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Corrosion Fundamentals

Materials Selection

Carbon Steel

Stainless Steel

Aluminum

Copper Alloys

Titanium

Carbon Steel

Weathering Steel StatueMost large metal structures are made from carbon steel-the world's most useful structural material. Carbon steel is inexpensive, readily available in a variety of forms, and can be machined, welded, and formed into many shapes.

This large statue by Pablo Picasso in front of the Chicago city hall is made from a special form of carbon steel known as weathering steel. Weathering steel does not need painting in many boldly exposed environments. Unfortunately, weathering steel has been misused in many circumstances where it could not drain and form a protective rust film. This has given the alloy a mixed reputation in the construction industry.

Where other means of corrosion control are not practical, other alloys can be substituted for carbon steel. This normally doubles or more the material cost for a structure, and other corrosion control methods must be considered before deciding on the use of more expensive alternates to carbon steel.

Some forms of carbon steel are subject to special types of corrosion such as hydrogen embrittlement, etc. It is common practice to limit the allowable strength levels of carbon steel to avoid brittle behavior in environments where environmental cracking may occur. High strength bolts cannot be galvanized for the same reason-a concern that they may hydrogen embrittle due to corrosion on the surface.

Protective coatings, cathodic protection, and corrosion inhibitors are all extensively used to prolong the life of carbon steel structures and to allow their use in environments such as the Kennedy Space Center where the environment would otherwise be too corrosive for their use.

Stainless Steels

Stainless Steel Body on Sports Car

The stainless steel body on this sports car is one example of how stainless steels can be used. The stainless steel is virtually immune to corrosion in this application-at least in comparison to the corrosion that would be experienced by conventional carbon steel or aluminum auto bodies.

Stainless steels are a common alternative to carbon steels. There are many kinds of stainless steels, but the most common austenitic stainless steels (300-series stainless steels) are based on the general formula of iron with approximately 18% chromium and 8% nickel. These austenitic stainless steels are frequently immune to general corrosion, but they may experience pitting and crevice corrosion and undergo stress corrosion cracking in some environments.

Aluminum

Aluminum alloys are widely used in aerospace applications where their favorable strength-to-weight ratios make them the structural metal of choice. They can have excellent atmospheric corrosion capabilities. Unfortunately, the protective properties of the aluminum oxide films that form on these alloys can break down locally and allow extensive corrosion. This is discussed further in the section on intergranular corrosion.Guardrail that was Subject to Corrosion

The highway guardrail shown on the right is located near the ocean in Florida. The aluminum alloy maintains a silvery shine except in locations where the passive film has suffered mechanical damage. The wear caused by the rail touching the wooden post at this location destroyed the passive film on the edges of the rail and allowed intergranular corrosion to proceed and cause the exfoliation corrosion shown above. While the corrosion above is very interesting and makes for an interesting web site, it is important to note that the railing is decades old and would have never lasted as long in this location if it were made of carbon steel.

Intergranular corrosion is a major problem on airplanes and other structures made from aluminum alloys. It frequently occurs at bolt and rivet holes or at cutouts where the small grain boundaries perpendicular to the metal surface are exposed.

Copper Alloys

Brasses and bronzes are commonly used piping materials, and they are also used for valves and fittings. They are subject to stress corrosion cracking in the presence of ammonia compounds. They also suffer from dealloying and can cause galvanic corrosion when coupled with steel and other structural metals. Most copper alloys are relatively soft and subject to erosion corrosion.

Dezincification

The dezincification shown above could have been controlled by using inhibited brasses which have been commercially available since the 1930's.

Titanium

Crevice Corrosion in a Titanium Aerospace PartTitanium is one of the more common metals in nature, but its limited use means that small-scale production operations result in a relatively expensive metal. In the United States it finds extensive use in the aerospace industry. The Japanese make extensive use of titanium in the chemical process industries.

There are two general types of titanium alloys-aerospace alloys and corrosion resistant alloys. The crevice corrosion of an aerospace alloy flange in a saltwater application is a classic example of how titanium gets misused.

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