Gray irons graphitic corrosion

Gray iron stagnation conditions Soils, many waters Iron (graphitic corrosion)... 

Graphitic Corrosion Graphitic corrosion usually involves gray cast iron in which metalhc iron is converted into corrosion products, leaving a residue of intact graphite mixed with iron-corrosion products and other insoluble constituents of cast iron. 

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). 

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... 

Dealloying is normally detectable by a color change. Brasses turn from yellow to red. Cast irons become dark from silver gray as a result of enrichment of graphite. Gray iron that has suffered such corrosion is like a sponge with virtually no mechanical strength. 

Graphitization gra-fe-t9- za-sh9n (1899) n. Corrosion of gray iron in which metallic... 

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