Monday, April 4, 2016

Corrosion, Corrosion

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!



Anonymous said...

Lead and cadmium in your water spigots?

Try this stuff next time:

Robert Salnick said...

No... I poured it out. The inside of the tubing is as pristine as it was before casting - I checked.

Is that moldable plastic an uninformed idle suggestion? Or have you actually used it for bending tubing? There is a world of difference between pourable molten metal and a soft sticky mass that you have to cram down a small tube, always keeping in mind that any gap in the fill *will* result in a kink.

Oh, and do you have copper plumbing in your house? Unless it was built recently, all the joints were made with lead.

Kyle said...

First off, thanks for the hint on Wood's metal. Never heard of it before, but now I have a few projects I think could benefit from it, one being a faucet for my camper build. I'm interested in the quenching for small grain size. That is completely correct, but it actually reduces ductility. To soften a metal and increase ductility, you need a slow cool (annealing process). I'm wondering if the small grain size though helps with a cleaner bend, and the difference in ductility is negligible for this application. Very interesting, I may have to play with this to learn more...

As far as your corrosion issue, my guess would be that you're carrying stray current in the seawater and it's completing the circuit when the tap is on and flowing. Since it's occurring on the inside of the bend, instead of the outside (if it were on the outside of the bend, I would suspect liquid impact or erosion as you suggested), it's likely an intergranular attack or crevice corrosion at the point where the tube was cold worked (bent). Cold working induces surface cavities, which are more prone to corrosion. Anyway, just thought I'd chime in with my $0.02, hopefully someone else can offer some more insight!

Anonymous said...

Hi Bob,
That's moldable plastic. I've used it for bending tubing by sealing the tube, pouring the pellets in, warming it, and gently compressing it from the open end using a dowel. It's good enough to get the job done in small tubes like you used and is readily removed with compressed air after warming. The low-tech way would be to do the same thing with sand.

I'm sure you got the bulk of the metal out, I'm just concerned about the inside surface contamination. Cadmium is nothing to play around with.

BTW, they've had lead-free solder for copper pipes for some time now...

Robert Salnick said...

Thanks Kyle for the very reasoned response!

You're right of course about grain size and ductility - silly me! Bending the filled tubing was easier than bending a solid 7/16" stainless rod would have been, but it still crushed the roller disk on my bending jig just the same.. I should have used something stouter than plywood (and I shouldn't have quenched!)

And thanks for your thoughts on the corrosion. Again, you are right - if it were erosion, it would appear on the outside of the bends first, not the inside. Good catch! I would expect intergranular corrosion at the outside bends too, since this is where the metal is stretched - it's compressed on the inside of the corner.

Given that the spout is isolated from other metals except for the seawater connection, that eliminates galvanic corrosion.

That leaves stray current. And like Holmes said, "When everything else has been eliminated..." But once again I am at a loss. The sink is not connected to anything electrically... all the water connections come via plastic tubing. And the spout too is isolated. I can't fathom a current path.

Thanks again for your thoughts!

Robert Salnick said...

Anon -

Yes, lead-free solders have been available for some time... actually it was the "Safe Drinking Water Act" of 1986 that effectively banned lead-based solders for sweating copper pipes in the US. So, if your house was built in the period following 1986 (approximately... things take a while to filter down the the actual guy with the propane torch in his hand...), then you probably have lead-free plumbing.

I don't know when copper plumbing became the gold standard for household water distribution. I do know that the house my parents built in 1953 used galvanized iron. But there were millions and millions of houses built in the years leading up to 1986, with copper piping. If you've ever sweated copper pipes, then you also know that small BB's of solder will end up on the inside of the pipe. Presumably these eventually end up in the low points in the plumbing. And are still there today.

But you correctly identify the cadmium as the real risk here, not lead.

Have you ever tried to solder to stainless steel? It is very, very difficult, and requires an extremely aggressive flux to cut thru the chromium oxide layer. Otherwise the solder just beads up on the surface. That's the way the Wood's metal acted - it would not "wet" the metal. When I poured it out, it left the surface completely, much the same as pouring water out of a teflon tube would.

I did briefly consider using sand as the filler. But I could not think of an effective way to seal both ends of the tube solidly enough such that the compression applied at the bend would not just force the sand out of the pipe. This is steel we are talking about here, not the much softer copper or aluminum. And if there were a void/bubble (which seems unavoidable) in the plastic that you suggest, then the pipe would kink at that point because the plastic is very soft compared to the steel, and air is compressible.

For the squeamish, there are other alloys (see Field's metal) that are free of lead and cadmium. But I didn't have any, and they are ridiculously expensive ($70 / 4 oz) since they indium based.

Ever handled cadmium plated fasteners? Welded galvanized steel? Then you got far more heavy metal exposure there.

Finally, how about Zicam? That's intentionally ingesting heavy metals...

dbostrom said...

I thought it was "boga," not boat yoga?

Great result with Woods metal, what a bright idea. It's great stuff.

Long ago, I was employed as a electronic technician w/Welex, a downhole logging firm. We used Woods metal for keeping photomultipliers cool while in hot holes; a mass of frozen metal surrounded the sensor and in combination with a temperature sensor, clock and a nomogram we could calculate whether we were about to cook an expensive tube. As long as any metal remained solid, the tube's temperature was within the melting point and thus safe. Maintenance free, too; pull the tool, let it cool and it was ready to go again.

Stainless Steel Flange said...


Syd Stapleton said...

The tube bending is interesting, but the discussion about corrosion misses a fundamental fact. Salt water is a conductor, so the aluminum tube, regardless of how it may have been isolated, is connected to your bronze (?) propeller by a saltwater "wire." Aluminum is so low on the galvanic scale that it is replacing zinc as an environmentally more acceptable sacrificial anode. Aluminum anything has no place in contact with saltwater, unless it is as a sacrificial anode. It's just another example of crappy modern boat manufacturers cutting corners...

Robert Salnick said...

Syd -
Oh no! The guy across the waterway with the aluminum boat is going to be sorry to hear that!

In all seriousness, tho. As you point out, the aluminum and any number of more noble metals, both on my boat and on nearby boats are connected with a saltwater wire ( great characterization!). But as long as those metals are not connected metal-to-metal, no galvanic corrosion will occur. Consider a battery.... Both electrodes are submerged in an electrolyte, but nothing happens until the circuit is closed with a metal-to-metal connection. Then the anode corrodes and current flows.

In my situation, the aluminum is completely isolated from metal-to-metal contact, the reason for my quandary.

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