Nodular cast iron, graphitic corrosion

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. 

Graphitic corrosion has two distinct features that are useful in distinguishing it from other forms of corrosion. First, it affects an unusually limited number of metals the only metals commonly affected are gray cast iron and nodular cast iron. Second, metal that has experienced graphitic corrosion may retain its original appearance and dimensions. Consequently, graphitic corrosion frequently escapes detection. 

The occurrence of graphitic corrosion is not location specific, other than that it may occur wherever gray or nodular cast iron is exposed to sufficiently aggressive aqueous environments. This includes, and is common to, subterranean cast iron pipe, especially in moist soil (Case History 17.1). Cast iron pump impellers and casings are also frequent targets of graphitic corrosion (Case Histories 17.2 through 17.5). 

Selective form of iron corrosion, primarily in gray cast iron but also less commonly in nodular cast iron, whereby the (anodic) iron matrix converts to iron oxide while the (cathodic) graphite remains intact. The casting retains it shape but loses all strength and can be cut with a knife. 

Austenitic cast irons (either flake graphite irons or nodular graphite irons) are produced by mixing in nickel from 13-30%, chromium from 1-5% and copper from 0.5-7.5 (to lower nickel-containing grades to augment the corrosion resistance at lower cost). 

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