The study further cites that the above figure does not include indirect costs resulting from corrosion - estimated to be equal to the direct costs detailed above. Indirect costs are difficult to assess, as they include the loss to parties other than owners and operators. For example, traffic delays due to bridge repairs and rehabilitation that are more difficult to turn over to the owner or operator of the structure (Figure 1). These become indirect costs to the user but can have a significant impact on the overall economy due to lost productivity. This is an important statistic because corrosion prevention is not only a matter of monetary savings but also has a significant impact on safety and health. Corrosion has caused bridges to collapse, ships to sink, pipelines to explode and even has been responsible for fatal airplane crashes. In conjunction with pollutants, corrosion has also contributed to public health hazards. Although corrosion is not a problem with a 100-percent effective cure, the use of current technology can diminish its damaging effects. Corrosion control measures will save our country's resources while in the process recovering billions of dollars presently being lost.
Metals corrode for a variety of reasons. Most metals are found in nature as ores and oxides, and the extraction of metals from their ores requires a considerable amount of energy. For example, iron is found in nature as ferrous oxide or iron ore. To convert the iron ore to some of the materials we know such as cast iron, stainless steel or carbon steel, requires that it be alloyed with other elements. Energy is required for the alloying process, and it is introduced in the form of heat. The resulting materials have a much higher energy level than when they were in their corresponding ores and will show a natural tendency to return to their natural state. The process whereby these metals return to their natural state as ores is known as corrosion.