Guide
Why do machined brass parts arrive tarnished or green, and how do finish and packing prevent brass corrosion?
Brass corrosion is the reaction of a copper-zinc alloy with air, moisture and contaminants, and on machined parts it ranges from a harmless darkening film (tarnish) to green corrosion products, pink dezincified patches and, in rare cases, cracks. Much of the discoloration found on brass at receiving is cosmetic tarnish or staining caused by residue, handling or packing rather than a defect in the alloy, but the cases that are real material problems must be separated out before a lot is accepted.
By Saola Manufacturing engineering team · Published · Updated
The practical question is rarely “does brass corrode.” A tray of turned C360 fittings or milled brass covers has been opened at receiving and the parts are dull, brown, spotted with fingerprints or dusted with green. Someone has to decide whether to accept, clean, rework or reject them, and then change something so the next lot arrives bright. The causes sort into three groups with different owners: cosmetic (normal oxidation and sulfur tarnish), process and packing (cutting fluid, wash water, sweat, condensation, sulfur-bearing or acidic packaging), and material (dezincification and stress corrosion cracking). This guide shows how to tell them apart, which finish or coating actually keeps brass bright, and what to write on the drawing and the purchase order.
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How do you tell cosmetic brass tarnish from real corrosion on received parts?
Tarnish is an even, thin film that darkens the surface from bright yellow toward gold, brown or near black and polishes off with no pits underneath. Green crust, pink or copper-red patches, or fine cracks mean something more than tarnish is going on and the lot should be held for diagnosis.
Freshly machined brass is bright because the cut exposes bare metal. Copper in the alloy begins to oxidize as soon as it meets air, and the color shifts gradually. That film is self-limiting in clean, dry air and does not change dimensions or strength in any way that matters for a mechanical part. It is a problem only when the surface is cosmetic, carries an electrical contact, or must be soldered or plated later, because the film then has to be removed first.
Three appearances are not ordinary tarnish. A green or blue-green crust is a copper corrosion product that formed where moisture combined with a reactive species such as chloride, acid vapor or ammonia. A pink, copper-colored or spongy area on a part that has been in water service usually means dezincification, in which zinc leaves the alloy and porous copper is left behind. Fine cracks, especially at threads, crimped sections or press-fit bores, can be stress corrosion cracking, which brass is prone to when residual or applied stress meets ammonia. Record which of these you see, on which faces, and on how many parts, before anyone cleans the lot.
What causes machined brass parts to tarnish or turn green?
Moisture trapped on the parts, from wash water, water-based coolant or condensation in transit, is behind most cases, together with fingerprints, sulfur from packaging or cutting oil, and acidic vapor from wooden crates. Dezincification and ammonia cracking are less common on new parts but are the ones that make parts unusable.
Moisture comes first because most of the other mechanisms need it. Parts that are packed warm or damp, or sealed in a bag with residual coolant, sit in their own humid microclimate for the whole voyage. Ocean containers and unventilated warehouses add condensation every time the temperature drops. Fingerprints are the next suspect: the Canadian Conservation Institute advises handling copper alloys with clean cotton gloves to avoid transferring the moisture, corrosive salts and oils from fingers (CCI Note 9/3, revised 2007), and on brass those salts show up as dark prints that later turn green.
Sulfur gives the dark brown to black tarnish. Materials that contain sulfur can release gases such as hydrogen sulfide or carbonyl sulfide that tarnish silver and copper; the institute names rubber, including latex gloves, elastic bands and O-rings, and protein fibers such as wool among the sources (CCI Note 17/5, 2017). Cutting fluids are a second sulfur route: oils with active sulfur additives can stain copper alloys (oil suppliers usually report copper staining with the ASTM D130 copper strip corrosion test), so a shop that runs a steel-cutting oil on brass may ship parts that are already stained. Acid vapor from wood is the third route in closed storage. The institute warns that wooden cabinets, new ones especially, give off acid vapors that attack many metals, and advises against oak (CCI Note 9/4, revised 2007). A study of lacquered copper-alloy coins from the Athenian Agora excavations, published in the Journal of the American Institute for Conservation, concluded that the lacquer apparently did not shield the coins from acetic acid emitted by their wooden storage cabinets (JAIC, 2003).
| What you see | Most likely cause | Confirming check before disposition |
|---|---|---|
| Even dulling from yellow to gold or light brown on every face; polishes off cleanly | Normal oxidation tarnish from time in air | Wipe with a mild brass cleaner; smooth bright metal underneath; compare age of lot with storage conditions |
| Dark brown to black tarnish, worst where parts touched a band, gasket, bag liner or divider | Sulfur gas from rubber, wool felt, adhesives or packaging; sometimes active-sulfur cutting oil | Pattern matches contact points; lead acetate paper test on the packaging material; ask which cutting fluid was used |
| Dark prints or smudges that later go green | Fingerprints: sweat salts and skin oils | Ridged print pattern visible under a magnifier; bare-hand handling after final wash |
| Water spots, rings or blotches, often on the bottom layer of a tray or in blind holes | Wash water, water-based coolant or condensation dried on the part | Spots collect where water pools; check whether parts were dried before bagging; look for moisture in the pack |
| Green or blue-green powder or crust on parts from a closed wooden crate or long storage | Acid vapor from wood or boards, or chloride contamination, with humidity | Crate or box material; green crust heaviest on faces nearest the wood; humidity history of the shipment |
| Pink, copper-red or spongy patches on parts that have been in water service | Dezincification of a brass not resistant to it | Section a sample and measure depth of dezincification; compare the alloy with the service water |
| Fine cracks at threads, crimps, bends or press-fit bores | Stress corrosion cracking from residual or assembly stress plus ammonia | Magnified inspection or dye penetrant; ammonia sources in service or cleaning; residual stress test on samples from the same lot |
How do you diagnose the cause at receiving inspection before cleaning or rejecting the lot?
Quarantine and photograph first, then map where the discoloration sits, wipe and magnify, and inspect the packaging itself. Only the green-crust, pink and cracking cases need a lab; the rest can be dispositioned at the receiving bench.
- Hold the lot as received. Photograph parts in the open pack, with the bag, liner, desiccant and dividers in view, before anyone handles them. The pack is evidence; once it is thrown away the sulfur and moisture causes can no longer be proved.
- Map the pattern. Even tarnish on all faces points to age and storage. Marks that follow contact points point to packaging. Prints point to handling. Spots in low points or blind holes point to trapped water.
- Check the pack for moisture. Note whether the desiccant is saturated or its humidity indicator card has changed color, whether there is visible condensation inside the bag, and whether the green or spotted parts sit at the bottom of the pack, where water collects.
- Wipe and magnify. Clean a few marks with a mild nonabrasive brass cleaner and look at the metal with a magnifier. Smooth metal underneath means the problem is cosmetic. Etched spots, pits or a rough copper-colored surface mean real attack.
- Test the packaging. Rubber bands, foam, felt and some liners can be checked with the lead acetate paper test, which turns the paper dark brown or black when hydrogen sulfide is released, as CCI Note 17/5 describes. Check whether wood touched the parts or shared a closed space with them.
- Ask the supplier three questions. Which cutting fluid was used, how the parts were washed and dried, and how long they were stored before packing. The answers usually confirm the mapping.
- Identify green deposits if the source is unclear. A lab can say whether a green product is a chloride, carbonate or acetate. Chloride points to sweat or salt, acetate to acid vapor from wood or board, and carbonate to ordinary damp exposure, which tells you whether to change handling, crating or drying.
- Send only the material cases to a lab. For pink or porous areas, section a sample and measure the depth of dezincification using the method of ISO 6509-1:2014. For cracks, confirm the crack under magnification, and if residual stress is suspected test sample parts from the same lot per ASTM B858, the ammonia vapor test for susceptibility to stress corrosion cracking in copper alloys.
The ASTM B858 test is an accelerated check for residual stress in wrought copper alloy products made from alloys known to crack in ammonia vapor; its scope says it is not intended for assemblies under applied stress. Use it on parts from the lot, not on a finished assembly, and keep the photos and the spot map so the supplier can find the step that failed.
How do you fix tarnished brass parts, and which finish or coating prevents it?
Cosmetic tarnish and stains are fixed by cleaning, drying and repacking; the long-term prevention is a finish matched to the part’s function, from a benzotriazole dip or wax for bare functional parts to clear lacquer for cosmetic faces and nickel plating for parts that must stay bright in service. Dezincified or cracked parts cannot be cleaned into conformance and need a material or design change.
- Normal tarnish on non-cosmetic parts. Accept it if the drawing does not control appearance. If it does, clean with a mild cleaner, rinse, dry fully and repack. Many metal polishes contain ammonia, which can dissolve copper under some conditions, and some leave an inhibitor film that makes later treatment difficult, as CCI Note 9/3 warns, so specify the cleaner on any rework order for parts that will be plated or soldered.
- Sulfur tarnish and fingerprints. Clean, then remove the source: replace rubber bands and wool felt with polyethylene, and require gloves after the final wash. Latex gloves are a sulfur source (CCI Note 17/5), so specify clean cotton or non-latex plastic gloves.
- Benzotriazole (BTA) dip. Benzotriazole is a specific corrosion inhibitor for copper and copper alloys and is widely used in industry to reduce their corrosion in both atmospheric and immersed conditions (Walker, Anti-Corrosion Methods and Materials, 1970). It suits functional parts such as fittings and threads that must stay bare. BTA is a chemical with handling hazards, so the finisher applies it, not the receiving dock.
- Microcrystalline wax. A thin wax film is a removable barrier that conservators use on brass. It wears off with handling, so it suits storage and display faces more than working surfaces.
- Clear lacquer. An acrylic lacquer, ideally one with an inhibitor such as BTA, keeps a cosmetic face bright, but CCI Note 9/3 notes that lacquer always alters the appearance, CCI Note 9/4 that a poorly applied coat can lead to corrosion later, and wherever it is scratched or thin the brass tarnishes locally, often more visibly than an even film would. Keep lacquer off threads, electrical contacts, sealing faces and press fits.
- Nickel or other plating. For parts that must stay bright in service, or carry wear, plating replaces the brass surface with a more stable one. Plating needs a clean, oxide-free substrate, so tarnish must be removed before plating.
- Dezincification. Change to a dezincification-resistant alloy suited to the service water, and specify the resistance test. A surface treatment is not a reliable fix.
- Stress corrosion cracking. Reject cracked parts, remove ammonia from cleaning and service where possible, and ask for stress relief of the brass after heavy cold work, verified on samples by ASTM B858.
Packing does as much as the finish. How fast copper corrodes depends on the atmosphere around it, and the international classification of atmospheric corrosivity, ISO 9223, rates environments by the first-year corrosion of standard copper, zinc, steel and aluminum specimens. Its typical indoor examples run from heated, dry offices up to unventilated sheds in subtropical and tropical zones, a useful reference point for a sealed carton in a hot warehouse or a container at sea, although the standard does not classify packaging.
| Category | Copper corrosion rate, first year (µm per year) | Typical indoor environment (ISO examples) |
|---|---|---|
| C1 Very low | up to 0.1 | Heated spaces with low relative humidity and insignificant pollution, such as offices |
| C2 Low | 0.1 to 0.6 | Unheated spaces with varying temperature and humidity, low condensation, such as storage |
| C3 Medium | 0.6 to 1.3 | Moderate condensation and moderate pollution from production, such as food-processing plants |
| C4 High | 1.3 to 2.8 | High condensation and high pollution from production, such as industrial processing plants |
| C5 Very high | 2.8 to 5.6 | Very high condensation or pollution, such as unventilated sheds in subtropical and tropical zones |
| CX Extreme | 5.6 to 10 | Almost permanent condensation or extreme humidity, with outdoor pollutants such as airborne chlorides |
What should the drawing, RFQ and purchase order say about finish and packing for brass parts?
State the alloy and specification, the finish or the words “bare, no coating,” the cosmetic acceptance for tarnish, and a packing note that keeps moisture, sulfur, acid vapor and bare hands away from the parts. Without those lines, a tarnished lot is not a nonconformance and the supplier has nothing to fix.
- Material. For example, “MATERIAL: C36000 FREE-CUTTING BRASS PER ASTM B16/B16M.” For potable water parts or water service, name the lead-free or dezincification-resistant alloy and the test instead of C360.
- Finish. Name it exactly: “BARE, CLEAN AND DRY, NO COATING,” “BENZOTRIAZOLE TARNISH INHIBITOR DIP,” “CLEAR ACRYLIC LACQUER ON SURFACES MARKED ‘C’ ONLY, MASK THREADS,” or a nickel plating specification. “Protect from tarnish” on its own is not a finish.
- Cosmetic acceptance. For example, “UNIFORM LIGHT TARNISH ACCEPTABLE ON NON-COSMETIC SURFACES. NO GREEN CORROSION, NO FINGERPRINTS, NO WATER SPOTS, NO PINK OR COPPER-COLORED AREAS ON ANY SURFACE.” Mark which faces are cosmetic.
- Process notes. “CUTTING FLUID SHALL BE NON-STAINING TO COPPER ALLOYS. WASH AND DRY COMPLETELY BEFORE PACKING. HANDLE WITH CLEAN COTTON OR NON-LATEX PLASTIC GLOVES AFTER FINAL WASH.”
- Packing. “PACK DRY IN SEALED POLYETHYLENE BAGS WITH DESICCANT. NO RUBBER BANDS, WOOL FELT OR LATEX. NO DIRECT CONTACT WITH WOOD; LINE ANY WOODEN CRATE WITH A BARRIER FILM.” If a volatile corrosion inhibitor (VCI) is used, require one rated for copper alloys; VCI products are formulated for particular metals, and not every formulation suits copper alloys.
- Stress and service. If the part is crimped, heavily cold worked or will see ammonia, ask for stress relief and an ASTM B858 test on samples, and write the service environment on the RFQ.
For brass touching steel or aluminum in a wet assembly, also check the galvanic couple; the galvanic corrosion guide explains how to read the chart.
Who does what on a Saola order for brass parts?
Brass parts are machined by CNC turning and CNC milling at our lead CNC plant in Vietnam. Plating and other finishing are handled by partner plants we manage, and the quote names the production site and the country of origin.
Electroplating is on the menu listed on the surface finishing page; name the plating, for example nickel, and its specification on the drawing, and the quote confirms whether it can be routed to a partner plant and where. Clear lacquer, a benzotriazole dip and wax are not on that menu, so a drawing that calls for one should flag it in the RFQ; the quote then states whether the step can be routed to a partner plant and where, or that it is excluded and left to the buyer.
The packing note, the glove and drying requirements and the cosmetic acceptance written on the purchase order are confirmed in the quote, and material certification for the brass bar can be requested. Send the drawing with the alloy, finish and packing notes as you intend them through the quote form. The receiving inspection guide covers sampling the lot when it arrives.
Frequently asked
Does tarnish weaken brass parts or change their dimensions?
Ordinary tarnish is a very thin surface film and does not weaken a brass part or change its fit in any practical sense. It matters for appearance, electrical contact resistance, soldering and plating. Green corrosion products, pink dezincified areas and cracks are different and should be investigated.
What is the green stuff on brass?
It is a copper corrosion product, typically a basic copper carbonate, chloride, sulfate or acetate depending on what reached the surface. On new parts it usually means moisture combined with sweat salts, acid vapor from wood or residues from cleaning, so look for the wet source before blaming the alloy.
Is clear lacquer or nickel plating better for keeping brass bright?
Lacquer keeps the brass color and costs less, but it scratches and tarnish starts at every break, so it suits cosmetic faces with light handling. Nickel plating changes the color to nickel but holds up to handling and wear. For bare functional parts, a benzotriazole dip and good packing are often enough.
Can tarnished brass parts be accepted after cleaning?
Usually yes, if the metal under the tarnish is smooth and the drawing allows rework. Clean with a cleaner that will not affect later plating or soldering, dry fully and repack to the corrected packing note. Parts with pitting, dezincification or cracks should not be accepted on cleaning alone.
Why do some brass parts crack in storage or service?
Brass is susceptible to stress corrosion cracking when residual stress from cold work or assembly stress meets ammonia, which can come from cleaners, some polishes, fertilizers or animal housing. ASTM B858 tests wrought copper alloy products for that susceptibility, and stress relief after heavy cold work reduces it.
Sources
- ISO 9223:2012, Corrosion of metals and alloys — Corrosivity of atmospheres — Classification, determination and estimation, International Organization for Standardization, 2012
- ASTM B858-06(2024), Standard Test Method for Ammonia Vapor Test for Determining Susceptibility to Stress Corrosion Cracking in Copper Alloys, ASTM International, 2024
- ISO 6509-1:2014, Corrosion of metals and alloys — Determination of dezincification resistance of copper alloys with zinc — Part 1: Test method, International Organization for Standardization, 2014
- ASTM B16/B16M-24, Standard Specification for Free-Cutting Brass Rod, Bar and Shapes for Use in Screw Machines, ASTM International, 2024
- The Cleaning, Polishing and Protective Waxing of Brass and Copper, CCI Notes 9/3, Canadian Conservation Institute, Government of Canada, revised 2007
- How to Test for Sulfur in Materials Using Lead Acetate Test Paper, CCI Notes 17/5, Canadian Conservation Institute, Government of Canada, 2017
- Basic Care of Coins, Medals and Medallic Art, CCI Notes 9/4, Canadian Conservation Institute, Government of Canada, revised 2007
- R. Walker, The use of benzotriazole as a corrosion inhibitor for copper, Anti-Corrosion Methods and Materials, vol. 17, no. 9, pp. 9–15, 1970
- A. B. Paterakis, The Influence of Conservation Treatments and Environmental Storage Factors on Corrosion of Copper Alloys in the Ancient Athenian Agora, Journal of the American Institute for Conservation 42(2), pp. 313–339, 2003