Studies in Wet Corrosion

We now examine three real corrosion problems the protection of pipelines, the selection of a material for a factory roof, and materials for car exhaust systems. The rusting of iron appears in all three case studies, but the best way of overcoming it differs in each. Sometimes the best thing is to change to a new material which does not rust but often economics prevent this, and ways must be found to slow down or stop the rusting reaction. 

Many thousands of miles of steel pipeline have been laid under, or in contact with, the ground for the long-distance transport of oil, natural gas, etc. Obviously corrosion is a problem if the ground is at all damp, as it usually will be, and if the depth of soil is not so great that oxygen is effectively excluded. Then the oxygen reduction reaction 

If the pipe is connected to a slab of material which has a more negative corrosion voltage (Fig. 24.1), then the couple forms an electrolytic cell. As explained in Chapter 23, the more electronegative material becomes the anode (and dissolves), and the pipe becomes the cathode (and is protected). 

Naturally, because the protection depends on the dissolution of the anodes, these require replacement from time to time (hence the term sacrificial anodes). In order to minimise the loss of anode metal, it is important to have as good a barrier layer around the pipe as possible, even though the pipe would still be protected with no barrier layer at all. 

An alternative way of protecting the pipe is shown on Fig. 24.3. Scrap steel is buried near the pipe and connected to it through a battery or d.c. power supply, which 

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