Guides / Alloys
Choosing Aluminum for Parts That Live in Salt Water
September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked
Salt water is not just wet. It is an electrolyte, and that turns ordinary corrosion into an electrical process that can eat an aluminum part from the inside of a crevice where nobody will see it until something fails.
If your part will live on or near the sea, the alloy choice is not a preference. It is the main thing keeping the part alive.
Why the 5000 series wins here
The 5000 series alloys are aluminum with magnesium as the main addition. They are not heat treatable, so they never reach the strength of a 6061 or 7075 part, but magnesium gives them genuine resistance to salt.
The 6000 series adds silicon and magnesium. The 7000 series adds a lot of zinc and some copper. Copper and zinc are exactly what you do not want in an electrolyte, because they set up small galvanic cells inside the metal that drive pitting.
| Alloy | Yield, typical | Marine behaviour | Machining |
|---|---|---|---|
| 5052-H32 | ~193 MPa | Excellent, standard for sheet and formed parts | Soft and gummy |
| 5083-H111 | ~228 MPa | Excellent, standard for structural plate | Soft and gummy |
| 6061-T6 | ~276 MPa | Good with an intact coating, pits if scratched | Very good |
| 7075-T6 | ~503 MPa | Poor, prone to stress corrosion cracking | Good |
5052 or 5083
They overlap, and for a lot of parts either works.
5052 is the sheet metal alloy. It bends without cracking, it welds, and it is what you reach for on enclosures, brackets, trays and anything formed from flat stock.
5083 is the plate alloy. It is stronger, it is the structural choice for hulls and load carrying parts, and it holds up better where thickness is doing the work. It is also the one marine classification societies name most often.
If the part is under 6 mm and might be formed, look at 5052. If it is thicker and structural, look at 5083.
The mistake that actually sinks parts
Alloy choice is only half of it. The other half is what you bolt the part to.
Put a stainless fastener through an aluminum bracket, add salt water, and you have a battery. Stainless is cathodic to aluminum, so the aluminum corrodes preferentially around the hole. The fastener stays perfect and the part around it turns to white powder.
Three things reduce it, and none of them are free:
- Isolate. Nylon or phenolic washers and sleeves break the electrical path.
- Coat. A thick anodize or a chem film under the fastener head slows the reaction.
- Sacrifice. Add a zinc anode that corrodes instead, as boats have always done.
Say in the RFQ what the part bolts to and whether it is submerged, splashed or just coastal. It changes which alloy a shop recommends and whether they suggest a coating.
What changes at the machine
The 5000 series are softer than 6061 and they do not break chips cleanly. The material smears, tends to build up on the cutting edge and leaves a duller as-machined surface. Shops handle it with sharper tools, higher surface speeds and more coolant, but it is real work.
Two consequences for your quote. Expect a slightly higher price per part than the same geometry in 6061, and do not specify a fine surface finish on a 5000 series part unless you actually need it, because getting there takes an extra operation that 6061 would not have needed.
Sources
Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.
Frequently asked questions
Is 6061 really unusable in salt water?
It is usable and plenty of boats have 6061 parts on them. It just pits faster than the 5000 series and it relies more heavily on its coating staying intact. For a part that is rinsed and maintained, 6061 is fine. For something permanently submerged or never inspected, the 5000 series is the safer specification.
Can I anodize 5083?
Yes, and it takes a clear anodize reasonably well. Colour consistency is less predictable than 6061 because of the magnesium content, so for a cosmetic coloured finish expect more variation between batches.
Which is easier to machine, 5052 or 6061?
6061, clearly. The 5000 series alloys are softer and gummier, they smear rather than break into chips and they need sharper tools and different feeds. Expect a slightly higher price per part and a slightly worse as-machined surface finish.
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