Guide
What pretreatment and prebake does powder coating zinc die castings need?
Powder coating zinc die castings is the process of cleaning a Zamak casting, giving it a conversion coating that suits zinc, spraying it with an electrostatically charged dry powder and fusing that powder in an oven. The coating itself is the same one used on steel and aluminum. What changes is the substrate: zinc reacts with the wrong cleaners, holds gas under its skin, and keeps changing metallurgically every time it goes through an oven.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical engineers and sourcing managers who buy powder-coated zinc die castings, such as housings, knobs, handles and brackets, and want the purchase order to control the steps that decide whether the film stays on. It covers pretreatment, prebake, the two kinds of blister, the test methods worth naming, and what to ask a supplier in Vietnam or anywhere else before the first lot.
A note on sources. The alloy is covered by ASTM B86, which lists Zamak 3 as Alloy 3, ASTM AG40A, UNS Z33525. No ASTM practice we could find is written specifically for powder coating zinc die castings, so this page leans on the test methods that apply to any organic coating on metal and says plainly where a practice was written for another zinc substrate. Where a figure appears it comes from the linked source, and cure temperatures are left to the powder data sheet on purpose.
On this page
Why is zinc harder to powder coat than steel or aluminum?
Zinc is chemically reactive in both acid and strong alkali, so a cleaning and phosphating line set up for steel can etch it or leave a weak, powdery conversion layer. Die-cast zinc also carries gas and trapped fluids below a dense skin, and its properties shift with temperature far below any cure oven.
Each of those three traits maps to a failure you can see. Aggressive cleaning leaves a smutted or etched surface, and the powder sticks to the smut instead of the metal, so the tape test pulls it off in flakes. Gas and pretreatment fluid in surface-connected pores push through the melting powder and leave pinholes or blisters. And the heat of the oven accelerates the natural aging of the alloy, which moves dimensions and slightly softens the casting.
The last point is easy to miss because it is invisible on the paint line. The North American Die Casting Association notes that rapid solidification locks zinc alloys into a crystal structure that is not stable at room temperature, that the alloys shrink slightly as they age, and that the mechanical properties of zinc alloys vary significantly with relatively minor changes in temperature (NADCA, zinc alloys). A powder cure is therefore also an uncontrolled heat treatment unless someone writes down how hot the metal got and for how long.
Which pretreatment works on Zamak 3 before powder coating?
Use a mild, inhibited cleaner followed by a conversion coating chosen for zinc, usually zinc phosphate, a trivalent chromium conversion coating or a zirconium-based treatment. Ask the finisher to name the chemistry on the quote rather than accepting “standard pretreatment”.
The pretreatment has three jobs on a zinc casting: remove die lubricant, release agent and machining fluid; leave the surface free of loose corrosion product; and give the powder a thin, tightly bonded layer to key into. Iron phosphate baths, the default on many steel lines, tend to produce little useful conversion on zinc, which is why a line that coats mostly steel may need a separate chemistry for zinc parts. Strongly alkaline or acid cleaners that suit steel can attack zinc, so the cleaner should be one its maker rates for zinc and die-cast alloys.
Legacy yellow chromate gave good paint adhesion on zinc, but it is a hexavalent chromium process. Hexavalent chromium is one of the ten substances restricted in electrical and electronic equipment sold in the EU under the RoHS Directive (European Commission, RoHS Directive), and in US workplaces it carries an OSHA permissible exposure limit of 5 µg/m³ as an 8-hour average (29 CFR 1910.1026). For a part going into an assembly with any electrical content, specify a trivalent or chromium-free conversion coating on the drawing and ask for a written statement that the finish contains no hexavalent chromium.
Two practical checks catch most pretreatment problems before powder goes on. A clean zinc surface holds an unbroken film of rinse water, while a surface with residual die lube lets it bead. And the final rinse and dry-off matter as much as the chemistry: rinse water left in pores or crevices is both a gas source in the oven and a source of soluble salt under the film. ASTM D7803 and D6386 describe surface preparation of zinc for powder and liquid paint, but both are written for hot-dip galvanized steel, not die castings; they are useful background for a conversation with the finisher, not a callout for a Zamak drawing.
Should zinc die castings be prebaked, and how hot?
Yes, a degassing prebake after pretreatment and before powder is the usual first defense against pinholes, and it should reach at least the metal temperature the cure will reach. The ceiling comes from the powder data sheet and from what the zinc part can tolerate, so write both prebake and cure as metal temperature and time on the purchase order.
The logic of a prebake is simple. Whatever gas and moisture will come out at cure temperature is driven out while the part is bare, so the powder later passes through an oven the casting has already survived. The trap on zinc is running the prebake much hotter than the cure “to be safe”. Every extra degree and minute adds to the aging the casting has already done, and on thin-walled parts it can distort features that were in tolerance before coating.
NADCA gives a useful sense of scale. Its published schedule for deliberately accelerating the aging of zinc die castings runs at temperatures well below a powder cure, which tells you that a cure oven completes much of the alloy’s aging in one pass. Of the total aging shrinkage NADCA quotes, most occurs in the first weeks at room temperature (NADCA, zinc alloys).
| Item | Value published by NADCA |
|---|---|
| Stabilizing option 1 | 3 hours at 212 °F (100 °C) |
| Stabilizing option 2 | 5 hours at 185 °F (85 °C) |
| Stabilizing option 3 | 10 hours at 158 °F (70 °C) |
| Total aging shrinkage | 0.0007 in./in. |
| Share occurring at room temperature within five weeks | 0.0005 in./in. |
| Time for ductility and tensile changes at room temperature | Approximately 10 years |
Three drawing and purchase-order habits follow from the table. First, if a feature is tight enough that aging shrinkage matters, inspect it after coating, not before, because the oven will have moved it. Second, prefer a powder whose cure window is at the low end of what the finisher offers, and ask for the product data sheet so the cure schedule is written down rather than remembered. Third, ask for an oven profile, a data-logger trace with a probe on the thickest section of a real part, at first article and whenever the rack loading or the oven changes. Air temperature on the oven controller is not metal temperature, and on a heavy zinc casting the two can differ for most of the cycle.
Zinc alloys also creep under sustained load, a point NADCA makes in its alloy comparison (NADCA, alloy selection). A powder film under a bolt head adds a second soft layer to the joint, so mask bolt-bearing faces and ground contact points and show the masking on the drawing.
What causes blisters on powder-coated zinc castings?
Blisters that appear in the oven come from gas escaping through the melting powder, while blisters that appear weeks later in humid service usually come from contamination or corrosion under the film. The timing tells you which side of the process to fix.
Oven blisters and pinholes repeat in the same place on every part, cluster over thick sections, and are worst on faces that were machined before coating, because the cutter removes the dense as-cast skin and opens porosity. Fixes sit upstream: gate and overflow placement that keeps porosity away from cosmetic faces, less machining on coated faces, a proper prebake, and a powder formulated to stay open a little longer so gas can escape. Where porosity is surface-connected, sealing by impregnation is sometimes used, but the sealant has to survive the pretreatment and the cure, so both suppliers must agree to it.
Late blisters are a different failure. Zinc corrosion products take up more volume than the metal they came from, and soluble salts left by a poor final rinse draw moisture through the film. Either one lifts the coating from below, often starting at edges, cut marks or thin film on sharp corners. These blisters do not show on a fresh part, which is why a humidity or salt-spray exposure on production parts, rated for blistering afterwards, belongs in the purchase order. ASTM D714 rates blister size and frequency against photographic references and leaves the permissible degree to be agreed between purchaser and seller.
Which tests should a purchase order name for powder-coated zinc castings?
Name a method, a sampling plan and a pass level for film thickness, adhesion, humidity or salt-spray resistance and blistering, and require the adhesion test on real production parts. The standards give the method and the rating scale; the pass level is yours to set.
The table lists the methods most useful on a zinc casting. The notes come from each standard’s published scope, so they say what the method covers, not what result a part should achieve.
| Item | Method | What the scope says | What to write on the PO |
|---|---|---|---|
| Alloy | ASTM B86 | Covers zinc and ZA castings; Alloy 3 is Zamak 3, AG40A, UNS Z33525 | “Zamak 3 per ASTM B86” and a chemistry report per heat or lot |
| Film thickness | ASTM D7091 | Eddy-current gauges for nonconductive coatings on nonferrous metals | Minimum and maximum film, number and location of readings |
| Adhesion | ASTM D3359, Method B | Method B is for coatings under 125 µm; the test shows whether adhesion is adequate, not how high it is | Minimum rating on production parts from each lot, not only on test panels |
| Salt spray | ASTM B117 | Defines the apparatus and conditions; does not set exposure time or pass criteria | Exposure hours, scribe or no scribe, and how the result is rated |
| Humidity | ASTM D2247 | Water resistance of coatings at 100 % relative humidity with condensation on all surfaces | Exposure time and the blister rating allowed afterwards |
| Blistering | ASTM D714 | Photographic references; permissible degree agreed by purchaser and seller | Permissible blister size and frequency after exposure |
| Bare conversion coat | ASTM B201 | Protective value of chromate coatings on zinc, usually by salt spray and abrasion resistance | Only if some faces stay unpainted with a chromate-type finish |
Two details make these tests mean something on zinc. Adhesion on a flat test panel says little about a casting with a machined face, a thick boss and a sharp edge, so test the part. And a dimensional check after coating belongs on the same inspection plan, using the tolerance basis already on the drawing; the ISO 2768 and NADCA tolerance guide explains how to state that basis for a cast part.
What should you ask a supplier in Vietnam before ordering coated zinc castings?
Ask where each step happens, which chemistry and powder are used, how the oven is verified, and which tests are run on production parts. Put the answers in the purchase order so the first lot and the tenth lot are measured the same way.
Many buyers moving zinc parts out of China into Southeast Asia focus on the casting and treat the finish as a detail. The finish is where a new supply chain most often slips, because the casting, the machining and the coating may sit in three different companies with three different ideas of “clean”. A short list of questions settles most of it:
- Are casting, machining and powder coating done at one site, or is the coating sent out? If it is sent out, who owns the result when the film fails the tape test?
- What is the cleaner and conversion coating by trade name, and is it free of hexavalent chromium?
- Which powder is used, and can you have its product data sheet with the cure window?
- Is there a prebake, and what metal temperature and time does it reach, shown by an oven profile on a real part?
- Which faces are masked, and how is masking shown to the inspector?
- Which of the tests in the table are run per lot, on how many parts, and what goes into the lot report?
At Saola, Zamak 3 die castings come from partner foundries and surface finishing is done by partner plants that Saola manages; powder coating is one of the finishes listed on the surface finishing page, and the quote names where each step is done. The die casting page covers the two alloys offered. Send a drawing through the quote form with the finish callout and any test requirements, and the answers to the questions above come back as part of the quote review.
Frequently asked
Can zinc die castings be powder coated without a conversion coating?
They can be coated, but adhesion and underfilm corrosion resistance depend heavily on the conversion layer. For anything beyond an indoor decorative part, specify one that is chosen for zinc.
Is the powder coat cure a problem for Zamak 3?
It is a heat treatment the part will feel. NADCA’s own stabilizing schedule runs well below a normal cure, so expect aging shrinkage to complete in the oven and inspect tight features after coating.
Why do pinholes appear only on the machined face?
Machining removes the dense as-cast skin and opens porosity below it. Keep cosmetic coated faces as-cast where possible, or agree a prebake and a powder suited to degassing.
Should I call out powder coat or plating on a zinc part?
Plating suits bright decorative and conductive finishes, powder suits color, texture and a thicker barrier. The finish comparison guide covers the trade-offs.
What belongs on the drawing?
Alloy per ASTM B86, finish type and color, conversion coating free of hexavalent chromium, masked faces, cosmetic faces, and the tests from the table with pass levels. The RFQ preparation guide has the full package list.
Sources
- ASTM B86-23, Standard Specification for Zinc and Zinc-Aluminum (ZA) Alloy Foundry and Die Castings, ASTM International, 2023 (scope)
- Zinc Alloys, NADCA Design, North American Die Casting Association, accessed September 2026
- Alloy Selection, NADCA Design, accessed September 2026
- ASTM D3359-23, Standard Test Methods for Rating Adhesion by Tape Test, ASTM International, 2023 (scope)
- ASTM D7091-22, Nondestructive Measurement of Dry Film Thickness, ASTM International, 2022 (scope)
- ASTM B117-19, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2019 (scope)
- ASTM D2247-15, Testing Water Resistance of Coatings in 100 % Relative Humidity, ASTM International, 2015 (scope)
- ASTM D714-02(2017), Evaluating Degree of Blistering of Paints, ASTM International, 2017 (scope)
- ASTM B201-80(2019), Testing Chromate Coatings on Zinc and Cadmium Surfaces, ASTM International, 2019 (scope)
- ASTM D7803-19, Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Surfaces for Powder Coating, ASTM International, 2019 (scope)
- ASTM D6386-22, Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Surfaces for Painting, ASTM International, 2022 (scope)
- RoHS Directive, European Commission, accessed September 2026
- 29 CFR 1910.1026, Chromium (VI), eCFR, accessed September 2026