Primary passivation potential

FIG. 28-9 Typical electrochemical polarization curve for an active/passive alloy (with cathodic trace) showing active, passive, and transpassive regions and other important features. (NOTE Epp = primary passive potential, Ecaa- — freely corroding potential). 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

The critical current and primary passivation potential will not appear on an anodic polarisation curve when the steady-state potential already is higher than In such a case the potentiostat is unable to provide direct data for constructing the full polarisation curve. If that portion of the curve below the steady-state potential is desired, then the potential has to be held constant at several points in this range and corrosion currents calculated from corrosion rates as determined from solution analyses and/or weight losses. 

Figure 2 Typical anodic dissolution behavior of an active-passive metal. ZJpp = primary passivation potential, iait = critical anodic current density, and ipass = passive current density. (After Ref. 71.)...

One of the unique features of a corroding metal undergoing passivation is a region of apparent negative resistance. Looking at Fig. 17M, we note that at potentials anodic to the primary passivation potential... 

Fig. 17M Schematic representation of the corrosion and passivation of iron in sulfuric acid. The primary passivation potential and the corresponding critical current density for corrosion i are shown. Breakdown of the passive film occurs at potentials more positive than E. ...

Pickering and coworkers [31, 34, 35] have demonstrated both experimentally and computationally that for systems that meet the criteria of the IR theory, lA is predicted. The amount of potential drop increases as one moves into the crevice because of the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances, the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabihzation of crevice corrosion according to this model. 

The potentiodynamic polarization technique is used to determine the potential region at which the alloy or the metal is passive when exposed to a particular environment. It estimates both the ability of the material to spontaneously passivate as well as the critical current density necessary for its passivation. Potentiodynamic polarization measurements identify the corrosion properties of passivating metals and alloys and are very usefirl in predicting how a material will behave when exposed to a corrosive environment. The method estimates the corrosion active region, the onset of passivation, the critical current density, the primary passive potential, the current in the passive region, and the voltage span of the passive region. 

Pourbaix diagram (electrode potential-pH diagram)—a graphic representation showing regions of thermodynamic stability of species in metal-water electrolyte systems, primary passive potential (passivation potential)— the potential corresponding to the maximum active current... 

Epa = Passive potential Ecorr = Corrosion potential Eo3 = Oxygen evolution potential Epp = Primary passive potential Ep = Pitting potential... 

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. 

The above case is exemplified by titanium in deaerated H2SO4 or HCl or Fe in H2SO4. Titanium has a more active primary passivation potential ( pp) than the reversible potential of... 

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