Passivation and Pickling: Why Stainless Corrosion Resistance Lives in the Finish
“Stainless” describes the passive chromium-oxide film, not the bulk metal. Machining, stamping and handling all damage that film — passivation restores it.
Bottom line: for most Anti-Galvanic Corrosion applications the decision comes down to load, environment and assembly method — the Anti-Galvanic Corrosion guide walks through the selection.
What passivation actually does
An acid bath (nitric or citric per ASTM A967 / AMS 2700) dissolves free iron smeared onto the surface during forming — from tooling contact, steel brushes, or grit from shared equipment. Free iron is corrosion initiation site number one; remove it and the chromium forms a continuous, self-repairing oxide.
When passivation is not enough
- Heat-tinted weld zones need pickling first — passivation cannot remove thick oxide scale.
- Severe service (marine, chemical) may warrant ASTM A967 Nitric 2 or citric-plus treatments verified by ferrite testing.
- Contaminated finishing lines can re-infect parts; dedicated stainless equipment matters.
Verification, not faith
Water-immersion and copper-sulfate tests confirm film quality cheaply; salt-spray validation closes the loop for critical parts. Topprecision passivates its stainless fastener range per A967 with per-lot test records, and advises customers on handling practices that keep the film intact between our plant and their assembly line.
Fastener material comparison
| Material | Tensile (MPa) | Density (g/cm³) | Best for |
|---|---|---|---|
| Steel 8.8 | 800 | 7.85 | General structural, machinery |
| Steel 12.9 | 1220 | 7.85 | Weight-critical, tooling |
| Stainless 304 (A2-70) | 700 | 7.93 | Indoor, food, corrosion-resistant |
| Stainless 316 (A4-80) | 800 | 7.98 | Marine, coastal, chloride |
| Aluminum 7075-T6 | 570 | 2.81 | Weight-critical aerospace |
| Titanium Grade 5 | 900 | 4.43 | Aerospace, medical, high cost |
Need help selecting the right fastener material?
Toprecision cold-heads and CNC-turns fasteners in steel (4.6–12.9), stainless 304/316/410, aluminum 6061/7075, titanium Grade 5 and engineering plastics. Send the service environment and load requirement — we will recommend the material and grade with tensile test data. Request material selection support.
