Did you ever do the thing in Science class in school, where you stick a copper nail and a galvanized nail into a potato (or lemon, or apple – anything damp and acidic)?
The ‘ol potato battery? You can measure a little voltage – positive at the copper, negative at the galvanized.
Dissimilar metals in an electrolyte, the “less noble” metal (the “anode”) tries to get rid of electrons and the “more noble” metal (the “cathode”) is trying to gain them. And since those wacky little electrons are negatively charged, there is electrical potential there (voltage). So if you connect the anode and cathode, you get current flow.
So far so good.
But consider this: metal oxidizes, corrodes. Some metals are more corrosion-resistant than others, but three things that dramatically speed corrosion are water, salt, and acid. Everything we have in our electrolyte. So either the copper nail or the galvanized nail of our school experiment would corrode faster alone in the potato than they would potato-free. The weird thing is that our battery – the galvanic cell – if it is connected and current is flowing, it prevents corrosion at the copper cathode, but speeds it up at the galvanized anode.
If “corrosion” is a kid, and copper and zinc are peas and carrots respectively, connecting this galvanic battery turns the copper into brussel sprouts, and zinc into cake. The kid won’t eat the brussels until he’s eaten ALL of the cake. The copper won’t corrode until ALL the zinc is gone.
That is why people put sacrificial chunks of zinc on boat bottoms in salt water: to turn the metal the boat is made of from peas to brussel sprouts, and giving the hungry kid named Corrosion some zinc cake to eat!
In this photo, you can see a wire with a terminal end at the left. And, there is actually a small bronze bolt tapped into the end of the long keel bolt. That wire, which people who do house wiring would recognize as a “ground” (they even use green-coated wire for it) used to be attached to the bolt – to every bolt and through-hull-fitting on the boat, a practice called “bonding”. And this used a sacrificial zinc to make a galvanic cell battery to protect all the bronze at the expense of the zinc. That sounds like a great thing, right? And the bolt in the picture is 84 years old, all of them spent in saltwater, and it is as sound as the day it was made.
But, there’s a but…
When you run electrical current through water, it breaks up some of the water molecules into their component hydrogen and oxygen. Little bubbles form of hydrogen and oxygen gas. In fact, you know how they say the car of the future will run off a “hydrogen fuel cell”? This is how they get hydrogen: by using electricity to “decompose” water into hydrogen and oxygen (hydrogen is actually usually produced by burning fossil fuels, but for it to be a green renewable power source, the hydrogen is produced by solar-electric electrolysis of water). Technically, this electrical decomposition of water is called “electrolysis” when it is powered by an external source, not when it’s the result of a galvanic cell – but, the word “electrolysis” is commonly used to refer to galvanic corrosion.
We still haven’t gotten to the “problem” part, but we’re close:
So, we have our bronze/zinc battery working, the boat is all grounded or “bonded” so it’s cranking out electricity, the bronze is well protected, and the zinc is dissolving. That current flow is also breaking-up some water, most of which just bubbles away as hydrogen and oxygen gas. But it’s salt water – so the H2O also contains NaCl (sodium chloride – table salt) and other minerals.
So the alphabet soup of busted-up molecules we have at the cathode – all the bronze bolts – includes Na (sodium), O (oxygen), and H (hydrogen). When those guys team up to form NaOH, things get “complicated”.
NaOH is Sodium Hydroxide. Back in the day, when your sink drain got clogged, you mixed some Drano crystals in water, the jar got hot, and you poured that down the drain – and it broke-up the proteins, made them all water-soluble, and cleared the drain. It worked so well that they don’t let you buy it anymore. And it was called “Lye”.
When we have soap-making classes at Mill Hollow Works, they are “cold process”: we use Sodium Hydroxide or Lye to “saponify” some sort of vegetable or animal fat.
In papermaking, they use Sodium Hydroxide on wood pulp to digest the plasticy glue that holds wood together – called lignin – and leaving the cellulose fibers.
In some movies and documentaries you’ve seen, they used Sodium Hydroxide, or Lye, or Caustic Soda to completely digest and dispose of the bodies of plague victims or murder victims.
Sodium Hydroxide is a very, very strong alkaline – a powerful base, like Chlorine Bleach but stronger. And a small amount of it is produced at the positive-charged bronze bolt cathode in our galvanic cell. And it does exactly what it does in papermaking: it digests the lignin in wood, leaving the cellulose. The result is that the wood immediately surrounding the protected metal gets white and furry. It’s not “rot”, it’s the destruction of one component of wood, caused by a chemical produced by the catholic protection against galvanic corrosion. The catchy name of this is “Electrolytic Delignification”.
If this is getting too complicated, yeah – I agree. This is way more complicated than it seems like it should be. But to say it as simply as I can: Using grounding wires and sacrificial zincs to protect metal fastening in a wood boat is good for that metal, but it dissolves the boat.
Having looked at the results over a few decades of bonding and cathodic protection, smarter people than I have concluded that Electrolytic Delignification represents a greater risk to a boat than the corrosion of copper alloy fastenings and hardware. So, we cut the bonding wires – that’s why the cable is disconnected in this photo. And, we limit the size and number of sacrificial zinc anodes on wood boats to the minimum – often just on the prop shaft to protect the propeller and shaft from corrosion.
One last thing on the photograph – the white ring around the bolt isn’t from the Delignification. If you want proof that wood degradation is caused by “an alkaline condition”, one way to test it – and to neutralize it – is to apply a mild acid. So this photo was taken just after I poured a small amount of vinegar on the area. The ring of wood right around the bolt, and a few tiny spots further away (where the alkali got to by capillary action) fizzed up into a white foam! To bring it all the way back around to middle-school Science Class, just like your baking soda & vinegar volcano!