Corrosion Types — Dry, Wet, Galvanic, Pitting and Prevention Methods

Corrosion is the gradual destruction of a metal by chemical or electrochemical reaction with its environment. Rusting of iron is the most common example:

At the anode (iron surface):

At the cathode (water droplet with dissolved oxygen):

The $\text{Fe}^{2+}$ ions further oxidise to form hydrated iron(III) oxide — rust ($\text{Fe}_2\text{O}_3 \cdot xH_2O$).

graph TD
    A[Corrosion Types] --> B[Dry/Chemical Corrosion]
    A --> C[Wet/Electrochemical Corrosion]
    B --> B1[Direct reaction with air/gas]
    B --> B2[Example: tarnishing of silver in H2S]
    C --> D[Galvanic Corrosion]
    C --> E[Pitting Corrosion]
    C --> F[Crevice Corrosion]
    D --> D1[Two dissimilar metals in contact]
    D --> D2[More active metal corrodes faster]
    E --> E1[Localised pits on metal surface]
    E --> E2[Caused by breakdown of protective layer]
    F --> F1[In gaps and tight spaces]
    F --> F2[Oxygen-depleted areas corrode]

When two different metals are in electrical contact in the presence of an electrolyte:

• The more reactive (lower reduction potential) metal becomes the anode and corrodes
• The less reactive metal becomes the cathode and is protected

Example: Iron pipe connected to copper fitting — iron corrodes faster because it is more reactive than copper. This is why we never connect dissimilar metals in plumbing without insulation.