¯ 597 ¯ 120

Abstract

Electrochemical theory of wet corrosion: fundamentals As with dry corrosion wet corrosion reactions are only possible if the free energy of the products of reaction is lower than the free energy of the reactants. This is the case however for the reaction of nearly all metals with water and oxygen to give metal hydroxides: M ·H 2 O´lµ M´OHµ sµ (1) for example ¯ for Mg: ∆G 597 kJ/mol ¯ and for Cu: ∆G 120 kJ/mol In addition, and unfortunately, the rate of wet corrosion may often be very high compared with dry corrosion on the same metal at the same temperature. There are two underlying reasons for this: ¯ the dipolar water molecule stabilizes the free (dissociated) metal ions in solution; ¯ the metallic structure and water in contact with it can both conduct electric current. This enables reaction 1 above to proceed through the coupling of two primary corrosion reactions: A : M M 2· ·2e (2) 1