Engineering Chemistry II: Metal Corrosion

Metal Corrosion and Its Prevention

• Metals and alloys are used as fabrication or construction materials in engineering, but they can deteriorate slowly by the action of atmospheric gases, moisture, and other chemicals if not properly maintained.
• Corrosion is defined as the gradual destruction or deterioration of metals or alloys by chemical or electrochemical reaction with their environment.

Origin of Corrosion

• Metals occur in nature in two different forms: native state and combined state.
• Native metals are non-reactive with the environment, have good corrosion resistance, and are typically noble metals like Au, Pt, and Ag.
• Combined metals are reactive, react with the environment, and form stable compounds like oxides, sulphides, chlorides, and carbonates, and exist in their form of stable compounds called ores and minerals (e.g., Fe2O3, ZnO, PbS, CaCO3).

Causes and Consequences of Corrosion

• Metals are extracted from ores and are reduced to their metallic states, but in the pure metallic state, they are unstable and have higher energy.
• As soon as metals are extracted from their ores, the reverse process begins, and they form metal compounds that are thermodynamically stable, with lower energy.
• When metals are used in various forms, they are exposed to the environment, and the exposed metal surface begins to decay, converting into more stable compounds.
• Corrosion leads to the loss of useful properties like electrical conductivity, ductility, and malleability.
• Consequences of corrosion include:
  • Loss of efficiency in machinery
  • Contamination of products
  • Frequent replacement of corroded equipment
  • Plant failure
  • Need for over design to compensate for corrosion
  • Release of toxic products, health hazards, and more
  • Loss of time and value of goods