The electrochemistry of corrosion Part 4: Passivity

There are few areas in corrosion science which are so confusing to the learner as the ability of certain metals to become passive. This phenomenon was discovered by Faraday about 1840 when he showed that although iron reacted vigorously with dilute nitric acid, it was totally “passive” in concentrated nitric acid. The exact mechanisms of the process of passivation are still somewhat obscure but the mechanics of the process are well enough understood for it to be used to protect large steel tanks against attack by sulphuric acid which would otherwise dissolve the tanks in a few days.