Professor David Kingery, author of the classic text, Introduction to Ceramics, said it well: “Corrosion, like death and taxes in our existence, is inevitable for all materials.” Metals, ceramics, and polymers all undergo change when exposed to the environment. Metals can exhibit electrochemical corrosion, while ceramics and plastics experience primarily chemical corrosion. Although “corrosion” often is used to describe the environmental stability (or instability) of metals, a host of other definitions have been used to characterize the changes that occur in ceramics and polymers.

Definitions can become an issue when scientists from various disciplines work together to solve a problem of national/international importance. One such example involves the description of the surface changes that occur when nuclear waste forms are exposed to an aqueous environment (as might occur in a geological repository). The observed surface changes have been referred to as corrosion, leaching, weathering, degradation, and alteration, to name a few. In early symposia on nuclear waste management, sponsored by the Materials Research Society and the American Ceramic Society, definitions and technical issues were discussed extensively by materials scientists, chemists, physicists, geologists, nuclear engineers, and other professionals. Because of this interdisciplinary interaction, significant advances were made (in spite of disagreements over terminology) in understanding the aqueous alteration of these materials. Much of this knowledge will be useful in other applications where environmental stability is important.

Although aqueous alteration is an important concept in materials corrosion, the topic of environmental stability is much broader. Other important subtopics are erosion, thermal stability, oxidation and radiation damage, and molten metal, glass, and salt corrosion. Finally, environmental instability need not always be bad. Indeed, there are numerous processes and products that work only because of the material's instability in specific environments.

The United States is at an important crossroads in its management of high-level radioactive waste (HLW). It is presently in the process of changing over from a strategy that was developed during World War II, which involves storing HLW in a relatively mobile liquid form, to a strategy of the 1990s, which involves removing and isolating potentially harmful radionuclides and immobilizing them into solid and inert forms, more specifically, borosilicate glass. The resulting waste glass products will then be permanently disposed of by deep burial, within stable geologic formations, where they become one element of a multibarrier waste-isolation system. This barrier system is designed to retain radionuclides so they can be permanently isolated from the public and from the accessible environment. Important contributing factors to the success of this strategy include the excellent stability and technical performance of waste glass forms and the ability of the glass, as well as waste package materials, to retain radionuclides even when exposed to potential leachants within a repository environment.

Polymers, the main ingredient of plastics, constitute the fastest-growing sector of materials—having been introduced only in this century. Their application, however, is plagued by inappropriate use and resulting premature failure. Conversely, polymers are also being attacked as indestructible contributors to the waste stream. Although objective discussion of their merits is complicated by the enormous financial interests involved, it is clear that their rapid use with a short-term experience base has led to many problems. On the positive side, they are amenable to good theoretical and experiential understanding because of the excellent organic and physical chemistry background underlying the field. Hence, we can expect to see even more widespread use of polymers and hopefully fewer problems, as our knowledge base grows, is disseminated, and is applied. This article will address the current field of polymer environmental stability and summarize a few select areas where major technical efforts are being directed.

The discovery of high Tc superconductivity in layered cuprate ceramics has so far led to the identification of about 35 distinct superconducting cuprate systems, the latest of which is the 133 K superconducting system Hg-Ba-Ca-Cu-O. For most of their proposed applications, high Tc ceramics have to be resistant to environmental degradation both with respect to atmospheric water vapor, e.g., in storage, and to liquid water (produced by condensation on warm-up from cryogenic conditions). The presence of CO2 is an important factor in both environments. Increasing environmental stability involves improving the processing methods to eliminate pores, cracks, and other macroscopic defects (e.g., highly leachable impurity phases) which are prevalent in materials prepared by solid-state sintering. Furthermore, protective coatings and hermetic seals are necessary in many applications involving films because of small film thickness. (Wires are usually drawn inside metal tubes, which provide protection.)

The environmental degradation of materials poses a serious limitation in the utility of engineering materials. The corrosion of metals, for example, has been estimated to represent a 4–5% decrease in the Gross National Product each year. To this, add losses involved in the replacement or restoration of ceramic structures such as buildings and transportation systems, i.e., the “infrastructure,” and the result is a significant sacrifice of economic strength.

Most of us are familiar with the consequences of exposing materials to environments in which the materials are chemically unstable and convert into substances that are unable to perform the function for which the original material was selected. The corrosion of metals into soluble or insoluble oxidation reaction products, chain scission or molecular mutation in polymers, even hydrolysis and leaching of silicious ceramic compounds represent behavior which ultimately limits the service applications of most engineering materials. For example, aluminum and its alloys are unsuitable for use in environments where oxide formation rates are high enough to represent a problem with respect to useful service life.

Oxide-based ceramics have long been used as linings in containment vessels for hot materials (metals, glasses, cement, etc.) and hot gases, at temperatures often in excess of 1500°C, because of their chemical compatibility with these hot materials and with the process ambient—conditions where metals and polymers simply can't perform. However, their low thermal conductivities and generally high thermal expansivities cause poor thermal shock resistance. In addition, their creep resistance (resistance to permanent deformation under load) is generally poorer than the more covalently bonded ceramic materials such as nitrides and carbides which also have excellent thermal shock resistance.

Recent breakthroughs in materials science and technology have led to improved dental restorative ceramics, metals, and resin-based composites. These breakthroughs have been most important since one of the major controversies during this period has centered on allegations of the physiological risks of mercury released from amalgam restorations. Dental amalgams have been used in the United States since 1832, yet not a single well-documented case of dental patient toxicity to mercury from amalgam restorations has been reported in the scientific literature.

A particularly extreme position has been adopted by the Swedish government, which has proposed the progressive elimination of amalgam as a dental restorative material by 1997. Among the materials that have been considered as the most biocompatible alternative to amalgam are dental ceramics. New ceramic products and technologies have gained considerable acceptance in Sweden even though their safety and efficacy have not been fully established.

Although dental ceramics constitute a much more costly alternative than some other materials, they offer the advantages of esthetics, inertness, durability, low thermal conductivity, and low diffusivity, as compared with amalgams. Although ceramics are generally considered to be inert in the oral cavity, there are certain phenomena associated with their use that may cause significant surface degradation of these materials and subsequent physiological side effects.