Surface treatment on metal parts is specified for three reasons — corrosion resistance, wear resistance, and appearance — and the right choice follows almost entirely from two inputs: the base metal and the service environment. Aluminium is anodised, carbon steel is plated or organically coated, and stainless steel is passivated rather than coated at all. The environment then sets the performance level, which is why an indoor swivel and a coastal wind-turbine anchor can share an alloy and still need different finishes. This guide covers anodising to MIL-A-8625, zinc and nickel plating, passivation to ASTM A967, salt-spray testing to ASTM B117 and ISO 9227, and how to write a finish call-out a factory can quote.
Finish is the most under-specified item on a typical drawing package; our material selection guide covers the base-metal decision underneath it.
Anodising for aluminium: what is the difference between Type II and Type III?
Type II is conventional sulphuric acid anodising for corrosion protection and colour; Type III is hard anodising for wear resistance. Both are defined by MIL-A-8625, which also lists Types I and IB (chromic acid), Type IC (a non-chromic alternative to Type I) and Type IIB (a thin sulphuric coating). Class 1 is undyed, Class 2 is dyed.
Anodising converts the aluminium surface itself into aluminium oxide rather than laying a film on top, so it cannot flake or peel. Type II is typically specified between roughly 2.5 and 25 microns (0.0001 to 0.001 in). Type III is thicker and much harder, commonly called out at about 50 microns (0.002 in), with a working band of roughly 25 to 75 microns.
Two consequences catch buyers out. First, dimensional growth: the oxide grows roughly half into the original surface and half outward, so about half the coating thickness is added per surface. A 50 micron Type III coating adds around 25 microns per surface and roughly 50 microns across a diameter — enough to seize a close-fitting pin or thread. Threads, bearing bores and bonding points need masking, since the oxide is an insulator.
Second, colour consistency. Dyed Class 2 colour depends on alloy, temper, bath chemistry, coating thickness and dye lot, so black on 6061 will not necessarily match black on 7075, and Type III darkens naturally toward bronze as it thickens. Where mixed-process parts sit side by side, agree a physical master swatch at sampling — anodising cannot reliably hit a Pantone or RAL number.
Which plating should you specify for steel safety hardware?
Plating is chosen by how aggressive the environment is and how much wear the surface sees:
- Zinc, electroplated at roughly 5 to 12 microns, is the low-cost sacrificial default for indoor and light outdoor duty, but it leans heavily on its passivate and topcoat: a passivated deposit of that thickness commonly reaches only around 120 hours of neutral salt spray before red rust.
- Zinc-nickel alloy, typically 12 to 15 percent nickel, is the serious corrosion answer. Common specifications expect 10 microns unpassivated to pass 500 hours or more before red rust, and passivated, topcoated systems routinely exceed 1000 hours.
- Electroless nickel is autocatalytic, so it deposits evenly inside blind holes, threads and complex castings where electroplating throws badly. High-phosphorus deposits (10 to 13 percent P) can exceed 1000 hours neutral salt spray, and baking near 400 C raises them from about 40 to 45 HRC toward hard chrome at 65 to 70 HRC.
- Hard chrome remains the wear benchmark for pins, shafts and rods, but is deposited from hexavalent chromium chemistry, whose EU regulatory position needs checking first.
One point specific to load-bearing hardware: electroplating charges hydrogen into steel. High-strength steel above roughly 1000 MPa tensile risks hydrogen embrittlement and needs a post-plating bake, commonly a few hours at around 190 to 220 C, per ASTM B850. On a connector body that belongs on the drawing.
What does passivation actually do to stainless steel?
Passivation chemically removes free iron and surface contamination from stainless steel and lets a uniform chromium oxide film re-form across the whole surface. It adds no measurable thickness, changes no dimension, and will not rescue the wrong grade.
It matters because machining, grinding and tumbling smear free iron onto the surface and embed particles from tooling; those sites rust, which the customer reads as a material defect. ASTM A967 specifies the chemical passivation treatments — nitric and citric acid variants — and the tests that verify them; ASTM A380 is the broader practice covering cleaning, descaling and passivation. On welded or heat-treated parts, descaling comes first.
Where do powder coating, e-coat, PVD and zinc flake fit?
These cover what anodising and plating cannot:
- Powder coating gives a thick, tough, fully colour-controlled film, typically 60 to 100 microns, easily matched to RAL. It builds on edges and threads, so masking matters.
- Cathodic e-coat deposits a very even film of 15 to 25 microns, reaching recesses a spray gun cannot see. It is normally a primer under a topcoat.
- PVD coatings such as TiN, CrN and DLC are a few microns thick, extremely hard and dimensionally near-neutral, at a higher cost per part.
- Zinc flake systems of the Dacromet type, applied to references such as ISO 10683 for fasteners, are non-electrolytic — no plating current, so no hydrogen embrittlement risk, hence their use on high-strength bolts.
What does a 480-hour salt spray result actually prove?
It proves a coating survived a defined laboratory exposure — a neutral 5 percent sodium chloride fog at 35 C, run to ASTM B117 or the equivalent neutral salt spray test of ISO 9227 — for that many hours before defined corrosion appeared. It does not mean the part lasts proportionally long in service.
Two things deserve precision. On zinc systems, hours to white rust (the zinc corroding) and to red rust (the steel corroding) are different numbers, so a quotation saying only 480 hours is ambiguous. The test was never designed to predict service life, and published agreement with field performance is poor. Treat it as a batch-to-batch consistency check, not a warranty period.
Do your surface treatments comply with RoHS and REACH?
If you sell into the EU you have to ask, because the traditional high-performance finishes are the ones under pressure. RoHS limits hexavalent chromium to 0.1 percent by weight in any homogeneous material, which rules out classic yellow and olive hexavalent chromate passivates. Under REACH, chromium trioxide sits on the Authorisation List and the EU process to move chromium(VI) substances toward restriction is still developing. The practical answer is trivalent chromium (Cr3+) passivates, plus zinc-nickel or zinc flake where a finish previously relied on hexavalent chemistry. Ask for the passivate chemistry by name and a current declaration.
How do you choose a finish for the service environment?
Work down from where the product lives:
- Indoor industrial, dry: zinc plating with a trivalent passivate, or Type II anodise on aluminium.
- Outdoor general use: zinc-nickel or zinc flake on steel; sealed Type II anodise or powder coat on aluminium.
- Coastal and marine: 316 stainless passivated to ASTM A967 wherever geometry allows; zinc-nickel plus topcoat on any steel that remains.
- Offshore and chemical plant: 316 or duplex stainless, or a duplex system such as e-coat over zinc-nickel.
- High abrasion, rope running over the part: Type III hard anodise on aluminium, electroless nickel or hard chrome on steel.
What should the drawing say so the factory quotes the right finish?
A finish call-out a factory can quote and inspect against contains five things: the specification and class (for example anodise to MIL-A-8625 Type III Class 2 black), the thickness with a tolerance, the colour reference plus a physical master swatch, the masking requirement with a marked-up view showing which threads and bores stay bare, and the corrosion criterion as hours, test method, and whether it is white rust or red rust. For high-strength steel, add the post-plating bake. Our three-stage quality control guide covers where finish is inspected.
Working with Power Honour
Power Honour is a Taiwan-based integrated OEM/ODM precision metal manufacturer with more than 30 years in climbing hardware, fall protection and industrial PPE, running surface treatment — anodising, plating, coating and passivation — alongside machining, forging, casting, stamping and heat treatment. Send us your drawing and the environment the product will live in, and we will tell you whether the specified finish is right and what it does to your dimensions. Initial reviews are no-commitment.