Corrosion that removes the more active element from an alloy and leaves a spongy shell of the more noble one. Dezincification of brass leaves red, porous copper plugs or layers; graphitic corrosion of gray cast iron dissolves the iron and leaves a graphite network that keeps the pipe's shape but cuts like pencil lead. Parts can hold pressure for years and then fail suddenly under a knock or a water hammer.
Old cast iron main looked fine until you scraped it, dealloyed right through, you could carve it with a pocketknife.
Tap old cast iron or scrape it with a knife before trusting it. Graphitized pipe sounds dull and cuts soft.
Also heard as
- selective leaching
- dezincification
- graphitic corrosion
Related terms
Galvanic corrosion
Accelerated corrosion of the more active metal when two dissimilar metals touch in a shared electrolyte.
Uniform corrosion
Corrosion spread evenly over a whole surface, thinning the metal at a fairly predictable rate.
Red water
Rust-colored tap or process water carrying iron corrosion products shed from pipes and mains.