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Lead Corrosion

The ease with which lead can be shaped and its resistance to corrosion make it valuable as the outer covering, for electric cables. It protects the insulated wires inside without making the cable too stiff to bend. As lead has become much more expensive, it is used for roofing less often than in the past, and copper or plastics such as polythene are preferred materials, because we have learned that lead is poisonous. Sheet lead is used to line tanks holding corrosive liquids, such as acids, which would eat through other metals.

Lead is one of the most stable metals. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.

The processes used in its production are essentially the same as those used for zinc. Lead sulfide ore is converted into lead oxide which is reduced to metallic lead by a continuous process in a blast furnace. Refining of blast furnace bullion yields 99.99% pure lead. The relatively low cost, easy workability and good environmental resistance of lead have made it in the past an obvious choice for sheathing purposes. however in more recent years its high toxicity, high density and its low creep strength have led to its substitution by plastics and metals such as aluminum, and its use is becoming restricted to applications where its special properties offer unique advantages to the end-product.

However, lead is corroded by alkalis at moderate or high rates, depending on aeration, temperature and concentration. It is attacked, for example, by calcium hydroxide solutions at room temperature, including waters that have been in contact with fresh Portland cement. In view of this, if Portland cement was used to prepare a concrete, the cement hydration reaction resulted in a high alkalinity which is maintained in the pore moisture of the bulk concrete. This alkaline electrolyte has caused the corrosion of the lead shower pan.

Lead is used extensively as a roofing material in the UK, particularly on historic buildings. Upon exposure to the outdoor environment lead develops a gray patina, resulting in the metal's characteristic appearance. This patina may be non-adherent, and so may result in 'run-off' staining of adjacent areas, thus spoiling the aesthetic appeal of a lead roof.