Carbon steel atmospheric rust films

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

The carbon dioxide normally present in air neither initiates nor accelerates corrosion. Steel specimens rust in a carbon-dioxide-free atmosphere as readily as in the normal atmosphere. Early experiments by Vernon showed that the normal carbon dioxide content of air actually decreases corrosion [16], probably by favoring a more protective rust film. 

In atmospheres with chlorides present the corrosion of carbon steel proceeds in local cells that resemble the sulfate nests. They may form around chloride particles deposited on the surface, where the concentrated local chloride solution destroys the passivating film of FeOOH. The chlorides are concentrated in the anodic areas formed by migration, while the surrounding area covered by rust acts as a cathode. 

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