Why do we worry about galvanic corrosion on stage bottle rigging but not on doubles ?

[Max Speed]

All instructions for stage bottle rigging stress isolating the stainless steel hose clamp from the bottle with webbing, inner tube, electrical tape, etc.

No one I have seen insulates their doubles from the stainless steel bands.

Urban myth?

 

[tbone1004]

vast majority of doubles are steel, most of them galvanized either hot or cold. Very different than a painted or raw aluminum tank

 

[waterpirate]

I cut circles of tank webbing to go under my bands to help with drainage. I.E. not trapping moisture. maybe not needed but I enjoyed the excersise.
YMMV
Eric

 

[Max Speed]

vast majority of doubles are steel, most of them galvanized either hot or cold. Very different than a painted or raw aluminum tank

I think this argument is flawed. Galvanizing (primarily zinc) is even more anodic (approx. 1.25) than aluminum (.75-.9) and therefore even further away from any of the common stainless steels (.5-.6?). Zinc should be much more likely to sacrifice itself when in contact with stainless. And if you insulate the aluminum with paint you should have even less material loss than with galvanizing.

I think the stainless hose-clamps in contact with aluminum tanks is way overblown. And perhaps we should be more concerned with tank bands on galvanized steel doubles.

 

[Brandon]

I think this argument is flawed. Galvanizing (primarily zinc) is even more anodic (approx. 1.25) than aluminum (.75-.9) and therefore even further away from any of the common stainless steels (.5-.6?). Zinc should be much more likely to sacrifice itself when in contact with stainless. And if you insulate the aluminum with paint you should have even less material loss than with galvanizing.

And that's why I subscribed to this thread... I was thinking the same darn thing. Waiting for one of the resident metallurgists to chime in with their learned opinion...

On my doubles, you can tell where the bands sit from the slight amount of discoloration underneath on the hot dipped zinc finish. It's more of a whitish gray than the rest of the cylinder, and may very well be a slight bit of corrosion (or its just that the cylinders weathered where they weren't covered by the bands). At any rate, it isn't much, and these doubles are 6+ years old. If they are corroding, it's not a quick process.

Does the surface area or mass of the zinc coated cylinders compared to the stainless steel bands matter?

Zinc plated crush washers on stainless bands = corroded after a weekend of diving. Tiny amount of zinc compared to large amount of stainless steel.
Stainless bands on hot dip galvanized cylinders = slow, if any corrosion. Huge amount of zinc compared to small amount of stainless steel.

Dunno, just a thought...

Regardless of the answer, covering the hose clamp with sleeving does have another benefit - it helps keep the clamp from gouging the painted finish.

 

[waterpirate]

I covered my stage clamps to protect my drysuit. Paint can be stripped. A leaky drysuit is.... well just leaky.
Eric
YMMV

 

[tbone1004]

I think the point is for the zinc to sacrifice itself to the bands so the tank doesn't, and it is a slow enough process that it isn't cause for concern. The metal/metal also provides a lot of friction which you need in the doubles to keep them from slipping whereas a stage bottle hose clamp should never see any vertical forces. Also covering the screw is most important since that saves your drysuit though. Can't use nylon webbing on the doubles bands because nylon is useless when it gets wet so it would slip like no other in that situation, tubing would cause water to sit around the bands causing more oxidation to the bands. A lot of the older bands had rubber pads on them or the whole thing was dipped in what looked like plastidip. Those have fallen out of use, likely to WKPP/DIR practices standardizing away from them. Could be wrong on that, but it wouldn't surprise me.

 

[Max Speed]

Agree with protecting the paint and covering the screw to prevent slicing things. I think a covering prevents cutting the rope as well.