A2 (304) VS A4 (316) Stainless Steel Bolts: Corrosion Resistance, Salt Spray Test Data & Industry Application Rules
Most procurement buyers cannot distinguish A2 and A4 stainless fasteners, which causes pitting corrosion within just 1 year of service on coastal and chemical industrial facilities. The core difference between the two alloys is molybdenum content, which directly determines resistance to salt, acid and chloride ion corrosion. This article provides lab test comparison data and clear matching guidelines for all operating environments across Russia.
Core Chemical Composition Differences Between A2 & A4 Alloy
- A2 (304): zero molybdenum content, only resistant to fresh water and dry indoor air
- A4 (316): 2–3% molybdenum alloy additive, suppresses chloride-induced pitting corrosion effectively
- Both grades contain chromium and nickel to form a self-repairing passive anti-rust film on metal surfaces
- Surface passivation improves service life for both grades, but cannot make up for lack of molybdenum in A2 steel
Service Performance Comparison Table A2 VS A4
| Technical Index | A2 (304) Fastener | A4 (316) Fastener |
| Molybdenum Alloy Content | 0% | 2.0–3.0% |
| Neutral Salt Spray Hours Before First Rust | 200–500 hours | 1000–2000 hours |
| Suitable Working Environment | Inland cities, fresh water equipment, dry indoor workshops | Coastal salt air, acid media, chemical reactor assemblies |
| Guaranteed Maintenance-Free Coastal Service Life | 1–2 years with visible rust spots | 8–12 years without pitting corrosion |
Strict selection rule: A2 stainless bolts are prohibited for port terminals, offshore platforms and chloride-containing chemical workshops. These high-corrosion projects require fully passivated A4 stainless fasteners with integrated ceramic thread anti-seize coating.