Industry News

Rusty screws? Choose material and finish right

2026-08-10

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

Why fasteners rust and where it starts

Carbon steel rusts in humid and salt-spray environments; weak coating or scratches are common causes. The mechanism is worth understanding because it explains why two identical-looking parts can behave completely differently. Iron in the presence of water and oxygen forms hydrated oxide, and the rate depends on three things: how long the surface stays wet, how much chloride is present, and how well the coating isolates the steel. Hong Kong is close to the worst case on the first two. Relative humidity stays above 70 per cent for most of the year, coastal chloride deposition is high, and the daily condensation cycle on a metal structure means many fixings are wet for hours every morning even when it has not rained. In that environment a bright zinc-plated fastener with a 5 to 8 micrometre coating will typically show white rust within three to six months on an exposed coastal site and red rust within one to two years. Four failure paths account for most cases. Coating too thin for the environment, which is a specification error rather than a product defect. Coating damaged during installation, since driving a screw with an impact driver or over-tightening a bolt shatters the plating at the contact faces where corrosion then begins. Crevice attack under a washer or inside a lapped joint, where water sits and cannot dry. And galvanic corrosion where a stainless or plated fastener contacts a dissimilar metal, which corrodes the less noble neighbour and looks like a fastener problem but is a design problem. The rate also depends strongly on exposure position: a fixing on the sheltered north face of a building may last three times as long as one on a rain-washed but salt-loaded seaward face, so one specification applied across a whole project rarely fits everywhere.

Anti-rust options compared by environment

Outdoor and coastal conditions call for stainless (304 / 316) or Dacromet and hot-dip galvanising; dry indoor work is adequately served by zinc plating. Working through the options by environment gives the clearest picture. Dry indoor service, air-conditioned or heated space — bright zinc plating at 5 to 8 micrometres is sufficient and the best value. Humid indoor service such as kitchens, laundries and plant rooms — zinc plating at 8 to 12 micrometres with a clear trivalent passivate, or stainless 304 where the parts are visible or hard to reach. Sheltered outdoor service with rain washing and low chloride — hot-dip galvanising at 45 to 60 micrometres, giving a service life measured in many years. Coastal outdoor within a few kilometres of the sea — hot-dip galvanising above 65 micrometres or stainless 316; zinc-nickel alloy at 8 to 12 micrometres is also a good option where the coating thickness available matters, since it delivers 600 to 1000 hours of neutral salt spray. Direct marine splash or permanent wetting — stainless 316 or 2205 duplex, with Dacromet as a lower-cost alternative for non-structural parts. High-strength bolts above 10.9 grade needing corrosion protection — Dacromet or zinc flake rather than electroplating, because the acid pickle and plating process introduces hydrogen that can cause delayed brittle fracture. Aluminium structures — stainless fasteners with isolating washers, never bare carbon steel. Swimming pool halls and chlorinated washdown areas — stainless 316 as a minimum, since even 304 will pit in that atmosphere. The general principle across all of these is that stainless solves corrosion by changing the material, while coatings delay it by buying time, and the choice between them is usually about access for future replacement as much as about corrosion rate.

Scenario by scenario selection

The same information read as a selection table from scenario to recommended specification and reason. Indoor machinery in a dry factory — zinc-plated 8.8 grade bolts, because the coating is adequate and the cost is lowest. Roof-level plant within sight of the sea — stainless 316 for brackets and enclosures, because chloride deposition is continuous and access is expensive. Structural steelwork exposed to rain — hot-dip galvanised bolts with a minimum 65 micrometre coating, since this gives decades of service with periodic inspection. Curtain wall and façade fixings — stainless 316 or 2205 duplex, because the fixings are concealed and cannot be replaced without disturbing the façade. Solar racking on a coastal rooftop — hot-dip galvanised structural bolts with stainless clamps, matching each part to its exposure and replacement difficulty. Vehicle undersides and washdown equipment — Dacromet or zinc-nickel, which tolerate road salt and frequent wetting and avoid the embrittlement risk of plating. Food and beverage plant with daily sanitising — stainless 316, since chlorinated cleaning agents attack even a good coating and the material must be cleanable. Temporary or short-life fixings — bright zinc plating or black oxide, because the cheapest finish that outlasts the service requirement is the correct engineering answer. Where a project mixes these environments, as most buildings do, the specification should be set by zone rather than by structure, since applying a single class everywhere either wastes money on the sheltered areas or leaves the exposed areas under-protected.

Pitfalls and inspection checklist

Three pitfalls cause most premature rust. Substituting a cheaper coating class at the buying stage, which passes a visual check at delivery and fails within a year in service. Damaging the coating during installation with an impact driver or an over-torqued bolt, so that corrosion starts at the contact faces that no one inspects. And assuming stainless means rust-proof, when a non-passivated or contaminated stainless part will spot-rust within weeks and a 304 part in a chloride atmosphere will pit regardless. Three further traps are worth noting: using a carbon steel wire brush or a grinding wheel previously used on mild steel on stainless parts, which embeds free iron and seeds rust; mixing materials in one joint without isolating washers, which drives galvanic corrosion; and treating rust as a cosmetic issue, when a corroded fastener has often already lost clamping load. At inspection, work through five checks. Look at the sheltered and crevice positions rather than only the visible surfaces, since that is where attack starts. Check the joint rather than only the fastener, looking for movement, cracked paint around the fixing and water staining that indicates repeated wetting. Verify the coating class against the original specification on any replacement, so the repair does not inherit the same fault. Confirm that isolating washers and bedding compound are present wherever dissimilar metals meet. And record the location and date of any rust found, so that the pattern across the site can be used to set a realistic replacement interval rather than fixing the same position repeatedly. Need anti-rust selection advice or a coating recommendation for a specific exposure? Contact JIALI Industrial (164.hk) with your environment and service life requirement.

Rusty screws? Choose material and finish right

Why fasteners rust and where it starts

Anti-rust options compared by environment

Scenario by scenario selection

Pitfalls and inspection checklist

Related Reading