Anodising, chromate conversion, black coating, bead blasting and passivation compared for heat sinks: thermal impact, corrosion performance, cost and lead time.
Heat Sink Surface Finish Options and Cost Impact
Direct answer: For aluminium heat sinks, clear or black anodising is the default choice, adding roughly 5-15% to part cost. Black anodising gives a small radiation benefit that matters in natural convection and almost nothing in forced air. Chromate conversion coating is cheaper but less durable. Bare milled aluminium has the best interface performance but no corrosion protection. Powder coating should generally be avoided on a heat sink because it is a thermal insulator.
Finish options compared
| Finish | Cost impact | Added lead time | Thermal effect | Best for |
|---|---|---|---|---|
| Bare / milled only | Baseline | 0 | Best contact performance; emissivity about 0.05 | Grease interfaces, potted assemblies, cost-critical parts |
| Clear anodising, 10 micron | +5-10% | 3-5 days | Emissivity about 0.8; negligible conduction change | General electronics, indoor use, most heat sinks |
| Black anodising, 15-25 micron | +8-15% | 4-7 days | Emissivity about 0.85-0.90; small gain in natural convection | Natural convection, outdoor, aesthetic products, LED |
| Hard anodising (Type III) | +20-35% | 7-10 days | Thicker layer raises interface resistance if on contact face | Wear resistance, harsh environments; mask the contact face |
| Chromate conversion (RoHS trivalent) | +3-6% | 2-3 days | Emissivity about 0.2-0.3; thin film, minimal impact | Cost-sensitive parts, EMI housings, as a paint base |
| Bead blasting / sand blasting | +3-8% | 2-4 days | Roughens surface, can slightly improve radiation | Pre-treatment before anodising, cosmetic uniformity |
| Powder coating | +8-15% | 5-8 days | 60-100 micron insulator; degrades performance measurably | Only where the heat sink is a decorative cover, not a thermal part |
| Electroless nickel | +20-40% | 7-12 days | Nickel is far less conductive than aluminium; thin layers only | Solderability, wear, specific chemical resistance |
Cost impacts are relative to an uncoated machined part and vary with part size, masking complexity and anodising batch size. They are indicative, not quoted.
Why black anodising is over-specified more often than not
Black surfaces radiate better than bare aluminium, and this fact gets repeated often enough that black anodising is specified by reflex. The physics is narrower than that.
Radiation heat transfer scales with the fourth power of absolute temperature and with emissivity. At the surface temperatures a typical electronics heat sink runs at, say 60-80 C, radiation is a modest fraction of total heat rejection when forced air is present. In natural convection with low airflow, it can be 20-30% of the total, and emissivity matters.
So: black anodising is worth paying for in natural convection designs, outdoor enclosures and LED fixtures. In a forced-air design with a fan moving real airflow, convection dominates and the colour is cosmetic. If the customer wants black for appearance, that is a legitimate reason, but call it what it is.
The contact face problem
This is the most commonly mishandled detail in heat sink finishing. Anodic layers are aluminium oxide: hard, corrosion resistant, and thermally insulating compared with the metal underneath. A 15 micron layer on the contact face adds a real resistance between the component and the heat sink.
Two options:
- Mask the contact face before anodising. Adds masking labour and handling cost, but keeps the interface bare metal. This is the correct choice when the interface uses grease or bare-metal contact.
- Anodise everything and rely on the interface material. Acceptable when using a thick gap pad at moderate conductivity, because the pad resistance dominates anyway. Not acceptable for high heat flux or bare-metal clamping.
If you choose masking, define the masked area on the drawing with a dimension, not a note. Masking boundaries are hand-applied and a note like "keep base bare" produces inconsistent results across batches.
Corrosion: what you are actually protecting against
Aluminium heat sinks in service face three corrosion mechanisms: galvanic corrosion at dissimilar metal contacts, pitting in chloride environments, and cosmetic staining from handling and humidity.
- Indoor, controlled environment: clear anodising is sufficient and is mostly cosmetic and handling protection.
- Outdoor or humid, with salt exposure: anodising at 15-25 micron, sealed, or chromate conversion plus a sealed anodise. Specify salt spray hours if you have a requirement; 500 hours neutral salt spray is achievable and should be stated if needed.
- Dissimilar metal contact: anodising helps only if it remains intact at the joint. Where a stainless screw contacts aluminium, use a coated fastener or an insulating washer; the coating on the heat sink will not survive the thread engagement.
- Copper or brass interface plates: this is a genuine galvanic couple. Isolate it or accept that the aluminium will sacrifice.
How to specify finish on the drawing
Write a complete finish specification, not a colour. A workable specification contains: process, standard, thickness class, colour, sealing, masked areas and any cosmetic acceptance criteria.
Example: "Clear anodise per MIL-A-8625 Type II, Class 1, 10-15 micron, sealed, contact face masked, no visible scratches on fin tips, colour to match supplied sample."
That sentence eliminates the three most common disputes: thickness arguments, colour mismatch and whether the base should be bare.
Frequently asked questions
Does anodising reduce heat sink performance?
On the fin surfaces, no: the layer is thin and the surface area benefit is unchanged. On the contact face, yes, if the layer is between the component and the metal. That is why masking exists.
Is black anodising better than clear?
Only in radiation-dominated situations, mainly natural convection and outdoor. In forced air the difference is negligible and the choice is cosmetic.
Can I paint a heat sink?
You can, but paint is an insulator and it will cost thermal performance. If you need a colour for appearance, anodising or a thin conversion coating is the better route.
How long does anodising add to lead time?
Typically 3-7 working days including queue, depending on colour and batch size. Black and hard anodising take longer than clear. In peak season, anodising is often the bottleneck, not machining.
Tell us the environment and the interface you are using and we will recommend the finish that meets the requirement without paying for properties you do not need.
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- Aluminum Heat Sink Price per kg: 2026 Cost Breakdown
- Custom Heat Sink MOQ and Lead Time by Process
- Heat Sink Extrusion Die Cost and Tooling Amortisation
- Heat Sink RFQ: What to Send for an Accurate Quote
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- Heat Sink RoHS, REACH and UL Compliance Explained
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- Heat Sink Prototype and Sample Service: What to Expect
- Heat Sink Packaging, Shipping and DDP Terms
- Heat Sink Supplier Audit and Second Sourcing
- Heat Sink Thermal Test Report: What the Data Package Contains
