Bracket material for a salt-water pontoon
A coastal boatyard must fabricate 40 identical mounting brackets for a floating pontoon. Each bracket is a 6 mm thick plate, 150 mm × 100 mm, with two 14 mm bolt holes. It is welded to a 50 mm × 50 mm × 4 mm square hollow section stub and carries a design load of 2.5 kN. The bracket is splashed with salt water daily, and the fixing bolts are A4 stainless. The yard has MIG sets for steel but no AC/DC TIG set. The stores hold S275 mild steel, hot-dip galvanised S275, 304 stainless, 316 stainless and 6082-T6 aluminium. Lead time is two weeks and the budget is tight. Previous 304 stainless brackets showed brown stains and pitting after six months, and some galvanised brackets rusted at drilled holes and weld edges.
1. Select the most suitable material for the brackets and justify your choice against corrosion, strength, weldability, cost and lead time.
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Hot-dip galvanised S275 is the best compromise. S275 has ample strength for a 2.5 kN bracket, is easy to MIG weld with a normal steel set, and is available within two weeks at low cost. The zinc coating gives sacrificial protection in salt splash, provided it is at least 85 µm thick for 6 mm steel to ISO 1461 and is not damaged. 304 stainless is unsuitable because chloride pitting has already occurred; 316 would resist pitting better but costs much more and needs matching 316 filler and tighter welding control. 6082-T6 aluminium is not a good choice: welding destroys the T6 strength in the heat-affected zone, it needs a TIG or pulsed MIG set, and it forms a strong galvanic couple with A4 stainless bolts. The choice is therefore not simply the most corrosion-resistant metal; it is the one that meets the load, the available process, the budget and the two-week lead time.
2. Describe how you would protect the galvanised brackets at the holes, cut edges, welds and stainless bolt contacts.
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Drill the 14 mm holes before galvanizing, because drilling after coating exposes bare steel. Weld before galvanizing, then clean the welds and remove all slag and spatter so the zinc can adhere. After galvanizing, inspect for bare areas and repair cut edges, weld damage and thread areas with a zinc-rich paint applied in two coats to the maker's thickness. Do not rely on the coating alone at the bolt contact: A4 stainless to zinc is a relatively small couple, but the bolt can damage the coating and trap salt water. Fit nylon isolating washers or a sealed washer under the stainless bolt head and nut, and use a sealing compound at the joint faces. Provide a drain or weep hole so water cannot stand in the hollow section, and avoid sharp edges that thin the coating.
3. Explain why the 304 stainless brackets failed and whether 316 stainless or aluminium would be more suitable if the budget were increased.
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304 stainless relies on a chromium oxide passive film. In salt splash, chloride ions break down that film locally and prevent it from repassingivating, producing pitting and brown rust stains. 316 contains molybdenum, which improves chloride resistance, so it would perform better, but it is not immune in a marine splash zone and it costs more. It would also need 316 filler and careful cleaning to avoid carbon pickup and weld decay. Aluminium 6082-T6 would resist salt water if properly protected, but welding reduces its strength in the heat-affected zone, and contact with A4 stainless bolts creates a galvanic couple in which aluminium becomes the anode. If aluminium were used, it would need isolating washers, a suitable filler such as 5356, and a design that accounts for the lower HAZ strength. Increasing the budget does not automatically make either option better than galvanised steel.