Nanotech 2011

A proposal of AC corrosion mechanism in cathodic protection

A. Brenna, M.V. Diamanti, L. Lazzari, M. Ormellese
Politecnico di Milano, IT

Keywords: alternating current, cathodic protection, corrosion, passive film

Abstract:

The research proposes an electrochemical mechanism of AC-corrosion in CP condition. Electrochemical tests were carried out on carbon steel in simulated soil conditions to identify AC corrosion risk related parameters during cathodic polarization, like AC and CP current density ratio and protection potential. A corrosion risk diagram (Figure) is proposed: laboratory results show that overprotection condition seems to be the most dangerous in the presence of AC-stationary interference due to the high pH that can be reached close to the metal. A two-steps corrosion model is proposed: AC causes the local breakdown of the nanosized passive film formed in cathodic protection condition (Step 1) and then corrosion can occur (Step 2) due to the high alkalinity in overprotection condition. Indeed, if the nanosized passive film is broken by AC, at high pH carbon steel corrosion is possible with a chemical mechanism that doesn’t depend on potential. The effect of AC interference on nanosized passive films was also studied by means of electrochemical tests on stainless steel with different localized corrosion resistance.
 

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