You just got a quote for 500 CNC machined aluminum parts. The machining cost looks good. Then the buyer asks: "What about the finish? Black anodize? Or hard coat?" And suddenly the per-part price jumps 30%. This is the reality of CNC machining finishing options — the surface treatment often costs as much as the machining itself, and picking the wrong one means rework, delays, or a part that fails in the field.

I see this all the time. Engineers nail the GD&T, spec the right material, then treat the finish as an afterthought. Then the anodizing shop calls and says the internal threads are too tight for the coating thickness. Or the black oxide rubs off in handling. Or the passivation step was skipped and the stainless steel pits in service. Let's fix that.

Anodizing: The Go-To for Aluminum Parts

Anodizing is electrochemical. It grows a controlled oxide layer on the aluminum surface. That layer isn't painted on — it's part of the metal. It won't peel or chip.

Type II (sulfuric anodizing). The standard. Produces a 5-25 micron coating. Can be dyed any color — black is the most common. Corrosion resistance is good, hardness improves moderately. Typical applications: automotive brackets, electronic housings, consumer goods. Cost: low to moderate.

Type III (hard coat anodizing). Thicker — 25-100 microns. Much harder (up to 60-70 Rockwell C). Used for parts that see wear: hydraulic components, valve bodies, sliding surfaces. The coating penetrates the surface and builds outward, so internal dimensions shrink. You need to pre-compensate in the machining. I've quoted a job where we machined threads 0.05mm oversize to account for hard coat build-up. Forgot to tell the machinist and had to rework 200 parts. Learn from that.

Clear anodize. Minimal color, just natural protection. Used when appearance matters but you don't want to hide the metal. Aerospace and architectural specs often call for clear anodize.

One thing most buyers don't know — anodizing is an electrical insulator. The coating doesn't conduct. If your part needs electrical grounding or continuity, you'll need to mask certain surfaces or leave uncoated zones. Plan for it in the print.

⚠️ Real example: A customer spec'd black anodize on a 6061 bracket with M4 threaded holes. The anodizing built up 0.02mm inside the threads. M4 tap drill is 3.3mm. After anodizing, the effective minor diameter shrank enough that M4 bolts wouldn't start. Solution: we now run a thread chase after anodizing, or pre-compensate by cutting threads slightly oversize before coating.

Plating: Electroplating and Electroless Options

Plating deposits a metal layer onto the surface. It changes the part's appearance, corrosion resistance, and sometimes its electrical properties.

Zinc plating. Cheap. Common for steel parts — brackets, fasteners, hardware. Adds 5-15 microns. Yellow or clear chromate conversion after plating improves corrosion resistance. Not great for high-temp environments. A standard choice for interior automotive hardware.

Nickel plating. Two types. Electroless nickel (ENP) deposits uniformly on complex geometries without external current — ideal for internal passages and blind holes. Electroplated nickel is brighter but has uneven build-up on edges. ENP offers excellent wear resistance and lubricity. Used in hydraulic fittings, fuel system components, and valve parts.

Chrome plating. Hard chrome for wear resistance on shafts, piston rods, cylinder bores. Decorative chrome for appearance — but that's usually plastic these days. Hard chrome is thick (50-300 microns) and requires grinding after plating to achieve final dimensions. Expensive and environmentally regulated.

Silver and gold plating. For electrical conductivity. Gold is standard on connector pins and PCB contacts. Silver is used in high-power electrical contacts and RF shielding. Both are expensive but necessary where conductivity and corrosion resistance are critical.

My honest take on plating tolerances: if your print says "plate to dimension," you need to specify whether the plating thickness is included in the final tolerance or if it's an addition. Don't assume the shop knows. A simple note on the drawing saves a lot of argument.

Passivation: Protecting Stainless Steel

Passivation isn't a coating — it's a chemical treatment. It removes free iron and other contaminants from the surface of stainless steel, allowing a thin chromium oxide layer to form naturally. That layer is what makes stainless steel "stainless."

If you skip passivation on 304 or 316 stainless parts, you risk surface rust. I've seen medical device parts that looked great coming off the machine but developed rust spots after a week in storage — because the passivation step was missed. Machining embeds iron particles from tooling into the surface. Passivation dissolves those particles and lets the chromium oxide regenerate.

Key specs: ASTM A967 and AMS 2700. Most shops can do nitric acid passivation (Type 2). Citric acid passivation (Type 1) is more environmentally friendly and increasingly preferred. Medical device buyers usually require citric acid per ASTM A967.

Passivation doesn't change dimensions. It doesn't change appearance (unless your part had embedded contamination, which shows as discoloration after treatment). It's cheap and fast — a few hours in the tank, a rinse, and it's done.

layer is what makes stainless steel "stainless."

If you skip passivation on 304 or 316 stainless parts, you risk surface rust. I've seen medical device parts that looked great coming off the machine but developed rust spots after a week in storage — because the passivation step was missed. Machining embeds iron particles from tooling into the surface. Passivation dissolves those particles and lets the chromium oxide regenerate.

Key specs: ASTM A967 and AMS 2700. Most shops can do nitric acid passivation (Type 2). Citric acid passivation (Type 1) is more environmentally friendly and increasingly preferred. Medical device buyers usually require citric acid per ASTM A967.

Passivation doesn't change dimensions. It doesn't change appearance (unless your part had embedded contamination, which shows as discoloration after treatment). It's cheap and fast — a few hours in the tank, a rinse, and it's done.

Black Oxide: Classic Finish for Steel Parts

Black oxide is a conversion coating. It produces a black iron oxide layer on steel surfaces — about 1-2 microns thick. It offers mild corrosion resistance and a matte black appearance. It also reduces light reflection, which matters for certain tooling and optical applications.

Black oxide is cheap but not durable. It needs oil or wax sealing to provide real corrosion protection. Without sealing, it's mostly cosmetic. I've seen black oxide parts that rusted in a humid warehouse within a week because the oil film dried out.

For high-volume steel parts that need a black finish and some rust protection, black oxide + wax is a budget-friendly option. For anything that needs real durability, go with zinc plating or an organic coating instead.

Powder Coating: Durable Color for Larger Parts

Powder coating applies a dry powder electrostatically, then bakes it into a hard finish. Thickness is typically 60-150 microns — much thicker than anodizing or plating. That means dimensions change noticeably.

Powder coating is great for large parts: frames, enclosures, covers, structural components. It offers excellent impact and chemical resistance. Available in almost any color and texture (gloss, matte, textured, metallic).

The catch: threaded holes need masking or re-tapping after coating. Part features that mate with other components need to be considered. And powder coating adds significant cost for small parts — the $50 minimum charge per batch can double the per-part price on a 10-piece run.

Choosing the Right Finish — A Quick Framework

Here's how I walk through finish selection with clients:

• What's the substrate? Aluminum → anodize or powder coat. Steel → zinc plate, black oxide, or powder coat. Stainless steel → passivation. Copper/brass → plate or clear coat.
• What's the environment? Indoor/dry → cosmetic finishes are fine. Outdoor/humid → anodize (Type III) or powder coat. Marine → 316SS + passivation, or heavy nickel plate. Chemical exposure → electroless nickel or specific powder coatings rated for chemical resistance.
• What's the function? Wear surface → hard anodize or hard chrome. Electrical contact → gold or silver plate. Corrosion barrier → anodize, nickel, or passivation. Sliding fit → electroless nickel or hard anodize with low coefficient of friction.
• What's the dimensional impact? Anodizing builds 5-25 microns (Type II) or 25-100 microns (Type III). Plating adds 5-50 microns depending on type. Powder coating adds 60-150+ microns. If the finish changes dimensions, your print needs to account for it.
• What's the budget? Passivation and black oxide are cheapest. Anodizing and zinc plating are moderate. Powder coating is cost-effective for large parts but expensive per-part for small runs. Electroless nickel, hard chrome, and precious metal plating are premium.

AOOM Technology offers complete post-machining finishing services. We coordinate with qualified finishing shops — anodizing lines, plating baths, powder coating booths — so your parts go from raw stock to finished product without multiple handoffs. Need black anodized 6061 brackets with masked threads? We handle it. Need electroless nickel on 4140 steel components with full thickness certification? Same deal.

Don't leave your finish to chance. Get in touch with AOOM Technology and we'll recommend the right surface treatment for your project. We'll review your part, your operating environment, and your budget — and give you a finish spec that works. Contact us today.