Corrosion is the most common reason fasteners get rejected on arrival, and one of the easiest to prevent at the specification stage. The problem is rarely "bad steel" — it is usually a coating or grade chosen for a different environment than the one the product will actually see. Here is the decision framework we use when reviewing drawings with procurement teams.
Start with the exposure class, not the price
Before comparing quotes, write down where the assembly will live:
- Dry indoor (furniture, cabinetry, indoor racking): zinc-plated carbon steel with a clear or yellow passivate is normally enough.
- Humid or outdoor, non-marine: hot-dip galvanising or a zinc-flake coating gives far better service life than electroplated zinc at a modest premium.
- Marine, de-icing salts, chemical exposure: A2 (304) or A4 (316) stainless. A4 adds molybdenum and is the right call when chlorides are present.
- High-strength structural bolts (8.8 / 10.9 / 12.9): be careful — these grades are susceptible to hydrogen embrittlement from acid pickling during electroplating. Zinc-flake or mechanical plating is safer.
A surprisingly common mistake is specifying 316 stainless for an indoor rack while using plain zinc bolts on an outdoor gate. Cost goes to the wrong place.
Read salt spray numbers with skepticism
Neutral salt spray testing (ISO 9227 / ASTM B117) is a comparative accelerated test, not a service-life predictor. A coating rated "240 h to white rust" does not mean two years outdoors. What the number is useful for is batch-to-batch consistency: if your approved sample survives 240 h and a production lot fails at 90 h, something changed in the plating line.
Ask suppliers for:
- The test standard and duration to white rust and to red rust (they are different endpoints).
- Coating thickness measured in micrometres, not just "zinc plated".
- Whether a passivation or topcoat is applied after plating.
Watch for galvanic couples
Joining dissimilar metals in a wet environment creates a battery. Stainless bolts in an aluminium bracket, or carbon-steel screws into a copper-alloy insert, will corrode the less noble metal faster than either would alone. Mitigations, in order of effectiveness:
- Use the same metal family for fastener and mating part.
- Isolate with nylon washers, sleeves or a non-conductive coating.
- Seal the joint so no electrolyte can bridge the two surfaces.
- Accept the couple but make the sacrificial part the cheap, easily replaced component.
Thread standards and tolerance classes
Metric ISO and inch UNC/UNF are not interchangeable, even when a bolt "feels" like it fits. A forced mismatch damages both threads and produces a joint that loosens under vibration. On export projects, confirm the destination market's prevailing standard and specify the tolerance class (e.g. 6g/6H) on the drawing. For automated assembly lines, consistent thread tolerance matters more than most buyers expect — it drives torque scatter.
A short incoming-inspection checklist
| Check | Method | Typical failure sign |
|---|---|---|
| Grade marking | Head stamp + mill certificate | Missing or mismatched property class |
| Coating thickness | Coating thickness gauge | Below specified µm |
| Dimensional | Go/no-go ring gauge | Threads out of tolerance |
| Corrosion | Salt spray sample per lot | Early white rust |
| Hydrogen embrittlement | Sustained-load test for high-strength parts | Delayed brittle fracture |
Keep the certificate against the batch number. If a field failure happens six months later, traceability is what lets you isolate a single lot instead of recalling everything.
FAQ: are stainless bolts always stronger?
No. A2-70 stainless is roughly comparable to a class 8.8 carbon-steel bolt in tensile strength, but stainless has lower yield strength and is more prone to galling (cold welding) during tightening. Use an anti-seize lubricant and controlled torque, or switch to a different fastener family if the joint is structural.
Wrapping up
The cheapest fastener is the one that survives the environment for the product's full life. Define the exposure class, pin down the coating with a measurable thickness and test standard, check for galvanic couples, and require batch traceability. Those four steps remove most corrosion surprises.
For standard and custom fasteners with material certificates and batch traceability, see Zimmor Hardware. Additional specifications and product categories are available at zimmorhardware.com.
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