Ask any supplier for an "aluminum mounting plate" and you will get one of two alloys: 5086 or 6061. Both look identical and both cost roughly the same per kilo. They are not the same in salt water — and the difference shows up in the second season, not the first.
Alloy 5086 is part of the 5xxx series — magnesium is the main alloying element (around 4% Mg). This gives it natural corrosion resistance in marine environments without any surface treatment. Even a bare, unpainted 5086 plate will last 10+ years in salt spray. This is the alloy that boat hulls, deck plates, and marine mounting systems are made from worldwide.
Alloy 6061 is part of the 6xxx series — it uses magnesium and silicon. It is stronger when properly heat-treated (T6 temper), machines beautifully, and is cheaper to produce. But it is not naturally corrosion-resistant in salt water. Left bare, 6061 will pit and develop white aluminum oxide within a season. It needs hard anodizing, powder coating, or alodine treatment to survive marine use.
Weldability: 5086 welds easily with 5356 filler wire — standard MIG aluminum welding. 6061 can be welded but loses 40-60% of its strength at the weld zone unless re-heat-treated (which is impractical for boat accessories). If a product is welded, 5086 is almost always the right choice.
Strength numbers (don't be fooled): 6061-T6 yield strength is ~276 MPa vs 5086-H32 at ~207 MPa. On paper 6061 looks stronger. In practice, boat accessories rarely operate at stress limits — they fail from corrosion and fatigue long before static load. The weaker 5086 outlasts the stronger 6061 in every real-world boat test we've run.
MARINAC uses 5086-H32 for all structural marine components: mounting plates, motor brackets, tackle organizers, fender boards. For decorative or strictly freshwater parts (indoor boat fittings, freshwater-only tackle boxes) we use 6061-T6 where strength per gram matters more than decade-long salt resistance.
How to verify what you're buying: ask the supplier for a material certificate (3.1 or 3.2 per EN 10204). Any serious marine manufacturer provides it. If the answer is vague or "it's just aluminum" — assume it's 6061 and plan to repaint every season.
Bottom line: for anything that touches salt water and needs to last more than two seasons, pay the small premium for 5086. For freshwater lake rigs or weight-critical racing applications where you can re-anodize yearly, 6061 is fine. For everything else MARINAC makes — 5086 is the default, and we publish it on the product page.