Lecture 15: Cathodic Protection

• Theoretical Basis

• Cathodic Protection Potential

• Sacrificial Anode Cathodic Protection

• Summary

• Reading Assignments

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Theoretical Basis

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

Cathodic Protection Potential

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 sacrificial anodes

• zinc
• magnesium
• aluminium

are used to protect

• buried pipelines
• offshore structures
• ship's hull

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

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Summary

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..

Reading Assignments

To reinforce learnings in this lecture read pages 375-393 (textbook)
To prepare yourself for the next lecture read pages 393-404 (textbook)

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