Corrosion is a 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:
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+O2→2MO
Sulfurization: Metal reacts with sulfur-containing gases.
M+S→MS
Halogenation: Metal reacts with halogens.
M+X2→MX2
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→Mn++ne-
Cathodic Reaction: The less reactive metal (cathode) gains electrons.
O2+2H2O+4e-→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).