Whitney Award Lecture — 1982: Improving the Ability to Predict Corrosion Behavior Through Corrosion Research

Laboratory research provides a unique opportunity to separate and investigate experimental variables and deduce mechanisms of behavior which must be understood before the solutions to materials behavior problems can be optimized. Unfortunately, such research tends to be both time consuming and expensive. Fortunately, there are methods such as electrochemical methods which provide many opportunities to improve the cost effectiveness of corrosion research. The electrochemical techniques offer particular advantages since they represent a “clean” way to accelerate the corrosion process. For example, they can be used to increase or control the oxidizing potential of an environment and thereby reveal specific characteristics of a material, such as passivity, without adding chemical oxidizing agents. Similarly, these methods can be used to measure corrosion rates without removing the specimen from the environment or even significantly disturbing it. While electrochemical methods can reduce dramatically the time and effort needed to define the “critical experiment,” it is still necessary to perform the critical experiment. A combination of thermodynamics with kinetics (as measured by electrochemical methods, for example), coupled with a much improved ability to characterize surface phenomena using modern scientific instrumentation, provides the corrosion engineer and scientist with a powerful array of tools for the solution of practical problems; the elucidation of corrosion mechanisms; the development of new materials; the prediction of behavior of materials in specific environments; and the identification of materials and processes which can control the rate of corrosion reactions. Pourbaix diagrams are now available for all of the metallic species of the periodic table. While these diagrams are of considerable qualitative and quantitative usefulness, they have important limitations. For example, the equilibrium diagram does not contain kinetic information, and second, only pure metals have been studied until recently. Experimental potential versus pH diagrams, based on electrochemical measurements can overcome many of the shortcomings of the calculated (equilibrium) Pourbaix diagrams by providing a basis for the prediction of corrosion behavior of engineering alloys in aqueous media. In extending to alloys techniques originally proposed by Pourbaix for pure metals, workers at the University of Florida have developed or refined techniques for: (1) Construction of three-dimensional Pourbaix diagrams (potential/pH/composition) for alloy systems. Such diagrams are useful in alloy development. (2) Inclusion of kinetic data on the experimental Pourbaix diagrams. Such data is useful in predicting corrosion and comparing the effects of alloy additions between members of alloy families. (3) Evaluation of the influence of crevices and other occluded cells on the corrosion behavior of alloys in engineering structures. (4) Prediction of the tendency for dealloying as a function of potential in pH. (5) Evaluation of the effect of environmental variables such as electrolyte composition, gas concentration, the presence of other ionic species, etc. (6) Computerized calculation and plotting of equilibrium diagrams at various temperatures for electrolytes containing several ionic species in addition to water and metal ions. A number of examples are presented showing application of the proposed techniques to various materials behavior problems.