![]() ![]() The chromium tends to collect near the surface, where it forms an oxide layer that protects the iron. For example, stainless steel is mostly iron with a bit of chromium. Other strategies include alloying the iron with other metals. ![]() As long as the paint remains intact, the iron is protected from corrosion. The layer of paint prevents the water and oxygen necessary for rust formation from coming into contact with the iron. One way to keep iron from corroding is to keep it painted. The speed of the spontaneous reaction is increased in the presence of electrolytes, such as the sodium chloride used on roads to melt ice and snow or in salt water. Once the paint is scratched on a painted iron surface, corrosion occurs and rust begins to form. Unlike the patina on copper, the formation of rust does not create a protective layer and so corrosion of the iron continues as the rust flakes off and exposes fresh iron to the atmosphere.įigure 2. The number of water molecules is variable, so it is represented by x. What we call rust is hydrated iron(III) oxide, which forms when iron(II) ions react further with oxygen.ĤFe 2+( aq) + O 2( g) + (4 + 2 x)H 2O( l) ⟶ 2Fe 2O 3 The electrons reduce oxygen in the air in acidic solutions.Ĭathode: O 2( g) + 2H +( aq) + 4e − ⟶ 2H 2O( l) E ° oxygen = +1.23 V Once exposed to the atmosphere, iron rapidly oxidizes.Īnode: Fe( s) ⟶ Fe 2+( aq) + 2e − E ° iron = −0.44 V The main steps in the rusting of iron appear to involve the following (Figure 2). Iron will rust when it is exposed to oxygen and water. Perhaps the most familiar example of corrosion is the formation of rust on iron. (b) Exposure to the elements has resulted in the formation of the blue-green patina seen today. (a) The Statue of Liberty is covered with a copper skin, and was originally brown, as shown in this painting. The formation of the protective layer is a form of passivation, which is discussed further in a later chapter.įigure 1. Fortunately, formation of the patina created a protective layer on the surface, preventing further corrosion of the copper skin. These three compounds are responsible for the characteristic blue-green patina seen today. As a result, sulfur trioxide, carbon dioxide, and water all reacted with the CuOĢCuO( s) + CO 2( g) + H 2O( l) ⟶ Cu 2CO 3(OH) 2( s) (green)ģCuO( s) + 2CO 2( g) + H 2O( l) ⟶ Cu 2(CO 3) 2(OH) 2( s) (blue)ĤCuO( s) + SO 3( g) + 3H 2O( l) ⟶ Cu 4SO 4(OH) 6( s) (green) Copper metal is oxidized to copper(I) oxide (Cu 2O), which is red, and then to copper(II) oxide, which is black.ĢCu( s) + \fracO 2( g) ⟶ 2CuO( s) (black)Ĭoal, which was often high in sulfur, was burned extensively in the early part of the last century. The oxidation-reduction reactions of copper metal in the environment occur in several steps. ![]() The copper that is the primary component of the statue slowly underwent oxidation from the air. It was brown, the color of its copper “skin.” So how did the Statue of Liberty change colors? The change in appearance was a direct result of corrosion. When this statue was first delivered from France, its appearance was not green. The Statue of Liberty is easily identified by its height, stance, and unique blue-green color (Figure 1). The Statue of Liberty is a landmark every American recognizes. ![]()
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