Combined Use with Coatings

The potential of zinc is less than that of magnesium ( )h in seawater = -0.8 V) hence, current output per anode is also less. High-purity zinc is usually specified in order to avoid significant anodic polarization with resultant reduction of current output caused by accumulation of adherent insulating zinc reaction products on commercial zinc. This tendency is less pronounced in zinc of high purity. 

A sketch of a magnesium anode rod installed in a steel hot-water tank is shown in Fig. 13.3. Such rods may increase the life of a steel tank by several years, particularly if the rod is renewed as required. The degree of protection is greater in waters of high conductivity, where the currents naturally set up by the magnesium-iron couple reach higher values than in waters of low conductivity (soft waters). 

The distribution of current in a cathodically protected steel water tank is not ideal—too much current may flow to the sides and not enough to the top and 

In the general application of cathodic protection using either impressed current or sacrificial anodes, it is expedient to use an insulating coating, and this 

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