Question

Please illustrate and explain why a metal or alloy will be more readily passivated if it has small critical anodic current density and an active primary passive potential.

Answer 1

Critical current Density :The maximum current density observed in active region for metal or alloy that exhibits an active –Passive behavior
Passive current density: The minimum current density required to maintain the thickness of film in the passive range

Passive Potential :The potential of an electrode where a change from an active to a passive state occurs

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.

Corrosion current is a kinetic value and potential is thermodynamic value. Corrosion rate is proportional to corrosion current (corrosion current density). Corrosion potential can shows corrosion tendency but not corrosion rate.

Irrespective of corrosion potential, corrosion rate can be evaluated based on corrosion current density value. Corrosion potential changes quickly with change in environment whereas corrosion current density changes slowly as the corrosion proceeds. Accumulation of corrosion products on surface also indicates lower values of corrosion potential but clear idea will be obtained by corrosion current density.