Aluminium Corrosion in a Deep Water Marine Environment

[paul234]

Could someone please provide some help and guidance towards the use of Aluminium alloys in the marine environment.

It's use would be primarily for a diving vessel.

I have consulted the Galvanic series and considered Hard Anodising the material but am concerned of the effect of scratch/damage occurring which would exposure the underlying aluminium.

I have recently been looking into Copper/Aluminium alloys but have no reliable data.

Regards

Paul Coughlin
Design Engineer

 

[CESSNA1]

PAUL..:

A source to consider is the National Association of Corrosion Engineers (

http://WWW.NACE.ORG).

You may have to join, but they a very good (but not the only) source of corrosion information.

Regards
Dave

 

[rnd2]

Paul, aluminium hulled boats last for years and years -Commercial fishing vessels 20 years old are still in good condition. Provided they are adequately protected with zinc anodes,
scrathes would not present a problem.

 

[AlanD]

From my thesis research work several years ago. The 2XXX series Aluminum-Copper alloys are not suitable for an environment where corrosion is a possiblilty. The copper is selectively leached during any anodizing process, particularly from the grain boundaries. As a result, even in extremely low copper concentrations, this leaching process leave cavities in the material which encourages crevice corrosion.

When comparing the anodized alloys to the unanodized alloys of the some copper concentration, the unanodized alloys behaved better in a humidity test than the anodized alloys. However, they behaved equally poorly in a salt spay test.

In the past, aluminum-copper alloys have been clad in a layer of pure aluminium, to enhance its corrosion properties. However, there are better modern, age hardenable / high strength aluminum alloys.

 

[stevenal]

I believe the 5XXX and the 6XXX series alloys are the ones usually used for marine use. Don't submersibles spend most of their time on deck, more like a trailered boat than the commercial boats rnd2 mentioned? If so, are anodes required? Of coarse zinc might be good for ballast.

 

[EdStainless]

The 50XX alloys are the most common in marine applications. The biggest trick is to be careful about the attachment of other metals.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rnd2]

Even if the submersible were to spend most of its time on deck, unless its carrier vessel operates on a millpond expect a regular drenching with salt water. The total time the submersible is "virtually underwater" may be considerably more than it is actually underwater.

 

[rnd2]

Also,
5083 Al plate is used for hulls. It is easier to form curves than 6000 series.

 

[stevenal]

If anodes helped with salt water drenching, folks in the the Midwest would be bolting them to their cars right now. Time in the splash zone with atmospheric O2 present is not the same.

 

[rnd2]

Ah yes, it would be hard to imagine, but, hypothetically, if the folks in the mid-west had 40'waves crashing over their cars every thirty seconds or so all day and night, day after day, they might consider zinc anodes positioned to protect the exposed metal bits.

 

[stevenal]

Paul,

Will your sub be subjected to to 40' waves every 30s all day and night day after day when on deck? (Where's this, Antarctic Ocean)? Or will it be subjected to the more severe duty of occasional drenching and drying and salt fog. Sacrificial anodes address the first situation, selection of materials and coatings address the second.