Material Selection for a Corrosive Marine Bracket
A bracket on a marina walkway must carry a 15 kN static load and is exposed to salt spray and occasional immersion. Three options are shortlisted: hot-dip galvanised mild steel, 316 stainless steel, and 6061-T6 aluminium. The bracket is a flat plate in tension with a 200 mm^2 minimum cross-section at the bolt holes. Mild steel yield is 250 MPa, 316 stainless is 205 MPa, and 6061-T6 is 275 MPa. The client wants a 20-year life with minimal maintenance and is worried about galvanic corrosion where stainless bolts pass through an aluminium bracket.
1. Check the stress in each material at the minimum section and judge whether strength alone eliminates any option.
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Stress = F/A = 15 000 / 200 x 10^-6 = 75 MPa in all cases, since the geometry is fixed.
Safety factors: mild steel 250/75 = 3.3; stainless 205/75 = 2.7; aluminium 275/75 = 3.7. All exceed a typical factor of 2, so strength alone does not eliminate any option. The decision must be driven by corrosion and maintenance, not static strength.
2. Recommend a material and justify it, addressing the galvanic corrosion concern.
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Hot-dip galvanised mild steel has a zinc coating that sacrifices itself to protect the steel, but in salt spray and immersion the coating is consumed over time and the 20-year maintenance-free target is unlikely without re-coating.
6061-T6 aluminium is strong enough and forms a passive oxide layer, but it is anodic to stainless fasteners; in wet salt conditions the aluminium around the bolts will corrode preferentially, and the small anode area relative to the cathode makes it worse. Insulating washers and sealant could mitigate this but add a maintenance dependency.
316 stainless is the best judgement: its molybdenum content gives good chloride resistance, it is cathodic to both other options so it will not be the sacrificial anode, and it needs no coating. The lower safety factor of 2.7 is acceptable for a static walkway load. Specify stainless fasteners to avoid a dissimilar-metal couple.