Cast iron graphitic residue

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. 

The presence of active sulphate-reducing bacteria usually results in graphitic corrosion and this has led to a useful method of diagnosing this cause of corrosion. The leaching out of iron from the graphitic residue which is responsible for the characteristic appearance of this type of corrosion leads to an enriched carbon, silicon and phosphorus content in the residue as compared with the original content of these elements in the cast iron. Sulphur is usually lost to some extent but when active sulphate-reducing bacteria are present, this loss is offset by the accumulation of ferrous sulphide in the residue with a consequent increase in the sulphur content of the residue out... 

The characteristic mode of corrosion of some alloys may be the formation as a corrosion product of a redeposited layer of one of the alloy constituents, as in the case of the brasses that dezincify, or of a residue of one of the components, as in the case of the graphitic corrosion of cast iron. Particularly in the case of the dezincified brass, the adherent copper is not likely to be removed with the other corrosion products, and therefore the mass-loss determination will not disclose the total amount of brass that has been corroded. This is especially important because the copper layer has very little strength and ductility and the extent of weakening of the alloy will not be indicated by the mass loss. In these cases, also, the mass-loss determinations must be supplemented by, or replaced by, mechanical tests or metallographic examination, or both, to reveal the true extent of damage by corrosion. Difficulties in obtaining accurate mass losses of heavily graphitised specimens have been reported... 

After a year or two in sea water, a residual layer of graphite forms on the cast-iron surface. If a cast-iron pump casing becomes graphitized, the potential relationship is reversed and the casing may become cathodic to the bronze impeller. The latter... 

Material welded construction, cast iron, spheroidal graphite cast iron, cast steel Filter system single or double filter Vacuum pump type of construction, air capacity, residual pressure which can be achieved... 

Bloom, P. R. and Tuovinen, O. H., "Characterization of Graphitic Corrosion Residue of Cast Iron in a Water Distribution System, Materials Performance, February 1984, p. 21. 

Graphitic corrosion This is the deterioration of cast iron in which the iron-rich matrix is selectively leached or converted to corrosion products and a weak residue of graphite flakes is left behind. 

Graphite flakes are cathodic to iron, and corrosion is localized to iron which starts leaching (becomes a porous mass) and leaves a rich residue of graphite flakes. Graphitization is observed on gray cast iron water pipes. 

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