Coastal Handrail Brackets: Alloy and Joint Selection

Selection: 316/316L (1.4401/1.4404) plate, A4-70 (316) fasteners, 316L (1.4430) filler wire - not 304 and not 308L.

Reasoning.

• Pitting and crevice corrosion. Chloride attack is the controlling mechanism in a marine splash zone. 304 has a pitting resistance equivalent number around 18; 316 with 2–2.5% molybdenum is around 25. The splash zone is the most aggressive part of the structure because surfaces are repeatedly wetted, concentrate chlorides as they dry, and are never washed down - so the passive film on 304 will break down at inclusions and scratch sites and pit.
• Crevices. Every bolt head, washer and weld toe is a crevice. Crevice corrosion is a chloride-driven, oxygen-starved attack and it is the most likely first failure of 304 here.
• The weld is the weak link. 308L deposits 304-type weld metal, so the weld and its HAZ would have the lowest molybdenum content on the whole assembly and would corrode first. 316L filler matches the parent metal. Use the low-carbon 'L' grade to avoid chromium carbide precipitation and sensitisation in the HAZ.
• Fasteners. A2-70 is 304; it would corrode in the bolted crevice and stain the bracket. A4-70 matches the parent metal and avoids a galvanic couple at the bolt.
• The inland precedent is irrelevant. Tea staining inland is cosmetic and driven by atmospheric deposits; it says nothing about chloride pitting at 80 m from salt water. Fifteen years with visual-only inspection and no repainting rules out both 304 and galvanised mild steel - the galvanised option would need coating maintenance every few years and would still rust at every weld and cut edge.
• If pitting appears in service, duplex 2205 (PREN about 35) is the upgrade path; 316 is the defensible minimum, not the ideal.

What the joint does wrong.

• Process problems. Zinc melts at 419 °C and boils at 907 °C; the arc vaporises the coating and the vapour causes porosity, heavy spatter and zinc oxide fume. Molten zinc can also penetrate austenitic grain boundaries and cause liquid-metal embrittlement and cracking in the stainless weld.
• Coating destruction. Welding burns the galvanising back from the joint, leaving bare steel, and the heat-affected coating around it is damaged and brittle.
• Galvanic couple. In the galvanic series, zinc is anodic to stainless steel. The stainless is a large cathode and the bare steel at the weld is a small anode, so the small anode corrodes very rapidly - the joint will rust through long before the surrounding coating is consumed, and the large cathode area accelerates depletion of the adjacent zinc.
• Weld dilution. 308L or 316L diluted with zinc and with carbon picked up from the steel gives unpredictable weld-metal composition and properties.

Least-bad detail if welding cannot be avoided:

1. Grind the galvanising back at least 50 mm each side of the joint, down to bright steel.
2. Weld with 309L (designed for dissimilar joints) or 316L, minimum heat input, stringer beads, no weave.
3. Remove all fume at source - this joint makes zinc fume inevitable.
4. Reinstate protection immediately: zinc-rich primer on the stripped steel, then a barrier coat over the joint and 50 mm beyond, so water cannot bridge the couple.
5. Detail for drainage: no upward-facing crevices, seal the joint faces with a neutral-cure sealant, and keep the anode area as large as possible relative to the cathode.
6. Better still, break the metallic path: a mechanically bolted connection with an insulating gasket and insulating washers removes the galvanic couple altogether and is usually cheaper than the corrosion repairs.

Specifying 316 delivers nothing if the surface is contaminated during fabrication. The controls are:

Segregation of tooling. Keep dedicated stainless tools: separate grinding discs, cutting discs, wire brushes, files and slings. A carbon steel wire brush or a disc previously used on mild steel embeds free iron in the surface, and those particles rust into brown bleeds that look like the stainless itself has failed.

Handling and environment. Do not grind or cut mild steel in the same bay while stainless work is open; airborne iron dust settles on the work. Handle with clean gloves, keep the parts off the floor, and protect finished surfaces in transit with chloride-free packing.

Cleaning. Degrease with chloride-free solvents only - never chlorinated degreasers, which are a classic source of stress-corrosion cracking in austenitic stainless. Remove all spatter and heat tint.

Post-weld treatment. Pickle and passivate the welds and HAZ to restore the chromium oxide film, either with pickling paste per the manufacturer's instructions or by brush or spray passivation to ASTM A380 practice, followed by a thorough rinse with clean, low-chloride water. Heat tint must be removed, not painted over: it is a chromium-depleted oxide that will pit.

Dimensional and finish checks. Verify weld leg length, bracket squareness and hole positions before drilling the plinths, and record the heat numbers of plate and filler so any corrosion complaint can be traced.

Site controls. Fit the A4-70 bolts with a torque wrench to the specified value; a slack bolt makes a crevice that holds water. Seal the bolt heads with a neutral-cure sealant, ensure the plinth top falls away from the post so water cannot pond at the base, and do not let site crews touch up the stainless with mild steel tools or ordinary steel wool. Issue the client a simple annual inspection sheet: look for brown staining at welds and bolt heads, and check for pitting with a magnifier.