What buyers should verify first
Anodizing, passivation and powder coating are not interchangeable cosmetic options. They apply to different substrates and change corrosion behavior, dimensions, appearance and inspection requirements.
Finish specifications to put on the drawing
| Finish | Typical substrate | Typical thickness | Main purpose | Reference |
|---|---|---|---|---|
| Sulfuric anodizing | Aluminum | Roughly 5 to 25 μm depending on type and class | Corrosion resistance, wear and controlled appearance | MIL-PRF-8625 or an agreed commercial specification |
| Hard anodizing | Suitable aluminum alloys | Often 25 to 75 μm | Higher wear resistance and electrical insulation | MIL-PRF-8625 Type III |
| Passivation | Stainless steel | Surface treatment; no intentional dimensional coating build | Remove free iron and improve corrosion resistance | ASTM A967/A967M |
| Powder coating | Steel, aluminum and suitable conductive substrates | Commonly about 60 to 120 μm | Durable color and environmental protection | Project-specific pretreatment and coating system |
Thickness ranges are planning values and must be confirmed with the processor. State color or gloss, pretreatment, masking, rack-mark location, cosmetic zones, salt-spray or adhesion testing when required, and whether critical dimensions apply before or after finishing. ASTM B921 concerns non-hexavalent chromium conversion coatings for aluminum; it is not an anodizing specification.
Anodizing for aluminum
Conventional sulfuric anodizing provides an oxide surface on suitable aluminum alloys, often specified around 5 to 25 µm for commercial applications. Hard anodizing commonly uses thicker films, often around 25 to 75 µm, when wear resistance is more important than decorative color. Specify the agreed MIL-PRF-8625 type and class or other applicable specification, thickness and sealing requirement; processing and alloy condition affect the result. Conventional anodizing is generally a moderate-cost option, while hard anodizing, precise masking and tight color matching increase cost.
Passivation for stainless steel
Chemical passivation treats a properly cleaned stainless surface and helps remove free iron contamination. It does not create a thick dimensional coating, so assigning it a paint-like thickness is misleading. ASTM A967/A967M identifies treatment and verification requirements, with the appropriate method selected for the stainless grade, such as 304 or 316. Relative cost is often low to moderate for simple clean parts, but cleaning, testing, handling and lot size can dominate the quotation.
Powder coating for a colored barrier
Powder coating adds an organic film over appropriately prepared steel, aluminum or other suitable substrates. A commercial planning range is roughly 60 to 120 µm, subject to the actual coating system and geometry. Select the resin and pretreatment for indoor or outdoor exposure, then agree thickness, adhesion, cure and appearance acceptance; decorative color alone does not define corrosion performance. Cost is generally moderate for an efficient batch, with extra masking, color changes and low quantities increasing the price.
Protect fits and approve the complete finish system
A coating applied on both sides of a slot reduces its clearance, while a coated grounding pad can interrupt electrical contact. Mark threads, bearing seats, seal faces and contact areas that require masking or controlled post-finish size. Use a representative part of the actual alloy for appearance approval, since a flat color chip cannot reveal rack marks or recess coverage. For outsourced finishing, retain lot identity and inspect the returned parts before final release.
Practical buyer scenario
A powder-coated bracket can fail assembly when coating is ignored on a slot or grounding pad. Drawing notes should identify masked areas and final dimensional requirements.
Topic-specific review checklist
- Base material and alloy
- Finish type, class, color and gloss
- Thickness range and measurement method
- Critical dimensions before or after coating
- Masking, rack marks and electrical contact areas
- Adhesion, corrosion or appearance acceptance
- Packaging that prevents finish damage
Frequently asked questions
Is passivation a coating?
It is a chemical treatment for stainless steel, not a thick decorative layer like paint or powder.
Will anodizing change dimensions?
It can affect critical fits; discuss process type and dimensional allowance with the finisher.
Why do anodized lots vary in color?
Alloy, temper, surface preparation, geometry and batch conditions influence appearance.
Can finishes be combined?
Sometimes, but compatibility and sequence must be engineered and validated.
Sources and further reading
- ASTM A967/A967M stainless steel passivation
- Anoplate anodizing process guidance
- ASTM B921 conversion coating scope
Discuss this drawing requirement with WEKO before releasing material or committing the production route.
