Corrosion

By Dr E. Ramanathan, Paint Consultant

Chemistry of Corrosion

Corrosion is a natural process that involves the deterioration of materials, typically metals, due to chemical reactions with their environment. The theory of corrosion is essential for understanding and preventing this phenomenon. Several theories and mechanisms explain the corrosion process, with the most widely accepted ones being the electrochemical theory and the chemical reaction theory. Here’s an overview of these theories:

Electrochemical Theory (Electrochemical Corrosion):

• This theory is the most widely accepted and applicable to most types of corrosion.
• It is based on the principle that corrosion is an electrochemical process that involves the flow of electrons.
• Corrosion occurs at anodic and cathodic sites on the metal surface.
• At the anode, metal atoms lose electrons and become ions, entering the solution as metal cations (e.g., Fe²⁺ for iron).
• At the cathode, reduction reactions occur, typically involving the reduction of oxygen or another oxidizing agent, which gains electrons.
• The overall reaction can be represented as a cell in which metal oxidation and reduction reactions take place.
• For example, in the corrosion of iron (rusting), the overall reaction can be represented as: 4Fe(s) + 3O₂(g) + 6H₂O(l) → 4Fe(OH)₃(s).

Chemical Reaction Theory:

• This theory focuses on the chemical reactions responsible for corrosion.
• It emphasizes the role of specific chemical reactions that take place between the metal and its environment.
• For example, the corrosion of aluminum is primarily due to its reaction with water to form aluminum hydroxide.
• The specific chemical reactions vary depending on the metal and the corrosive environment.

In addition to these fundamental theories, there are different types of corrosion, including uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, and stress corrosion cracking. Each type of corrosion has specific mechanisms and conditions that influence its occurrence.

Corrosion prevention and control strategies include the use of protective coatings, inhibitors, cathodic protection, alloy selection, and proper design and maintenance practices. Understanding the underlying theories of corrosion is crucial for developing effective strategies to mitigate the negative effects of corrosion on various materials and structures.

Corrosion Quiz

1. What is the primary cause of corrosion in metals?
   a) Mechanical stress
   b) Chemical reactions
   c) Thermal expansion
   d) Electrical conductivity
2. Corrosion is most accurately described as a:
   a) Physical change
   b) Chemical reaction
   c) Biological process
   d) Mechanical deformation
3. Which theory of corrosion emphasizes electrochemical processes?
   a) Chemical Reaction Theory
   b) Galvanic Theory
   c) Electrochemical Theory
   d) Rust Theory
4. In the electrochemical theory of corrosion, where does oxidation occur?
   a) Anode
   b) Cathode
   c) Electrolyte
   d) Metal surface
5. What type of corrosion is characterized by localized, small pit formations on a metal surface?
   a) Pitting corrosion
   b) Uniform corrosion
   c) Galvanic corrosion
   d) Crevice corrosion
6. Which metal is prone to “rust” or iron corrosion?
   a) Copper
   b) Aluminum
   c) Iron
   d) Zinc
7. In the corrosion of aluminum, what compound is typically formed?
   a) Aluminum oxide
   b) Aluminum hydroxide
   c) Aluminum chloride
   d) Aluminum sulfide
8. What is the term for a protective coating applied to metal surfaces to prevent corrosion?
   a) Paint
   b) Inhibitor
   c) Passivation
   d) Galvanizing
9. Which of the following is not a common method of corrosion prevention?
   a) Cathodic protection
   b) Alloying
   c) Oxidation
   d) Coatings
10. What type of corrosion occurs when dissimilar metals are in contact, leading to one metal corroding faster than the other?
    a) Pitting corrosion
    b) Galvanic corrosion
    c) Crevice corrosion
    d) Stress corrosion cracking
11. Stress corrosion cracking is most likely to occur in which type of environment?
    a) Dry, low-humidity conditions
    b) High-temperature and high-pressure conditions
    c) Highly acidic solutions
    d) Normal atmospheric conditions
12. Which of the following metals is known for its excellent corrosion resistance and is often used in chemical and marine applications?
    a) Copper
    b) Brass
    c) Stainless steel
    d) Aluminum
13. What is the primary cause of crevice corrosion in metal structures?
    a) High temperature
    b) Low oxygen concentration
    c) Low humidity
    d) High pH
14. Which factor is not critical in the corrosion of metals?
    a) Temperature
    b) Humidity
    c) Pressure
    d) Time
15. The overall corrosion reaction of iron is often represented as the formation of what compound?
    a) Iron sulfide
    b) Iron oxide (rust)
    c) Iron carbonate
    d) Iron chloride
16. What is the term for the protective layer that can form on the surface of some metals, reducing further corrosion?
    a) Oxidation layer
    b) Passivation layer
    c) Erosion layer
    d) Galvanic layer
17. Which of the following is not a common corrosion inhibitor?
    a) Phosphoric acid
    b) Sodium nitrate
    c) Sodium chromate
    d) Potassium chloride
18. What is the process by which sacrificial anodes are used to protect metal structures from corrosion?
    a) Passivation
    b) Cathodic protection
    c) Oxidation
    d) Galvanizing
19. The corrosion of a metal in a specific environment can be accelerated if the pH is:
    a) Neutral (pH 7)
    b) Basic (pH > 7)
    c) Acidic (pH < 7)
    d) Independent of pH
20. Which type of corrosion is characterized by a more or less uniform removal of material from the metal surface?
    a) Pitting corrosion
    b) Uniform corrosion
    c) Galvanic corrosion
    d) Stress corrosion cracking

Corrosion in Copper

Corrosion on Mild Steel Plates

Salt Spray Resistance Tested Panel