Galvanized Walkway Brackets at a Coastal Site
A coastal municipal walkway uses 6 mm hot-dip galvanized mild-steel angle brackets to support handrails. The brackets were cold-bent to an inside radius of 8 mm after galvanizing, then fixed with A4 stainless-steel M12 bolts through 14 mm holes. The site is 40 m from the sea and receives salt spray with alternate wetting and drying. After eight months, maintenance reports red rust streaming from the bolt holes and along the outside of the bends; white rust is present on horizontal ledges. Coating thickness on a removed bracket averages 85 µm away from bends, but only 35 µm at the bend crowns. The bolt threads show no corrosion, but the galvanized steel around the holes is deeply pitted. The client proposes simply painting the affected areas and replacing the bolts with larger stainless bolts. There is no isolation washer or sealant at the connections.
1. Explain the corrosion pattern and assess whether the client’s proposed paint-and-bolt repair is appropriate. Recommend a durable connection detail.
Read the worked answer
The pattern is not uniform atmospheric rusting; it is crevice and galvanic corrosion at the stainless/galvanized connection. In salt spray the zinc coating is the anode and the A4 stainless bolt is a relatively large cathode. At a bolt hole the coating is damaged by drilling and the exposed steel is small in area, so corrosion is intense and pitting is deep. The bend crowns have only 35 µm coating because the coating cracked and flaked during post-galvanizing bending; red rust appears there. White rust on ledges is zinc corrosion product from repeated wetting. Painting over rust will not stop the cell; it may trap moisture. Larger stainless bolts increase the cathode area and make the galvanic effect worse. A durable detail would use hot-dip galvanized bolts with galvanized washers, or stainless bolts isolated by nylon/neoprene washers and bushings, sealed with a neutral-cure sealant, with clearance holes deburred and drain paths. The bracket should be bent before galvanizing and given a zinc-rich repair coat at any damaged area.
2. The bend radius is 8 mm on 6 mm plate. Evaluate the bending sequence and propose a manufacturing change, including inspection criteria.
Read the worked answer
For 6 mm plate, 8 mm inside radius is 1.33t. That may be acceptable for the steel, but not for a brittle zinc coating on the outside of the bend; the coating cracks in tension, and the measured 35 µm at the crown confirms it. Bending after galvanizing is therefore the process error. Options: (a) bend before galvanizing, then galvanize the finished bracket, so the coating covers the bend; or (b) bend after galvanizing only with a larger radius, roughly 2.5t to 3t (15–18 mm), and repair the coating with zinc-rich paint. Inspection should include coating thickness per ISO 1461 on the finished part, visual check for cracks and bare steel, adhesion/bend test on samples, and checks that the bend line is transverse to the rolling direction where possible. Also control springback and use a bend radius that avoids excessive thinning.
3. If replacement is required, compare hot-dip galvanized brackets with powder-coated aluminium brackets for this location, considering maintenance access and a 20-year life.
Read the worked answer
For a 20-year life in a coastal C4/C5 environment, hot-dip galvanized steel at 85 µm is a known system if the coating is intact and the connection is detailed to avoid crevices. Powder-coated aluminium is lighter and avoids the zinc/stainless couple, but any scratch or cut edge becomes a small anode in chloride and can pit deeply; powder coating at bends and bolt holes is also difficult to maintain. Aluminium needs a thick anodized or powder-coated finish and stainless or isolated fasteners. Maintenance access matters: galvanized brackets can be inspected and touched up with zinc-rich paint; aluminium damage is harder to repair on site. My choice would be hot-dip galvanized brackets, bent before galvanizing, with isolated stainless fasteners and sealed crevices, because it gives a sacrificial coating that tolerates minor damage. Aluminium would be acceptable only with a specified coating system and a maintenance regime.