What Does Critical Pitting Potential (Epit) Mean?
Critical pitting potential (Epit) is the least positive potential at which pits can form. It is the potential at which the metal salt of the aggressive ion in solution is in equilibrium with the metal oxide.
Such equilibrium may be achieved when the activity of the anion in the pit is increased by the potential difference between the inside and outside of the pit. This can successfully predict the critical pitting potentials for the following:
However, it cannot predict critical pitting potentials for the following:
Critical pitting potential is also known as breakdown.
Corrosionpedia Explains Critical Pitting Potential (Epit)
Pits form and propagate at and above the critical pitting potential. Most specifically, pitting for a given material is characterized electrochemically by the critical pitting potential (Epit). Epit lies between the noblest potential, where no grown pit is found, and the least noble potential, where pitting occurs. Epit is the most negative potential above which pits nucleate and grow, which approximates the noblest potential below which a pit never forms.
Critical pitting potentials are usually determined from steady-state anodic polarization curves. The most reliable approach is to use a potentiostatic technique in which a constant potential is applied, and the current is recorded as a function of time.
Conditions favoring pitting are as follows:
- Stagnant solutions
- Acidic solutions
- Presence of Cl- and Br- (halides)
- High temperature
- Rough surface finish
- Presence of surface deposits
- Rupture of protective coatings
The value of critical pitting potential for a given metal depends on chloride concentration. For a given chloride concentration, the more positive the critical pitting potential, the more resistant the metal or alloy to pit initiation.
Epit shows a significant active shift with active chloride contents. Due to the formation of strong and more resistant passive oxide film, the high alloy stainless steel has a much higher Epit than conventional stainless steels.