When finishing is underspecified, the factory decides. This guide covers 10 industrial metal finishing processes, anodizing, electroplating, passivation, and more, with standards, thickness ranges, and a 12-process comparison table.
Most metal finishing process failures aren't manufacturing errors. They're specification errors. When a drawing says "surface finish: TBD" or just specifies a color without a process, the factory chooses. Whats the consequence? You will get whatever surface finish factory has ready. Which may or may not match your environment, tolerance, or performance requirements.
The metal finishing process is the final step in sheet metal fabrication processes and CNC machining. It determines whether a part resists corrosion for 3 years in a marine environment or corrodes in 3 months. If it survives a cleanroom inspection or gets rejected at goods-in. Or if it bonds to a primer coat or repels it.
Understanding the options (and how to specify them) is the difference between a drawing that produces repeatable parts and one that produces expensive rework.
The metal finishing process is a surface treatment applied to metal parts after the metal fabrication process to improve corrosion resistance, wear resistance, appearance, or adhesion for secondary operations like painting or bonding.
It's not cosmetic. Finishing determines whether a stainless steel surgical instrument passes ISO 13485 validation, whether an aluminium enclosure survives ASTM B117 salt spray testing, or whether a zinc casting bonds reliably to a powder coat.
Most custom metal parts require at least one finishing operation. How you specify it on the drawing determines what comes back.
Metal finishing processes fall into 2 categories:
Additive and altering processes change the surface by depositing material or chemically modifying its composition. Use these when the goal is corrosion resistance, electrical conductivity, aesthetics, or surface preparation for bonding or painting.
Examples of metal finishing processes:
Subtractive and refining processes improve the surface by removing imperfections, contaminants, or small amounts of material. Use these when the goal is reduced surface roughness, burr removal, or a consistent matte or polished texture.
Examples or substractive and refininsh processes:
Some projects use both: abrasive blasting prepares the surface, then powder coating is applied on top. The sequence matters. Additive processes almost always require a subtractive preparation step first.
Every metal finishing process, regardless of type, follows 3 core steps:
The part is cleaned to remove machining oils, oxides, scale, and contamination. Common methods: alkaline degreasing, acid etching, abrasive blasting, ultrasonic washing. No finish bonds reliably to a contaminated surface (this step is not optional!).
The chosen finish is applied through electrochemical deposition (electroplating, anodizing), chemical reaction (passivation, phosphate coating), electrostatic spraying and heat curing (powder coating), or mechanical action (abrasive blasting, buff polishing).
The finished part is rinsed, sealed (anodized parts), cured (powder coating), or passivated again (post-plating passivation). Surface finish is verified by visual inspection, Ra measurement, salt spray testing per ASTM B117, or coating thickness measurement.
Sidenote. When submitting an RFQ for finished custom parts, specify which step you want the factory to control - surface preparation method, finish specification, and inspection criteria. Leaving any of these open gives the factory discretion you probably don't want.
The 10 most common metal finishing processes used in custom manufacturing cover both additive and subtractive categories. Each suited to specific base materials and functional requirements.
Anodizing is an electrochemical process that grows a dense, corrosion-resistant oxide layer on aluminium by using the part as the anode in an acid electrolyte bath. The oxide layer is integral to the metal - it can't flake or peel. It can be sealed and dyed in most colors.
Standard anodizing (Type II, per MIL-A-8625): Layer thickness 5-25 µm. Suitable for corrosion protection and decorative finishes. The porous oxide accepts anodising dyes - black, blue, red, natural (clear/silver). Sealing with deionised water closes the pores and finalises corrosion resistance.
Hard coat anodizing (Type III, per MIL-A-8625): Layer thickness 25-150 µm, surface hardness 60-70 HRC equivalent (varies by aluminium alloy and process parameters). Use for high-friction, high-wear applications: valve seats, hydraulic components, firearm components. Typical colors are dark bronze or black - the thickness limits dye options. Small-batch hard coat anodizing carries a fixed setup cost regardless of quantity.
Compatible metals: Aluminium primarily. Also effective on magnesium and titanium. Not applicable to steel or stainless steel.
Electroplating is a process that deposits a thin layer of metal onto a substrate using an electric current. The part acts as the cathode in a plating bath containing dissolved metal ions.
The deposited metal changes the part's surface properties without altering its geometry significantly (typical thickness: 2.5-25 µm depending on metal and specification).
Choose the plating metal based on what property you need:
Compatible metals: Steel, stainless steel, copper, aluminium (with zincate pretreatment). Also applicable to plastics with specialist pretreatment.
Electroless plating deposits metal through a chemical reaction rather than an electric current. The part is immersed in a reducing chemical bath - no external power supply, no racks or electrical contacts required.
The result is an extremely uniform coating regardless of part geometry. Electroless nickel (EN) is the most common variant: it coats internal bores, blind holes, and complex cavities with the same thickness as external surfaces. Typical thickness: 5-50 µm.
Why it matters for complex parts: Rack electroplating produces uneven deposits on complex geometries - thicker at corners and edges, thinner in recesses. Electroless plating eliminates this problem.
Compatible metals: Steel, stainless steel, aluminium, copper, and some plastics.
Powder coating applies a dry polymer powder to the metal surface using an electrostatic charge, then cures it in an oven (typically at 160-200°C) to produce a hard, continuous film.
Passivation is a chemical treatment that removes surface iron contamination from stainless steel and restores the metal's natural chromium oxide passive layer.
Stainless steel's corrosion resistance comes from its chromium oxide layer. Machining, grinding, and forming can embed free iron particles from tooling into the surface. These iron particles corrode preferentially, causing localised rust spots - even on 316L stainless steel in supposedly inert environments.
Passivation (per ASTM A967 or AMS 2700) dissolves the free iron using nitric acid or citric acid without affecting the steel's appearance, dimensions, or base material. A correctly passivated 316L part in a cleanroom environment resists corrosion far better than an unpassivated 304 part.
Chromate conversion coating (also called chemical film or Alodine) creates a thin corrosion-resistant film on aluminium, zinc, and magnesium through a chemical reaction with the metal surface.
The film is extremely thin (0.5-3 µm) and does not change part dimensions. This makes it appropriate for precision parts where powder coating or anodizing would be too thick.
Its primary application is as a pre-treatment before painting or bonding. Chromate conversion dramatically improves paint adhesion on aluminium surfaces that would otherwise fail adhesion testing. RoHS-compliant Type II formulations (trivalent chromium, Cr3+) are the standard for most industrial and defence applications.
Phosphate coating (phosphatisation) applies a thin layer of iron, zinc, or manganese phosphate crystals to the steel surface through a chemical reaction. The coating creates a porous base that dramatically improves adhesion for subsequent paint, powder coat, or oil.
Manganese phosphate + oil is a specific variant used on mechanical components (gears, bolts, engine parts) where the oil-impregnated porous phosphate layer reduces friction and prevents galling during break-in, per MIL-DTL-16232.
Electropolishing is the electrochemical reverse of electroplating. An electrical current removes metal ions from the part surface, eliminating microscopic burrs, peaks, and surface contamination to produce a smooth, brightened finish.
Typical material removal: 5-25 µm per pass. Ra surface roughness values typically improve by 30-50% per pass. The process also removes microburrs in areas inaccessible to mechanical polishing - internal bores, laser-cut edges, complex geometries.
Abrasive blasting propels abrasive particles at high velocity against the metal surface, cleaning contaminants, removing scale, and creating a uniform matte texture.
It's the most common surface preparation step before painting or powder coating. The blasting creates a surface profile (anchor pattern) that mechanically bonds to subsequently applied coatings. Without this profile, adhesion tests will fail on smooth machined surfaces.
Hot blackening (black oxide) applies a thin black oxide layer (1-3 µm) to steel through immersion in a series of chemical tanks at 141°C.
The finish provides mild corrosion resistance when sealed with oil or wax. It's primarily decorative and used where a black matte appearance is required without significant dimensional change. Common applications: firearms, hand tools, automotive components, military hardware.
Hot blackening is typically run in batches of small parts. The process adds essentially zero dimensions - suitable for tight-tolerance parts where powder coating or plating would alter fits.
Choosing the right metal surface finish depends on 3 factors: (1) base material - some finishes only work on specific metals; (2) functional environment - corrosive, abrasive, high-temperature, or sterile; (3) post-processing requirements - bonding, painting, sterilisation, or soldering.
|
Scenario |
Recommended Process |
Reason |
|
Aluminium part needing corrosion protection + colour |
Anodizing Type II (MIL-A-8625) |
Integral oxide layer, dyeable, precise thickness control |
|
Aluminium in high-friction or wear application |
Hard coat anodizing Type III |
Hardness 60-70 HRC, thicker protective layer |
|
Stainless steel for medical, food, or pharmaceutical use |
Electropolishing + passivation |
Removes burrs, improves cleanability, restores passive layer |
|
Stainless steel with iron contamination from machining |
Passivation (ASTM A967) |
Removes embedded iron, restores corrosion resistance without adding thickness |
|
Aluminium or zinc part before painting or bonding |
Chromate conversion coating |
Improves adhesion without dimensional impact |
|
Steel structural parts needing durable colour finish |
Phosphate coating + powder coat |
Phosphate improves adhesion; powder coat provides durable colour and protection |
|
Precision complex part needing uniform coverage |
Electroless nickel plating |
Even coating on all surfaces including internal bores |
|
Decorative part requiring high gloss |
Electroplating (nickel or chrome) |
Smooth, reflective finish with corrosion layer |
|
All parts before painting or powder coating |
Abrasive blasting |
Creates anchor profile for coating adhesion |
|
Ferrous part needing black matte finish with minimal dimension change |
Hot blackening + oil seal |
1-3 µm process, tight-tolerance safe |
|
Process |
Compatible Metals |
Typical Thickness |
Key Standard |
Corrosion Resistance |
Cost Tier |
|
Anodizing Type II |
Aluminium, Mg, Ti |
5-25 µm |
MIL-A-8625 |
Good |
Low-medium |
|
Hard coat anodizing Type III |
Aluminium |
25-150 µm |
MIL-A-8625 |
Very good |
Medium-high |
|
Electroplating (zinc) |
Steel, copper |
5-25 µm |
ASTM B633 |
Good (sacrificial) |
Low |
|
Electroplating (nickel) |
Steel, stainless, Cu, Al |
5-25 µm |
ASTM B689 |
Good |
Medium |
|
Electroless nickel |
Most metals + plastics |
5-50 µm |
ASTM B733 |
Very good |
Medium-high |
|
Powder coating |
Steel, Al, Zn |
40-120 µm |
ISO 12944 |
Very good |
Low-medium |
|
Passivation |
Stainless steel, Al |
0 (no added layer) |
ASTM A967 |
Excellent (restores) |
Low |
|
Chromate conversion |
Al, Zn, Mg |
0.5-3 µm |
MIL-DTL-5541 |
Moderate |
Low |
|
Phosphate coating |
Steel, cast iron |
5-20 µm |
MIL-DTL-16232 |
Moderate (+ primer) |
Low |
|
Electropolishing |
Stainless, Cu, Al |
Removes 5-25 µm |
ASTM B912 |
Excellent (improves) |
Medium |
|
Abrasive blasting |
All metals |
N/A (surface profile) |
ISO 8501-1 |
None (prep step) |
Low |
|
Hot blackening |
Ferrous metals |
1-3 µm |
MIL-DTL-13924 |
Low-moderate (+ oil) |
Low |
The 5 main purposes of metal finishing are: improving corrosion resistance, increasing wear and abrasion resistance, improving adhesion for coatings or bonding, achieving specific aesthetic requirements (color, gloss, texture), and removing surface defects or contamination.
Electroplating is the most widely used industrial metal finishing process. Zinc electroplating alone is applied to billions of fasteners, brackets, and structural parts annually for sacrificial corrosion protection. Powder coating is the most common organic finishing process.
Anodizing converts the aluminium surface into an oxide layer - the finish is part of the metal itself. Electroplating deposits a separate metal layer on top of the substrate. Anodizing only works on aluminium (and a few other non-ferrous metals). Electroplating works on most metals and plastics.
Yes, most processes add thickness that affects fit and tolerance. Electroplating and anodizing Type II add 2.5-25 µm per side. Hard coat anodizing Type III adds 25-150 µm. Powder coating adds 40-120 µm. Passivation and chromate conversion add negligible thickness. Factor this into your drawing tolerances before specifying a finish for tight-clearance features.
Call out the process name, applicable standard, and inspection criteria. Examples: "Anodize per MIL-A-8625 Type II Class 2, Black"; "Passivate per ASTM A967, Method C (nitric acid), verify by water break test"; "Zinc plate per ASTM B633, Type III SC4, minimum 25 µm." Vague callouts like "surface finish: black" or "anodize - natural" give the factory too much discretion.
These are mechanical grinding finishes for stainless steel sheet. A #3 finish is a coarser directional grain (80-120 grit equivalent), used in architectural applications. A #4 finish is a finer, brushed directional grain (150-180 grit equivalent), the standard for food-contact and light industrial equipment. Neither is an electrochemical process - they're mechanical surface conditions.
Depends on the process. Electroless nickel and passivation coat all surfaces uniformly including internal bores. Rack electroplating struggles with internal geometries. Powder coating requires line-of-sight for the electrostatic spray - internal features are typically left uncoated. Abrasive blasting is difficult on internal surfaces without specialist tooling.
Specifying finishing correctly in your RFQ prevents the most common category of non-conforming parts from Chinese factories. Whether your parts go through sheet metal fabrication, CNC machining, or casting, finishing requirements must be written into the drawing before the quote goes out - not discussed after the fact.
When you submit an RFQ through Haizol, you can include surface finish requirements alongside your CAD file - the process, standard, and inspection criteria. Finishing capability is pre-screened across the CNC machining services and fabrication factory network: ISO 9001, ISO 13485, and IATF 16949-certified factories available for anodizing, powder coating, electroplating, and passivation. 90% of RFQs receive quotes from 8 or more verified factories within 24 hours - each priced with finishing included, not as an afterthought.
If you're ready to test it, submit a sourcing inquiry on Haizol with your CAD file and finishing specification. Compare what factories quote - and how their finishing credentials stack up side by side.
