Ali cylinder exterior corrosion

[Paul Devane]

Hey guys.
I wanted to post this to see if anyone else has had similar problems.

The pictures are of my ali cylinder bought new in February 2011 and hardly used really!
The cylinder is Luxfer and is made from aluminum alloy 6061-T6.

The cylinder has also a brushed finish and is left untreated or painted.

I always clean down my gear with fresh water at the end of a days diving.
I was diving last week in Portroe and had to remove the stage atrap fromt he bottle so I it could be used as a single tank on a bcd. Thsi is when I noticed the corrosion!

To make a long story even longer.. I used a small wire brush to remove the corrosion as is did get a proper rince in portroe and I have it protected for now with ACF-50.

What do ye guys think regarding the photos? Teh pitting is approx 1mm deep in the worst place.

I had a stage strap with stainless jubilee and backed with the correct rubber so no way could it be down to dissimilar metal reaction.

Paul.


Paul.

[Stephen McMullan]

No expert but it really does look like electrolysis. Despite the rubber backing on the jubliee clips perhaps the salt water moisture was completing the circuit?

[Paul Devane]

Hi Stephen,

That is exactly what I suspected and I cant rule it out. I had the jubilee tightened on the nylon strap but the rubber backing on the jubilee was keeping them seperated.
If you look at the photo, you can see the line of the rubber of the jubilee. This is where it seems that salt deposited as the water evaporated.

I was not expecting this to happen! Luxfer warranty the cylinder for 15 years!!

To prevent this for gettign worse or happening again, Im going to spray the cylinder where the jubilee will be seated with U-pol acid 8 etch primer (I use this on maring engines)
I will also place the nylon strap inside a small rubber/silicone hose about 10mm either side of the jubilee. This should stop any contact betewwn the stainless clip and ali cylinder by water..

Paul.

[Sidemount Ninja]

Hi Paul, thanks for posting this. I'm amazed it happened so quickly, definitely galvanic corrosion. Must have been a salt bridge between the two metals but even then its a bit of a shocker just how quickly it happened.

I'll wait until the morning to check my cylinders, don't want nightmares. 

[markb]

Hi paul,thanks for the post,i like Aaron will be checking mine in the morning!!!

ps:maybe a bit of filler and silver paint would do the trick 

[Paul Devane]

Interseting bit of info regarding galvanic corrosion  http://corrosion-doctors.org/Forms-galvanic/galvanic-corrosion.htm

[MartinT]

Just had a look at mine Paul (it being from the same order) and theres nothing like that sort of corrosion although I have a different stage kit on mine. There is a little minor residue underneath the jubilee clip but nothing I'd be worried about.

[Paul Devane]

Martin, do you rinse your gear shortly after?

[MartinT]

Ya generally within 24hrs of the dive

[emmbee]

Hi Paul,

There's various versions of this "Luxfer's scuba cylinder visual inspection guide" available here or here.

It states in section 4C "External surface corrosion":

CONDEMN all cylinders with one or more external corrosion pits over 0.060 inch (1.53 mm) deep in the sidewall, crown, and/or base.
CONDEMN all cylinders with external line or broadspread corrosion when one or more pit, or the broadspread corrosion, is over 0.030 inch (0.76 mm) deep in the sidewall or crown.
CONDEMN all cylinders with external line corrosion over 6 inches (152 mm) long.

I'm not an expert, far from it, but if I were you I'd get this examined by somebody who is (as in, get it VIP'd) before someone gets hurt.

Hate to say this, maybe you already have, you didn't say...

Where did you get this jubilee and rubber thing?

Cheers,

Matthieu

[Paul Devane]

Hi Matthieu.

At the moment the cylinder is empty until I I have it inspected.

I am not taking a chance on a cylinder filled with 3016psi!! 

Paul.

[Paul Devane]

Are them visual inspection guids for the older type aluminium? I know there was a different alloy used on older alu cylinders that is not as good as the current 6061-T6

Read below and look at table B.2

http://www.nhzjj.com/asp/admin/editor/newsfile/2010317154753690.pdf

[emmbee]

Luxfer seems to have changed to the new alloy in '87/'88. The guide I linked to is from '98. AFAIK, it's still current: http://www.luxfercylinders.com/support/care-maintenance/411-luxfer-s-guide-to-scuba-cylinder-inspection.

Table B2 in the doc you linked says:

If a total length of corrosion in any direction exceeds the diameter of the cylinder and the depth exceeds 10 % of the original wall thickness
or
if the wall thickness is less than minimum guaranteed thickness

Like I said, I'm not an expert. Not even an amateur. I think the minimum guaranteed thickness is the design minimum wall thickness written on the cylinder, so 12.5 mm for the Luxfer Ali 80 next to me. But to know how much you have left, you'll need to do an inspection...

Cheers,

Matthieu

[gordy]

hi, one way around this problem is to cut a wide snoopy loop , say 60mm , and slide up the tank to desired position , then you need a
thin strip of lead , say 25 mm x dia of tank , wrap lead on top of snoopy loop and put jubilee clip over and tighten up . depending on the amount of diving you do will depend on how often you need to change out the lead strip , about every yr or so , another way that helps but isnt as good is to swap stainlees jubilee for galvanised and use snoopy under as well .
a good friend of mine told me about this set up afew yrs  back , he was a RN  diver .
hth
gordy,,,

[Graham Cowley]

I bought my 2 in around 2008, used for a year and stored on a shelf for nearly 2 with the odd dive, rinsed and kept dry, but I noticed bubbling of the paint around where the jubilee clip and material tubing was, I stripped it off and decided to respray them, so I tried a mild sandpaper and a plastic scrapper but not all the paint was affected, some bits would flake off (around the neck and bottom) and some was solid, so I got a paint remover called Strippit, which was recommended for use on Aluminimum and it worked a treat, some scrapping was needed but generally took all the paint off, then I used an Etch primer, which is needed to stop further corrosion and also is a good basis for primer and paint.
I have the cylinders ready now for spraying, using primer and main coat for boat hull, got from Sutton Marine, should have enough paint for 4/5 cylinders.
With that kind of corrosion Paul Id have a technician check the wall of the cylinder with an eddy current tester, to make sure its not going to fail its next test.