Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Passivation Potential

Last updated: August 14, 2018

What Does Passivation Potential Mean?

Passivation potential is the potential that divides decreasing current from the increasing current. It can be best described by looking at a highly negative electrode potential where passivation film has been built up. It is equivalent or relatively positive compared to the equilibrium potential of the formation phase of the film. Typically, corrosion current travels through the most at passivation potential.

The forward motion of polarization scans features that, when the current rapidly increases along with the potential close to corrosion potential, the current obtains maximum value and drops rapidly.


Corrosionpedia Explains Passivation Potential

Passivation potential is mostly exhibited by nickel-based or iron-based alloys in sulfuric acid, for instance. This potential is usually the outcome of an alloy surface going through a valency change or passivation process as potential increases.

The passivation of metal is commonly studied with electrochemical testing. If the metal potential is controlled and moved to a more positive, or anodic, direction, the required current to promote a shift typically varies. However, if the required current to achieve the shift has been generally polarized, the metal can be regarded as active-passive and can be protected anodically.

The corrosion rates of active-passive metals are reduced significantly by potential shifting so the metal stays within the value of the passive range. The current needed to promote potential shifting following the positive direction from corrosion potential can be numerous magnitude orders higher than the required current needed to keep potential passive. With this, the current can peak the value of passivation potential, which can be represented by Epp.

For passivation to occur, the critical current density should be surpassed. Anodic or positive potential should be maintained within the passive spot, not letting it fall back into the active or transpassive area where anodic film can become deformed and fail. Therefore, a small amount of critical current density is required to promote passivation.

Passivity can also be readily achieved without external passivating potential through the utilization of oxidants to manage environments of redox potential like the use of nitric acids and other acid types to build passivation.


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