It took many of the metals several minutes to begin to react. Once the reaction was initiated, several of these metals continued to react quite vigorously. What was the most probable cause of this delayed reaction? (Hint: The metal getting hot isn't the only reason.)

Passivation of Oxide layers of the metals.

Explanation:

Passivation is a non-electrolytic finishing process that makes most metals rust-resistant. The prosses removes free iron from the surface by using either nitric or citric acid. When this happens, it results to an inert, protective oxide layer that is very slow or less likely to chemically react with air and cause corrosion.

Passivity caused many of the metals several minutes to begin to react. Once the finishing process that makes metals less likely to react was eroded, reaction was initiated vigorously.