Corrosion Unwanted Voltaic Cells

The reactions occurring in voltaic cells (batteries) are important sources of electricity, but similar reactions also underlie corrosion processes. First, we will consider the electrochemical basis of corrosion, and then we will see how electrochemical principles can be applied to control corrosion. 

The potential difference for these two half-cell reactions is 

Fcdi is even greater than 0.841 V. Corrosion is especially significant in acidic solutions, in which the reduction half-cell reaction is 

The pink color results from the indicator phenolphthalein in the presence of base the dark blue color results from the formation of Turnbull s blue KFe[Fe(CN)6]. Corrosion (oxidation) of the nail occurs at strained regions (a) the head and tip and (b) a bend in the nail, (c) Contact with zinc protects the nail from corrosion. Zinc is oxidized instead of the iron (forming the faint white precipitate of zinc ferricyanide). (d) Copper does not protect the nail from corrosion. Electrons lost in the oxidation half-cell reaction distribute themselves along the copper wire, as seen by the pink color that extends the full length of the wire. 

Some metals, such as aluminum, form corrosion products that adhere tightly to the underlying metal and protect it from further corrosion. Iron oxide (rust), however, flakes off and constantly exposes fresh surface. This difference in corrosion behavior explains why cans made of iron deteriorate rapidly in the environment, whereas aluminum cans have an almost unlimited lifetime. The simplest method of protecting a metal from corrosion is to cover it with paint or some other protective coating impervious to water, an important reactant and solvent in corrosion processes. 

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Corrosion Unwanted Voltaic Cells 19-7 Electrolysis Causing... 

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