron are shown in Fig. 5.24 the environment is 1 N H2SO4 at 25 °C (Ref 21). Iron and chromium are body-centered-cubic metals, and the alloys are solid solutions having this structure. The passivation potential (Epp), the active peak current density (icrit), and the passive state current density (ip) are decreased... 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

For the formation of passive layers, at least 10.5% Cr is necessary. The activation potential of iron-chromium alloys in sulfuric acid changes, depending on the chromium content. Alloys with chromium... 

Ferrous ion, a product of the corrosion reaction in Eq. (12.6), reacts with nitrate immediately to form a barrier oxide film through Eq. (12.13). The resulting potential would then be the potential of Fe203 in water. The anodic potential shift is due to protective film surface coverage. The observed performance improvement of chromium-containing alloys suggests that the inhibitor helps stabifize both the iron and chromium oxide layer. Nitrite ions act as anodic inhibitors by increasing barrier oxide film formation rate. 

The stainless steels (types 303, 316, and 316LVM) as well as the cobalt-nickel-chromium-molybdenum alloy MP35N are protected from corrosion by a thin passivation layer that develops when exposed to atmospheric oxygen and which forms a barrier to further reaction. In the case of stainless steel, this layer consists of iron oxides, iron hydroxides, and chromium oxides. These metals inject charge by reversible oxidation and reduction of the passivation layers. A possible problem with these metals is that if the electrode potential becomes too positive... 

The addition of a more passive metal to a less passive metal normally increases the ease of passivation and lowers the Flade potential, as in the alloying of iron and chromium in 10 wt. % sulphuric acid (Table 10.31) . Tramp copper levels in carbon steels have been found to reduce the corrosion in sulphuric acid. Similarly 0 -1 palladium in titanium was beneficial in pro-... 

Table 10.31 Effect on critical current density and Flade potential of chromium content for iron-chromium alloys in lOwt.% sulphuric acid (after West )...

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