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The fact that corrosion does occur should not be cause for surprise. Almost all materials should be expected to deteriorate with time when exposed to "the elements." Corrosion is a perfectly natural process, as natural as water flowing downhill. If water flowed uphill or remained stationary on a hillside, there may be cause for surprise, yet our human ingenuity can accomplish this by putting water in a closed container (pipe) and closing the bottom end, or by merely freezing it.
Similarly, if iron or steel was exposed to air and water, rust would be expected to develop within a matter of minutes or hours. In fact, it would be surprising if the exposed iron did not corrode or rust. Of course, if copper, brass, aluminum, or stainless steel was substituted for iron, a given degree of corrosion might take longer, but some corrosion would still be anticipated. Instead of forming rust (a form of iron oxide), some oxides of copper, aluminum, or chromium may form a protective coating on the bare metal. This oxide coating, even if extremely thin, may provide a partial barrier to continued attack and slow down the rate of corrosion.
A surface layer formation, whether it is oxide, carbonate, sulfate, or any other compound, is a major factor in corrosion resistance, particularly if the layer effectively separates the underlying metal from its environment. Such a naturally formed coating must be diffusion and moisture resistant to be effective. Ordinary iron does not naturally form an effective barrier; naturally occurring rust is porous and does not prevent oxygen and moisture to penetrate and continue rusting. Thus, unless precautions are taken, failure will eventually occur.
Other metals and alloys, such as stainless steel, titanium, or aluminum, are frequently left unpainted. 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 so thin as to be invisible to the eye, they can be detected and their presence verified.
Precautions to prevent iron and its alloys from corroding constitute a major effort in corrosion control. It is here that we often resort to the use of protective coatings over the metallic surface to prolong its useful life. Other techniques such as anodic and cathodic protection can provide very efficient corrosion control in the most demanding environments. Some environments are more corrosive than others. While 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
Hot water is more corrosive than cold water
Polluted air is more corrosive than clean air
Acids are more corrosive than bases (alkalies) to steels
Salt water is more corrosive than fresh water
Stainless steel will outlast ordinary steel
No corrosion will occur in a vacuum, even at very high temperatures
While it may be a surprise to some, there are instances when all of the above statements, including the last one, are incorrect. This would indicate that broad, categorical statements regarding corrosion should be considered to be suspect. There is essentially no statement regarding corrosion or the use of a material that does not have an exception.
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