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Corrosion Fundamentals


Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface. It can also occur by redeposition of the noble component of the alloy on the metal surface.
Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast iron.

Dezincified Brass

Degraphitized Cast Iron Pipe

The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe shown on the right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface. The rust tubercules or bubbles are also an indication of pitting corrosion.

The bottom photo shows a layer of copper on the surface of a dealloyed 70% copper-30% nickel cupronickel heat exchanger tube removed from a ship. Stagnant seawater is so corrosive that even this normally corrosion-resistant alloy has corroded. Virtually all copper alloys are subject to dealloying in some environments.

Dealloying of a Zinc Copper Alloy