Chromium alloys experimental observations

Extensive studies have been carried out by Giggins and Pettit and by Vasantasree and Hocking on a range of nickel chromium alloys with up to 50% alloying addition. Generally the principles outlined above can be used to interpret the experimental observations, where the thermodynamics of the reaction are a major factor determining the rate of attack, depending upon whether oxide or sulphide is the stable phase. 

The nature of passive films grown on Fe-Cr alloys has been reviewed [73—75]. Iron is passivated when alloyed with chromium by the formation of electronically conductive passive films. The corrosion rate of iron drastically decreases from 0.08 mm/year to 0 when chromium content in the alloy increases from 8% to 13% [9]. The Flade potentials of chromium-iron alloys in 4% NaCl solutions increase from —0.57 V in the absence of chromium to +0.17 V in an alloy with 12% chromium [10,11]. The critical passivating current for Cr-Fe alloys at pH = 7 reaches a minimum of 2 X 10 mA/cm at 12% chromium [76]. The small critical passivation current density observed for Fe-Cr alloys explains why these alloys are easily passivated in aerated aqueous solutions. Later experimental studies identified the existence of critical chromium concentration on the passivation behavior of the alloys [76]. 

The equation (6.13) is plotted in Figure 6.20 with the (arbitrary) assumption r = 8. The model permits to rationalize the observed chromium enrichment in terms of the relative rates of oxidation and dissolution reactions of alloy components, but the agreement with experimental data is mediocre. One reason is that the model ignores the fact that the passive film composition on Fe-Cr alloys is not uniform and stationary, but varies as a function of depth and time [20]. 

Indeed, numerous experimental studies have been performed to study the evolution of the enviromnent in a crevice. Most data were obtained in actively corroding artificial crevices or pits, either by sampling the solution or by direct pH and Cr concentration measurements. Table 1 summarizes significant results that confirm the foregoing trends. In crevice solutions, the drop of pH depends on the hydrolysis constants of the metal cations. On stainless alloys, chromium and molybdenum are considered to be the cause of the very low, sometimes negative, pH observed. Iron, nickel, and aluminiun exhibit much less acidic hydrolysis reactions and the pH values in the crevices are higher values of 3 to 5 are reported for iron, values of 3 to 4 for the aluminum alloys. 

---