Metals corrode, and 16 to 20% of the total iron content effectively disappears. Chemical corrosion is an oxidation process and occurs mainly in the presence of water and oxygen. Copper, aluminium and chrome are relatively resistant to corrosion. Metals are also attacked by acids: carbonic acid from carbon dioxide and water, and sulphuric acid. Iron, aluminium and magnesium are those most affected. Materials containing lime can attack metals, particularly aluminium, zinc and lead. Electro-corrosion can occur with certain combinations of metals.
Steel structures in heavy sections are usually easy to disassemble; and as they are most often produced in standardized dimensions they are quite easy to re-use. In reinforced concrete, where the steel content can be up to 20%, material recycling is the only alternative and the process is relatively expensive and complicated. Copper in electric cables and tin in tin cans make it near impossible to recycle the steel in these products. Another issue is that waste metal often has a surface treatment that can lead to complications.
Metals and alloys that are finally melted from waste can be recycled by being added to new products in varying proportions, from 10 to 100% depending upon the end product and its quality requirements.
Steel and aluminium alloys can only be used for similar alloy products; whereas copper, nickel and tin can be completely reclaimed from alloys in which they are the main component. Copper, for example, is removed from brass through an electrolytic process.
The technology for smelting is relatively simple. Breaking down alloys electrolytically and further refining, casting or rolling techniques require much more complex machinery.
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