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Stainless bolts still rust? Passivation, corrosion testing and procurement essentials

2026-10-20

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why stainless bolts rust anyway - the passive film does the work

The corrosion resistance of 304 and 316 stainless steel comes from a nanometre-thin, chromium-rich oxide film on the surface, not from the bulk metal. Cold heading, turning and polishing can press free iron particles from tooling into the surface; incomplete pickling, tumbling media shared with carbon-steel parts, or a sanding belt previously used on mild steel all leave free-iron contamination. Those iron spots rust first and act as pitting initiation sites - a classic reason why brand-new bolts show rust stains straight out of the box. ASTM A967 and AMS 2700 define passivation as a chemical treatment that removes free iron and promotes a uniform chromium-oxide film. Properly passivated A2/A4 fasteners show markedly better salt-spray performance than merely pickled ones. ISO 3506-1 only covers mechanical properties and corrosion-resistance grades; the passivation condition is a separate technical requirement that must be written into the purchase contract - and it often decides whether an A2-70 bolt lasts three years or three months in coastal or chloride-laden service.

Choosing a passivation process: nitric vs citric acid

Two main routes dominate. Nitric acid processes per ASTM A967 Type II (20-40% HNO3) or Type IV (nitric plus sodium dichromate), typically 20-50 degrees C for 20-30 minutes, remove free iron robustly with decades of service data, but nitrate-bearing waste streams raise treatment costs. Citric acid processes per ASTM A967 Type VIII (4-10% citric, 40-70 degrees C, 20-30 minutes) or AMS 2700 Method 2 are environmentally benign and safe for both austenitic and martensitic grades, and have become the default in aerospace and medical supply chains, though they demand more rigorous pre-cleaning. A practical grade-based selection: A2/A4 austenitics accept either route; free-machining 303 needs low-concentration nitric to avoid over-etching; martensitic 410/420 suits nitric-dichromate. The process chain is fixed: degrease, rinse, pickle, passivate, two-stage counterflow rinse (residual chloride is a hidden pitting trigger after passivation), then hot-air dry - no intermediate rinse may be skipped.

Acceptance and procurement: write the passivation condition into the spec

First checkpoint is free-iron contamination: the water-immersion test of ASTM A967 (an unbroken water film indicates a pass) or the high-persulfate copper sulfate test, sampled per batch. Second, corrosion verification: neutral salt spray per ISO 9227 (typical A4-80 requirement 480-1000 hours with no red rust; A2-70, 96-480 hours depending on service), with ASTM G48 ferric chloride or ASTM A262 intergranular tests added where pitting or welded 316 is a concern. Third, documentation: process type, temperature, time and test results recorded in an EN 10204 3.1 certificate or a standalone passivation report. Three procurement rules: specify 'condition: passivated per ASTM A967' in the contract; run a water-immersion test before goods-in; and for coastal, chloride or high-humidity projects, prefer passivated A4-80 over A2 plus light oiling. Need passivated A2/A4 stainless fasteners with batch-verifiable condition and 3.1 certification? Contact Garly Industry (164.hk) with your specification and service environment for a matched solution and quotation.

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