Material Choice for a Salt-Spray Exposed Bracket
A coastal marina asks for 500 brackets to hold navigation lights on a steel groyne. The brackets are 180 mm long, L-shaped, bolted through two 10 mm holes, and must carry a 6 kg light plus wind load (design 250 N at the tip). The environment is salt spray with daily wetting. Three options are quoted: hot-dip galvanised mild steel (yield 250 MPa, $14 each, 5-year life with maintenance), 316 stainless steel (yield 205 MPa, $58 each, 20-year life), and 6061-T6 aluminium (yield 240 MPa, $22 each, but galvanic corrosion risk against the steel groyne). The client's budget is tight and they ask for the cheapest option that will last 20 years.
1. Check the strength of the lightest section for each material and compare 20-year costs.
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Bending moment at the bracket root: M = 250 N × 0.18 m = 45 N·m = 45,000 N·mm. Required section modulus Z = M/σ_allow. Using a safety factor of 1.5 on yield:
Galvanised steel: σ_allow = 250/1.5 = 167 MPa, Z = 270 mm³. 316 stainless: σ_allow = 205/1.5 = 137 MPa, Z = 328 mm³. 6061-T6: σ_allow = 240/1.5 = 160 MPa, Z = 281 mm³.
All three are easily met by a modest 30×6 flat, so strength does not decide it - corrosion does. 20-year cost: galvanised needs replacement or repaint at year 5, 10, 15. Even ignoring labour, 4 sets × 500 × $14 = $28,000. Stainless: 500 × $58 = $29,000 once. Aluminium: 500 × $22 = $11,000, but galvanic isolation (insulating washers, isolating coating) adds cost and risk.
On materials alone stainless and repeated galvanised are close; once maintenance labour and access costs on a groyne are included, stainless is cheaper. The judgement is that the 'cheapest' option depends on whether maintenance access is costed - and on a marine groyne it must be.
2. Explain the galvanic risk for the aluminium option and how it changes the recommendation.
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Aluminium is anodic to steel in the galvanic series, so in a wet, salty environment the aluminium bracket becomes the anode of a galvanic cell with the steel groyne as cathode. The small anode (bracket, especially at the bolt holes) relative to the large cathode (groyne) concentrates corrosion at the fasteners and contact faces, which are also the load path. Pitting at the holes could cause premature failure well before the 20-year target, and the failure would be hidden behind the fixing.
Mitigation requires insulating washers and sleeves, a dielectric coating on the contact faces, and regular inspection - all of which erode the $11,000 saving and add a failure mode. Given the inaccessible location, the aluminium option is not robust. Recommend 316 stainless, which is passivated in salt spray, or hot-dip galvanised steel if the client accepts a scheduled maintenance regime with realistic access costs. The decision should weigh lifecycle cost and failure consequence, not sticker price.