Start with the machined surface
Before adding any coating, know what a well-machined metal part already gives you. On a good CNC machining setup, an as-machined aluminum part can hold Ra 0.8–1.6 µm (32–64 µin). That is smooth enough for many brackets, housings, and heat sinks. Steel comes out rougher, closer to Ra 1.6–3.2 µm, unless you spec a finishing pass with a small stepover. If corrosion, wear, or color is not an issue, leaving the part as-machined is almost always the cheapest and fastest option.
The catch is that raw machined aluminum fingerprints, oxidizes unevenly, and looks inconsistent under showroom lighting. Steel will rust in humid environments within days if left unprotected. So the finish decision is really a functional one: corrosion, wear, electrical insulation, appearance, or biocompatibility.
Anodizing: not just for looks
Anodizing is an electrochemical conversion of the aluminum surface into aluminum oxide. It is not a layer painted on top, so it adds dimensional growth from the base metal. For every 25 µm of coating thickness, roughly half grows into the surface and half grows outward. A part with tight holes or threads needs that growth budgeted in.
Type II vs Type III
Type II sulfuric anodize gives a 5–15 µm coating, good dye absorption, and modest wear resistance. Typical for brackets, enclosures, and cosmetic parts. Lead time is usually 2–4 days once the lot is at the finisher.
Type III hardcoat anodize builds 25–75 µm of dense ceramic. It dramatically improves wear and can approach Rockwell 60C on the surface. The trade-offs are thicker dimensional growth, slightly gray appearance, and reduced fatigue resistance on very thin sections. Cost is typically 1.5–2× Type II.
Anodizing only works on aluminum alloys that form a stable oxide film. High-silicon die-cast alloys like ADC12 anodize poorly and often look blotchy. If you need a black cosmetic finish on a die-cast part, powder coating is usually the safer path.
Electroless nickel and hard chrome
For steel and some aluminum applications, electroless nickel (EN) deposits a uniform 5–25 µm layer without the current-density issues of electrolytic plating. It adds hardness, corrosion resistance, and a degree of lubricity. We use it on mold cavities, pump housings, and parts that need a solderable or RF-shielding surface. Cost is moderate, but masking is labor-intensive if only selected areas need coating.
Hard chrome plating gives extreme wear resistance and a low coefficient of friction. It is common on shafts, pins, and hydraulic components. The environmental handling makes it expensive and slower, with lead times of 1–2 weeks. For prototype quantities, we often suggest EN or DLC-like coatings instead unless the spec explicitly calls for chrome.
Powder coating and wet painting
Powder coating applies a dry polymer powder electrostatically and bakes it at 180–200 °C. The resulting film is 60–120 µm thick, tough, and consistent. It is excellent for outdoor parts, consumer enclosures, and any part that will see abrasion. The high bake temperature rules out some castings with trapped porosity; outgassing can create pinholes in the finish.
Wet painting is slower and softer but handles low volumes and color matching better. For 10–50 prototype parts, wet paint can be more economical because there is no powder-booth setup charge. Both finishes require masking of threads and precision bores.
Passivation and chemical film
For stainless steel and some aluminum parts, passivation removes free iron from the surface and restores the chromium oxide layer. It adds almost no thickness and is required for many medical and food-grade components. Chemical film (Alodine / chromate conversion) on aluminum provides mild corrosion protection and a conductive surface, often used as a base for paint or as a standalone finish for electrical enclosures.
Cost ranking and lead time reality
Here is how the options stack up for a typical prototype lot of 20–100 small aluminum or steel parts:
- As-machined: $0 added cost, 0 days added. Best when corrosion and cosmetics are not concerns.
- Passivation / chemical film: Low cost, 1–2 days. Functional corrosion protection, minimal dimensional change.
- Type II anodize: Moderate cost, 2–5 days. Good cosmetic range, color options, moderate wear.
- Electroless nickel: Moderate-to-high cost, 3–7 days. Uniform thickness, wear and corrosion protection.
- Powder coating: Moderate cost, 3–7 days. Thick, durable, color choice, requires masking.
- Type III hardcoat: Higher cost, 4–7 days. Maximum wear resistance on aluminum.
- Hard chrome / specialty plating: Highest cost, 1–3 weeks. Reserved for extreme wear or legacy specs.
DFM notes that save money
Finish costs explode when a part has complex masking, internal passages, or tight tolerances on coated surfaces. Design for the finish from the start: leave relief grooves next to bearing surfaces, avoid deep pockets that trap powder, and specify coating thicknesses that match your tolerance budget. If a 12 µm Type II coating will push a press-fit hole out of spec, either adjust the hole size or choose a thinner coating.
On die casting parts, remember that porosity can outgas during powder coating and anodizing, creating cosmetic defects. For pressure-tight castings, impregnation before finish is often necessary. On CNC aluminum parts, a well-planned finish turns a raw machined component into a product-ready part without a second machining pass.
