Plastic Rods for CO2 Pipes?
One of my complaints with my previous three warships is that the vinyl CO2 lines seem to have a way of always getting tangled and pinched. I have started making a nice aluminum frame for Nassau's six turrets and I've been thinking about running hard lines in/on/through it to help organize things. The Clippard fittings are all brass so I started wondering what my options were while avoiding galvanic reactions.
Galvanic Compatibility
I found this great page on galvanic compatibility. In case the page goes away here is a summary of what I learned: For two materials to not have corrosion problems due to galvanic reaction in a wet environment there should less than a 0.15V difference in the anodic index of the two materials. Even in a dry environment the difference in the anodic index of the two materials should not be more than 0.25V.
Brass has an anodic index between 0.35V and 0.45V. Aluminum has an anodic index between 0.75V and 0.95V. At best this is a 0.3V difference. So even in a nice dry boat (which Nassau will not be) I am over the limit. This means that anywhere aluminum and brass are in contact with each other the metals will corrode.
How I Arrived At Plastic
The galvanic incompatibility of brass and aluminum means that the hard lines can not be aluminum. If they were then the brass Clippard fittings will corrode and seize into the lines. However, if I make the hard lines in brass and paint them wherever the contact the aluminum frame I should no problem.
So I started shopping for brass rod I could use. The 10-32 thread the Clippard fittings use will require around a 0.1610" diameter hole. McMaster-Carr has some brass rod with a 0.1520 ID and 0.25" OD (7782T611/7782T613). This seemed about right but I wanted to see how 0.25" piping would fit.
I happened to have some 0.25 hollow plastic rod laying around... which got me to thinking: Could I use that? I honestly did not expect that I could. However, I threaded the ends of a small piece capped one end and a hose barb to the other.
Results
To my surprise it held at 150 psi. I will have to rig up a higher pressure test and try it when the plastic is very cold and very hot. However, that it worked at all surprised me. (Enough so that I thought I should post it up 
)
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Comments
Re: Plastic Rods for CO2 Pipes?
my first thought is that over time they'd become brittle and more likely to break. How thick are the walls (compared to those in accumulators)? What type of plastic IS it?
Re: Plastic Rods for CO2 Pipes?
It is one of those white plastic rods they sell at hobby shops. I assume it is styrene but I don't know. It is just something I had laying around. I agree about it probably becoming more brittle over time but I can not remember the last time I bought small plastic rod like this... it is at least two years old already and may be a lot more... Lion's masts are made of the same stuff (this may be scrap from that project) and after almost ten years in the sun seem fine.
I didn't measure the walls but where I drilled out the ends they would be about 0.045" thick and threaded! (0.25" OD minus 0.1590" drill) So the stuff has a pretty impressive tensile strength for el-cheapo hobby shop plastic.
I probably will not use it as this size brass is relatively cheap - but it is neat that it held up at all. It takes no time to test I will probably do high and low temp testing - mostly because I am curious now. I also want to jack up the pressure to 200+ psi. However, if it fails with a hiss instead of a bang I might try it out.
Re: Plastic Rods for CO2 Pipes?
What about "gage line"? I believe Parker should sell it in many sizes and ratings but most should be WELL above any of the requirements. It might require the purchase of a few tools and fittings to make the flared connections normally used by Parker.
The "gage line" and fittings I used to work with were rated for 15kpsi and I can personally attest to them holding with out any leaks what so ever at ~11kpsi.
http://www.parker.com