What effect does oxygen have on boiler metal?

Oxygen has a significant negative impact on boiler metal, primarily through the process of corrosion. When oxygen enters the boiler system, it reacts with water and metal surfaces, leading to the formation of rust and scale. This process often manifests as pitting, which involves localized corrosion that can create small holes or cavities in the metal. Pitting can weaken the structural integrity of the boiler over time, leading to potential leaks and failures if not managed properly.

In contrast, other options suggest advantageous effects, which are not correct in the context of oxygen's interaction with boiler metal. For instance, corrosion prevention implies a positive effect that does not occur in the presence of oxygen. Increased thermal efficiency and improved fuel combustion similarly suggest enhancements that would not result from oxygen exposure. Instead, oxygen exposure typically leads to detrimental effects on boiler operation and lifespan.