Published: Jan 2012
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THE PREVENTION OF CORROSION BY SURFACE processing enjoys significant economic leverage, and, as evidence, one may cite the widespread use of coatings, films, and inhibitors for metals and semiconductors in many service environments. All engineering metals used in modern technological societies are unstable with respect to corrosion, and the result is a loss of properties. Natural oxide films provide protection against continued attack for some metals, and alloying extends the life of other metals by developing highly stable passive films. Where metals may not be protected by oxide films, other modification methods have been developed to reduce corrosive attack. In reality, the improvement of corrosion resistance of metals by modification of the surface has been practiced since the invention of metal tools. Some of the earliest techniques to prevent corrosion involved coating with greases or natural oils. More modern methods were developed in the 19th and 20th centuries and include multiple coatings, zinc galvanizing, electroplating of other pure metals, and vacuum physical vapor deposition of mostly pure metal coatings by electron beam and sputtering techniques. The metal coatings are better barriers than organic films because of the lower permeability of the former to moisture, oxygen, and ions. Inhibitors or conversion coatings and primers for paints are cheaper than metal coatings and are used widely by paint manufacturers even though they remain highly proprietary in nature. The use of organic coatings to protect metal surfaces is practiced widely. Much of the use is for atmospheric exposure of motor vehicles as well as for structural units such as bridges and buildings. The successful implementation of existing technologies has greatly reduced the effects of corrosion of automobiles, for example, in the past decade in response to consumer demand. Despite many recent advances, coating technologists and scientists acknowledge that much is unknown and that new processes and understanding are the keys for further progress . Defects in the metal substrate and in the overlayers are among the primary concerns because they are the source of localized corrosion phenomena. Defects may occur on length scales from atomic-level lattice vacancies to arrays of defects at grain boundaries (for crystalline materials) or to random pores or cracks (for example, in noncrystalline films). Avoiding such defects by proper quality control is a major concern in coatings science and technology. In the discussion that follows, aspects of corrosion that involve thermodynamics and kinetics will be developed as a basis for the description of the specific nature of corrosion of metals under protective films and overlayers. Some emphasis will be given to protection of thin metal films and micro-structures that are particularly sensitive to corrosion and whose successful protection provides a basis for advancing protection technology in general.
Smyrl, William H.
Professor, University of Minnesota, Minneapolis, MN