Finishes & Coatings for Machined Parts: Anodize, Chem Film, Passivation, Plating, Powder Coat

Finishes do one or more of four jobs: corrosion protection, wear or hardness, appearance, and function (electrical conductivity or insulation, paint adhesion, lubricity, reflectivity). Which job you need decides the finish; the finish decides the spec; and the spec's type and class decide the thickness. That thickness is the part the drawing most often forgets.

Some finishes are conversion coatings that grow into and out of the surface (anodize, chem film, black oxide, phosphate): anodize adds roughly half its thickness outward, so a 0.002" hard coat grows a surface by ~0.001" per side and closes a hole by ~0.002" on diameter. Others are deposited (plating, paint, powder) and add their full thickness. Either way, if a feature carries a tolerance tighter than the coating build-up, the drawing must say whether the dimension applies before or after finish, and the shop will machine undersize or mask accordingly. The default convention (ASME Y14.5) is that dimensions apply after finish unless the drawing says otherwise, which surprises a lot of first-time anodize customers.

Anodizing grows a controlled aluminum-oxide layer electrochemically. The oxide is hard, porous (so it takes dye), and electrically insulating. The governing spec is MIL-A-8625, and the callout has a Type (the process chemistry) and a Class (undyed vs dyed):

Class 1 is undyed (clear/natural), Class 2 is dyed (black is by far the most common, then red, blue, gold, clear). Color varies by alloy: 6061 anodizes evenly; 7075 and 2024 come out darker and less uniform because of their copper and zinc; castings and 2000-series can look blotchy. Sealing (hot water, nickel acetate, or dichromate) closes the pores after dyeing and should be assumed unless the drawing says "unsealed" for a later bonding or PTFE step. The aluminum alloy guide notes which alloys anodize well.

Chemical conversion coating, universally called chem film or by the trade name Alodine (also Iridite), is a thin chromate or trivalent-chromium layer deposited by immersion or brush. It is almost dimensionally invisible (under 0.00005"), so it never affects tolerance, it is electrically conductive (unlike anodize), and it is the standard primer base under paint and powder coat on aluminum. Spec: MIL-DTL-5541. Type I is hexavalent chromium (gold/yellow, the traditional Alodine 1200), Type II is trivalent (clear to faint blue, RoHS-friendly and now the default in most new designs). Class 1A is maximum corrosion protection; Class 3 is the thinner, low-resistance film for electrical bonding and grounding surfaces. Chem film is cheap and fast (often same-week) and the right answer for an aluminum housing that needs corrosion protection and conductivity but not wear resistance or color.

Machining stainless steel smears free iron from the tooling and from the alloy's own sulfides across the surface; left alone, that iron rusts and the part looks like it is failing even though the bulk alloy is fine. Passivation dissolves the free iron in nitric or citric acid and lets the chromium-oxide passive layer re-form. It adds no thickness, changes no dimension, and costs little. Specs: AMS 2700 (aerospace; Method 1 nitric, Method 2 citric, with Types by bath chemistry) and ASTM A967 (commercial). Free-machining 303 and 416 need the gentler citric or the specific nitric-dichromate bath because their sulfides etch; the processor will know, but the drawing should name the spec and the method if your customer cares. Passivation of titanium per AMS 2700 or ASTM F86 removes iron contamination for implant and aerospace parts. The stainless guide covers which grades need it most.

Electroplating deposits a metal layer from solution; electroless plating deposits it chemically, which gives uniform thickness even inside holes. Plating adds its full thickness to every plated surface, so tolerance and masking notes matter. The common callouts on machined steel and aluminum:

Black oxide (MIL-DTL-13924, Class 1 for carbon/alloy steel) is a chemical conversion that blackens steel with a magnetite layer about 0.0001" thick (dimensionally negligible) and seals it with oil or wax. It gives a uniform black appearance and mild indoor corrosion resistance; it is not a substitute for plating outdoors. Common on tooling, fixtures, gauges, and hardware where looks and a little protection matter and tolerance cannot move. Zinc or manganese phosphate (MIL-DTL-16232) is a heavier grey-black crystalline conversion coating that holds oil and acts as a paint base or break-in lubricant on sliding steel parts; it adds 0.0002–0.0006" and is commonly called out on firearms, gears, and threaded fasteners.

Powder coating electrostatically applies a dry polymer (polyester, epoxy, hybrid, or TGIC) that is then cured in an oven at ~350–400°F, producing a thick (0.002–0.004"), tough, chip-resistant film. It is the standard cosmetic and protective finish for enclosures, brackets, frames, and panels. Wet paint (primer + topcoat, often MIL-PRF-23377 epoxy primer with MIL-PRF-85285 polyurethane topcoat on aerospace work, or commercial enamels) gives thinner films, finer color matching, and works on parts that cannot take the cure oven. For either, the callout needs a color (RAL, Federal Standard 595, Pantone, or a sample), a gloss, a thickness range, the pretreatment (chem film on aluminum, phosphate or blast on steel), and masking. Because paint and powder are thick, every threaded hole, bore, and datum face that is not masked will be out of tolerance; see the next section.

Every finish needs the part held (racked) somewhere, and every rack point is a bare spot. Every finish that adds thickness needs the features that cannot grow masked (tape, plugs, caps, lacquer) or machined undersize by the coating allowance. Both cost money and the processor has to be told. A finish note should say: which features are masked, where rack marks are acceptable (usually a non-cosmetic face or the inside of a hole), and whether threads are masked, plugged, or chased after plating. Class 3 chem film on ground pads, bare contact patches under an anodize, unplated threads: these are all normal, but only if they are on the drawing.

A complete finish note names the process, specification, type and class, color (if any), thickness (if it matters), masking, and whether dimensions apply before or after. Examples that a processor can execute and an inspector can verify:

If the drawing has no finish note, the part ships as machined, deburred, with a light oil on steel. If it has a finish but no spec, we will quote the common interpretation and state it on the quote. The RFQ guide has the full secondary-operations checklist, and AMS vs ASTM vs SAE explains the numbering behind the specs.