Brass Corrosion Guide

1. Uniform tarnish & patina

The brown-then-green colour change visible on exposed brass over months and years is a thin layer of copper carbonate / copper acetate / basic copper sulfate formed when copper in the brass reacts with atmospheric oxygen, carbon dioxide and moisture. This is purely cosmetic — the patina layer is self-limiting (it slows further attack rather than accelerating it) and the underlying mechanical strength is unaffected.

The Statue of Liberty's iconic green colour is patinated copper, formed over a century with no structural damage to the underlying metal. The same chemistry happens on brass, with the addition of zinc oxides and zinc carbonates.

When patina matters: Only for cosmetic reasons. Brass fittings on visible architectural hardware, lighting, decorative trim and instruments require a lacquered surface (Incralac, acrylic) or a plated surface (nickel, chrome) to prevent visible patina formation. See the surface finishes guide.

2. Dezincification — the dominant risk for leaded brass in water

Dezincification is the corrosion mode that drives the majority of in-service brass-fitting failures in plumbing systems. It is a selective form of corrosion: the zinc atoms preferentially dissolve out of the brass lattice, leaving behind a porous copper skeleton that retains the original shape but has negligible mechanical strength.

The mechanism

In brass alloys with Zn > ~15%, the zinc preferentially enters solution under specific water-chemistry conditions:

• Low pH (< 7.0)
• Elevated chloride concentration (> ~100 mg/L)
• Elevated temperature (> 60°C accelerates the rate)
• Stagnant or slow-flow conditions (the corrosion products aren't washed away)
• Soft water (low mineral content, less protective scale)
• Oxygenated conditions (presence of dissolved oxygen)

Two patterns develop:

• Layer-type — a uniform layer of dezincified material forms parallel to the surface. Often cosmetic at early stage; eventually penetrates the wall.
• Plug-type — localised deep penetration creates a "plug" of porous copper. More dangerous because the rest of the part still looks intact. Pressure-bearing fittings can fail catastrophically through a plug, releasing water under pressure.

Which alloys are at risk

The CW602N, CW724R and C6802 alloys are all "DZR" (Dezincification Resistant) qualified per ISO 6509-1. They are the correct choice for any installation that combines water contact, elevated temperature, and chloride exposure.

The ISO 6509-1 test

The standard accelerated dezincification test for copper alloys per ISO 6509-1:

1. Cross-sectional sample, metallographically polished to expose grain structure
2. Immerse in 1% CuCl₂ solution at 75°C for 24 hours
3. Measure maximum depth of dezincification using optical microscopy
4. Pass criterion: max DZ depth ≤ 200 µm (and no plug-type attack)

3. Stress corrosion cracking ("season cracking")

Stress corrosion cracking (SCC) is a brittle failure of brass under the simultaneous action of:

1. Tensile stress (applied or residual from cold working)
2. A specific corrosive environment (ammonia, amines, or moisture with sulfur dioxide)
3. A susceptible alloy (any brass with Zn > ~15% is at risk; higher Zn = higher risk)

The historical name "season cracking" comes from the late 19th century in India, where British brass cartridge cases stored in horse stables during the monsoon would crack spontaneously. The cracking was eventually traced to ammonia vapour from horse urine plus the residual stress from cold-drawing the cartridge cases. The phenomenon is now formally tested per ASTM B858 — the ammonia vapour test.

Where SCC matters

• Refrigeration / HVAC — ammonia is a refrigerant; brass with residual stress can crack in service. Anneal-after-forming reduces the risk.
• Marine atmospheres — coastal humidity + atmospheric ammonia from biological sources
• Agricultural environments — animal effluents, fertiliser storage
• Pickling / cleaning lines — amines in cleaning agents

Prevention

• Stress-relief anneal brass parts at 250–300°C for 1 hour after cold drawing or cold rolling
• Low-zinc alloys for high-risk environments (CW724R, C6802, C11000)
• Avoid cold-forming brass that will see ammonia exposure
• Plate or lacquer for additional protection

4. Galvanic corrosion

When brass is in electrical contact with a dissimilar metal and both are wetted by an electrolyte (water, even atmospheric humidity), an electrochemical cell forms. The less noble metal corrodes preferentially.

5. Pitting corrosion

Localised attack creating discrete pits, typically caused by chloride ions concentrating in a small surface defect or scale fissure. Most relevant for brass in:

• Seawater (high Cl⁻)
• De-icing salt environments
• Chlorinated swimming pool service water
• Polluted urban water

The pit grows because the local environment inside the pit becomes more aggressive than the bulk fluid (the pit becomes acidic and high-Cl⁻ as corrosion products accumulate). The surrounding bulk metal stays protected by its oxide film while the pit penetrates.

Prevention: select aluminium brass (C68700, ~22% Zn, 2% Al) for seawater service — the aluminium forms a protective passive layer that resists chloride pitting. For freshwater service with chlorination, CW602N (DZR, arsenic-inhibited) is the standard choice.

6. Erosion-corrosion

High-velocity flow physically removes the protective oxide film from the brass surface, exposing fresh metal to corrosion. The result is a smoothly-eroded depression — often shaped like a horseshoe — usually downstream of a flow disturbance (elbow, valve, partial obstruction). Common in:

• Plumbing systems pressurised above design (velocity > 2 m/s for hot water)
• Cooling water lines after a partially-closed valve
• Marine seawater coolers

The CIBSE design guide for the UK limits hot-water velocity in copper alloy systems to 1.5 m/s (continuous) to prevent erosion-corrosion. Aluminium brass (C68700) and admiralty brass (C44300) have much better erosion-corrosion resistance than standard CW617N and are the right choice for high-velocity marine cooling.

7. Microbiologically influenced corrosion (MIC)

Certain bacteria — sulfate-reducing bacteria (SRB), iron-oxidising bacteria — colonise brass surfaces and create local environments far more aggressive than the bulk fluid. MIC is responsible for many "mystery" failures of brass fittings in installed water systems, particularly those that have sat dormant. The dark sulfide tubercles on the inside of a failed brass valve are a classic MIC signature.

Copper's antimicrobial property means brass naturally suppresses most planktonic bacteria, but biofilms can still form. Disinfection (chlorination), regular flow, and avoiding stagnation are the practical defenses.

8. Prevention by alloy choice + design

9. Standard tests & inspection

Frequently asked questions

10. Sources & references

Last reviewed: June 2026. For procurement-critical decisions verify against the current published edition of the cited standard. This guide is general engineering reference only.