Graphite 57 Iron rusting

Rates of reaction vary from those which seem to be instantaneous, e.g. reaction of H30+(aq) with OH (aq), to those which are so slow that they appear not to occur, e.g. conversion of diamond to graphite. Intermediate situations range from the slow oxidation of iron (rusting) to a typical laboratory experiment such as the bromination of an alkene. But in all cases the reactant concentration shows a smooth decrease with time, and the reaction rate describes how rapidly this decrease occurs. 

Carbon, graphite Iron(III)oxide (rust, scale)... 

Examples Diamond converts to graphite (infinitesimally slow rate). Iron rusts in air (very slow, but noticeable change). 2H2(g) + 02(g) -> 2H20(g) (very fast reaction or explosion with an external spark). 

Figure 17.5 Brittle fracture through pipe. The gray material is graphitically corroded cast iron. Unaffected pipe wall metal is orange from normal rust.

Local corrosion or pitting is more important for practical purposes than the rate of general corrosion, and may proceed 10 times or so more rapidly than this. Inasmuch as certain types of cast iron are liable to suffer graphitic corrosion, whereas steel does not, steel might theoretically be expected to show to some advantage when used for buried pipelines. In practice, however, a cast-iron pipe has to be of stouter wall than a steel pipe for equal strength, and it is doubtful whether any distinction between the rust resistance of the two materials in the soil is justified. 

Some pigments promote corrosion owing to their content of soluble salts, their reactivity, or their electrochemical action, and thus should be avoided. Rust of the spotted type can be the consequence of their presence in a paint, especially the hrst coat, e.g. of graphite (noble to steel), some red oxides of iron, gypsum, ochre or lamp black. 

Chemical kinetics is the study of how fast reactions take place. Many familiar reactions, such as the initial steps in vision and photosynthesis happen almost instantaneously, whereas others, such as the rusting of iron or the conversion of diamond to graphite, take place on a timescale of days or even millions of years. 

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