Corrosion: Graphitic Corrosion
After operating for six months, a pump impeller (of nickel-containing cast iron) showed considerable corrosion. Cross sections showed substantial penetration of the wall thickness without loss of material. The observed supercooled structure implied low strength but would not affect corrosion resistance. Etching of the core structure showed a selective form of cast iron corrosion (spongiosis or graphitic corrosion) which lowered the strength of the cast iron enough that a knife could scrape off a black powder (10.85% C, 1.8% S, 1.45% P). Analysis showed that some of the “sulfate” found in the scrubbing water was actually sulfide (including hydrogen sulfide) and was the main cause of corrosion.