Pony bottle and Galvanic corrosion

[drbill]

This is an issue I brought up here and on a few other boards several years ago when I noticed my Al pony bottles were showing signs of what appeared to be electrolytic corrosion. I also dive a steel 120. My latest pony doesn't appear to be having much of a problem, perhaps because I wash off both tanks more carefully.

 

[pescador775]

An aluminum bottle will be subject to galvanic corrosion when in wet contact with a 'nobler" metal which covers a wide range, but typically stainless steel and brass. The principal source of corrosion will be the brass valve and I have seen ponies with a section of paint in the neck area peeled off. Moreover, there have been problems with air leaks due to corrosion of the neck O ring shelf. It takes a couple years for this to happen. Corrosion under the metal bands could occur but a thin strip of insulating material (dielectric) under the pony bands will help prevent this. Use of rubber strips has been mentioned and has been standard practice in the industry when fitting bands on SCUBA cylinders.

The neck problem which occurs with aluminum tanks can be repaired. If leaks occur, then the neck can be dressed much like the procedure for a "valve job" done for an engine. Instructions can be found on the web.

 

[muddiver]

From an engineering view point, it is unlikely that you will get any galvanic corrosion unless you plan to hang your gear in salt water for weeks at a time. If you don't rinse with fresh water after you dive, yes you will get some corrosion due to the salts in sea water (assuming you dive salt water and not fresh water).

I wrap my commercial bail out with electricians tape where the stainless steel bands of the bell backpack contact to cylinder. It keeps the original paint from getting chipped and prevents crevice corrosion under the paint. You can do the same with your pony. Rinse after every dive, have your gear serviced every year, don't bang it on the deck and you should be ok.

 

[carlcz]

Also check under the tape periodically (remove tape once in a while and rinse with fresh water), because certain types of pitting corrosion can occur with salt build up if the tape allows ANY salt water to enter and there is even a scratch where paint has been removed. Not so much an issue with any galvanized material, as the zinc coating will provide a certain amount of anodic protection for a scratch.

 

[pescador775]

From an engineering point of view, most corrosion is galvanic in nature. Corrosion will often occur due to dissimilar elements represented by different components of a system or in reference to those individual elements contained in an alloy such as tank metal. Corrosion is typically seen after salt water has intruded into gaps which are formed by joints such as a tank valve which has not been torqued. Salt is hydrophilic meaning that a bit of salt trapped in a joint or interface will attract moisture even if only exposed to air and even if a superficial wash has been performed. This is because the salt has been driven into the joint under pressure.

 

[BobbyT22]

Thanks all for the informative responses. I'll look at slinging first, but want to also explore back mounting as an alternate option with a wrap of some sort. I knew this was the place to ask!