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Metal finishing by base material: how each substrate takes a finish
The finish is only as good as the preparation under it, and preparation depends on the base material. This page lists the common substrates, the finishes each one takes well, the pretreatment it needs and the specs buyers call out. Name the alloy and its condition on the RFQ and we match it with up to 3 shops that run that preparation.
Substrates, finishes and preparation
Typical combinations, not promised. The spec on your drawing and the process control of the shop decide the result. The ASTM preparation guides listed describe how each base metal is cleaned and activated before plating.
| Base material | Common alloys | Finishes that suit | Preparation to expect | Specs often called out |
|---|---|---|---|---|
| Aluminum, wrought | 6061, 6063, 5052, 7075, 2024 | Anodize Type II and III, chem film, electroless nickel, powder coat | Etch and desmut; zincate before any plating | ASTM B253 (prep), MIL-PRF-8625, MIL-DTL-5541 |
| Aluminum, cast | A356, A380, ADC12 | Chem film, powder coat, electroless nickel; anodize looks grey and uneven | High silicon and porosity limit anodize; impregnation for leak-tight parts | MIL-DTL-5541, customer powder spec |
| Carbon and alloy steel | 1018, 1045, 4140, 12L14 | Zinc, zinc-nickel, zinc flake, electroless nickel, phosphate, black oxide, e-coat, powder coat | Degrease, pickle or blast to remove scale and rust | ASTM B183 (prep), ASTM B633, ASTM B841 |
| Hardened steel | Over 39 HRC: springs, tool steels, case hardened parts | Zinc flake or mechanical zinc, plating with relief bake, PVD, black oxide | Avoid long acid pickling; hydrogen relief bake after electroplating | ASTM B242 (prep), ASTM B850 (bake) |
| Stainless steel | 303, 304, 316, 410, 420, 17-4PH | Passivation, electropolish, PVD, plating over a nickel strike | Nickel strike (Wood's nickel) to activate before plating | ASTM A967, AMS 2700, ASTM B254 (prep), ASTM B912 |
| Zinc die-cast | Zamak 3, Zamak 5, ZA-8 | Copper-nickel-chrome, trivalent conversion, e-coat, powder coat | Copper strike first; avoid strong acids and alkalis that attack zinc | ASTM B252 (prep), ASTM B456 |
| Magnesium | AZ91D, AM60B, AZ31B | Conversion coat, magnesium anodize, e-coat, powder coat; plating with special prep | Conversion coat within hours of cleaning; isolate from steel fasteners | SAE AMS-M-3171, ASTM B480 (prep) |
| Copper and brass | C110, C360, C260, phosphor bronze | Nickel, tin, silver, gold over nickel, bright dip and lacquer, PVD | Bright dip or acid clean; nickel barrier under gold | ASTM B281 (prep), ASTM B545 (tin), ASTM B700 (silver), ASTM B488 (gold) |
| Titanium | Grade 2, Grade 5 (Ti-6Al-4V) | Color anodize, PVD, passivation, bead blast | Fluoride etch; plating needs special activation and is rarely the first choice | AMS 2488 (anodize), ASTM F86 (implant surfaces) |
| Plastics | Plating grade ABS, PC/ABS | Decorative copper-nickel-chrome, PVD or vacuum metallizing, paint | Chemical etch, catalyst, electroless copper or nickel before electroplating | ASTM B604, customer appearance spec |
Hardened steel over 39 HRC
Any electroplating on steel over about 39 HRC needs a hydrogen embrittlement relief bake soon after plating, for example to ASTM B850. Name the hardness and the bake on the drawing, or choose a finish without hydrogen such as zinc flake. See the relief bake guide.
How each family behaves in the shop
The points that decide whether a finish sticks, looks even and keeps the part in tolerance.
Aluminum
The oxide on aluminum reforms in seconds, so plating needs a zincate step and anodize needs a clean etch. Alloy decides anodize color and hardness: 6061 anodizes evenly, 7075 and 2024 darker and softer, cast alloys grey. Anodize grows about half its thickness into the part. See hard anodizing.
Carbon and alloy steel
The easiest base to plate once scale, rust and oil are removed. Zinc and zinc-nickel protect by corroding first. Free machining leaded grades can leave smut that needs extra cleaning. See zinc-nickel plating.
Stainless steel
Stainless resists corrosion on its own once free iron from machining is removed by passivation. The same passive film stops plating from sticking, so a nickel strike comes first. 400 series and 17-4PH need milder passivation baths than 300 series. See passivation.
Zinc die-cast
Zinc dissolves in the acids and strong alkalis most lines use, so castings get a mild clean and a copper strike before nickel and chrome. Surface porosity and cold shuts show through bright plating, so cosmetic areas need a sound casting skin.
Magnesium
The most reactive structural metal. It corrodes fast against steel or in salt, so it is conversion coated soon after machining, often then e-coated or powder coated. Steel fasteners need isolating washers or a coated fastener.
Copper and brass
Plate well after a bright dip or acid clean. Gold needs a nickel barrier underneath, or the copper diffuses into it. Leaded brass such as C360 can outgas and blister under plating, so name the alloy. See contact plating guide.
Titanium
Usually left bare, color anodized for identification or PVD coated for wear. Titanium anodize is a thin interference film, so color depends on voltage, not dye. Plating titanium is possible but needs special activation.
Plastics
Plating grade ABS and PC/ABS are etched to give the metal a grip, then metallized with electroless copper or nickel. Decorative chrome on plastic is common on consumer and automotive trim. Other resins usually get PVD, vacuum metallizing or paint.
Mixed assemblies
Finishing an assembly of two metals in one tank rarely works, because each base needs its own prep. Finish the parts before assembly, or choose a finish that both metals accept, such as powder coat over the right conversion coat for each.
What to put on the RFQ about the base material
These lines decide which shops can take the job. Missing items come back as questions.
- Alloy and temper6061-T6, 304, AZ91D or Zamak 3, not only "aluminum" or "stainless".
- HardnessAlways for heat treated steel. Over 39 HRC triggers the relief bake.
- Process routeMachined, stamped, cast or welded. Castings and welds change the prep.
- Surface conditionOily, rusty, scaled, already coated or needing a strip first.
- Dissimilar partsInserts, pressed bearings or bonded parts in another material.
- Critical dimensionsBefore or after finish, so the shop knows what the build-up must fit.
Related pages
Finish pages give typical thickness and specs per finish. The guides go deeper on single substrates.
- All finishes and specs
- Electroless nickel plating
- Anodizing 6061, 7075 and cast aluminum
- Passivation vs electropolish
- Metal finishing spec cross reference
- Finishes by application
Background on how metal coatings are deposited: electroplating.
Questions about base materials
For how matching works, see the home page.
Why does the base material matter so much for the quote?
Each base metal needs its own cleaning and activation before the finish will stick: zincate on aluminum, a nickel strike on stainless, a copper strike on zinc die-cast. Many shops only run the pretreatment lines for some substrates. Naming the alloy and its condition lets us send the RFQ only to shops that run that preparation, up to 3.
Can cast aluminum be anodized?
It can, but the result is usually grey, blotchy and less protective, because the high silicon in alloys such as A380 and ADC12 does not form a clean oxide. Chem film or powder coat over chem film is the more common choice for die castings. A356 anodizes somewhat better. Approve a sample if the look matters.
What is different about hardened steel?
Steel over about 39 HRC can be embrittled by hydrogen absorbed during acid cleaning and electroplating, and parts can crack under load hours or days later. Electroplated parts need a relief bake soon after plating, for example to ASTM B850, or a non-electrolytic finish such as zinc flake. Put the hardness and the bake requirement on the drawing.
Can my zinc or magnesium die castings be powder coated?
Yes, over a conversion coat. Die castings often hold gas in their porosity, which can blister the powder during cure. Shops reduce this with a degassing bake before coating and an outgassing-grade powder. Tell the shop the alloy and whether the casting has been impregnated.
Can I send an RFQ for plating on plastic parts?
Yes. Choose Other as the base material on the RFQ form and name the resin. Plating on plastic needs plating grade ABS or PC/ABS and a dedicated line, so fewer shops run it and you may get fewer quotes. Wall thickness, gate position and mold surface all show through the plate.
One RFQ, up to 3 matched shops
Free for buyers. Send the drawing, base material, finish, spec and lot size. We reply within 24 business hours with matched shops or a clear no.
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