Industry News

Marine fastener selection: 316 / 2205 stainless steel in salt-spray environments

2026-09-01

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

Why marine fasteners must resist salt spray

Salt concentration at sea is far higher than inland, and chloride ions penetrate platings to attack the base metal. Zinc-plated carbon steel often shows red rust within 72–96 hours in ASTM B117 salt-spray testing, and lasts even shorter in marine service: on an open deck with regular spray, a white-zinc plated M10 bolt can show red rust in three to six months, and once the coating is breached the corrosion spreads along the thread and into the joint. The failure is not gradual in the way an engineer expects. A corroded fastener loses its clamping load quietly, and by the time rust staining appears on the surrounding paintwork the joint has often been loose for a season. The cost pattern is what really drives the specification. A set of replacement 316 deck bolts for a 20 metre vessel may cost a few thousand Hong Kong dollars in material, but the labour to remove seized fixings, repaint the affected area and re-seal the deck can run ten to twenty times that, before any question of lost operating days. For marine fasteners, go straight to 316 stainless steel or the higher-grade 2205 duplex — solving corrosion at the material level and avoiding frequent replacement that affects voyage safety. The one exception worth noting is interior dry spaces such as the wheelhouse and accommodation joinery, where 304 stainless is entirely adequate and saves real money.

316 vs 2205 duplex — a selection comparison

The choice between the two main marine grades follows load and chloride severity rather than preference. 316 (A4) contains 2–3% molybdenum, resisting seawater pitting and crevice corrosion, suitable for deck and cabin joints, handrails, cleats, hatch frames and general topside fittings. 304 (A2): fine for interior dry areas, cabinetry and below-deck dry compartments, but it will pit in splash zones and should never be used on an open deck. 2205 duplex (S32205): roughly twice the strength of 316 with a yield strength of at least 450 MPa, and markedly better resistance to chloride stress-corrosion cracking, so it suits propeller shafting, ballast tanks, rudder assemblies, keel bolts, engine beds and other high-load, high-corrosion zones. Silicon bronze or copper alloys: chosen for traditional hull fastening and for fittings where a non-magnetic, gallvanically compatible material is needed below the waterline. Titanium: used on racing yachts and where weight and absolute corrosion resistance justify the premium. Where a joint combines high load and permanent immersion, duplex is usually cheaper over the life of the vessel than a larger number of 316 fasteners, because fewer and smaller bolts carry the same load. Match marine bolts, nuts and washers in the same material family to reduce galvanic corrosion; for high-strength jobs, order to drawing and request material certificates.

Galvanic corrosion and the dissimilar-metal trap

The most common marine failure is not a wrong grade selection but a correct grade used against the wrong neighbour. When two dissimilar metals sit in a wet, conductive environment, the less noble one corrodes preferentially, and seawater is an excellent electrolyte. Three pairings cause most of the trouble: stainless fasteners in an aluminium structure, where the aluminium around the bolt hole corrodes and the fixing works loose; stainless fittings against a carbon steel hull or frame, which rusts around the contact point; and bronze or copper components touching aluminium, an aggressive couple that can destroy the aluminium in a single season. Practical countermeasures are straightforward once the risk is recognised. Fit isolating washers and sleeves so the fastener shank never touches the dissimilar metal directly, and use a bedding compound or sealant to keep the electrolyte out of the joint. Keep the anode area small relative to the cathode where a couple is unavoidable, since the corrosion rate is governed by that ratio. Maintain the vessel's sacrificial anodes on schedule, because a depleted anode shifts the electrochemical balance and accelerates attack on structure and fixings. And wash down with fresh water after every passage where salt has been deposited, which is the cheapest and most effective single measure available to a boat owner.

Surface treatment, pitfalls and acceptance points

For even higher resistance, choose Dacromet-coated parts or passivated stainless steel, and note that passivation is not optional on machined stainless components — free iron left on the surface from tooling produces rust spots within weeks in a marine atmosphere, so ASTM A967 passivation should be a standard step. Where dissimilar metals meet, add insulating washers or a coating barrier. Four pitfalls recur on marine work. First, assuming all stainless is equal: a 304 fastener bought at a hardware shop for a splash-zone repair will pit within a year, and mixed batches of unknown origin are common. Second, ignoring thread galling, which is a serious issue for stainless running against stainless — apply an anti-seize compound and control tightening speed, or the nut will seize halfway down before reaching the required torque. Third, reusing fasteners after removal, since corroded or stretched bolts will not hold their clamping load a second time. Fourth, ordering by size alone without stating the grade, so the delivery may satisfy the dimension but not the corrosion requirement. At acceptance, check the material certificate and grade marking on a sample of bolts, confirm the passivation or coating treatment, request a salt-spray report where the specification calls for one, and verify that isolating washers and bedding compound were used at every dissimilar-metal joint. Need a marine fastener selection, a grade substitution reviewed, or 316 and 2205 stock with certificates? Contact JIALI Industrial (164.hk) with your application, salt-spray class and load requirements for fast confirmation of material and size.

Related Reading

• Fastener purchase contracts and drawing tolerances: getting the quality requirements right the first time
• Stainless bolts still rust? Passivation, corrosion testing and procurement essentials
• JIALI launches Stainless Steel zone with full 304 / 316 range

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