Metallurgical aspects of iron and steel

For any analysis of corrosion results of a special metal, another factor of prime importance is the structure of the investigated metal. The metallurgical treatment must be taken into account. This complex nature of corrosion is best explained for the element iron. Iron can exist in two main modifications. Below 911 °C the stable modification is a-iron with a body centered cubic (bcc) structure, above 911 °C the stable modification is y-iron, the face centered cubic (fee) structure. A third modification is found at higher temperatures (5-iron) for a bcc structure with different crystallographic parameters. 

At lower temperatures the austenitic phase decomposes into ferrite and cementite. The lowest point of stability of austenitic crystals is at a concentration of 0.8% carbon and a temperature of 723 °C. At this point the austenite transforms to a eutectoid mixture of ferrite with 0.02% carbon and cementite with 6.67% carbon. This eutectoid point of the solid solution is adequate to the eutectic point of a melt where the melt decomposes into a eutectic mixture. The eutectoid mixture of ferrite and cementite is called pearlite. 

The process of heat treatment and different quenching rates of iron with carbon causes further structural differences. One product is martensite, an oversaturated (metastable) solid solution of carbon in ferrite. Martensite is obtained when the steel is heated to temperatures where austenitic steel is formed and then rapidly quenched. 

This was a very short sketch of the metallurgical aspects of iron and steel. For details one must consult special literature.All the different modifications of iron and steel have 

Another reaction investigated in some detail was copper dissolution. It was assumed that anions like chloride stabilize the one-valent ad-atom. The old results by Bockris andEnyo could then be described by the following two-step mechanism with Cu as intermediate. 

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