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I know only as much about chemistry as I can remember from a year in college. So, please speak slowly and use short words.

I'm trying to decide on an appropriate material for a heat-transfer coil that is meant to remain submerged for extended periods in salt water, within a few feet of other metal parts.

I'm designing a solar still that will use a cooling/condensing coil to take advantage of the heat differential between the solar-heated evaporation chamber and nearby (cooler) water temperature. It will be mounted on a sailboat. The coiling coil will probably be about 12-18 inches long and 2-3 inches diameter.

Impulse was to use copper, but I'm not sure how this will react in a salt-water environment over time. I know that sea life will stay off of it, since copper is the active element in many bottom paints designed to minimize subsurface growth.

Any help is appreciated. Maybe the answer is to use a sacrificial anode on the coil? Or, maybe some other material would be more suitable for long-term use?

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My advice is to use copper and protect it with a sacrificial anode. Copper would be the choice in a non corrosive environment and if you can protect it go for it.

However, if the coils will contact parts of the boat that are important than the coils should be aluminum. Copper would cause the other metal parts to corrode faster.

Why not use rubber or plastic, something like the pipes used in drip irrigation?

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  • $\begingroup$ I've leaned away from rubber/plastic for two reasons. First, those materials are way less conductive, with thermal conductivity numbers in the range of about 0.2 W/(m K), as compared to copper at around 400 W/(m K). But, secondarily, such a surface would need to be coated with a protective paint to reduce slime and barnacle adherence. And, such a coating would further reduce thermal conductivity.Would zinc be a good choice for an anode? Or would that be the opposite of a good choice? Zinc is used elsewhere to protect steel parts. $\endgroup$ – Foswick Jan 18 '15 at 15:57
  • $\begingroup$ Take a look at a table of standard reduction potentials. They are written as an ion being reduced to metal. You want the most negative number for an anode, the more positive number for what is being protected. Copper is .34 (volts), zinc -.76, aluminum is -1.71. Aluminum can protect zinc and zinc can protect copper. It sounds like protecting copper is your best option. $\endgroup$ – Brinn Belyea Jan 18 '15 at 16:01
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have you thought about using cupro-nickle, which is used exensively in marine engineering. I am aware it is expensive, but further research should reveal its suitability. I hope that you have found this helpfull.

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  • $\begingroup$ Wow. You're not kidding about it being expensive. Looks like somewhere around $100 USD for what I would need. Though, it may save money in the long run. Thanks for the tip! $\endgroup$ – Foswick Jun 3 '15 at 3:26

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