1. Introduction

Metals react with the environment, producing corrosion products similar to the original ore from which the metal was obtained. Corrosion processes are electro-chemical reactions taking place at the surface of the metal.

Related Items:

Read details on Zinc coatings

Go to the IZA primer on zinc-or metallic-coated steel sheet

Corrosion products (rust) may act as a barrier between the metal and its surroundings, slowing down the corrosion rate. In some cases this barrier very effectively retards corrosion. This is called passivation. Passivation can increase the corrosion resistance of metal remarkably.

Dry corrosion

At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the atmosphere. Increase of temperature may cause formation of a heavier layer, or the layer may detach.

Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in thickness very slowly. Aluminium carries a thin, highly protective oxide layer.

Some corrosion takes place even under completely dry conditions.

Wet corrosion

Wet corrosion takes place in environments where the relative humidity exceeds 60 %. The corrosion may be uniform destruction of the metal surface or localised destruction (pitting, stress corrosion cracking). The corrosion can be concentrated adjacent to a more noble metal or at points where the oxygen supply is limited.

Wet corrosion is an electro-chemical phenomenon. When two metals are in contact with water solution containing salts, an electric potential is formed between two different metals or the surfaces of the same metal with different surface conditions.  This causes the dissolution of the less noble metal. The more noble metal remains protected but the less noble metal corrodes.

Wet corrosion is most efficient in waters containing salts, such as NaCl (e.g. marine conditions), due to the high conductivity of the solution. Chlorides also may increase the corrosion rate of metals.

E-mail this article