Electrolysis: reminders of cathodic protection
Introduction
The protection against the corrosion of immersed structures (piers, offshore oil drilling platforms, ships…) is often carried out by cathodic protection.
Definition
Cathodic protection allows controlling the corrosion of a metal surface by transforming that surface into the cathode of an electrochemical cell.
With this technique, the electric potential of the metal to be protected is decreased to an inferior value of EP (a value of the protection potential under which the oxidation reaction of metal becomes negligible). This decrease in the electric potential is obtained by passing a current from the surface to be protected (cathode) to an auxiliary electrode (anode).
It can be carried out in two different ways:
-
by sacrificial anodes (galvanic coupling between the sacrificial material and the structure to be protected)
-
by an imposed current (use of a non consumable anode and a current generator)
| Cathodic protection … | Principle |
|---|---|
|
by sacrificial anode (reactive anode) |
An electrochemical cell is created, which imposes the direction of the electrons motion in order to prevent the corrosion reaction. The anode gets dissolved (it is sacrificed), while the cathode remains stable. |
|
by imposed current (active anode) |
The direction of the electrons motion is imposed by establishing a difference of potential between the part and the environment with a current generator. |
Electro-chemical cell
An electrochemical cell is a heterogeneous system which comprises:
- at its ends, electronic conducting materials (metal…)
- between the extremities, an ionic conducting material (electrolyte…).
The electrochemical reaction describes the phenomenon that occurs at the interface between two conductive systems (electronic and ionic) during the transfer of the electric charge consisting of one or more electrons.