Cast iron continued structure

If austenite is cooled slowly toward ambient temperature, the dissolved carbon in excess of 0.022 weight % comes out of solid solution as cementite, either in continuous layers of FeaC (pearlite) or as layers of separated FeaC grains (bainite). In either case, the iron is soft and grainy, as with cast iron. If, on the other hand, the hot austenite is cooled quickly (i.e., quenched), the 7-Fe structure goes over to the a-Fe form without crystallization of the interstitial carbon as cementite, and we obtain a hard but brittle steel known as martensite in which the C atoms are still randomly distributed through the interstices of a strained a-Fe lattice. Martensite is kinetically stable below 150 °C above this temperature, crystallization of FesC occurs in time. 

It has been shown above that there must be a source of current to supply electrons to the areas of the metal which is corroding. In a metal buried in ground, anodic areas corrode by release of electrons and if an equal number of electrons are not introduced from an external source, the metal would continue to corrode. An external anode which supplies such current is called auxiliary anode in the electrochemical cell and referred to as anode in a cathodic protection system. Electrodes of graphite, cast iron, platinum and titanium act as conductors of electricity and supply the desired current to the structure to be protected. The conductors are energized by a DC source. The rate of consumption of anode electrodes... 

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