2: Galvanic Corrosion

Definition: Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte, forming an electrochemical cell.
Mechanism: This connection generates a potential difference, similar to a battery, resulting in a flow of electrons that accelerates corrosion of one metal (anode).

Involves two metals: one becomes the anode (oxidation occurs) and the other the cathode (reduction occurs).
Redox Potential: Metals have different redox potentials, which influence their behavior in pairs:
- The metal with lower redox potential (higher tendency to oxidize) acts as the anode.
- The other metal, with higher redox potential, acts as the cathode and is protected from corrosion.

Set Up: Metallic samples can be paired with a standard electrode (often using sodium chloride solution) to measure redox potentials.
Illustration:
- Anode: where oxidation occurs (metal consumed).
- Cathode: where reduction occurs (receives electrons and is protected).

Organizes metals according to their tendency to oxidize or reduce.
Active Metals: More likely to corrode when paired with noble metals (more resistant to corrosion).
Noble Metals: Less likely to oxidize; include precious metals like gold, silver, platinum, and certain alloys.
Stainless Steel Variants:
- Austenitic stainless steels (most noble) versus ferritic and martensitic types (lower on the series).
- Austenitic types have passive oxide layers that increase corrosion resistance.

Environmental factors influence galvanic corrosion:
- Severity of environment: e.g., outdoor, marine (high salt content), or industrial settings (presence of solvents or chemicals).
- Electrolyte Presence: Inhibits electron flow; without it, galvanic action is halted.
- Compatibility of Metals: Guidelines exist, e.g., MIL-STD-889B, which evaluates metal combinations based on environmental conditions.

Cathodic Protection: Involves coupling a less noble metal to protect a more noble one; examples include using zinc with steel:
- Galvanized Steel: Zinc coating protects steel from corrosion.
- Zinc Blocks: Attached to ship hulls to sacrificially corrode in protection of the hull.
Electrical Insulation: Stop electron flow between dissimilar metals using non-conductive materials.
Avoiding Electrolytes: Intervene with dry conditions or inert environments to prevent corrosion.

Surface Area Ratio: Large anode surface area relative to cathode limits corrosion rate:
- Example 1: Stainless steel plates with carbon steel bolts (high corrosion rate in the bolts).
- Example 2: Carbon steel plates with stainless steel bolts (limited galvanic impact on corrosion).

Understanding and managing galvanic corrosion is crucial for material integrity in diverse environments. Proper planning, material selection, and protective strategies mitigate risks effectively.