Microstructure gray cast iron

Graphitic corrosion has been observed on buried pipelines after many years of service. Gray cast iron has a continuous graphite network in its microstructure that is cathodic to iron and remains behind as a weak, porous network as the iron is selectively removed... 

Gray cast iron such as ASTM A48 material is plain cast iron, composed of ferrite containing graphite stringers. Figure 21.5 shows the microstructure. It is relatively brittle and is used for applications in which toughness is not a concern, including utility services, as pump cases, and valve bodies. 

The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%, respectively. For most of these east irons, the graphite exists in the form of flakes (similar to corn flakes), whieh are normally snrroimded by an a-ferrite or pearlite matrix the microstructure of a typical gray iron is shown in Fignre 11.3a. Because of these graphite flakes, a fractured surface takes on a gray appearance—hence its name. 

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

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