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Metal to be protected is made the cathode
(1) by coupling a more reactive metal sacrificial anode
(2) applying an external current (impressed current)
E-pH diagram can be used to formulate ways to protect iron:
(a) increase pH of the environment so that Fe is shifted into the passivation region
(b) potential is made more negative so that Fe is shifted into the immunity region
E < -800 mv (CSE) copper-copper sulphate electrode
(c) potential is made more positive so that Fe is shifted into the passive region (formation
of
insoluble corrosion products
Empirical potential range for steel : -800 mV/-900 mV CSE
Typically: -850 mV CSE
Applied potential causes polarisation; polarisation affects corrosion rate.
Cathodic polarisation always suppresses rate of anodic reaction (rate of metal dissolution or corrosion)
Sacrificial Anode Cathodic Protection
Common sacrifical anodes
are used to protect
Combined use of cathodic protection with paints is ideal for steel structure as
(1) defects in paints will be cathodically protected against corrosion
(2) most surface area is painted thus the required cathodic protection current is small
Cathodic protection is the only method that can reduce the corrosion rate to zero (in theory). In practice, a polarisation of 200 mV below the corrosion potential for steel structures is all that needed to reduce the corrosion rate to a negligible level. This cathodic polarisation can be achieved by the use of sacrificial anodes or impressed current..
To reinforce learnings in this lecture read pages 375-393
(textbook)
To prepare yourself for the next lecture
read pages 393-404 (textbook)
