Question

Pure iron ordinarily rusts quickly, but steel does not corrode nearly as fast. How does steel resist corrosion?

Short answer

Steel resists corrosion better than pure iron due to the formation of a protective passive layer (mainly composed of chromium oxide), lower carbon content, and the presence of regions with different electrochemical potentials providing self-protection against galvanic corrosion.

Step-by-step solution

Understand the Composition of Steel and Pure Iron

Steel is an alloy made up of iron and a small percentage of carbon, usually less than 2%. The carbon, along with other elements such as chromium, manganese, and nickel, are added to change the properties of the iron, making it stronger and more resistant to corrosion. Pure iron, on the other hand, is composed of only iron atoms.

Formation of a Protective Layer

The presence of other elements, particularly chromium, in steel can lead to the formation of a thin, protective layer on the surface called a passive layer. This layer, which is composed of chromium oxide, is adherent and insoluble, preventing the further reaction of the steel surface with oxygen and water. This reduces the rate of corrosion significantly. Pure iron does not form such a passive layer, making it more susceptible to corrosion.

Effect of Carbon Content

The amount of carbon present in steel also influences its ability to resist corrosion. Lower carbon content in steel (such as in stainless steel) improves its corrosion resistance. This is because carbon, when combined with iron, forms iron carbide (Fe3C), which is more prone to corrosion than iron. With low carbon content, the formation of iron carbide is minimized, and hence, the steel is more resistant to corrosion compared to pure iron.

Galvanic Corrosion Protection

Steel, due to the presence of different elements, can have multiple regions with different electrochemical potentials. This creates a self-protecting mechanism against corrosion as the regions act as anodes and cathodes. The anodic region corrodes while the cathodic region is protected. In contrast, pure iron has a uniform and single electrochemical potential which contributes to its faster corrosion as its whole surface is subjected to the same level of corrosion. In conclusion, steel resists corrosion better than pure iron due to the formation of a protective passive layer, the effect of lower carbon content, and the presence of regions with different electrochemical potentials.