The Corrosion and Preservation of lron Antiques Robert Walker Department of Metallurgy and Materials Technology, University of Surrey, Guildford, Surrey The corrosion of iron to form rust is a familiar process. This reaction occurs because iron is an unstable metal and combines with moisture in the environment to form hydrated iron oxide, Fe20j HzO, i.e., rust. This product is usually porous so that it is unprotective and water can penetrate through to the underlying metal where corrosion continues until the article is completely converted to a weak layer of oxide which disintegrates. The extent of the attack does depend upon the composition of the metal and the particular environment. The Iron Pillar near Delhi in India is one of the metallurgical wonders of the world and is 7.2 meters high and either 130001 1600years old. The reasons for the preservation of this ancient iron have been discussed by Wranglen ( I ). General Corrosion Reactions All metals except gold form an oxide film when exposed to air a t room temperature. This layer of oxide is protective against corrosion, hut i t can be mechanically damaged or dissolved or broken down by acids. In practice the oxide may be quite thick because most articles are heated during manufacture so that more oxidation can occur. Corrosion is an electrochemical process and involves: (1) anodic areas where oxidation occurs and electrons are pro-
duced, M
-
+
Mn+ ne
(2) cathodic areas where reduction occurs and electrons are con-
sumed,
+
2Ht 2e On + 2Hz0 + 4e 2H2 + 2e
--
Hz 4(OH)2(OH)- Hz
+
(31 a rnrrall~r
