Flade potential passive film formation

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. 

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]. 

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. 

The lower portion of the anodic curve (nose of the curve) exhibits a Tafel relationship up to icritical which Can be considered as the current required to generate sufficiently high concentration of metal cations such that the nucleation and growth of the surface firm can proceed. The potential corresponding to icritical is called the primary passive potential (lipp) as it represents the transition of a metal from an active state to a passive state. Because of the onset of passivity, the current density (log i) starts to decrease beyond pp due to the oxide film formation on the metal surface. Beyond pp the current continues to decrease until at a certain value of potential, it drops to a value orders of magnitude lower than icritical- The potential at which the current becomes virtually independent of potential and remains virtually stationary is called the flade potential (fip). ft represents the onset of full passivity on the metal surface due to film formation. The minimum current density required to maintain the metal in a passive state is called passive current density (ip), ft is an intrinsic property of oxidation. 

Because the metal dissolution is an anodic process, for example, Fe(s) Fe +(aq) + 2e , the current of the process is assumed to be positive. When potential increases from Mez+zMe lo f (passivation or Flade potential), the current is increasing exponentially due to the electron transfer reaction, for example, Fe(s) -> Fe +(aq) + 2e", and can be described using Tafel s equation. At a E the formation of an oxide layer (passive film) starts. When the metal surface is covered by a metal oxide passive film (an insulator or a semiconductor), the resistivity is sharply increasing, and the current density drops down to the rest current density, 7r. This low current corresponds to a slow growth of the oxide layer, and possible dissolution of the metal oxide into solution. In the region of transpassivation, another electrochemical reaction can take place, for example, H20(l) (l/2)02(g) + 2H+(aq) + 2e, or the passive film can be broken down due to a chemical interaction with environment and mechanical instability. Clearly, a three-electrode cell and a potentiostat should be used to obtain the current density-potential curve shown in Figure 9.3. 

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