Please HELP someone. I have exam coming up.....?

A steel pipe standing vertically in salt water rusts quite rapidly. The rust appears just above the waterline while the pitting of the steel surface occurs below the waterline. Explain why this is so.

The formula of rust is Fe₂O₃•xH₂O.
Obviously, the three reactants to form rust are iron, oxygen and water.

Steel is an alloy of iron, and thus steel supplies iron for reaction. At the waterline of the steel pipe, there are sufficient amounts of iron, oxygen and water to form rust, and thus the rust appears just above the waterline. There is a small amount of oxygen dissolved in water, and such small amount of oxygen can react with iron and water to form pitting (small spot of rust) of steel surface below the waterline.


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Explain how attaching a block of zinc to this steel pole could protect it from corrosion.

In the electrochemical series, zinc or higher than iron. (OR: Zinc is more reactive than iron.) Therefore, zinc is corroded first before iron, and thus the steel pole is protected from corrosion.

Thank you so much 胡雪8° for helping me....You are an angel:)..