Powder coating aluminium parts: finish and adhesion

Aluminium shows no rust, so a part with a failed pretreatment looks perfect at dispatch and blisters around every fastener by the second monsoon. What decides whether powder coating aluminium parts survives is not the colour or the gloss; it is the 20–60 mg/m² conversion coating between the metal and the polymer, and the maximum metal temperature the temper will tolerate.

Three numbers frame the job. A standard polyester powder needs 180–200 °C metal temperature for 10–15 minutes. 6061-T6 was age-hardened at roughly 175 °C and starts to soften above 190 °C. A sharp laser-cut edge holds 40–60% of the face film thickness. Get any of those three wrong and the part passes a visual check and fails salt spray.

The sections below give the pretreatment sequence an aluminium part actually needs, the temper limits that decide your cure schedule, the film and adhesion numbers to write on the drawing, and the five defects that trace back to the alloy rather than to the booth.

Why aluminium needs a different pretreatment from CRCA

Aluminium needs an alkaline clean, a deoxidising etch and a non-ferrous conversion coating; the iron phosphate a CRCA line uses builds no usable conversion layer on it. Running an aluminium panel through a steel sequence produces a patchy grey film that wipes off with a thumb, and the powder then bonds to oxide and rolling lubricant rather than to metal.

Three mechanisms explain most of it.

Rolling oil and magnesium-rich oxide. Hot-rolled and cold-rolled aluminium sheet arrives with 0.5–3 g/m² of rolling lubricant and a magnesium oxide layer that thickens with storage time. An alkaline cleaner at 50–60 °C and pH 9–11 removes the oil and etches 1–3 g/m² of surface, which is deliberate: it takes the oxide with it. The smut left behind is copper, iron and silicon from the alloy, and it has to come off in the deoxidiser, not the rinse.

Rinse quality. Drag-out from an alkaline tank sits in laps, hemmed edges and blind holes, and shows up as a ring of blisters after cure. A final deionised rinse under about 30 µS/cm conductivity removes the dissolved salts; a tap-water rinse at 400–800 µS/cm does not. Drying before powder matters as much. A part that enters the booth with visible moisture produces pinholes along the low edge.

Substrate separation. Aluminium runs to its own schedule on the same line, never mixed into a steel basket. An acidic deoxidiser that is correct for CRCA will etch aluminium aggressively and leave a smut; a chromate bath loaded with aluminium soon loses its free-acid balance. Keep the two schedules apart and never let steel parts sit in an aluminium deoxidiser. The seven-tank pretreatment article sets out what each tank does in order.

Chromate conversion versus zirconium

Chromate conversion, at 0.3–1.0 g/m² coating weight, produces a film that self-heals at scratches and holds scribe creep to about 1 mm after 480 h neutral salt spray. Zirconium and titanium conversion films are an order of magnitude thinner — 20–60 mg/m² against 300–1,000 mg/m² — and do not self-heal; the same test typically shows 2–3 mm of creep unless the edge is radiused and the film is at the top of the range.

The trade is regulatory. Hexavalent chromium is restricted under REACH and excluded by most automotive and electronics declarations, so the industrial default is now chromium-free conversion with better edge geometry carrying the corrosion duty. If a customer specification still permits chromate, ask for it in writing before the first batch, because switching chemistry mid-programme means re-qualifying the salt spray panel.

What 80 µm costs in powder

Film build is bought by weight. At a powder density of about 1.5 g/cm³, an 80 µm coat on 1 m² is 80 cm³ of cured film, or roughly 120 g. At 65% transfer efficiency in an electrostatic booth, the gun has to deliver about 185 g per square metre of coated area, and every square metre of masking is 185 g not spent. That is why the masking schedule on a part like a genset control box with twenty cut-outs can change the coating price by 15–20% without any change in film specification.

Which temper limits your cure temperature

The cure schedule is set by the powder, but the alloy sets the ceiling. Standard polyester and polyester-TGIC powders flow and crosslink at 180–200 °C metal temperature for 10–15 minutes; a 1.5 mm panel reaches that in about 4 minutes in a convection oven and holds it for the remaining time. Thicker sections lag: a 6 mm bracket with heavy bosses can be 20–30 °C cooler than the oven setpoint when the timer starts.

That matters because aluminium tempers are heat treatments.

• 6061-T6 and 6082-T6 are artificially aged near 175 °C. Holding at 200 °C for 15 minutes overages them. Published yield strength of 6061-T6 drops from about 276 MPa toward 240 MPa, roughly 5–13% depending on section and hold time.
• 5052-H32 and 5083-H111 are work-hardened, not aged. Exposure above 150 °C starts to recover the strain hardening; a 200 °C cure can cost around 10% of yield strength.
• 6063-T5 extrusion used for covers, trims and light frames has enough margin that a standard cure is acceptable.
• 2024 and 7075 in the T3/T6 tempers should never see 200 °C if the part carries structural load.

The decision rule is simple. If the part carries a calculated load, specify a low-temperature cure powder at 140–160 °C metal temperature for 20–30 minutes and state the temper you are protecting. If it is a cover, a louvre frame or a decorative extrusion, run the standard 200 °C schedule and take the better chemical resistance that comes with a full cure.

One more arithmetic point for long panels. Aluminium expands at about 23 × 10⁻⁶/K, so a 2,000 mm panel heated from 20 °C to a 200 °C metal temperature grows by 23 × 10⁻⁶ × 2,000 × 180 = 8.3 mm; at the 180 °C of a standard cure it grows 23 × 10⁻⁶ × 2,000 × 160 = 7.4 mm. If the part is bolted into a rigid fixture while it is coated, that movement has nowhere to go and turns into a bow; if it hangs free on the rack, it comes back flat.

The numbers to put on the drawing

The salt spray column needs care. Aluminium performs differently from steel in neutral salt spray: it does not produce red rust, so a coated panel can look intact while the film has lifted from the scribe. Measure creep from the scribe with a blade, in millimetres, and record the worst point rather than the average.

Where powder coating aluminium parts actually fails

Blisters in a ring around holes. Drag-out from alkaline cleaner or deoxidiser trapped in a 6.5 mm hole or a hem. The ring is 2–5 mm across and arrives within one cure cycle. Racking the part so holes drain, adding a 30-second dwell over a drain tray, and checking rinse conductivity prevents it.

Pinholes on extrusion and castings. Outgassing, as described above. Castings with visible porosity are the worst case: gas escapes through the pore, and the film bridges over it and then ruptures during service. Blasting a casting to close the surface does not remove the gas.

Filiform corrosion at scratches and cut edges. This is specific to aluminium. At 80–90% relative humidity with chloride present, a scratch or a poorly coated edge starts filaments 0.1–0.5 mm wide that travel under the film for 10–30 mm. Chromate conversion suppresses it best; with chromium-free conversion the practical controls are edge radius, a full 80 µm face film and a tape test at the edge after 240 h.

Peel at sheared and lasered edges. Burr above 0.1 mm and residual lubricant on the cut face leave a line of poor adhesion. Deburring before pretreatment, or a radius on the laser path, removes the initiation site.

Chalking and colour shift. Over-cure, not under-cure, is the usual cause of a matt finish that reads chalky within a year. A ΔE above 2 and a gloss drop of 10 units at 60° against the master are the usual rejection limits. Check oven profile with a data logger on the part, not the oven display.

How to approve the first production batch

Approve the sample, not the powder. Take five coated parts from the first hour of the run and measure film thickness at nine points per part: four corners, four mid-edges and the centre. The low reading, not the average, is what fails outdoors. Do the cross-cut on a masked-edge zone of a production part, and fall back to a scrap tab from the same batch only where that edge cannot be reached, then run a 240 h salt spray on two panels and read the scribe creep with a blade. Finish with a 24 h water immersion at room temperature: a film that blisters there will blister in a Mumbai July regardless of its salt spray hours.

Masking, threads and electrical bonding

Powder is an insulator, and on aluminium that has consequences beyond appearance. A 60 µm film on an M6 thread reduces the minor diameter by about 0.12 mm and will strip the first time a bolt is run in. Use silicone plugs or caps on all threaded features and state it on the drawing as a masked zone.

Earthing and bonding faces need bare metal. Mask a 25 mm diameter area around every earth stud and specify a maximum 20 µm film on masked faces, because a plug never seals perfectly at its edge. On aluminium, a coated joint is worse than on steel: the oxide re-forms within minutes and the powder adds a 1.8–3 kV dielectric barrier at 60 µm. Use an external-tooth washer under the fastener and keep the masked area at least 1.5 times the washer diameter.

Thermal interfaces matter on heat sinks and heat-spreader parts. Polyester has a thermal conductivity near 0.2 W/m·K, so a 60 µm film adds about 0.0003 m²·K/W. On a 100 cm² pad carrying 20 W that is only 0.6 K, which is acceptable; on a 10 cm² pad it becomes 6 K, and that face should be masked instead.

Aluminium on the Ganesh line, tank by tank

Aluminium parts are cut on the 12 kW fibre laser with its 40 ft × 16 ft bed, or on the 6 kW and 3 kW support machines for thinner gauge, and formed on the 200 T 5-axis press brake with a 4,450 mm bed, which handles aluminium up to 10 mm. Coating runs through the seven-tank pretreatment line, the 18 ft × 14 ft electrostatic powder booth, the wet paint booth and the curing oven, with film thickness and cross-hatch adhesion checked in-house. The plants hold ISO 9001:2015 and ISO 14001:2015, and coated aluminium assemblies for genset, elevator and material-handling programmes go through that line on the aluminium schedule.

We do not anodise or hard-coat, and we will not claim chromate-level salt spray performance for a chromium-free conversion that has not been tested on the part. If the application needs anodic protection, say so at quotation and the part goes to an anodiser.

What to put on the drawing

1. Alloy and temper, for example 6061-T6, with the thickness and the grain direction where bending matters.
2. Finish system: powder chemistry (polyester, polyester-TGIC, polyurethane), colour reference, gloss target in GU at 60°, and face film range in µm.
3. Maximum cure metal temperature, if the temper cannot take 200 °C.
4. Pretreatment class and the conversion coating type, plus the chromium declaration.
5. Masked zones: threads, earth studs, mating faces, thermal pads, with the maximum film allowed.
6. Adhesion and corrosion acceptance: cross-cut grade, impact, and salt spray hours with a creep limit measured from the scribe.
7. Edge condition: radius, deburr or break, because the edge is where the film is thinnest.

If you are still choosing between powder and a wet system, the coating cost estimator turns film build, part area and masking into a per-part comparison. The surface treatment hub collects the pretreatment and failure-diagnosis articles, and degreasing before powder coating covers the tank chemistry that precedes everything above.

When powder coating is the wrong route

Three cases where we will steer a customer elsewhere.

Abrasion or electrical insulation requirements. Anodising builds a 5–25 µm ceramic oxide, and hard anodising 25–50 µm, both harder than any powder and electrically insulating. We do not anodise, and a powder coat specified in place of hard anodising on a wear face will polish through in weeks.

Parts larger than the booth. The powder booth is 18 ft × 14 ft, about 5.5 m × 4.3 m. A long extrusion or a single-piece canopy roof beyond that needs a different line, not a longer rack.

Class A appearance on thin, large panels. Below 0.8 mm, a 1,200 mm aluminium panel with a 200 °C cure will oil-can and show a visible wave. Wet paint at a lower bake, or a heavier gauge, solves it; a thicker powder film hides nothing and makes the orange peel worse.

For everything else, the pretreatment route and the numbers in the table are the whole specification. Send the drawing through the paint shop capability page or straight to request a quote with the alloy, temper, finish and annual quantity, and the route can be reviewed before any tooling or sample is discussed.