Theories of corrosion

 Theories of Corrosion is given below

• Theories of Corrosion are natural process that results in the deterioration of materials, usually metals, through chemical or electrochemical reactions with their environment.
• The primary theories of corrosion include acid corrosion, dry (chemical) corrosion, and galvanic (electrochemical) corrosion

1. Acid Corrosion (Electrochemical Corrosion in Acidic Environment):

Mechanism:

• Occurs when a metal reacts with an acidic environment, leading to the metal dissolving into ions.

Example Reaction:

$M→”M ^(2+)+2e ^-$

Common Metals Affected:

• Iron, steel, and other reactive metals.

Characteristics:

• Localized corrosion, often forming pits or holes.
• Accelerated in the presence of chloride ions (e.g., in seawater).

Prevention:

• Use of corrosion inhibitors.
• Coatings and protective films.
• Selecting more acid-resistant materials like stainless steel.

2. Dry Corrosion (Chemical Corrosion):

Mechanism:

• Involves direct chemical reactions between metal and non-liquid substances like gases at high temperatures.

Types:

• Oxidation: Metal reacts with oxygen to form an oxide layer.
  • $2M + O_2 \rightarrow 2MO$
• Sulfurization: Metal reacts with sulfur-containing gases.
  • $M + S \rightarrow MS$
• Halogenation: Metal reacts with halogens.
  • $M + X_2 \rightarrow MX_2$

Characteristics:

• Usually forms a surface layer of corrosion products (e.g., rust on iron).
• Can be protective if the oxide layer is adherent and non-porous.

Prevention:

• Use of heat-resistant alloys.
• Application of protective coatings.
• Controlled atmospheres in industrial processes.

3. Galvanic Corrosion (Electrochemical Corrosion):

Mechanism:

• Occurs when two dissimilar metals are electrically connected in the presence of an electrolyte (e.g., water).

Process:

• Anodic Reaction: The more reactive metal (anode) loses electrons and corrodes.
  • $M \rightarrow M^{n+} + n e^-$
• Cathodic Reaction: The less reactive metal (cathode) gains electrons.
  • $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$
• The anode corrodes faster while the cathode is protected.
• Common in marine environments and in structures with multiple metal types.

Prevention:

• Using metals with similar electrochemical properties.
• Applying insulating materials between dissimilar metals.
• Use of sacrificial anodes (e.g., zinc anodes for steel structures).

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