How do we prevent the corrosion of an oil rig

Oil rigs are often built to survive in some of the most inhospitable climates in the world. For example, the oil rigs in the North Sea between the UK and Scandinavia 

Oil rigs are made of steel. The sea in which they stand contains vast quantities of dissolved salts such as sodium chloride, which is particularly aggressive to ferrous metals. The corrosion reaction generally involves oxidative dissolution of the iron, to yield ferric salts, which dissolve in the sea  

If left unchecked, dissolution would cause thinning and hence weakening of the legs on which the rig stands. 

One of the most ingenious ways in which corrosion is inhibited is to strap a power pack to each leg (just above the level of the sea) and apply a continuous reductive current. An electrode couple would form when a small portion of the iron oxidizes. The couple would itself set up a small voltage, itself promoting further dissolution. The reductive current coming from the power pack reduces any ferric ions back to iron metal, which significantly decreases the rate at which the rig leg corrodes. 

Clearly, we want the net current at the iron to be zero (hence no overall reaction). The rate of corrosion would be enhanced if the power pack supplied an oxidative current, and wasteful side reactions involving the seawater itself would occur if the power pack produced a large reductive current. The net current through the iron can be positive, negative or zero, depending on the potential applied to the rig s leg. The conserver of the rig wants equilibrium, implying no change. 

---