Aluminum doubles

[Luis H]

I am thinking about setting up a set of double aluminum 50’s and I am debating between using tanks with a brushed aluminum finish or black painted aluminum tanks. My concern is with galvanic corrosion with the stainless steel bands. I guess I could always put a thin rubber strip between the bands and the tanks, but I was wondering how much of a concern it is.

I have seen plenty of dissimilar metal corrosion in a marine environment and even after rinsing the tanks, there is plenty of potential for electrolyte salt water being trapped between the bands and the tanks.

I would like to know what kind of experience people have with brushed aluminum tanks and SS bands. Do painted tanks work better or about the same?

Has anyone tried thin rubber or similar material for isolation in this situation?


Thanks

 

[ClayJar]

On my pony bottles, I use strips of rubber to isolate the stainless steel clamp bands from the aluminum cylinders, but I don't have a set of doubles at present. (I don't recall seeing "insulated" band doubles, but I'm sure someone will speak up.)

 

[Spimon]

I wouldn't have thought it would be that much of a problem due to the relative size of the tank (anode) to bands (cathode) (large anode to cathode ratio means corossion will be slow). If the cathode was much bigger than the anode it would be much more of a concern.

What about using the plastic 'waffle pattern' tank covers cut down to the length of the tank bands? That way you would have some insulation between the bands and the tanks, but also be able to wash off the majority of the salt and get more air flow?
I guess a solid rubber band (piece of a car inner tube or similar) would keep more water/salt out, but if it does get under there it would be harder to remove...

I'm sure someone who has first hand experience with this situation will be along soon to comment.

 

[Luis H]

What about using the plastic 'waffle pattern' tank covers cut down to the length of the tank bands? That way you would have some insulation between the bands and the tanks, but also be able to wash off the majority of the salt and get more air flow?
I guess a solid rubber band (piece of a car inner tube or similar) would keep more water/salt out, but if it does get under there it would be harder to remove...

I can get thin silicon rubber film with sticky back from McMaster Carr that I can easily line the inside of the bands with. It will not be visible and it may help to seal any water out and isolate the different metals, but I was wondering what other may have done…if anything.


This is my concern with that idea:
"I guess a solid rubber band (piece of a car inner tube or similar) would keep more water/salt out, but if it does get under there it would be harder to remove..."



Thanks

 

[DA Aquamaster]

I have a set of bands with what amounts to a rubber slip cover between the tanks and bands. In the past, rubber dip coated bands were also used and accomplished the insulation needed.

I agree that the tank may serve as a large anode, but my concern would be that most of the sacrifice of the anode would occur directly under the bands resulting in a pit.

 

[Luis H]

I don’t think the size of the anode or cathode makes much difference since the tanks actually spend very little time in salt water (the electrolyte). Most of the time they are stored dry (after they have been rinsed with fresh water).

My first concern is localized pitting under the bands due to galvanic corrosion (not electrolysis).

I know galvanic corrosion and electrolysis are kind of related, but my understanding is that they are different. Galvanic corrosion is localized to the contact area between dissimilar metals and does not require any external current or eddy currents. Were electrolysis occurs in an electrolyte bath and with either induced eddy currents (could be naturally occurring) or applied electrical currents. Any errors in this explanation please feel free to correct them.

In any case it seems that for starters the use of an isolating rubber shield between the bands and the tanks is a good idea. The question still is if it would be OK to use brushed aluminum or are the black painted ones superior?

How about the shot blasted aluminum tanks? Those look really cool...at least when they are new.

I think that I remember reading somewhere that the shot blasted aluminum tanks were bad news because the surface finish adds a large amount of surface area for corrosion to start…specially pitting. Has anyone else seen anything about this or did I imagine it?


I haven’t tried contacting Luxfer yet (and I don’t remember seen anything related in their FAQ), but I may end up sending them an email. I don’t know how responsive they are.


Thanks again for any help.

 

[ClayJar]

Actually Luxfer's new "natural" finish tanks (i.e. not shot-blasted, brushed, or painted -- just the natural mottled "ugly" oxide layer) would likely be an excellent idea. Not only are they potentially the most durable, but they don't look all shiny and inexperienced when you buy them.

 

[Doc Intrepid]

I had a set of double Luxfer 50s and a set of Catalina 63s, both were painted black and both used the black plastic 'waffle weave' sleeve over each tank as Spimon spoke of above.

The 50s I bought in the PX on Guam in 1983, and I sold them in 2003. Over the 20 years I used them they worked great, rinsed well, and I never had any pitting or corrosion. The plastic sleeve kept them in great condition. I took the 63s apart to use them as deco bottles about a year after I doubled them up, but they also suffered no ill effects from being doubled this way.

I love a small set of doubles. They made it easy to dive out of small boats and Zodiac-type inflatables, they were excellent for providing a bit more gas for longer dives in the 50'-70' range, they provided redundant regs for the weird types of shallow overhead diving I was doing, and they were handier for me to work with than larger single tanks in terms of logistics.

Go for it, and have fun with your small set of doubles.

Doc

 

[Flightlead]

I think that I remember reading somewhere that the shot blasted aluminum tanks were bad news because the surface finish adds a large amount of surface area for corrosion to start…specially pitting. Has anyone else seen anything about this or did I imagine it?

I seriously doubt that there is any significant increase in surface area. However, shot blasting MAY leave a residual compressive stress at the surface which could serve to inhibit corrosion.

 

[Couv]

Luis,

Be the first to have a "Gel Gasket" such as the latest gaskets used on aircraft antenna installations. The gel gasket will just about eliminate the corrosion between the bands and tanks by occluding water altogether. I know this would not be "vintage" but who would know? ;-)

Only half kidding, it just might be a viable alternative to a rubber sleeve that may actually trap water.

Check this out: http://www.avdec.com/images/Image007.jpg

c