Passivating potential decreased with chromium

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]. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

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 technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

Those alloying additions which decrease icritical are effective in increasing the passivating tendency. Consider alloying additions of Mo, Ni, Ta and Cb to Ti and Cr. The critical current density of Ti and Cr is reduced on addition of Mo, Ni, Ta or Cb. The potential of the above elements is active and their rate of corrosion is low. Generally, those alloying elements are useful which show low corrosion rates at the active potentials. Alloying with metals which passivate more readily than the base metal reduces icritical and induces passivity. Elements, like chromium and nickel, which have a lower (critical and Epassive than iron, reduce the... 

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