The document discusses corrosion mechanisms and corrosion cells. It explains that corrosion processes are electrochemical in nature, involving an anodic reaction where metal ions are produced and a cathodic reaction where electrons are consumed. It describes three types of corrosion cells: 1) dissimilar metal cells where two different metals contact, 2) concentration cells where electrodes contact solutions of different compositions, and 3) differential temperature cells where electrodes are at different temperatures. The document provides examples of corrosion mechanisms for iron rusting and zinc reacting with hydrochloric acid.

1. Corrosion Mechanism & causes
of Corrosion Cells
Presented To:
Sir Zaheer Hussain
Presented By:
Aijaz Ahmed
(K-17PG10)
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

2. Introduction
 There are three primary reasons for concern about and study of corrosion:
 safety,
 economics,
 and conservation
 Premature failure of structures due to corrosion can result in different losses.
 By The National Institute of Standards and Technology (NIST):
 estimated the annual cost: $9 billion to $90 billion.
 These figures were confirmed by various technical organizations, including the National
Association of Corrosion Engineers (NACE).
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

3. Losses
 Economic Losses
 Indirect Losses
 Shutdown
 Contamination
 Loss of Product
 Loss of Efficiency
 Environmental Damage
 Overdesign
 Conservation Losses
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

4. Corrosion Mechanisms
 Chemical reactions are those in which elements are added or removed from a chemical species.
 Electrochemical reactions are chemical reactions in which not only may elements may be added
or removed from a chemical species but at least one of the species undergoes a change in the
number of valance electron.
 Corrosion processes are electrochemical in nature
 Example:
 Attack of zinc by hydrochloric acid
Zn + 2HCl ——> ZnCl2 + H2
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

5. Zn + 2HCl ——> ZnCl2 + H2
 Oxidation
In the above reaction oxidation (Anodic reaction)
Zn ——> Zn+2 + 2e
 Reduction
 The reduction reaction (Cathodic reaction)
2H+ + 2e ——> H2
During metallic corrosion, the rate of oxidation will equal to the rate of reduction.
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

6. Corrosion Mechanism of Iron oxide (Rust)
Formation
 Now consider when an iron is immersed in water or sea water which is exposed to the atmosphere.
Now corrosion will occur due to the anodic reaction is
Fe —–> Fe+2 + 2e
 And also the medium is exposed to the atmosphere, it contains dissolved oxygen. Both water and sea
water are nearly neutral, thus the cathodic reaction takes place as follows
O2 + 2H2O + 4e —–> 4OH–
 Now, remember that the sodium and chloride in the sea water do not participate in the reaction. The
reaction is only between the iron and water. The reaction can be rewritten as follows
2Fe(OH)2 + H2O + ½ O2 —–> 2Fe(OH)2
 Now the final product is familiarly called as “Rust”.
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

7. Corrosion cells
1. Dissimilar electrode cells
2. Concentration cells:
a) a. Salt concentration cell
b) b. Differential aeration cell
3. Differential temperature cells
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

8. 1. Dissimilar electrode cells
 Dissimilar electrode cells may be formed when two dissimilar metals are in contact or due to the
heterogeneity of the same metal surface.
 The Daniel cell is a common example
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

9. 2. Concentration cells:
Concentration cells are formed when the electrodes are identical but are incontact with solutions of
differing composition.
 Salt concentration cell
 A salt concentration cell forms when one electrode is in contact with a concentrated solution and the
other electrode with a dilute solution.
 On short-circuiting, the electrode in contact with the dilute solution will be anodic.
 The local variation of composition of the process stream inside the pipeline in a chemical plant may lead
to such a situation in practice.
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

11. Differential aeration cell
 A differential aeration cell forms when the identical electrodes are exposed to solutions of
identical chemical composition that differ in oxygen content.
 The electrode in contact with the less aerated or oxygenated solution will act as the anode.
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's

13. 3. Differential Temperature Cells
 Differential temperature cells are formed when electrodes of the same metal, each of which is at
a different temperature, are immersed in an electrolyte of the same initial composition.
 Such a situation may arise in practice in components of heat exchangers, boilers, and similar heat
transfer equipment.
 Polarity developed in an electrode varies from system to system.
 For a copper electrode in a copper sulfate solution, the electrode at the higher temperature is
the cathode; but for lead, the situation is just the reverse.
 For iron immersed in dilute aerated sodium chloride solutions, the hot electrode is initially anodic
to the colder metal, but the polarity may reverse with the progress of corrosion.
Department of Petroleum and Natural Gas Engineering, MUET SZAB Campus Khairpur mir's