CAthodic Protection

[ronweiser]

Hi,

I am currently looking into Cathodic protection of submersed equipment for my employer. In one of the papers i was reading, the following statement is made:

"corrosion occurs at the annode but not at the cathode (unless the metal of the cathode is attacked by Alkali)"

The paper does not go on to explain the effect of Alkali on the cathode, which I assume is because this could make an entire paper in itself.

Can anyone shed light on the effects an Alkali has on the cathode, and why it changes the corrosion on the annode? Or has anyone got any suggestion son where to find information on this?

Regards

Ronny

 

[SJones]

One of the results of passing current through an aqueous electrolyte is the generation of hydroxyl ions (alkali) or the reduction of hydrogen ions (increasing pH) at the cathode. Therefore, cathodes - the protected structure - tend to sit in an alkaline environment. In seawater this shift in pH brings about the deposition of calcareous deposits which can form a protective film. The alkaline environment influences the polarisation of the cathode which in turn affects the driving potential between anode and cathode in a sacrificial system.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[ronweiser]

Thank you for your help!

 

[fdac]

The paper is referring to a generalised galvanic corrosion cell. As posted above, normally the anode corrodes and the cathode is protected.

Steel is protected from corrosion under alkaline conditions which is why CP is widely used. What the paper is saying is that some other metals, notably aluminium, corrode in alkaline conditions. This means that if over-protected or excessively polarised cathodically, they corrode. One has to be careful applying CP to these metals.