FeCr alloys dissolution

Figure 10.9 Potential dependence of the partial current densities of iron (spheres) and chromium (rectangles) dissolution from FeCr alloys in sulfuric acid (a) FeCrg gj, Tafel slope 40 mV and (b) FeCr j, Tafel slope 100 mV. (Reproduced with permission from Ref. [17], 1980, Elsevier.)...

Another example is the active dissolution of FeCr alloy (Figure 10.9). At low chromium content the observed Tafel slopes are similar to pure iron (40 mV). At higher chromium content the Tafel slopes are similar to pure chromium (100 mV). The surface layer changes from an iron rich to a chromium rich phase. 

The simple model of a homogenous passive layer of Figure 5.6 becomes more complicated if a second alloy component is present as shown in Figure 5.30. The composition of the passive layer is then determined by the oxidation rates of the components A and B at the metal surface, fheir transfer rates through the film, and their transfer across the passive layer-electrolyte interface, i.e., their individual corrosion rates in the passive state. The reaction rates at both interfaces may be decisive for the layer composition. One example is the preferential dissolution of Fe " ions due to the extremely slow cation transfer of Cr " ions at the surface of the film, which leads to an accumulation of Cr(lll) wifhin the film for FeCr alloys. Another example is the preferential oxidation of A1 of an A1 alloy containing 1% Cu. Cu does not enter the film and is accumulated at the metal surface while an AI2O3 film is formed. These examples are discussed in defail in the following. 

XPS studies of the reduction of a film formed at E = 0.96 V in phthalate buffer pH 5.0 on Fe-15A1 show characteristic compositional changes. Galvanostatic reduction with i = -20 pA cm yields a decrease of Fe(III) with an increase of Fe(II) to a maximum after 20 s and a decrease to a constant value of 12% after 40 s [117]. The Al(III) content stays constant till 40 s and drops afterwards to a constant value of 15%. Apparently, the Al(III) oxide remaining at the surface protects the remaining Fe(II) oxide against dissolution. The A1 enrichment in the center of the passive layer is displaced to the surface due to dissolution of iron after its reduction to Fe(II). The oxidation of Fe(II)-to-Fe(III) and its reduction with appropriate changes of the potential remain the same compared to that of the passive layers formed on pure iron and FeCr alloys as described above. 

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