Corrosion Basics: Corrosion and the Environment

Polluted air is generally more corrosive than clean air.

Corrosion Basics don’t get any more basic than the definition of corrosion as the deterioration of a material, usually a metal, that results from a chemical or electrochemical reaction with its environment.

The fact that corrosion does occur should not be cause for surprise. Almost all materials should be expected to deteriorate, to some extent, with time when exposed to the elements. Corrosion is a perfectly natural process, as natural as water flowing downhill. If water were observed flowing uphill or remaining stationary on a hillside, there might be cause for surprise; yet our human ingenuity can accomplish this by putting water in a container (pipe or tank) and closing the bottom end or merely freezing it.

Similarly, if iron or mild steel were exposed to water or moist air, rust bloom (a preliminary form of iron oxide) would be expected to develop within a matter of hours. In fact, it would be surprising if the exposed iron did not corrode. Of course, if copper, brass, aluminum, or a more resistive alloy of iron (e.g., stainless steel [SS]) were substituted for iron, a given degree of corrosion might take longer, but some corrosion would still be anticipated. Some oxides of copper, aluminum, and chromium typically form very slowly and coat the underlying metal. Even if they are extremely thin, these oxide coatings could form a partial barrier to continued attack and slow the rate of corrosion almost to a standstill.

Formation of a surface layer, whether it is based on an oxide, carbonate, sulfate, or any other compound, is a major factor in corrosion resistance, particularly if the layer effectively isolates the metal substratum from the environment. Such a naturally formed coating must be diffusion- and moisture-resistant to be effective. Ordinary iron does not naturally form an effective barrier; its rust permits oxygen and moisture to penetrate and continue rusting. Thus, unless precautions are taken, such as applying a protective coating over the surface, failure will eventually occur.

Some metals, such as SS, titanium, or aluminum, are frequently left unpainted when exposed to the atmosphere. This is not because these metals are inert, but because oxygen in the air helps develop a protective oxide layer on the metallic surface. Although these oxide layers are often so thin as to be invisible to the naked eye, they can be detected and their presence verified.

Some environments are more corrosive than others. Although there are exceptions, the following statements are generally accepted as facts:

  • Moist air is more corrosive than dry air.
  • Hot air is more corrosive than cold air.
  • Polluted air is more corrosive than clean air.
  • Hot water is more corrosive than cold water.
  • Salt water is more corrosive than fresh (low chloride content) water.
  • Acids are more corrosive than bases (alkalis) to steels.
  • SS will outlast ordinary steel.
  • No corrosion will occur in a vacuum, even at very high temperatures.

Although it may seem surprising, there are instances when all of these statements, including the last one, are incorrect. This would indicate that broad, categorical statements should be considered to be suspect. There is essentially no statement regarding corrosion or the use of a material that does not have at least one exception.

This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 13-14.

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