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THE MANY FACES OF ATMOSPHERIC CORROSION

1.1 DR. VERNON'S LEGACY

Thousands of years ago, humanity wrested materials from beneath the surface of Earth and processed them into spear points, rudimentary tools, and ornamental objects, which immediately began to corrode and have been corroding ever since. As technology has evolved and our atmosphere has come to contain increasing levels of acid gases, the rates of corrosion have increased. Everyday corrosion claims its victims-electronic connectors, towering bridges, and unique statuary. The forces opposing these processes are composed of corrosion scientists and engineers, whose war plan must, of necessity, be based on anticipating, understanding, and overcoming the enemy.

The science of atmospheric corrosion-corrosion that occurs in materials exposed to the ambient air-is less than a century old. Beginning in the 1920s, W.H.J. Vernon in England began systematic experiments in atmospheric corrosion. Except for some increased sophistication in instrumentation, his experiments were very similar to those of today: he cleaned metal samples, exposed them to specific concentrations of gases, such as SO2 and CO2, or to natural outdoor environments, and determined corrosion rates and the major corrosion products.

Vernon's work took place some 80 years ago. Werner Heisenberg was just inventing the uncertainty principle of quantum physics, the neutron was not yet discovered, polymer chemistry was barely thought of, continental drift was an unsupported speculation, and the DNA double helix would not be discovered for 30 years. Today, quantum physics is a mature specialty, insight into the atomic nucleus has resulted in the use of nuclear power, polymers are ubiquitous, Earth science has been revolutionized by plate tectonics, and biological scientists have sequenced the human genome. Meanwhile, Vernon's experiments are still cited in the corrosion science literature as relevant, at least occasionally. What has caused atmospheric corrosion science to stagnate while other scientific fields were forging ahead in great leaps and bounds?

One answer is that other fields are conceptually more straightforward and more highly specialized, while atmospheric corrosion is enormously complex and interdisciplinary. To understand quantum physics, one only needs the atom, its nucleus, and its electrons, for DNA only the molecule, although characterized by a highly complex structure. For atmospheric corrosion, however, one needs to understand a degraded solid phase, a very thin and transitory liquid phase, and a changing gas phase all at once and all without the ability to monitor everything during the time in which corrosion is actually occurring.

A second answer is that many applied investigations in corrosion science have had as their main emphasis the determination of the corrosion rate of a given metal in a given atmospheric environment. In these investigations, the corrosion products formed are to be removed from the metal by some chemical stripping treatment. However, this procedure not only determines the rate by removing the corrosion products, it also removes all the information hidden in the corrosion products that could tell something of what was going on during the corrosion process.

A third answer is that atmospheric corrosion has not traditionally attracted scientists performing fundamental research. In contrast, during the last three-four decades, a substantial amount of fundamentally orientated work in corrosion science has been devoted to understanding the chemical composition and atomic structure of passive films. Both atmospheric corrosion and passivity are research fields with enormous economic consequences. Yet, the efforts made in passivity have far outnumbered the efforts made in atmospheric corrosion. The main reason is simple: it is easier to set up and perform a well-defined laboratory experiment for fundamental passivity studies than for fundamental atmospheric corrosion studies. The former only needs two phases, the passivating metal and the liquid environment, whereas the latter needs three phases, the solid material, the atmosphere and a thin liquid film in between, and a thorough understanding of an intricate and rapidly changing atmospheric chemical environment.

1.2 CONCEPTS AND CONSEQUENCES

Atmospheric corrosion is the result of interaction between a material-an object made of a metal, a calcareous stone, a glass, or a polymer or covered by paint-and its surrounding atmospheric environment. The mechanisms that govern the corrosion or degradation of these materials differ greatly. The scope of this book has therefore been limited to the atmospheric corrosion of metals and alloys, whereas other types of materials only will be discussed occasionally. As opposed to the situation when the material is immersed in a liquid, atmospheric corrosion occurs during unsheltered exposure to rain or in rain-sheltered exposure indoors or outdoors.

Most frequently, atmospheric corrosion is triggered by atmospheric humidity, which forms a very thin water layer on the object. Depending on the humidity conditions, the water layer exhibits different thicknesses, resulting in various forms of atmospheric corrosion. In dry atmospheric corrosion or dry oxidation, the water layer is virtually absent. A common example of dry oxidation is the tarnishing of copper or silver, which can proceed without any humidity in the presence of reduced sulfur compounds. In damp atmospheric corrosion, humidity and traces of atmospheric pollutants result in a thin, mostly nonvisible, water layer. Wet atmospheric corrosion requires rain or other forms of bulk water together with atmospheric pollutants and results in a relatively thick water layer, often clearly visible to the eye.

The consequences of corrosion on our society are enormous. In the United States, for example, the total costs for all forms of corrosion have been estimated to be around 1000 US$ per capita per year. A substantial part of that amount is due to atmospheric corrosion. To estimate the costs for repair of corrosion-induced failures of our infrastructure, including bridges, elevated highways, railway, or subway systems, is tedious but can be done with a certain accuracy. It is more difficult to estimate the costs of direct or indirect consequences caused by atmospheric corrosion of electronic components or systems and how these can affect the reliability of security systems, aircraft, automobiles, or industrial processes. It is likewise difficult to estimate costs related to the loss of our cultural heritage. International concern has increased over the last decades as it has become evident that acid deposition through rain, snow, fog, or dew has resulted in substantial deterioration of artistic and historic objects, including old buildings and structures of historic value, statues, monuments, and other cultural resources.

1.3 THE EVOLUTION OF A FIELD

Developments in our understanding of atmospheric corrosion have been closely linked with society's need to gain more information about a visibly important process. During the first decades of the twentieth century, systematic field exposure programs were implemented in the United Kingdom and the United States when it became obvious that commonly used metals, particularly steel, copper, zinc, and aluminum, suffered from corrosion when exposed in heavily polluted atmospheric environments. The environments were categorized into rural, marine, urban, and industrial, and it was recognized that the metals exhibited different corrosion behaviors in these environments. In the 1920s and 1930s Vernon performed his pioneering work that transformed the field from art to science. He investigated the effect of relative humidity in combination with SO2 and discovered a rapid increase in atmospheric corrosion rates above a critical relative humidity.

In the decades to come, many important contributions were made by distinguished scientists, including U.R. Evans, J.L. Rosenfeld, and K. Barton, who, among others, could demonstrate the importance of electrochemical reactions in atmospheric corrosion. Further improvements were made by W. Feitknecht, who took into account the chemical properties of the solid products of the corrosion process. Electrochemical techniques thus became common tools for exploring the underlying mechanisms. The success was only partial, however, because of the obvious difficulties of reproducing the actual atmospheric exposure situation in an electrochemical cell in which the sample is completely immersed in an aqueous solution or covered by a relatively thick aqueous layer.

In the 1960s and 1970s, atmospheric corrosion effects on electronic components and equipment were recognized. One of the first observations was made in the electronics of American aircrafts in the Vietnam War, which were not adequately protected from the tropical conditions of high humidity and high chloride concentration. It was soon recognized that even very small amounts of corrosion effects, detectable only by highly sensitive analytical techniques, could have detrimental effects on the reliability of electronics. This coincided with the advent of surface analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy, capable of providing information on the chemical composition of the outermost atomic layers of a corroded material. A new set of tools was thus available for the understanding of atmospheric corrosion mechanisms. They were complementary to the electrochemical techniques and able to provide more specific chemical...