Tuesday, July 12, 2016

Corrosion, Corrosion

This post originally appeared on Windborne in Puget Sound

For a boat on salt water, corrosion is an omnipresent demon.

Even inside.  This is the spout on our galley sink which is piped to a saltwater foot pump. And to the cooling water discharge from our 12V refrigeration system, meaning that it has saltwater flowing out of it whenever the refrigeration compressor is running, as a telltail. Look closely at the inside of the right-hand bend... yup, the aluminum has corroded thru. I don't understand this... aluminum is supposed to be reasonably proof against saltwater.  The pipe is clamped to the sink in a plastic fixture, and is connected below the sink via vinyl tubing...  ruling out galvanic corrosion.  The entire refrigeration system is 12V, so stray 110V current cannot be an issue.  The compressor is powered by an external motor thru a V-belt. 


The motor and compressor are mounted on the same metal plate, and there are some pressure switches to control the motor mounted on the compressor.

Is that enough to cause stray current corrosion, tho there is no direct connection between the refrigeration unit and the aluminum tubing except via the saltwater itself?

Or is the corrosion simply the result of flowing saltwater washing away the protective oxide layer on the inside of the aluminum tubing?  I am very interested in what the net.wisdom has to say about this...

Regardless, this is the second spout that I have installed there, and they have gotten ridiculously expensive.  I am not planning to buy a third one.

Two pieces of 7/16" stainless tubing
Instead, I bought some thin-gauge 316 stainless tubing from Online Metals.  Now, if you've ever attempted to bend tubing, and especially thin-gauge tubing, you know that it requires special tooling to prevent kinking.  The tooling constrains the tube so that it can't collapse and kink while it is being distorted.  I looked up what a tubing bender for 7/16" tubing costs on the Interwebs, and Oh. My. Gosh.

OK, a Plan B is needed.

It is also possible to prevent collapse/kinking if the tubing is filled solidly with something incompressible.  Apparently some people have used ice (fill with water; freeze), but I was concerned that I'd never get the tubing bent before the ice started to melt.  This is where Wood's metal comes in.

This is Wood's metal - it is a eutectic alloy of 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight.  It melts at 158°F
I just happened to have some. 

Wood's metal foundry
For a foundry, I purpose-bought a can of tomato paste (69¢), and froze the tomato paste, retaining the can - just the right size.  I put it in a pan with some water and brought the water to a boil - 212°F, or about 50° of superheat.  I then poured the molten metal into the tubing (I had previously blocked one end of the tubing by pushing it into a wine cork - we seem to have plenty of these).  I then immediately plunged the filled tubing into a container of cold water - I had read that quenching creates a fine crystal structure in the Wood's metal, making it more ductile (read: easier to bend).

OK, now to bend.  I created a bending jig and lag-bolted it to a 4x4 in our shed:

Homemade bending jig
Yup, it bent just fine - no kinking, no collapse.

Recovering the Wood's metal
All that remained was to reheat the bent tubing in another boiling water bath to remelt the Wood's metal and pour it out.

And since our galley sink has two of these spouts (one for salt water and one for fresh water, foot-pumped from the tanks), I made another spout.  Gotta be symmetrical, don't you know.

(Clever camera angle conceals dirty dishes in the sink)
A little boat yoga, and the galley sink looks better than it ever has!


  1. Undoubtedly a clever fix. While reading that blog post, I wondered about someone other than a metalworking expert replicating the results. Then I began thinking about PVC/CPVC pipe, and if appearance is a concern, then some silver spray paint as well. While it does offend the "Stainless Gods", it agrees with the KISS and practical DIY principles, etc.

  2. Yoda -
    Yes, that could work... if the OD of the PVC/CPVC pipe was right. I didn't show the details of the sink fittings that the tubing fits into: a seal is made to the outside of the tubing. You can get an inkling in the first picture.

    Also, you'd want to do a decent job of bending the pipe - I think you'd have to heat it uniformly (in a oven?) and then cool it in some kind of jig to ensure a clean curve.


  3. The simplest method that I've seen described involves filling the PVC pipe with sand. Some people are advocating heating the sand. That seems like a bad idea to me -- if the sand is hot enough to melt the pipe, it will also fuse with it. The better idea seems to fill the pipe with unheated sand, using it only to add internal reinforcement against pinches/collapse, and apply external heat to bend the pipe (heat gun, etc).


    1. It's the same principle isn't it? Put something incompressible in the pipe to keep it from collapsing. But if the sand simply squeezes out the end of the pipe during bending, then it fails at its purpose. I believe you'd have to close the ends of the pipe to keep the sand in place.

      The purpose of heating the sand (not to the melting point of the PVC please!) is to ensure uniform heating of the plastic thru it's thickness by applying heat both from outside and inside. You could eliminate it if you were willing to wait longer for the pipe to soak in the heat of the oven.



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