Corrosive substances include those with a low (< pH 2.5) and high (pH >12.5) pH value. They can cause acute damage to skin and irreversible damage to eyes.
Common corrosive substances include hydrochloric acid, sulfuric acid, and sodium hydroxide.
When diluting concentrated forms of these substances, always add the corrosive substance (acid or base) to water, not the other way around! This will minimize the danger from splashes.
If you get a corrosive substance on your skin, wash immediately with lots of water. This will minimize the burn. Concentrated base will sometimes not burn right away, but will still be doing damage to your skin!
If you get some in your eyes, flush your eyes in an eyewash for at least 15 minutes, removing any contact lenses. Get medical attention.