Corrosion is now becoming more widely recognised as a menace to industrial plant and plant components. Corrosion which occurs inside process plant may often be difficult to detect until deterioration is well advanced. Such corrosion can destroy equipment, contaminate products and shut down production if it is not identified at an early stage and monitored. This fact was recognised by the Hoare Committee, reporting on Corrosion Protection, as long ago as 1972. The Hoare Committee concluded that the annual cost due to corrosion in the U.K. amounted to some 3% of the Gross National Product, but that savings of one-third were possible by better or wider use of existing corrosion protection and monitoring techniques.