Why are thick structural sections most susceptible to corrosive attack?

• A. Because they are more likely to have variations in their composition, particularly if heat-treated during fabrication
• B. They Are More Likely to Have Surface Finish Flaws
• C. They Are Less Exposed to Moisture
• D. They Have a Higher Resistance to Corrosion

Answer: (A)

The key idea is that internal differences in composition or microstructure, which can be introduced or amplified by fabrication heat treatments, create sites that corrode more readily. In thick structural sections, heat treatment can cause variations such as carbide precipitation or segregation and, in some alloys, sensitization of grain boundaries. Those localized changes form microgalvanic cells or anodic paths within the metal, so corrosion can start internally or along boundaries and propagate, making the section as a whole more prone to attack than a more uniform piece. This is why thick sections tend to be more susceptible when fabrication involves heat treatment.

Surface finish flaws and moisture exposure matter for corrosion, but they’re not inherently tied to thickness in the same way; better surface conditions don’t negate the internal composition-related vulnerabilities, and the idea that thick sections resist corrosion is directly contradicted by the tendency for internal microstructural variations to drive localized attack.