Which form occurs when two different metals are electrically coupled in an electrolyte?

• A. Uniform attack
• B. Pitting
• C. Crevice corrosion
• D. Galvanic corrosion

Answer: (D)

When two different metals are in electrical contact inside an electrolyte, they form a galvanic couple. The more active metal acts as the anode and dissolves as it gives up electrons, while the more noble metal acts as the cathode and is relatively protected. The electrolyte allows ions to move, completing the circuit and enabling continuous oxidation and reduction reactions. This combination—electrochemical coupling in an electrolyte—produces galvanic corrosion, with the corrosion rate governed by the potential difference between the metals and the relative surface areas of the anode and cathode. In contrast, uniform attack, pitting, or crevice corrosion arise from other mechanisms (general metal loss, localized breakdown of protective films, or differential oxygen availability) and do not require dissimilar metals in electrical contact.