That's a great observation! You're absolutely right. It's a fascinating phenomenon known as passivation.
Here's a breakdown:
* Concentrated Sulfuric Acid: At very high concentrations, sulfuric acid can form a protective layer, often iron sulfate, on the surface of steel. This layer acts as a barrier, preventing further corrosion. It's like the acid is "passivating" the steel.
* Dilute Sulfuric Acid: At very low concentrations, the acid is simply too weak to effectively attack the steel.
* Intermediate Concentrations: This is where the problem lies. The acid is strong enough to initiate corrosion but not strong enough to form the protective layer. This leads to rapid corrosion of the steel.
So, while concentrated and dilute sulfuric acid can be relatively benign to steel, it's the intermediate concentrations that pose the greatest risk of corrosion.