Small Tanks Corroding on the AP Valves BCD Commando, anybody has this too?

[ChangeAgent]

PS
Does anybody know what Luxfer does when I buy a 80 cft tank from them? What is the outside treated with?

 

[ChangeAgent]

slather it in silicone without the and

slather it in silicone without the and

before you slather it in silicone without the and

several of your sentences end with the word and. I am not sure how to interpret this. what do you mean?

 

[Eric Sedletzky]

Can someone PLEASE post a photo so I can understand what’s going on??
I might be able to help.

 

[Tanks A Lot]

What you describe is very typical of painted aluminium cylinders.

Aluminium also forms rust, aluminium oxide AL₂O₃. This is not commonly called rust, but it is essentially the same as a piece of iron rusting, it oxidizes. Unlike iron oxide, aluminium oxide does not flake off the base material. It forms a protective layer on top of the aluminium underneath which is just 4nm thick. The underlying aluminium does not come in contact with oxygen anymore. Therefore, it cannot form any additional aluminium oxide. Aluminium is protected by a layer of rust from rusting. This forming of a protective layer is also called making the metal passive. Passive in the sense that the underlying material will not react with oxygen anymore. This principle is at work in stainless steels, where more than 11% of the alloy's composition is made up of chromium atoms. These chromium atoms shield the underlying iron atoms from further oxidization by creating a thin layer of chromium oxide (Cr₂O₃). Again, it is only the outermost layer of the base material which comes into contact with the oxygen. Making a metal passive, can be achieved by several different methods. Painting a metal surface sufficiently is also a form of passivating the material. It prevents the base alloy coming into contact with the oxygen from the surroundings. All of the different methods have in common that they put a barrier between the base alloy and environment.

The oxygen atoms essentially bounce off the layer of AL₂O₃. They cannot react with the base alloy to create more AL₂O₃.

Unfortunately, these protective layers only work if they are sound and in place. Scratching the paint of a piece of iron will of course lead very rapidly to rust. The same is true if the formation of the passive layer is inhibited. A tight-fitting boot on an aluminium cylinder will inhibit the formation of this layer, so do the rather shoddy paint jobs. The aluminium oxide can't stick to the base alloy, leading to more and aluminium oxide formation. A tight-fitting cylinder net on an aluminium cylinder does the same. An aluminium cylinder is always left best without a net and without a boot and without any paint. The paint can chip and let oxygen and water get to the base alloy, while inhibiting the formation of a protective layer of aluminium oxide. The result will be a slow buildup of powdery aluminium oxide underneath the paint. This leads to the formation of small "bubbles" underneath the paint and the eventual chipping off of that very same paint.

A tight fitting boot, net or shoddy paint all inhibit the formation of the passivation layer.

Long story short, get unpainted cylinders or just strip yours.

 

[ChangeAgent]

Can someone PLEASE post a photo so I can understand what’s going on??
I might be able to help.

 

[ChangeAgent]

What you describe is very typical of painted aluminium cylinders.

The paint can chip and let oxygen and water get to the base alloy, while inhibiting the formation of a protective layer of aluminium oxide. The result will be a slow buildup of powdery aluminium oxide underneath the paint. This leads to the formation of small "bubbles" underneath the paint and the eventual chipping off of that very same paint.

Long story short, get unpainted cylinders or just strip yours.

this is exactly happening to mine. reading this I do not understand why AP Diving paints these tanks.

so back to practical. what would the best way to strip the paint in your humble opinion?

 

[Eric Sedletzky]

this is exactly happening to mine. reading this I do not understand why AP Diving paints these tanks.

so back to practical. what would the best way to strip the paint in your humble opinion?

Aluminum does better if left completely open so it can breathe and dry out. Trapped moisture and especially salty moisture is death for aluminum under paint.
Strip it off with paint stripper or sand it off with fine grit sandpaper and polish it out.
You will see a grain in the aluminum surface once you strip off the paint. As long as you don’t sand past the low spots of the grain you’ll be fine.
We’re only talking about 1000ths in measurement so carefully removing the paint and polishing won’t affect the integrity of the tank at all.
If you want to repaint it then it will still need to completely stripped and sanded, then acid washed with alumaprep 33, then treated with alodine, then a proper true epoxy primer, then topcoat.
Personally, I would just leave it bare and polish it out.

 

[ChangeAgent]

Aluminum does better if left completely open so it can breathe and dry out. Trapped moisture and especially salty moisture is death for aluminum under paint.
Strip it off with paint stripper or sand it off with fine grit sandpaper and polish it out.
You will see a grain in the aluminum surface once you strip off the paint. As long as you don’t sand past the low spots of the grain you’ll be fine.
We’re only talking about 1000ths in measurement so carefully removing the paint and polishing won’t affect the integrity of the tank at all.
If you want to repaint it then it will still need to completely stripped and sanded, then acid washed with alumaprep 33, then treated with alodine, then a proper true epoxy primer, then topcoat.
Personally, I would just leave it bare and polish it out.

Thanks Eric, very helpful. I think I strip it and leave it. What kind of stripper would you recommend or just any old stripper form the DIY store? And when you say 'polish it out'. Do I do this with a polishing agent? or how to best do this. I am here in new territory for me, hence the questions.

 

[Eric Sedletzky]

Thanks Eric, very helpful. I think I strip it and leave it. What kind of stripper would you recommend or just any old stripper form the DIY store? And when you say 'polish it out'. Do I do this with a polishing agent? or how to best do this. I am here in new territory for me, hence the questions.

I don’t know where sco is, but here in California good aircraft striper is a thing of the past.
Best I can say is go to the hardware store and get some jasco or whatever they have and try that. It may need to sit on there a while. Between that and sanding the rest off with 220 you need to get all the paint off. Then you can go to 400 then up to 1000 grit sand papers and maybe even 1500 grit.
Don’t oversand with the courser grits.
Once you have the surface where you want it you can use a metal polish by hand. Or for a stellar job, get a cotton wheel buff that goes onto a drill mounted on a mandril and use a little brown tripoli rouge bar that you spin the wheel on to take up some rouge. All this stuff you can get at a Harbor Freight store.
And then also pick out all the paint out of the stamped numbers with something like an awl.
For maintenance just use a little hand metal polish once in a while to keep it bright.
Salt water isn’t the culprit in deadening the surface of bare aluminum, chlorinated pools are.

 

[ChangeAgent]

Thanks Eric for engaging with your knowledge. Much appreciated.
BTW, Sco is Scotland. It is the official abbreviation . I updated my profile for others that might not know.
Thanks I will give it a go. I need to see what I can get here, but am sure I find something.
To polish could I also use (in your humble opinion) the grit paste I use to polish and settle the valves of a cylinder head?