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Steel Corrosion

Iron and steel, the most commonly used metals, corrode in many media including most outdoor atmospheres. Usually they are selected not for their corrosion resistance but for such properties as strength, ease of fabrication, and cost. These differences show up in the rate of metal lost due to rusting. All steels and low-alloy steels rust in moist atmospheres. In some circumstances, the addition of 0.3% copper to carbon steel can reduce the rate of rusting by one quarter or even by one half. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. Some excerpts of that document are used here.

Steel cathodic protection

The elements copper, phosphorus, chromium and nickel have all been shown to improve resistance to atmospheric corrosion. Formation of a dense, tightly adhering rust scale is a factor in lowering the rate of attack. The improvement may be sufficient to encourage use without protection, and can also extend paint life by decreasing the amount of corrosion underneath the paint. The rate of rusting will usually be higher in the first year of atmospheric exposure than in subsequent years, and will increase significantly with the degree of pollution and moisture in the air.

Ordinary steels are essentially alloys of iron and carbon with small additions of elements such as manganese and silicon added to provide the requisite mechanical properties. The steels are manufactured from a mixture of pig iron and scrap, which is treated in the molten state to remove excess carbon and other impurities. The steel may be continuously cast into strands or cast into individual ingots. The final product is then produced by rolling, drawing or forging. During hot rolling and forging the steel surface is oxidized by air and the scale produced, usually termed millscale. In air, the presence of millscale on the steel may reduce the corrosion rate over comparatively short periods, but over longer periods the rate tends to rise. In water, severe pitting of the steel may occur if large amounts of millscale are present on the surface.

Stress corrosion cracking (SCC) in passivating environments

Carbon and low alloy steels can suffer from SCC in a wide range of environments that tend to form a protective passivating film of oxide or other species. Cracking will not normally occur when there is a significant corrosion rate (note that this is not the case for hydrogen embrittlement - see below). A wide range of environments have been found to cause SCC, including strong caustic solutions, phosphates, nitrates, carbonates, and hot water. The problems are important for both economic and safety reasons. (reference)

Caustic cracking of steam-generating boilers was a serious problem in the late 19th century (the necessary strong caustic solution was produced by evaporation of the very dilute solution inside the boiler as it escaped through leaks in the riveted seams) and boiler explosions led to significant loss of life. More recently gas transmission pipelines have cracked in carbonate solutions produced under protective coatings as a result of cathodic protection systems. In this case the crack runs along the length of the pipe, and may propagate for very long distances by fast fracture. If the gas cloud that is released ignites, the resultant fireball is devastating.

High strength low alloy steel (HSLA) or Weathering steel

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See also: Equilibrium reactions of iron in water, Iron corrosion products, Iron species and their thermodynamic data, Pourbaix diagram of iron, Rust chemistry, Rust converters, Steel corrosion

Or: Aluminum, Aluminum alloys, Brass, Bronze, Cadmium, Chromium, Cobalt, Copper, Gold, Iron, Lead, Magnesium, Molybdenum, Nickel, Nickel alloys, Silver, Stainless steels, Steel, Tantalum, Tin, Titanium, Zinc, Weathering steel