Why outdoor chairs lose their bolts before their frames

Frames rarely fail outdoors. On our own warranty data for terrace and poolside seating, close to 70 per cent of claims are fasteners rather than tubes or welds: a seized bolt, a rust run down a leg, or a joint that worked loose inside one season.

Published by JPChairs · Chair manufacturer in Foshan, China · 2026-10-22

What the fastener is up against

The bolt is the smallest cross-section in any outdoor joint, and it is always the first part to reach the end of its life.

• Wetness: a terrace chair sits wet for 1,500 to 2,500 hours a year in a temperate climate.
• Salt load: within 2 km of the sea, deposition runs 30 to 300 mg/m²/day depending on wind and shelter.
• Section: an M6 shank is about 28 mm² against 90 to 150 mm² of tube wall around it, so the same corrosion depth costs the bolt far more of its strength.
• Crevices: attack starts in the 0.1 to 0.3 mm gap under a bolt head, where the water never refreshes its oxygen.
• Trapped water: a sealed tube end outlasts an open one by years, because a blind hole never dries out.
• Pools: chlorinated water is harder on 304 stainless than sea air is; we have seen 304 stain in 6 months around a pool.

Mixing metals: the galvanic bill

Put two metals together with water between them and the weaker one pays. Aluminium frames with stainless bolts are the classic mistake.

• Aluminium to 316: roughly 0.4 to 0.5 V of potential difference, with the aluminium acting as the anode and pitting around the hole.
• Aluminium to mild steel: 0.6 to 0.8 V, and it fails inside a single coastal season.
• The fix: not a bigger bolt but a break in the circuit, using a 1.5 mm nylon or EPDM washer plus a nylon sleeve through the tube.
• Galling: stainless into naked aluminium thread seizes on assembly; above M8 we use a Helicoil or an M6 rivet nut with a 1.0 to 3.0 mm grip range.
• Head recesses: use capped or plastic-filled heads and a 0.5 mm shoulder clearance so water cannot sit in the socket.
• Cost: isolation hardware adds about USD 0.30 to 0.60 a chair, against USD 12 to 25 to replace a frame leg in year two.

Coatings and the salt spray numbers

Salt spray hours to ASTM B117 are a rough comparison tool, not a warranty, but they rank the options honestly.

• Zinc plated, 8 to 12 µm: 96 to 200 hours to white rust and 300 to 400 to red rust. Fine indoors, wrong outdoors.
• Dacromet or Geomet, 8 to 10 µm: 480 to 1,000 hours, and our default for steel fasteners on covered terraces.
• 304 stainless: 500 to 1,000 hours to first red rust, and it stains long before it actually weakens.
• 316 stainless: 1,000 to 2,000 hours, and worth the 20 to 40 per cent premium only within 2 km of salt water or at poolside.
• Powder coat over the joint: adds 60 to 80 µm, but it chips at the bolt head in year one and then holds water against the steel.
• Test caveat: a 1,000 hour cabinet result is not 1,000 hours outdoors; treat it as a ranking, not a life.

Size, grade and torque

Most loose joints come from under-specifying the grade, not the diameter.

• Diameter: M6 is enough for a seat-to-frame joint; M8 for anything carrying a back or an arm.
• Grade: A2-70 inland, A4-80 coastal, with the head marked so a replacement is never guessed at.
• Dry torque: M6 A2-70 about 6 Nm, M8 about 15 Nm, M10 about 30 Nm; drop these by 20 to 25 per cent if threads are greased.
• Preload loss: expect 20 to 30 per cent in the first season as coatings and washers bed in.
• Locking: nylon insert nuts lose grip after 3 to 5 years of UV exposure, so use all-metal lock nuts or serrated flange nuts outdoors.
• Serviceability: threadlocker on a joint that has to come apart is a mistake; a lock nut and a torque mark is not.

Details that buy five more years

None of these cost more than a few cents, and together they are the difference between a five year chair and a ten year one.

• Drainage: a 6 mm hole at the lowest point of every leg and at the bottom of every sealed rail.
• Clearance: shoulder bolts with 0.5 mm clearance, so the joint clamps the tube rather than bottoming on the shank.
• Caps: an open 25 mm tube end collects 30 to 50 ml of water a year and holds it against the weld.
• Isolation: one nylon washer per side at every dissimilar metal joint.
• Position: keep bolt heads out of the splash zone and off the floor contact line.
• Avoid blind holes: through-bolts with a rivet nut survive removal and refit; tapped holes in aluminium do not.

Spec lines for the enquiry

Six lines are enough to stop the hardware being quietly downgraded between sample and bulk.

• Material: A4-80 within 2 km of the coast, A2-70 inland, marked on the head.
• Coating: Dacromet 8 to 10 µm minimum on steel fasteners, or 316 stainless.
• Isolation: nylon washer plus sleeve at every aluminium-to-stainless joint.
• Drainage: 6 mm drain hole at the lowest point of each tube.
• Torque: M6 at 6 Nm and M8 at 15 Nm, re-torqued at six months and then annually.
• Spares: 2 per cent of the fastener count shipped loose in a service kit.

The frames these fasteners go into are shown on the outdoor chair frames; warranty and care questions are answered on the warranty and care FAQ.