Principle of Cathodic protection
Corrosion is the spontaneous chemical reaction of a metal against its surroundings.
Particularly, in sea water this chemical reaction proceeds by an electrochemical
mechanism involving oxidation of the metal. This results in metal loss. When we
have a plate, which is not coated, its surface becomes rough, whereas on a coated
one, the surface turns into pitting appearance, at the spots where the coating
wasn't good enough to protect it.
The part of the metal surface in the electrochemical mechanism (electron release)
is called anodic. On the other hand, the part of the metal surface where the electron
consumption takes place is called cathodic. If we can supply electrons from an
external source, then the electron consumption (cathodic) will speed up and the
electron release (anodic) reaction will slow down. As a consequence, the rate
of iron dissolution will slow down and the electrode potential will decrease.
From the procedure above, the Principal of cathodic protection can be derived:
'By supplying electrons into the metal from an external source, we slow down its
dissolution, i.e. it's rusting away.'
Applications of anodic protection
The delivery of the electrons to the metal surface to be
protected can be achieved in two ways. Firstly, by using an impressed current
technique and secondly, by using sacrificial anodes
The impressed current method
By this method the electrical current is delivered to the
metal surface from a D.C. power source through an auxiliary anode i.e. the current
is forced in or impressed.
The metal surface will act as a cathode in
the shell that formed.
Typical galvanic anode for cathodic protection
of a ship's hull (anodes alongside the hull are normally mounted
alternately on both sides of dilge keels
The sacrificial anode method
By this method a galvanic cell is formed between
the structure to be protected and the sacrificial anode. The electrons
pass spontaneously from the anode to the metal surface (cathode). Thus,
the source of the electrons (the sacrificial anodes) must have a more
negative electrode potential than the metal surface. Metals and conducting
materials commonly used are listed below in such order that each normally
act as anode with respect to all the materials which follow it.
MAGNESIUM, ZINC, ALUMINUM IRONANDSTEEL, LEAD, BRASS,
COPPER, GRAPHITE, COKE E.T.A.
This explains why magnesium, aluminum and zinc alloys
are used to protect steel today.