Glossary

Electrochemical corrosion is a type of metallic corrosion that occurs when a metal deteriorates through an electrochemical reaction involving anodic oxidation and cathodic reduction—essentially forming a tiny battery or galvanic cell on the metal’s surface. This process takes place when a metal, an electrolyte (like water with dissolved salts), and oxygen or another oxidizing agent are all present, allowing electrical current to flow between different areas of the metal.

In simple terms, corrosion happens because metals naturally want to return to their more stable, oxidized state, just like they exist in nature as ores. The electrochemical process allows this to occur through a redox reaction, where one area of the metal loses electrons (oxidation) and another gains them (reduction).

Here’s how it works:

1. Anodic reaction (oxidation):
At certain microscopic sites on the metal surface—called anodic regions—metal atoms lose electrons and dissolve into the surrounding electrolyte as metal ions. For example, in the case of iron:
Fe → Fe²⁺ + 2e⁻
This is where corrosion (metal loss) physically occurs.

2. Cathodic reaction (reduction):
The released electrons flow through the metal to another region—the cathode—where they are consumed by a reduction reaction, usually involving oxygen and water. In a neutral or basic environment, this is typically:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

3. Electrolyte conduction:
The electrolyte (such as rainwater, seawater, or moisture on the surface) allows ionic movement between the anodic and cathodic areas, completing the electrochemical circuit.

4. Corrosion product formation:
The metal ions produced at the anode often react with oxygen or hydroxide ions in the electrolyte to form corrosion products—for example, iron oxide (rust) in steel.

Over time, this continuous cycle of oxidation and reduction leads to the gradual degradation of the metal. The process can be localized (as in pitting or crevice corrosion) or uniform, depending on environmental conditions, surface composition, and the presence of impurities or protective coatings.

Electrochemical corrosion is influenced by several factors, including:

- Electrochemical potential differences between microstructures or different metals.

- Electrolyte conductivity (higher salt content accelerates corrosion).

- Temperature and pH of the environment.

- Oxygen availability, which controls the cathodic reaction rate.

A classic example is galvanic corrosion, where two dissimilar metals (e.g., steel and copper) are electrically connected in a conductive environment. The more active metal (anode) corrodes faster, while the more noble metal (cathode) is protected.

To prevent electrochemical corrosion, engineers use methods such as protective coatings (paint, plating, galvanizing), cathodic protection (sacrificial anodes or impressed current systems), corrosion inhibitors, and material selection based on galvanic compatibility.