Machined aluminum parts come off the mill bright, bare and vulnerable. Left unfinished, 6061 forms a thin natural oxide that resists some corrosion but scratches easily, stains from handling and looks nothing like a product. Anodizing and powder coating are the two default fixes, and teams pick between them constantly — sometimes correctly, sometimes by habit. They protect by completely different mechanisms, and that difference drives everything: dimensions, edge quality, cost and where each finish fails.
Anodizing Converts the Surface; Powder Coating Covers It
Anodizing grows an oxide layer out of the aluminum itself using an electrolytic bath. Type II sulfuric anodize — the standard for cosmetic and corrosion parts — runs roughly 8 to 25 microns thick. Type III hard anodize builds 25 to 80 microns for wear surfaces, reaching surface hardness in the range of 350–550 HV. Because the layer is grown rather than deposited, it cannot peel or chip off; it is the metal.
Powder coating is the opposite philosophy: an electrostatically charged polymer powder sprayed on and cured in an oven, forming a sacrificial skin typically 60 to 120 microns thick. It sits on top of the metal, so it adds real thickness, absorbs small tool marks, and comes in nearly any color or texture.
The Dimensional Catch on Precision Parts
Anodizing's oxide grows roughly half outward and half inward. A Type II layer specified at 15 microns therefore raises each surface by about 7 microns — usually negligible. A 50-micron hard anodize on a bearing bore is not negligible: each side of the bore shrinks by roughly 25 microns, which will eat most of an H7 tolerance. Two honest options exist: mask critical bores and threads before anodizing, or machine them to final size after. Just don't leave the decision to the shop.
Powder coating adds its full 60–120 micron thickness everywhere it lands. That rules it out for threads, press fits and anything that once measured ±0.02 mm — masking with plugs and tape is mandatory, and even then sharp internal threads collect powder at the roots.
Edges, Corners and Where Each Finish Fails
- Sharp corners burn in anodizing — current concentrates on edges and pits the coating. Break edges with 0.5 mm radii minimum, more for Type III.
- Powder pulls back on the same sharp edges (electrostatic fields wrap poorly into corners), leaving the thinnest film exactly where chips start. Same fix, same radius.
- Anodize is hard but brittle; a sharp impact can crack the oxide layer, though it stays bonded. Powder absorbs impact well but cuts through to bare metal once chipped, and corrosion then creeps under the film.
- UV behavior splits them: natural clear anodize is permanently stable outdoors, while dyed anodize fades within a few seasons. Exterior-grade polyester powders hold color for years, which is why architectural aluminum is almost always powder coated.
Alloy Matters More Than Teams Expect
6061 and 7075 anodize to a clean, uniform, slightly yellow-silver finish and take dyes evenly. High-silicon casting alloys such as A380 and ADC12 are a different story: silicon particles resist the bath, so anodized die castings come out mottled gray — acceptable on hidden brackets, unacceptable on cosmetic housings. That is why die cast parts that must look finished are usually powder coated or painted instead. If you are spec'ing a finish on a casting, our die casting team can flag which surfaces will anodize cleanly before you commit.
Electrical and Thermal Behavior
Anodize is a genuine electrical insulator — useful for enclosure interiors, awkward for grounding points and EMI gaskets, which need masked contact areas. It still conducts heat reasonably well, anodized heatsinks being a standard example. Powder coating insulates both electrically and, to a larger degree, thermally, which occasionally matters on heatsink-adjacent covers.
Cost, Batch Size and Lead Time
Both processes carry minimum lot charges because parts are racked and run in batches. Anodizing is priced by rack area and jumps for Type III and dye colors; powder coating costs rise with color changes, which waste powder and oven time — batching same-color parts together is the single easiest saving. Outsourced turnaround for either typically lands between three and seven working days, so it belongs on the schedule from the start, not as an afterthought after assembly has slipped.
A Short Decision Rule
- Wear surfaces, slides and friction fittings: Type III hard anodize.
- Outdoor color and weather exposure: exterior-grade powder coating.
- Tight tolerances on bores or threads: anodize with masking, or powder with plugs — specify which surfaces are exempt.
- Metallic look, thin film, heat conduction: clear anodize.
- High-silicon castings needing cosmetic color: powder, not anodize.
Neither finish is universally better; they solve different problems. The cheapest mistake is treating finishing as a late checkbox — the right call is made when the part is still in CAD. If you want machined blanks finished to a coherent spec, our CNC machining team quotes anodizing and powder coating alongside the parts themselves, with masking mapped before the first chip flies.
