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Electrochemical Reactions & Galvanic Corrosion Explained

Chief Mate
MEO Class 2
MEO Class 4

times asked: 38; times asked
questions: 7; questions
last asked: 2026; last asked

The slides, written out

AI-drawn illustration. Simplified to explain the idea, not to scale.

Slide 1 · ELECTROCHEMICAL

ELECTROCHEMICAL REACTIONS & GALVANIC CORROSION Understanding Oxidation, Reduction and Corrosion Prevention at Sea e⁻ e⁻ ANODE (Oxidation) Electron Flow CATHODE (Reduction) Ions Zinc Anode (Corrodes) ELECTROLYTE (Seawater) Propeller (Protected)

Slide 2 · HOW CORROSION

HOW CORROSION HAPPENS Electrochemical reactions occur when electrons flow from the anode to the cathode through an electrolyte (seawater). THE CORROSION CELL ANODE (Oxidation) e⁻ CATHODE (Reduction) Zn Ions ELECTROLYTE (Seawater) OXIDATION (ANODE REACTION) Loss of electrons Metal goes into solution as positive ions (cations) Occurs at the anode Metal is destroyed (corroded) EXAMPLE: Zinc oxidation (sacrificial anode) Zn → Zn²⁺ + 2e⁻ REDUCTION (CATHODE REACTION) Gain of electrons Substance in the electrolyte accepts electrons Occurs at the cathode No metal loss (protected) EXAMPLES: 1. Oxygen reduction (neutral/alkaline seawater) O₂ + 2H₂O + 4e⁻ → 4OH⁻ 2. Hydrogen evolution (acidic / oxygen-starved environments) 2H⁺ + 2e⁻ → H₂ EXAM TIP – REMEMBER THIS! ANODE = OXIDATION = METAL LOSS (CORROSION) CATHODE = REDUCTION = NO METAL LOSS (PROTECTED) The whole process is a continuous electron transfer cycle.

Slide 3 · GALVANIC CORROSION

GALVANIC CORROSION WHY IT HAPPENS Galvanic corrosion occurs when two dissimilar metals are electrically connected in a conductive electrolyte (seawater). THE GALVANIC CORROSION PROCESS ANODE (Less noble metal) CORRODES e⁻ Electron Flow CATHODE (More noble metal) PROTECTED ELECTROLYTE (from anode) + (Seawater) Oxidation occurs at the anode. Metal dissolves into the electrolyte as ions. Reduction occurs at the cathode. No metal loss. The cathode is protected. THE GOLDEN RULE SMALL ANODE + LARGE CATHODE = VERY RAPID CORROSION A small anode connected to a large cathode produces a high corrosion rate at the anode because the large cathode demands lots of electrons. Always try to make the anode area large relative to the cathode. TYPICAL GALVANIC SERIES IN SEAWATER (Most Anodic → Most Cathodic) MAGNESIUM ALUMINIUM ZINC STEEL BRONZE STAINLESS STEEL (PASSIVE) Most Active (Anodic) Most Noble (Cathodic) GALVANIC CORROSION REQUIRES 3 THINGS Dissimilar Metals Electrical Contact Conductive Electrolyte (Seawater)

Slide 4 · PREVENTING

PREVENTING GALVANIC CORROSION Practical methods to break the galvanic cell and protect ship structures and equipment. 1 ELECTRICAL INSULATION Break the electrical connection between dissimilar metals. Non-metallic Gaskets Insulating Sleeves Insulating Washers Commonly used in flanges, bolted joints, and connections (e.g., aluminium superstructure to steel hull). 2 KEEP ELECTROLYTE OUT A galvanic cell cannot exist without the electrolyte. Seal joints and crevices Keep surfaces clean and dry Maintain paint systems and sealants Keep it dry, clean and well protected. 3 PROTECTIVE COATINGS Isolate metals from the electrolyte with high-quality protective coatings. EPOXY COATING MARINE PAINT Coat both metals where practical If only one can be coated, the more noble (cathodic) metal is generally preferred Inspect and maintain coatings regularly Good coatings dramatically reduce galvanic corrosion. 4 TRANSITION JOINTS Use explosion-bonded transition joints for permanent dissimilar metal connections. Aluminium Steel Explosion-bonded interface No crevices Can be painted and protected Ideal for steel–aluminium connections 5 CATHODIC PROTECTION Use sacrificial anodes to protect all metals in a galvanic couple. Anodes are less noble than both metals They corrode sacrificially Protect steel, bronze, and other noble metals ZINC (Zn) ALUMINIUM (Al) MAGNESIUM (Mg) KEY TAKEAWAYS Oxidation occurs at the anode. Metal loss. Reduction occurs at the cathode. No metal loss. Galvanic corrosion requires dissimilar metals, electrical contact and an electrolyte. Good design, insulation, coatings and cathodic protection prevent corrosion. Prevention is always better than repair – protect the ship, protect the mission.

In exams

Asked 38 times across 7 different questions, most recently in 2026. Asked most in Chief Mate (19), MEO Class 2 (11), MEO Class 4 (8).

Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.

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