Anodising, zinc plating, blackening — surface finishes for CNC parts

A part taken off the machine is rarely ready for assembly. Unprotected steel rusts, aluminium dulls and scratches, and the end customer expects a specific colour. That is why the surface finish of CNC parts is a normal part of the specification — and a frequent source of misunderstandings, because a coating can change the dimension of a part by more than its tolerance.

In this post we go through the five most common finishes: anodising, zinc plating, blackening, stainless steel passivation and powder coating. For each — what it does, which material it is for and what it does to the dimension. At the end: how to write the requirement correctly on the drawing, so the coated part still fits into its hole.

Why a coating at all

A coating usually serves one of four functions: protection against corrosion, increased hardness and abrasion resistance, appearance (colour, uniformity) or special functions such as electrical insulation or low friction. One coating rarely does everything at once — so the first question is not "which coating" but "what is this surface supposed to do". It is the same logic we apply to tolerances: the requirement should follow from the function.

Selection starts with the working environment: a dry shop floor, oil mist, chemical washdown, outdoor conditions — each sets a different minimum level of protection. The second filter is the base material, because not every coating can be applied to everything: anodising applies to aluminium, blackening to carbon and tool steels, and passivation only makes sense on stainless steel. Only the third criterion should be appearance. If a part combines several functions — for example a housing that must look good on the outside and carry fits on the inside — the solution is often masking or splitting the functions across two parts.

The second point: coating is a separate process, usually at a specialised subcontractor. It extends the lead time and adds a cost item, so it is worth putting into the enquiry right away — just like the material and the quantity. What else should be included in the package is described in which files to send for a CNC quote.

Overview of CNC part finishes

Anodising (aluminium). Electrochemical oxidation of the surface — the oxide layer grows partly into the material and partly above the original dimension. Natural anodising (an indicative 5–25 µm) provides corrosion protection, scratch resistance and the option of dyeing. Hard anodising (an indicative 25–60 µm) significantly increases surface hardness and abrasion resistance — typical for guides, small pistons and milled parts working in sliding contact. The layer is an electrical insulator; if the part must conduct at a contact point, that spot has to be masked.

Zinc plating (carbon and low-alloy steels). The basic corrosion protection for steel — an electrolytic zinc coating, usually with clear or yellow passivation, with an indicative thickness of 5–12 µm. Cheap, repeatable, good for machine parts working indoors. Watch out for fine-pitch threads and fits: a few micrometres on every surface can change the character of a sliding fit.

Blackening (black oxide) of steel. A chemical conversion layer around 1–2 µm thick — dimensionally practically negligible, which is why it is popular for fitted parts, gauges and turned components with narrow tolerances. Corrosion protection is limited and requires oiling; it is a "workshop-and-storage" finish plus aesthetics (uniform black), not a substitute for zinc outdoors.

Passivation of stainless steel. Not a coating but a chemical treatment: removal of iron contamination (e.g. from tools) and restoration of the natural passive layer. It changes neither dimension nor appearance, but it completes the corrosion resistance — in the food, chemical and pharmaceutical industries it is treated as standard. More about the specifics of this material in machining stainless steel.

Powder coating. Electrostatic application of powder paint followed by curing — a thick (an indicative 60–120 µm), mechanically resistant coating in any RAL colour. Best for covers, housings and structures; worst for fitted surfaces and threads, which must be masked or machined after painting. Sharp edges hold powder poorly, so it pays to design in radii.

Coating vs tolerances and fits

Here lies the most common documentation error: the drawing states Ø20H7, and a separate note says "zinc plate". The question is — H7 before or after coating? With zinc (a few to a dozen or so µm per side) an H7 hole can end up outside the tolerance zone after plating. With hard anodising the effect is even bigger: the layer grows in both directions, indicatively half the thickness above the original dimension, so Ø20 after 50 µm anodising is no longer the same diameter.

Practical rules: fitted surfaces and precision threads are masked or machined with a dimensional offset for the expected coating thickness; critical dimensions are unambiguously marked as applying after coating; and with blackening and passivation the problem practically disappears, because the dimensional change is negligible. The choice of coating can therefore be an argument in material selection — sometimes it is cheaper to start from stainless steel without a coating than from carbon steel with a coating and masking. A material comparison can be found in C45 steel, stainless or aluminium.

Two special notes. First, hardened and spring parts may require baking after zinc plating because of the risk of hydrogen embrittlement — a standard treatment, but it has to be planned into the lead time. Second, electrolytic coatings grow unevenly: on edges and corners the thickness tends to be greater than in holes and recesses, so a measurement "on the flat" does not tell the whole story about a thread or a small internal diameter.

The thickness values are typical, textbook figures — specific ranges are agreed with the plating or painting shop for a given part.

How to write the coating requirement on the drawing

A good finish specification contains five elements:

• the coating type with a standard or an unambiguous description (e.g. zinc plating with passivation, hard anodising),
• the thickness or thickness range, if it is functionally relevant,
• the colour or appearance (e.g. RAL for powder, black anodising),
• the masked surfaces — indicated on a view, not just described,
• a declaration: toleranced dimensions apply before or after coating.

One sentence in the requirements can save a whole complaint batch: "fitted dimensions apply after coating, mask the threads". If you do not know which option is cheaper in your case — say so directly in the enquiry, the supplier will calculate both variants.

Plan the sequence of operations too. Fine threads are often cheaper to cut after coating than to mask; with powder coating, fitted surfaces are sometimes easier to finish-machine after curing, provided the geometry of the part allows it. Such decisions are best made at the quoting stage, together with the supplier — changing the sequence of operations after production has started costs far more than one extra sentence in the specification.

Summary

The finish is chosen by function: anodising for aluminium (hard — when abrasion matters), zinc for steel in typical conditions, blackening where the dimension is sacred and appearance secondary, passivation to complete the resistance of stainless steel, powder for colour and covers. Every coating except passivation adds micrometres that must fit within the tolerance — which is why the note "before or after coating" is just as important as the thickness itself.

Do you have a part with a required coating, or are you unsure which one to choose? Send the drawing via the contact form — you will get a quote within 48 hours together with advice on how the coating will affect the fits and the lead time.