How it works
An alloy made of elements with different positions in the galvanic series can behave like a microscopic galvanic couple. In a suitable electrolyte the more active element dissolves preferentially — the same driving force described for galvanic corrosion, operating inside a single material. What remains is a sponge of the noble constituent, often mixed with corrosion products, with a fraction of the original strength and ductility.
The main forms
| Form | Alloy | Element removed | What remains |
|---|---|---|---|
| Dezincification | Brasses, especially higher-zinc grades | Zinc | Porous copper |
| Graphitization | Gray cast iron | Iron | Graphite network with iron oxides |
| Dealuminification | Aluminum bronzes | Aluminum | Porous copper |
Dezincification appears either as a uniform layer or as localized plugs. The affected brass turns from yellow to copper-red. It is favored by stagnant or slow-moving water, high chloride content and elevated temperature. Inhibited grades — admiralty and naval brasses with small additions such as arsenic, antimony or phosphorus — and dezincification-resistant brasses are used where the risk is known.
Graphitization affects gray iron because its flake graphite forms a continuous network that stays behind as the iron matrix corrodes. Pipe, valve bodies and pump casings in soil, seawater or brackish water can look sound and keep their dimensions while the wall has lost most of its strength.
Dealuminification attacks aluminum bronzes with an unfavorable microstructure, particularly in seawater. Alloy additions such as nickel and appropriate heat treatment improve resistance.
Why it is hard to detect
Thickness gauging is unreliable, since dimensions are retained. Clues include the color change on brass, a soft surface that can be scraped or cut with a knife on graphitized iron, a dull sound when struck, and leaks through a wall that measures full thickness. Confirmation comes from metallographic sections or hardness testing. Any part found affected should be assessed for remaining strength rather than judged by appearance.
What to do about it
Select alloys resistant to the specific form for the service, using inhibited or resistant grades, ductile iron or coated iron where appropriate. Avoid stagnation and deposits. Where dissimilar materials meet, check the couple against the galvanic series and the anodic index. Coatings and linings remove the electrolyte from the surface and are used to protect cast iron components that cannot be replaced.