Electrochemical reactions involve the transfer of electrons between atoms or ions and occur where electrical energy is produced or consumed during a chemical process. On ships, these reactions mainly drive corrosion and battery operations.

• Oxidation: This reaction involves loss of electrons. The metal atom at the anode loses electrons and becomes a positive ion.
  • Example: Fe→Fe2++2e−
  • (iron atom in steel hull loses electrons and dissolves into seawater at the anode).
• Reduction: This reaction involves gain of electrons. Electrons from the anode travel to the cathode, where another substance (like oxygen) gains these electrons.
  • Example: O2+2H2O+4e−→4OH−
  • (oxygen dissolved in seawater is reduced at the cathode).
• At the anode: Oxidation occurs (loss of electrons, metal corrodes).
• At the cathode: Reduction occurs (gain of electrons, metal is protected).

Galvanic corrosion is the accelerated attack on a metal due to electrical contact with a more noble metal in the presence of an electrolyte (such as seawater). When two dissimilar metals (e.g., steel hull and brass propeller) are joined, the less noble metal acts as the anode and corrodes faster, while the more noble metal remains protected.

Standard Marine Prevention Procedures :

• Use sacrificial anodes (zinc, aluminum, magnesium) attached to hulls or fittings. These are consumed instead of the hull or propeller.
• Employ Impressed Current Cathodic Protection (ICCP) systems to keep the hull cathodic.
• Apply coatings (paint or epoxy) to isolate metals from seawater and each other.
• Use insulating gaskets or sleeves to prevent direct contact between dissimilar metals.
• Choose compatible metals for fittings, minimizing galvanic potential difference.