red rot
The chemical dezincification of brass alloys, where the zinc in the brass leaches out and leaves behind fragile or powdery copper metal.
red rot: dezincification that turns brass to crumbling copper
Red rot is the selective corrosion of brass alloys in which zinc dissolves into the surrounding environment, leaving behind a brittle, often reddish or powdery mass of copper and copper oxides. The result is structural failure: a pipe fitting, valve, or coupling that looks intact from the outside but crumbles under mechanical stress or internal pressure. The copper skeleton left behind is mechanically weak and offers almost no corrosion resistance, making the component unusable.
The mechanism involves both the zinc and the copper in the brass lattice. When certain water chemistries are present (particularly water with low pH, high chloride content, or high dissolved oxygen), the zinc preferentially dissolves because it is more anodic than copper. The copper does not follow into solution; instead it redeposits locally as a porous, spongy structure. This process can occur at a microscopic scale throughout the brass cross-section, not just at the surface.
When and where red rot occurs
Red rot is common in brass fittings exposed to soft water, acidic water, or water with high ammonia content. It occurs faster at elevated temperatures and in stagnant conditions where oxygen gradients develop. Brass grades high in zinc (naval brass, for instance, at around 40 percent zinc) are more vulnerable than lower-zinc brasses. Dezincification resistant brasses, typically containing small additions of arsenic or selenium, are much less prone to attack.
The term 'red rot' originates from the reddish or rust-colored appearance of the degraded brass surface, and from the loose, powdery texture that resembles rot. It is found in plumbing systems, cooling water circuits, heat exchangers, and anywhere brass components contact corrosive or chemically aggressive water over extended periods. Once dezincification begins, it is essentially irreversible; the corroded section must be replaced.
Prevention relies on material selection (use dezincification-resistant brass when water chemistry is questionable), water treatment (raise pH, reduce chloride or oxygen), and in some cases protective coatings or linings applied to the internal brass surfaces.