Degreasing brass parts before coating

A brass cover that blisters along its fold after 240 hours of neutral salt spray was not coated badly. It was cleaned for steel. Brass strip arrives with 0.5–2 g/m² of drawing lubricant, buffed parts carry abrasive wax in the grain, and laser-cut brass carries an oxide that no alkaline bath will touch. Degreasing brass parts is the whole of the adhesion story, because the one thing that makes powder stick to steel — a phosphate crystal layer — does not form on brass at all.

The arithmetic is unforgiving. One gram per square metre of oil is a film about 1 µm thick. A 70 µm powder coat is seventy times thicker, so the film is a small fraction of the section and still lifts the entire coating, because the failure happens at the interface rather than through the paint. On mild steel the conversion stage buries a thin oil film in crystal growth and the part survives. On brass there is no conversion stage, so whatever is left on the surface when the part reaches the booth is what the powder bonds to.

What follows is the cleaning sequence brass actually needs, the alkaline condition that damages it, the reason it has to be kept away from a steel phosphating bath, and the checks that tell you a batch is ready to coat.

Why does powder lift off brass when the same powder holds on steel?

Because steel is given a surface to bond to and brass is not. Zinc or iron phosphating grows a crystalline layer 1–3 g/m² in weight that keys mechanically into the powder and also passivates the steel underneath. Brass — a copper-zinc alloy, typically 63/37 for sheet and 60/40 for free-machining rod — does not phosphating under normal line conditions. Copper in the alloy inhibits the reaction, and the zinc that does dissolve contaminates the bath instead of coating the part.

That leaves a 5–20 nm oxide on the brass as the bonding surface. It is thin enough to be useful if it is uniform and clean, and thin enough to be destroyed by one fingerprint. Powder flows onto it, cures, and holds by a combination of mechanical key into a lightly etched surface and secondary bonding. Every part of the process exists to keep that oxide even and oil-free.

The consequence for the drawing is simple: the surface condition is a specified requirement, not an assumption. A note that says "degrease before powder" tells a coater nothing, because degreasing is the operation that fails most often and the one nobody measures.

The degreasing brass parts sequence, stage by stage

The table below is the condition we work to for brass parts that will be powder coated, whether they were pressed, laser cut or machined. Values are for a racked batch on an immersion line; a spray line needs more dwell and more mechanical action to reach the same result.

Two rows in that table cause most of the trouble. The deoxidise stage is the one coaters skip when the part looks clean, and it is the only stage that removes the oxide left by laser cutting or by a hot press. The final rinse is the one that gets topped up from a hose; at 300 µS/cm the dissolved salts dry onto the part and every one of them becomes a blister nucleus.

What strong alkali does to a brass surface

Brass is not uniformly corrosion resistant in alkaline solution. The zinc phase dissolves preferentially at high pH and at temperature, leaving a copper-enriched layer on the surface. It looks like colour change — pink, salmon or orange — and it feels slightly rough under a glove. On a decorative part it is a reject on appearance alone. On a functional part it is worse, because the smut is porous and weakly bonded.

The safe window is a cleaner at pH 9.5–11 run at 55–60 °C with 3–5 minutes of dwell. That is enough to saponify drawing oil and emulsify buffing wax without stripping zinc. Two other conditions matter as much as pH:

• Silicate. Silicated cleaners leave a glassy film on non-ferrous metals that is hard to rinse off and stops the powder wetting the surface. Use a non-silicated grade.
• Ammonia and ammonium salts. Brass is attacked quickly by ammoniacal cleaners, and the attack shows as fine cracking along grain boundaries on stressed parts. Ammonia has no place near a brass cleaning bath.

Part geometry matters too. A pressed brass housing with a 4 mm deep hem holds cleaner in the fold and then holds rinse water in it. The part comes out of dry-off with a wet fold, the powder bridges the gap, and the trapped moisture blows a blister at cure. Racking the part fold-down so it drains is a two-minute change that removes a whole failure mode.

The four soils and how each one is removed

Brass arrives dirty in four different ways, and each one needs a different stage.

Drawing and stamping lubricant. A mineral or ester-based oil applied at 0.5–2 g/m², heavier on the blank edges where the lubricant is pushed during blanking. Alkaline degrease removes it, provided the bath is not already saturated. Bath life is measured in square metres of part per litre of bath, not in days, and a bath that has processed 20 g/m² of oil into a 1,000-litre tank is carrying 20 kg of oil in emulsion.

Buffing and polishing compound. A wax or tallow binder carrying abrasive grit, typically 5–20 µm thick and packed into surface scratches. Alkaline cleaning softens the binder but does not lift the grit, so a buffed part needs mechanical action or an ultrasonic stage followed by deoxidise. A part that has been buffed and then only degreased will show a faint grey haze under the powder at 30× magnification.

Laser oxide and dross. Laser cutting brass produces a dark oxide on the cut edge and, on thicker sections, a bead of dross. Oxide along a cut edge is where coating fails first, because it is a different chemistry from the face and the powder thins over it. Dross has to be removed mechanically; no bath dissolves it.

Handling soils. Cutting fluid from a secondary operation, adhesive from a protective peel film, and skin oil from inspection. Peel-film adhesive is the worst of the three: it is a pressure-sensitive acrylic that resists alkaline cleaning and needs a solvent wipe or a stronger deoxidise.

Why brass has to be kept out of the steel phosphating bath

A seven-tank pretreatment line is built around a phosphating stage, and that stage is a zinc bath that steel parts run through for 5–15 minutes. Put brass in it and the acid dissolves copper and zinc from the surface. Copper accumulates in the bath, and above roughly 20 ppm it starts to plate out on steel as a loose, dark smut that ruins adhesion on parts that had nothing to do with the brass.

The practical consequence is a scheduling rule rather than a chemistry problem. Brass is processed as its own batch, at the end of a shift, so the bath can be checked before steel goes back in. Brass does not need the phosphate stage, and it should not have it: the acid attack does more harm to the brass than the missing coating does.

That also means a coater who runs one line for everything will either refuse the brass or run it badly. Ask the question before the quote, not after the first reject.

Coating thickness, cure and the heat the brass sees

Brass conducts heat roughly four times faster than mild steel, so a rack of brass parts reaches oven temperature faster than a rack of steel and can also lose heat faster on the way to the booth. Both effects change the cure.

The powder needs to reach 180–200 °C at the metal for 10–15 minutes. Below 180 °C the film is under-cured: it passes a thumbnail test, then loses adhesion after a week in service because the resin never cross-linked. Above 200 °C on thin brass, the part can distort and the film can yellow.

Measure the cure with a thermocouple on a sacrificial part rather than the oven display, and record it per batch. Oven air temperature and part temperature differ by 20–30 °C during the ramp on a dense rack.

Film thickness on brass is best held at 60–80 µm. Thinner than 60 µm loses corrosion resistance at the edges, where the film is naturally thinnest. Thicker than 100 µm builds stress in the film and chips more readily on assembly, particularly on a part that flexes or takes a clip.

What Ganesh runs for brass and coated parts

Brass parts reach the booth through the same plant as the steel work: laser cut on the 12 kW fibre machine with its 40 ft × 16 ft bed or pressed on the 20–300 T presses, formed on the 200 T 5-axis brake, then finished on the seven-tank pretreatment line ahead of the 18 ft × 14 ft electrostatic powder booth, with a separate wet paint booth and a curing oven. Dry film thickness is checked with a gauge and adhesion by cross-hatch in house, and the route is described under the paint shop capability.

Brass forming and blanking is covered by our copper and brass fabrication capability, which is where the alloy and temper questions belong. For the material side of a brass design, the copper and brass pressed components article sets out die clearance and work hardening, and degreasing before powder coating covers the steel sequence this article deliberately departs from.

Cost is driven by rack density and by the number of stages, not by the metal. The powder coating cost estimator will show how much of a quoted price is pretreatment once you enter the part area and the batch size, and the wider set of coating articles sits in the surface treatment and coating hub.

What to put on the brass part drawing

1. Alloy and temper, for example CuZn37 half-hard sheet, so the coater knows what heat and chemistry the part will tolerate.
2. Lead content and any RoHS declaration, because free-machining brasses carry 1–3% lead and the declaration belongs with the drawing rather than in an email.
3. The finish specification: powder chemistry, colour, gloss, DFT in µm and the cure window.
4. Masked areas — threads, earth contact faces, bearing bores and any soldered joint — marked on the drawing, not described in a note.
5. Corrosion acceptance as salt-spray hours to ISO 9227 and the allowed creep from a scribe, typically 2 mm at 240 h.
6. Handling and packing, since a coated brass part that rubs against another in a carton arrives with a scuffed edge and no argument about who caused it.

When powder on brass is the wrong finish

Decorative bright brass should be lacquered, not powder coated. A clear powder over polished brass changes the colour, is 60 µm thick over a mirror finish, and shows every speck of dust under it. We do not polish or lacquer brass, so that work belongs with a finishing specialist.

Parts with soldered joints are also a poor fit for a 200 °C cure. Soft solder starts to creep well below that temperature, and a joint that was sound at goods-in can fail in the oven.

Finally, if the brass part is an electrical contact or an earthing component, the coating has to stop at the contact face. Masking is possible, but a masked edge is never as clean as bare metal, so the drawing needs to show exactly where the mask goes and how much bare area is required.

Send the brass drawing with the alloy, the finish and the salt-spray hours through the coating quote route, and the reply will list the stages the part needs before the price.