How to Select the Right Marine Aluminum Bar for Boat Repairs

2026-02-20 16:36:41

Most boat-repair guides focus on "strength" first. A more useful way to choose a marine aluminum bar is to think like a corrosion accountant: your real cost is paid over time in galvanic corrosion, cracking, and rework, not at the checkout. The "right" bar is the one that survives your specific marine environment and repair method with minimal future maintenance.

Below is a quick, practical selection guide you can use at the dock or in the workshop.

1) Start With the Repair Job: What Is the Bar Actually Doing?

Before picking an alloy, identify the bar's role:

  • Structural / load-bearing (brackets, engine mounts, transom supports): needs high strength and good fatigue resistance.
  • Machined parts (bushings, fittings, spacers, custom blocks): needs excellent machinability and stable dimensions.
  • Welded repairs (frames, rails, cracked supports): needs weldability and post-weld corrosion resistance.
  • General non-structural fixes (tabs, backing plates, light brackets): prioritize corrosion resistance and ease of fabrication.

This step prevents the common mistake of choosing a "strong" alloy that later cracks or corrodes prematurely.

2) Choose the Alloy by Environment First (Not Just Strength)

Best "Marine-First" Choices

5083 / 5086 (5xxx series)

  • Why choose it: Excellent seawater corrosion resistance; strong enough for many structural repairs; great for marine service.
  • Best for: Brackets, plates, bars near salt spray, hull-related components, general repairs.
  • Watch-outs: Machining is okay, but not as clean as 6061; availability in certain bar sizes can vary.

6061-T6 (6xxx series)

  • Why choose it: Balanced-good strength, widely available, good machining, decent corrosion resistance when protected.
  • Best for: Machined parts, structural brackets, repairs that require drilling/tapping.
  • Watch-outs: In true saltwater exposure, it benefits from coating/anodizing and good isolation from dissimilar metals.

Specialty / "Use With a Clear Reason"

6082 (common outside the US)

  • Similar use-case to 6061; often chosen for availability and strength.

7075 (7xxx series)

  • Why people want it: Very high strength.
  • Why it's risky on boats: Lower corrosion resistance; not ideal for welding; can be a long-term headache in wet, salty areas.
  • Use only for: Dry, well-protected, non-welded, high-load parts with proper surface protection and isolation.

2024 (2xxx series)

  • High strength but poorer corrosion resistance; generally not recommended for typical marine exposure.

Practical rule:
If the part is regularly wet with seawater, start with 5083/5086. If it's a machined component in a less aggressive location, 6061-T6 is usually the most practical.

3) Welding Changes the "Right Choice"

If your repair involves welding, alloy selection becomes more specific:

  • 5xxx alloys (5083/5086) are typically excellent for welding and marine durability.
  • 6061-T6 is weldable, but remember: the heat-affected zone loses strength (locally closer to T4-ish properties). Design around that with thicker sections, gussets, or geometry changes.

Also choose the right filler wire:

  • Common options: ER5356 or ER5183 (often used with 5xxx, and also with 6xxx depending on requirements).
  • The "right" filler depends on base alloy, strength target, and service temperature-if you tell your supplier what you're welding, they can match it properly.

4) Select Temper Like You Select "Risk Level"

  • T6: higher strength (common in 6061-T6) for machined/bolted parts.
  • H116 / H321 (common in marine 5xxx plate products): optimized for marine corrosion performance and stability.

Tip: For marine bar repairs, ask explicitly for marine-grade certification and mill test reports (MTRs)-especially for parts that will be hard to access later.

5) Bar Shape Is a Design Decision (Not a Catalog Choice)

Pick the shape that reduces stress concentration and water traps:

  • Flat bar: good for backing plates and brackets; easy to drill; watch for edge corrosion-radius edges when possible.
  • Round bar: best for pins, shafts, spacers; fewer crevice sites.
  • Square/rectangular bar: great for machining blocks and mounts; be mindful of sharp corners (stress risers).

Corrosion accountant tip: Avoid designs that create crevices (tight overlaps, unsealed seams). Crevice corrosion often beats "alloy choice."

6) Don't Ignore Galvanic Corrosion (Fastest Way to Lose a "Good" Alloy)

Your aluminum bar doesn't corrode "alone"-it corrodes in a system.

  • Stainless fasteners + aluminum in saltwater = galvanic risk
  • Carbon steel contact = severe corrosion risk
  • Copper-based anti-seize on aluminum = can worsen galvanic issues

Simple protections that work:

  • Use isolation washers/bushings (nylon, acetal, phenolic)
  • Apply marine sealant between mating surfaces
  • Choose the right primer + topcoat or hardcoat anodize (when appropriate)
  • Prefer aluminum or coated fasteners where feasible, or isolate stainless properly

7) Quick Selection Cheat Sheet (Most Common Boat Repairs)

  • Welded structural bracket in salt spray: 5083/5086 (marine grade), correct filler wire, avoid sharp corners
  • Bolt-on engine/seat mount you'll machine and tap: 6061-T6 (coat/anodize if exposed)
  • Backing plate under hardware on deck: 5083/5086 or 6061 with good sealing; prioritize crevice prevention
  • Custom spacer/pin in relatively dry area: 6061-T6 round bar
  • High-load part but always wet: Resist the urge to pick 7075-use a marine 5xxx design solution instead (thicker, gusseted)

8) What to Ask Your Supplier (Saves Time and Rework)

  1. "Is this marine-grade and suitable for seawater exposure?"
  2. "Can you provide MTR/traceability for the alloy and temper?"
  3. "Is it appropriate for welding, and what filler do you recommend?"
  4. "What's the exact size tolerance (for machining/fit-up)?"

Bottom Line

Selecting the right marine aluminum bar isn't just picking the strongest alloy-it's choosing the bar that will stay stable in saltwater, match your fabrication method, and avoid galvanic traps. For many real-world boat repairs, the safest defaults are:

  • 5083/5086 for marine exposure and welded durability
  • 6061-T6 for machined, bolt-on practicality (with proper protection in saltwater)

Related Blog