Corrosion of iron (rusting): pick the correct statement that best describes the real cause-pathway among the choices below, relevant to civil materials and maintenance. Select one correct statement.

Introduction / Context:
Rusting deteriorates steel bars, structural members, and fixtures. Understanding the environmental triggers and general pathway is essential for prevention strategies like coatings, cathodic protection, and detailing to avoid moisture retention.

Given Data / Assumptions:

• Iron requires an electrolyte (moisture) and an oxidizer (oxygen) to rust.
• Carbon dioxide in air can form weak carbonic acid in water films, accelerating corrosion.
• Precise intermediate species depend on pH, oxygen availability, and ions such as chlorides.

Concept / Approach:

The broadly correct statement is that rusting is driven by the combined presence of oxygen and moisture; dissolved CO2 can acidify the thin water layer and speed the process. The detailed intermediate compounds vary: common steps include formation of Fe(OH)2, further oxidation to Fe(OH)3, and dehydration to hydrated ferric oxide (rust). Universal formation of “ferrous bicarbonate” and “ferric bicarbonate” as required intermediates is not correct across conditions.

Step-by-Step Solution:

Verification / Alternative check:

Corrosion references emphasize oxygen/moisture as essential; intermediates like bicarbonates are environment-specific, not universal necessities.

Why Other Options Are Wrong:

Ferrous/ferric bicarbonate pathway (B, C) is not universally applicable. Statement D presumes CO2 liberation as a necessary final step, which is not generally stated in standard rusting sequences.

Common Pitfalls:

Overgeneralizing one mechanistic pathway; ignoring the role of chlorides and pH in real structures.

Final Answer:

Rusting is caused due to the combined action of air (oxygen), moisture, and carbon dioxide.