Page 366 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

344 CONSEQUENCES OF CORROSION

Figure 5.21 Close-up view of tie rods showing corroded threads, nuts, and washer. (Repro-
duced by permission of the Society of Petroleum Engineers (20).)

Galvanic Cell Formation When dissimilar metals are in contact in an electrolyte,
a galvanic cell is established because of the electrochemical potential difference (24).
The consequence of the galvanic cell is the corrosion of the less noble metal. For
example, when copper is in contact with iron in the presence of an electrolyte, the
iron anode will corrode to generated Fe 2+ ions, which on further reaction with oxygen
forms brown iron oxide while oxygen reduction occurs at the copper cathode. The
reactions are

Fe → Fe 2+ + 2e −
Fe 2+ + 2H O → Fe(OH) + 2H +
2 2
2Fe(OH) + ∕ 2O → Fe O + 2H O
1
2 2 2 3 2
−
1
Cathodic reaction ∶ ∕ 2O + H O + 2e → 2OH −
2 2
The reduced pH in the anode area accelerates the corrosion of iron.
Temperature Difference The severe corrosion that occurs in heat exchangers and
heat circulation pump locations is explained by the temperature effect. A potential