Stationary passive current density

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

Metal dissolution also occurs in the passive state, but at a much smaller rate. The extremely small passive current densities are a consequence of a thin, solid, poreless, and adherent film that covers the metal surface. For iron, the stationary passive current density is potential independent and equals ip = 6pA cm in 0.5 M H2S(34. It is smaller in more alkaline solutions and solutions without complexing properties. The film thickness grows linearly with the potential and reaches ca. 5nm for a ca. 1V increase above the passivation potential. Any dissolution phenomenon involves the transfer of cations across this layer and thus a transfer through the solid state. 

The passive current density is influenced by the action of anions even for conditions where pitting does not occur. Passive iron in 0.5 M H2SO4 shows stationary current... 

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. 

Otterstedt et a/.also studied waves in the oscillatory regime during Co dissolution. The oscillations possess a relaxationhke character, which is typical for oscillations between the active and the passive state of metal dissolution reactions. They are characterized by long, quasi-stationary periods of vanishing current density, followed by a sharp... 

When the applied current density reaches the critical value of 100 niAcm, the potential of the electrode swings between two stationary state values corresponding to a passivated and a non passivated state. When the applied current density is over this... 

Stationary state. The linear relation between the thickness and potential applied was reported by several authors. In Fig. 13 the current density cd) was also plotted, which was taken after 1 h oxidation. It is seen that the passive oxide grows with anodic potentials and the cd remains constant in the passive potential region, regardless of the potential value, and, however, it is dependent on the solution pH. The pH dependence of the stationary state cd was reported in the pH lower than five to be... 

Figure 15. Steady current density in the passive potential region as a function of solution pH. The cd reached the stationary value in the solution at pH lower than 5 in 1 h oxidation at each potential, however, it does not reach at pH higher than 5 in which the cd was plotted after Ih oxidation. Reprint from N. Sato and T. Noda, Ion Migration in Anodic Barrier Oxide Films on Iron in Acidic Phosphate Solutions , Electrochim. Acta, 22 (1977) 839, Copyright 1977 with permission from Elsevier Science.

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