Technical reference
Materials & finishes
A selection guide for the two decisions that determine corrosion life, friction and appearance — and that are usually made last, when they should be made first.
Materials
Substrate selection
Strength, corrosion and weight trade against each other. The right answer depends on the mating part, the cleaning chemistry and the service life.
| Material | Standard | Tensile (MPa) | Corrosion | Max temp | Magnetic | Typical use |
|---|---|---|---|---|---|---|
| SUS304 | A2 / 1.4301 | 500 – 700 | Very good | 400 °C | Slightly | General electronics, housings |
| SUS316L | A4 / 1.4404 | 480 – 680 | Excellent | 450 °C | No | Medical, marine, non-magnetic |
| SUS410 | 1.4006 | 440 – 640 (hardened 800+) | Moderate | 400 °C | Yes | Self-tapping, wear parts |
| SUS420 | 1.4021 | 650 – 850 (hardened) | Moderate | 350 °C | Yes | Pivot pins, blades |
| SWCH18A | — | 400 – 550 | Needs coating | 300 °C | Yes | High-volume internal screws |
| SCM435 | — | 800 – 1,200 (QT) | Needs coating | 300 °C | Yes | 10.9 / 12.9 structural |
| C3604 | — | 350 – 500 | Good | 200 °C | No | Inserts, standoffs |
| Ti-6Al-4V | Grade 5 | 900 – 1,100 | Excellent | 400 °C | No | Weight-critical, non-magnetic |
| Al 6061-T6 | — | 290 – 310 | Good (anodised) | 150 °C | No | Spacers, lightweight |
| PEEK | — | 90 – 100 | Excellent | 250 °C | No | Insulating, high temperature |
Tensile values are typical ranges for the supplied condition. Hardened martensitic grades reach higher strength at lower ductility.
Finishes
Coating systems
Finish is a specification, not a colour. It sets corrosion life, friction coefficient and appearance — and it changes thread dimensions.
| Finish | Thickness | Salt spray | Friction (µ) | Hydrogen risk | Colour | Best for |
|---|---|---|---|---|---|---|
| Zinc trivalent | 5 – 12 µm | 96 – 240 h | 0.18 – 0.25 | Yes (bake 10.9+) | Blue / clear | Economic steel protection |
| Zinc-nickel | 8 – 15 µm | 480 – 720 h | 0.15 – 0.20 | Yes (bake 10.9+) | Silver / black | Automotive, outdoor |
| Electroless nickel | 3 – 8 µm | 240 – 480 h | 0.18 – 0.22 | Low | Bright silver | Uniform coverage |
| Black oxide | 1 – 2 µm | 24 – 72 h (oiled) | 0.20 – 0.28 | No | Black | Cosmetic, internal |
| Dacromet / Geomet | 8 – 12 µm | 720 – 1,000 h | 0.14 – 0.18 | No | Grey-silver | Highest corrosion, safety parts |
| Passivation | 0.005 – 0.02 µm | Application dependent | 0.20 – 0.30 | No | Natural | Stainless only |
| TiN PVD | 2 – 4 µm | 96 – 240 h* | 0.20 | No | Gold | Wear, decorative gold |
| CrN PVD | 2 – 5 µm | 96 – 480 h* | 0.22 | No | Silver-grey | Corrosion plus wear |
| DLC | 2 – 3 µm | 96 – 240 h* | 0.10 | No | Near-black | Low friction, black |
| Zinc flake | 8 – 14 µm | 720 – 1,000 h | 0.13 – 0.17 | No | Silver / black | Automotive class 10.9+ |
* PVD salt spray assumes a nickel or CrN interlayer. PVD alone is a wear and appearance coating, not a sacrificial corrosion barrier.
Compatibility
Which finish works with which substrate
Some combinations are technically possible but commercially or electrochemically wrong. These are the pairings we recommend and the ones we advise against.
| Substrate | Recommended finish | Avoid | Reason |
|---|---|---|---|
| SUS304 / 316 | Passivation, electropolish, DLC | Zinc plating | Zinc on stainless creates a galvanic couple and looks poor |
| SUS410 / 420 | Passivation, CrN PVD | Long salt spray expectation | Martensitic grades pit more readily than austenitic |
| SWCH18A | Zinc trivalent, nickel | Bare | Uncoated carbon steel rusts within days |
| SCM435 (10.9/12.9) | Zinc-nickel, Dacromet, zinc flake | Thick electroplating | Hydrogen embrittlement risk rises with coating thickness |
| Brass | Nickel, bare with anti-tarnish | Zinc | Zinc on brass is sacrificial but visually inconsistent |
| Titanium | Passivation, anodise | Electroplating | Adhesion is poor and galvanic risk is high |
| Aluminium | Anodise | Steel plating | Galvanic corrosion with steel in wet environments |
Environment
Selecting by service environment
Start from where the product lives, not from a material preference. The environment determines the minimum acceptable system.
| Environment | Recommended material / finish | Expected life | Notes |
|---|---|---|---|
| Indoor, controlled (office, consumer device) | SUS304 passivated or zinc trivalent | 5 – 10 years | Standard electronics specification |
| Humid indoor (kitchen, bathroom) | SUS304 or zinc-nickel | 5 – 8 years | Avoid bare carbon steel |
| Outdoor, sheltered | Zinc-nickel or SUS316 | 5 – 10 years | Specify 480 h salt spray minimum |
| Outdoor, exposed / coastal | SUS316 or Dacromet on steel | 10 – 15 years | Chlorides attack 304; use 316 |
| Automotive under-hood | Zinc-nickel or zinc flake on SCM435 | 10 – 15 years | Thermal cycling plus salt |
| Medical sterilisation | SUS316L passivated, dry-film lube | Device life | Galling risk in stainless-on-stainless |
| Food processing | SUS316L, FDA-compliant lubricant | 10+ years | Cleaning chemicals require 316 |
The two traps
Galvanic corrosion and hydrogen embrittlement
These two mechanisms account for most of the field failures that are blamed on "bad plating". Neither is visible at incoming inspection, and both appear months after the product ships.
01
Galvanic corrosion
When two dissimilar metals are joined in the presence of an electrolyte, the less noble one corrodes. A zinc-plated steel screw in an aluminium housing will sacrifice the zinc, then the steel, while the aluminium remains intact. Isolate the joint or choose a compatible pair.
02
Hydrogen embrittlement
Acid cleaning and electroplating can introduce hydrogen into high-strength steel. Under sustained load the part fractures suddenly, sometimes days later. Class 10.9 and above requires a controlled bake within a defined window after plating — and a record proving it happened.
03
Thread fit after coating
A 10 µm coating adds up to 20 µm diametrally. For Class 6f fits this can turn a clearance into an interference. Gauge after coating, or mask the thread, or specify the coating thickness as part of the fit calculation.
Quick rule
If the part is visible, specify appearance and process separately
"Black" is not a finish. DLC, black oxide, black zinc and matte PVD look different, age differently and cost differently. Name the process, then add the cosmetic requirement as a second line on the drawing.
Need a recommendation?
Send the environment and the mating material.
Questions
Material and finish FAQ
Common questions from design and sourcing teams.
Is stainless always the best corrosion choice?
Why does coating thickness matter at the thread root?
What friction coefficient should I specify?
Can I mix materials in one assembly?
Do you provide material certificates?
Send the environment, we will specify the system
Tell us where the product lives, what it touches and how long it must last. We will propose a material and finish combination with the test evidence behind it.