Monday, March 31, 2014

Aluminum + Copper: A Horror Story

Q:  My friend has a metal boat.  I had him over for beers last night, and all he talked about was electrolytic corrosion.  I love boat talk as much as the next sailor, but I nearly threw him overboard.  What is it with you metal boat people and your corrosion issues?*

A:  Electrolytic corrosion is the worst.  The worst!  It is a creeping horror ready to eat away our hulls and leave us sad and boatless.  I'm sorry your imaginary friend bored you, but this is a real concern for us.  Why?  Because a penny and a little saltwater could send us to the bottom of the sea.

For those of you in need of a chemistry review, galvanic corrosion (or electrolytic corrosion - same thing) happens when two metals are in contact in an electrolytic solution, like salt water.  Essentially, one metal will corrode preferentially to save the other.  The bad news for us is that Aluminum likes to give up its electrons to almost every other metal.

"Really?" asked Stylish after we explained for the 8,793rd time why Copper pennies do not make good toys.  "Can we try it?"
Hurrah!  Science day!
Erik found a couple of small pieces of Aluminum.  The girls gave the surface a light sanding.
 We retrieved two pennies from The Evil Copper Jar.  I scooped up some marina water from beside the boat - neither the cleanest nor the saltiest, but it's the water we're in - and we were ready to go.
"Wait!" said Erik as I was putting the lid on the Tupperware pot.  He ran over and plunked two stainless steel washers on the aluminum.
I stared down into the pot.  "You can't do that!  That isn't a proper experiment!  Now we have three metals fighting it out in there.  That is terrible science."
"We are testing a hypothesis, Daddy," Stylish chimed in.
Erik made a noise.  "These are the things that fall in the bilge.  You're modelling a real-life situation now."
I just shook my head.  Engineers, I tell you.
Day 0. Washers present under protest from the scientific community aboard.
And here we were, Day 0.  The water looks clean, the Aluminum looks clean, and everything is shiny.

That didn't last long.
Day 3 - looking fuzzy.
Within a day, the water started to cloud.  By day 3, there was a definite snowdrift in the pot.  And what is that snow made of?  Yep.  Aluminum.
Day 19 - ugly, very ugly.
By day 19, things were downright terrifying.  The aluminum pieces had discolored, and the sprinkling of Aluminum snow was big enough to build a tiny snowman.
The Aluminum drift up close.
That was less than three weeks in only brackish water, with no worries about stray currents turning the whole operation into a serious battery.  Even with the Copper/stainless steel issue (sigh), you can still see that this is Not Good.  We all agreed to pull the plug on the experiment, because no one wanted to see how much worse it would get.  Well, maybe Indy, but the damage was already obvious, and I needed my pot back.

And so, dear reader, that is why metal boat owners get a little twitchy about maintaining their zincs and staying away from electrical currents.  Have a little pity on us.  And, in return, we promise to listen to you and your osmosis woes.

* Yes, I totally made up this question myself.


Dave said...

Ah, yes, the old galvanic chart. Many years ago I was chief engineer at a custom aluminum yacht builder, for the big boys. We had one captain who insisted we use copper based bottom paint on the megayacht. Only after I wrote a detailed explanation to the vessel's owner did the captain see the errors of his ways. Not stupid - you can't fix that. Just ignorant and teachable, sort of.

Amy Schaefer said...

Dave, that honestly made my hair stand on end. Although a certain perverse part of me would kind of like to see the bottom of a hull dissolve away completely...

Maybe if it were a victimless crime. We should all pitch in to put copper antifouling on an aluminum boat that is due to be scuppered, and film the results. Who's with me?

Anonymous said...

Interesting. Good to learn something new.

Also I love the disclaimer*


Sean said...

Happy with my fiberglass!