Heat sink surface finish and anodising: corrosion, emissivity and contact

What a heat sink finish actually does

A heat sink finish has three jobs: protect the aluminium from corrosion, set the surface emissivity and survive handling and cleaning. It can also be a cosmetic choice, which matters on a visible housing but not on a hidden plate.

The finish does not change the conduction through the metal. It changes the surface where heat leaves to the air and the interface where heat enters from the component. Those are different surfaces, and they often need different treatments.

The most common mistake is to treat the whole part with one finish and then wonder why the thermal resistance is higher than the calculation. The calculation usually assumed bare metal at the interface and a black surface at the fins.

Bare aluminium and corrosion

Aluminium forms a thin natural oxide in air. That layer gives some protection but it is not enough for a wet, salty or industrial atmosphere. Bare fins can pit, and a pitted fin loses area and gains thermal resistance.

An unfinished heat sink is acceptable in a dry, clean enclosure with no condensation. It is a poor choice outdoors, near the sea, in a washdown area or where the part will be handled with bare hands.

The alloy also matters. A casting with high silicon can show a mottled surface after cleaning, and a wrought extrusion can show extrusion lines. Neither is a corrosion failure, but both affect the look of a visible part.

Clear anodising

Clear anodising builds a controlled oxide layer on the surface. It improves corrosion resistance, adds wear resistance and leaves the metal colour visible. The layer is thin, typically in the range of 10 to 25 micrometres for a decorative or protective Type II finish.

The oxide is harder and more brittle than the aluminium. It can crack at a sharp edge or a tight bend, and a cracked coating is a corrosion start point. Radius the fin tips and the edges before anodising if the part will be handled.

Clear anodising raises emissivity compared with bare polished aluminium, but the surface still looks bright. In a visible natural convection product, a clear finish is often chosen for appearance rather than for the last few percent of radiation.

Hard anodising, sometimes called Type III, builds a thicker layer than a decorative finish. It gives better wear resistance on a part that will be handled or cleaned often, but the thicker oxide adds more thermal resistance at any surface where it is not masked. It is also more likely to crack at a sharp edge. Use it when the wear duty is real, not as a default upgrade.

The pre-treatment matters as much as the anodising itself. A poorly degreased surface can show patches, streaks or a weak bond. A casting with exposed silicon can anodise to a darker, mottled colour that is difficult to match across batches. Agree a visual sample and state the acceptable variation before the first production run.

Black anodising and emissivity

Black anodising adds a dye to the oxide layer. The dark surface raises emissivity, which matters when radiation is a meaningful part of the heat path. That is common in natural convection, in enclosed cabinets and in low airflow designs.

Emissivity values vary with the process, the alloy and the surface preparation. A typical black anodised surface is often quoted around 0.85 to 0.9, while bare polished aluminium is often below 0.1. Treat these as approximate and ask the finisher for a measured value if the thermal model depends on it.

Colour alone is not a guarantee. A black paint can have high emissivity but add a thick, low conductivity layer. A black anodised layer is thinner and part of the metal surface, which is why it is preferred on fins.

Radiation does not rescue a design with no airflow. In forced convection, the convection coefficient usually dominates and the colour has less effect. Use black where the physics supports it, not as a default.

Anodising and the thermal interface

The oxide layer is a ceramic, and its thermal conductivity is much lower than aluminium. A 20 micrometre layer sounds thin, but it sits exactly where the heat has to cross, and it adds a resistance that the interface material cannot remove.

If the component mounts directly to the heat sink with paste or a pad, the mounting area should be masked before anodising. The mask leaves bare aluminium for the interface. The same applies to a threaded hole, a press fit boss or a bonding stud.

Masking adds cost and handling. It also creates a boundary between coated and bare areas where corrosion can start. Design the mask as a simple shape with a clean edge, and specify the minimum bare area rather than an exact outline that the finisher cannot hold.

If the whole part must be coated for corrosion, use a thermal interface material that is filled to bridge the oxide, or move the interface to a separate machined insert. Both are compromises, so decide early.

Conversion coating and powder coat

A chromate conversion coating is much thinner than anodising and gives good corrosion protection with little effect on contact resistance. It is a sensible choice for a hidden heat sink that needs protection but not wear resistance or high emissivity.

Powder coat is thick, usually 60 to 100 micrometres. On a fine fin field it can close the gap between fins and add a thermal resistance over the whole surface. It is better suited to a housing shell than to a dense heat sink.

Painting and e-coating have similar issues. If the part is visible, a coating on the outside and bare or thin coated fins is often the practical answer. That means masking again, so the cost belongs in the quote.

How to specify the finish

Name the process, the standard, the thickness range and the colour. State which areas are masked and which must remain conductive. Add the corrosion test if the environment demands it, and the emissivity value if the thermal model uses one.

For an anodised part, give the alloy and the temper. Some alloys anodise to a darker or patchier finish, and a casting can show silicon particles. A sample agreed before production avoids a colour argument later.

Finally, say what the finish must not do: it must not close the fin gap, crack at the fin tip or cover the thermal interface. That turns a cosmetic note into a functional one.