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What Is Corrosion? (Continued from Previous Page) from These Forms of Corrosion Is Both Necessary and Possible battery; the dissolution reaction at the anode supplies electrons for the reduction reaction at the cathode. A short circuit is the electrical connection made by a conductor between the two physical sites, which are often separated by very small distances. Thus, the study of corrosion processes involves the use of many of the same tools that electrochemists studying batteries, fuel cells, and physical and analytical electrochemistry use. The application of mixed potential theory to corrosion was originally presented by Wagner and Traud6 and discussed later in the Journal of The Electrochemical What is Society by Petrocelli.7 In 1957, Stern and Geary theoretically analyzed the shape of polarization curves providing the basis for the primary experimental technique Corrosion? (electrochemical polarization) used in electrochemical studies of corrosion.8 The by Barbara A. Shaw and Robert G. Kelly formation of surface oxide fi lms is critical in mitigating the rate of metal dissolution, Corrosion is degradation of materials’ properties due to interactions so corrosionists have much in common with those studying dielectrics for other with their environments, and corrosion of most metals (and many purposes. It is these thin (< 10 nm) native materials for that matter) is inevitable. While primarily associated oxide fi lms that make the technological with metallic materials, all material types are susceptible to use of metallic materials possible by degradation. Degradation of polymeric insulating coatings on serving as barriers to dissolution. wiring has been a concern in aging aircraft. Even ceramics can Traditionally, corrosion is classifi ed into eight categories based on the undergo degradation by selective dissolution. Like death and morphology of the attack, as well as taxes, corrosion is something we hope to avoid; but ultimately it is the type of environment to which the something we must learn to deal with. The fundamental cause or material is exposed.9 Uniform or general corrosion is the most prevalent type of driving force for all corrosion is the lowering of a system’s Gibbs corrosion and examples of this type of energy. As Fig. 1 illustrates, the production of almost all metals corrosion abound; including rusting of (and engineering components made of metals) involves adding steel bridges, rusting of underground pipelines, tarnishing of silver, and patina energy to the system. As a result of this uphill thermodynamic formation on copper roofs and bronze struggle, the metal has a strong driving force to return to its statues. Anyone who has left a piece native, low energy oxide state. This return to the native oxide of unprotected steel outside is familiar with uniform corrosion. Fortunately, state is what we call corrosion and even though it is inevitable, uniform corrosion is predictable and can substantial barriers (corrosion control methods) can be used to be controlled by various methods such as slow its progress toward the equilibrium state. Thus it is the rate of painting the surface or applying a layer of a sacrifi cial metal like zinc to steel. This the approach to equilibrium that is often of interest. This rate is sacrifi cial corrosion of the zinc surface controlled not only by the nature of the metal surface, but also by layer to protect the underlying steel is the nature of the environment as well as the evolution of both. actually a form of galvanic or bimetallic corrosion. In this case, like in a battery, we are using corrosion to our advantage. The In light of the thermodynamic basis process, it is also not surprising that the surfaces of some metals (like aluminum, for corrosion it is not surprising that costs Corrosion Division is one of the oldest stainless steel, and titanium) are protected associated with corrosion are high. Several divisions within ECS. The Division was from uniform corrosion by an extremely studies over the past 30 years have shown established in 1942, but corrosion has thin oxide fi lm that forms naturally. Many that the annual direct cost of corrosion to been an important topic in the Society practical applications of materials depend an industrial economy is approximately since 1903. Reviews of the early literature on the presence of this protective oxide. 3.1% of the country’s Gross National and history of the Division were prepared We would not be able to use planes (or just Product (GNP). In the United States, this by Uhlig for the 50th and 75th anniversaries about any other structures for that matter) amounts to over $276 B per year.1 As of the Society2,3 and a centennial review made from aluminum if it were not for Fig. 2 reveals, the highest segments of by Isaacs was published more recently.4 this thin protective fi lm. Unfortunately, the cost of corrosion are associated with Readers looking for a good overall source this fi lm can breakdown locally, resulting utilities, transportation, and infrastructure. of corrosion information are encouraged in forms of corrosion like pitting of The Department of Defense alone has to consult Uhlig’s Corrosion Handbook.5 aluminum plates, crevice corrosion of corrosion costs of $20 B. Because of the Most corrosion processes involve at least stainless steel fasteners, or stress corrosion signifi cant economic, safety, and historical two electrochemical reactions (one anodic cracking of pipes in nuclear reactors. impact of corrosion on society and because and one cathodic). A corroding surface Protection of structures and equipment corrosion of metals is an electrochemical can be thought of as a short-circuited (continued on next page) 24 The Electrochemical Society Interface • Spring 2006 What is Corrosion? (continued from previous page) from these forms of corrosion is both necessary and possible. Approaches available for controlling corrosion include the application of protective coatings to metal surfaces to act as a barrier or perhaps provide sacrificial protection, the addition of chemical species to the environment to inhibit corrosion, alteration of an alloy chemistry to make it more resistant to corrosion, and the treatment of the surface of a metal to increase its resistance to corrosion. Organic coatings are FIG. 1. Why does steel corrode? typically the first line of defense against corrosion and items such as automobiles and aircraft employ involved coating $17.6 billion Production & systems comprised of several layers and Manufacturing mechanisms of protection. These coatings 12.8% are used to isolate the metal from the $20.1 billion $47.9 billion environment and their barrier properties Government Utilities are of specific interest. Some species when 14.6% 34.7% added to an electrolyte preferentially migrate to anodic and/or cathodic sites $22.6 billion slowing the corrosion process. Chromates, Infrastructure 16.4% phosphates, nitrates, molybdates, and $29.7 billion various organic compounds provide Transportation corrosion protection to piping systems in 21.5% power plants, chemical processing plants, and the oil and gas industry. You add an inhibitor to your automobile radiator FIG. 2. Costs of corrosion in the U.S.1 whenever you add coolant. Likewise, you are inhibiting corrosion of steel when you spray WD-40 TM on a surface. Stainless (a) (b) steels are the classic examples of alloying to improve corrosion resistance. In most environments, the ubiquitous rusting of steel is alleviated by the addition of at least 12% chromium. The chromium alters the composition of the naturally forming oxide film on the surface of the metal and at chromium concentrations FIG. 3. (a) Dealloyed nanoporous CoCr alloys film fabricated on silicon substrates. The film was prepared greater than 12%, the film contains by removing the less noble A1 from a deposited CoCr-A1 matrix. High magnification insets show details of enough chromium oxide to significantly the nanoporous morphology. (b) Left, commercial polymer-coated drug-eluting stent showing the polymer film cracking after stent expansion. Coating failure and delamination are serious issues due to the potential for reduce corrosion. The protective films thrombus formation and embolisms. Right, the dealloyed nanoporous metal coating on an expanded stent on other metals (like aluminum and (with a different strut design) shows no signs of failure. Both micrographs are at the same magnification. magnesium) can also be improved through alloying additions. Because While chromate is a powerful inhibitor of chemical, electrical, information), as well corrosion usually originates at the surface corrosion on many metallic surfaces, it is as economists. Accurate models require of metal, many successful corrosion carcinogenic. Thus, the search has been accurate representations of processes control methods involve treatment or to find an inhibitor that is as effective as over ten orders of magnitude in size (< alteration of the metal surface. These chromate, but environmentally benign. 0.1 nm to 10 m) and over twenty orders treatments can be as mundane as shot- This requires interdisciplinary work of magnitude in time (1 ps to 50 years or peening the surface of a metal to improve involving chemistry, electrochemistry, more). A compelling example of a grand its corrosion fatigue resistance; or they surface science, and metallurgy. challenge in this area is the prediction can be high-tech like laser-surface melting As the infrastructure of industrialized of corrosion behavior of the canisters to provide a smooth, compositionally countries continues to age, more and selected to hold high-level radioactive altered surface. more failures due to corrosion are waste for 10,000 years or more. Future trends in corrosion occurring. Replacing all the bridges Corrosion can be a good thing. research include the development of and pipelines (gas, oil, water) would be One can use dissolution to selectively environmentally benign inhibitors, prohibitively expensive, and unnecessary remove one component from a material accurate prediction of structure service as most are in good condition and can (known as dealloying), leaving a porous life, and finding ways to make corrosion provide many more years of service.
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