Stainless Steel Material Guide - A2 vs A4
A2 vs A4 Stainless Steel - Which Should You Use?
Every Simfix fastener is A2 (304) or A4 (316) stainless steel. Both are corrosion-resistant - but they handle very different environments. This guide will tell you which to pick in 30 seconds.
The short answer
- A2 (304) - indoor, outdoor, workshop and general engineering. The everyday workhorse.
- A4 (316) - marine, coastal (within ~5km of the sea), salt-spray exposed, or chemical environments.
If you're not sure, A4 is always the safer choice - it'll handle anything A2 handles, plus salt water. It costs a little more, but for marine or coastal work it's the difference between "looks new in 10 years" and "rust streaks in 18 months".
The science (briefly)
Stainless steel is corrosion-resistant because of an invisible layer of chromium oxide on the surface. As long as that layer stays intact, the metal beneath doesn't rust. The difference between A2 and A4 is what can break that layer.
| A2 (304) | A4 (316) | |
|---|---|---|
| Chromium | 18% | 16% |
| Nickel | 8% | 10% |
| Molybdenum | - | 2% |
| Resists chloride pitting | Limited | Excellent |
| Magnetic? | Slightly | No |
| Typical applications | Indoor, outdoor, workshop, food contact | Marine, coastal, chemical, swimming pools |
That 2% molybdenum in A4 is the key - it dramatically increases resistance to chloride pitting from salt water. Without it (A2), chlorides can locally break down the protective oxide layer, allowing rust to start in pinholes that then spread.
When A2 is fine
- Inside a building (kitchen, garage, workshop)
- Outdoor in normal UK weather, away from the coast
- Decking, fencing, gates, sheds - anywhere > 5km from the sea
- Engineering, automotive interior, electrical enclosures
- Food contact (A2 is food-safe - both grades are)
When you need A4
- Marine applications - boats, jetties, pontoons, rigging
- Coastal exteriors - within 5km of the sea, exposed to salt spray
- Swimming pools - chlorinated water is hard on A2 over time
- Salt-spread environments - coastal roads, salt-storage areas
- Chemical / industrial processing exposed to chlorides
How to spot the difference visually
You can't - they look identical to the eye. The grade is stamped or laser-etched on most fasteners in our range, and we never mix A2 and A4 stock. The product page always declares the grade clearly, and every Simfix product label has an A2 or A4 badge on the front.