Surface protection—purpose of galvanising iron sheets What is the principal purpose of galvanising iron sheets?

Introduction / Context:
Galvanising applies a protective zinc coating to steel or iron to combat corrosion. This is ubiquitous in roofing, automotive panels, and outdoor structures. Understanding the primary protection mechanism informs maintenance practices and suitability for various environments.

Given Data / Assumptions:

• Zinc coating is continuous and intact at installation.
• Service exposure includes atmospheric moisture and oxygen.
• Role of sacrificial action and barrier protection are both relevant.

Concept / Approach:
Corrosion of iron requires oxygen and moisture. A zinc layer first acts as a physical barrier that separates steel from oxygen in air. Second, zinc is anodic relative to iron; when the coating is breached, zinc preferentially oxidizes, protecting steel cathodically. While water facilitates corrosion, the fundamental chemical reaction is oxidation; oxygen access to iron is critical. Thus the essential purpose is preventing oxygen attack on the steel surface, with water presence accelerating the process but not being the oxidizing agent itself.

Step-by-Step Solution:

Verification / Alternative check:
Galvanic series shows zinc more active than iron, explaining sacrificial protection. Field performance of galvanized steel in atmospheric exposure supports oxygen-blocking and sacrificial behavior.

Why Other Options Are Wrong:

• Harden the surface: galvanising does not primarily harden the base metal.
• Increase glossiness: appearance is secondary and varies with spangle; not the engineering purpose.
• Prevent the action of water: water assists corrosion, but oxygen is the reactant reduced at the cathode; the core target is oxidation prevention.

Common Pitfalls:
Attributing corrosion control solely to water exclusion. Even in humid or marine environments, oxygen reduction drives rust formation; zinc addresses the oxidation pathway.

Final Answer:
prevent the action of oxygen.