Flade potential chromium

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 )...

Chromium (%) Crilical current density ( crii,. Am- ) Flade potential ( f. V)... 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

The Flade potential for iron (+0.63 V) indicates that only very strong and concentrated oxidizing agents will form passive films on its surface. However, even weak oxidizing agents form thin and very stable corrosion-resistant surface films on chromium. The 12-30% chromium content in stainless steel gives excellent corrosion resistance properties to steel due to formation of a stable chromium oxide passive film on its surface. Figure 4.2 shows the standard Flade potential measured for stainless steels with different chromium contents. 

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... 

This reproducible Flade potential and its 0.059 pH dependence is characteristic of the passive film on iron. A similar potential-pH relation is found for the passive film on chromium, for Cr-Fe alloys, and for nickel, for which the stan-... 

When activated cathodically, the Flade potential of chromium and stainless steels follows the relation n(0.059pH), where n may be as high as 2. For self-activation, n is 1 [8],... 

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

From the standard Flade potential for iron, calculate the apparent free energy of formation of the passive film per gram-atom of oxygen. Do the same for nickel and chromium. 

Chromium = -0.74 V) is more active in the Emf Series than is iron (< )° = -0.44V), but chromium has a strong tendency to become passive (< )p = 0.2V). Hence, the potential of chromium in aqueous media is usually noble to that of steel. However, in galvanic couples of the two metals, especially in acid media, chromium is polarized below its Flade potential, so that it erfiibits an active potential. Hence, the corrosion potential of chromium-plated steel, which is always porous to some degree, is more active than that of either passive... 

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