I remember my lovely Nana used to say to me “You learn something every day.”
Well – here is your something for today! That is unless you know everything there is to know about anodes already?
And if you don’t even know what an anode is, well – today is your lucky day! You are going to learn two things today!
Very simply (and in my own words I might add…) – an anode is a sacrificial bit of metal that attracts all the rusty, corroding stuff that is lurking in salt water – and it gets eaten up before the rest of the expensive bits on your boat – like propellers, sail drives and engines.
If you own a boat, then you need to know about these kinds of things. If you don’t have a boat – please don’t feel obliged to read on – but should you ever get asked what an anode is in a quiz question, you can thank me later. To be honest I still don’t really understand all the chemistry behind it. But according to the Ocean Yachtmaster syllabus, I need to know – hence this post. I hope you find it super interesting… hmmm
My first interaction with an anode was many years ago when I was scrubbing the hull of Natural Magic for my good friend Ivan Atkinson. I had my scuba gear on and was fumbling around with a bit of scratchy carpet trying to clean the slime off the hull in the murky water. Fun times. At the bottom of the keel I came across something I presumed to be a paua (abalone shell fish) stuck to the hull. I nearly broke my knife trying to scrape it off. When I got back to the surface Ivan was pleased I wasn’t successful with my scraping, as the thing I was scraping was the anode… whoops!
My second memorable anode experience was when my good friend Dave lost all the skin off his knuckles attempting to replace the anode on the back of Wildwood’s motor – located in the most awkward position and super tightly stuck in place. There was lots of swearing involved, poor Dave’s blood was shed and the anode didn’t budge. Sorry Dave…
Another more memorable experience with corrosion or electrolysis was just recently when we carefully left Wildwood on her mooring after being away for a few weeks, only to get a phone call in the morning to say she was drifting around the harbour – the shackle pin on the middle chain was completely missing. Whether it was corroded away or just not moused in properly I will never know. But either way – corrosion is BAD!!!
Galvanic corrosion is caused by two dissimilar metals being in contact with each other, in the presence of an electrolyte, such as seawater. Electrolysis is caused when an external current, called a stray current, finds a path between two metals in the presence of an electrolyte.
According to the experts, galvanic corrosion or electrolysis can damage or destroy underwater metal parts of boats, dock hardware and other equipment. When two different metals are touching each other or are electrically connected by a conductor, and are immersed in an electrically conductive fluid, like salt water, an electro-chemical reaction can occur. One of the metals (the “least noble” metal, called the anode) will corrode faster than it normally would, and the other (the “most noble” metal, or the cathode) will dissolve more slowly.
Least Noble Metals
Most Noble Metals
When two metals are close to each other on the scale, they have a lessor tendency to corrode.
Metals start to dissolve when they are immersed in salt water. The metals that are at the top of the table above will corrode more quickly than the metals at the bottom of the table. If you attach an anodic metal beside a more cathodic metal, then it will effectively attract the corrosion away from the other metals.
You should replace your anodes when they are 2/3 gone. If you leave them to disappear completely then the next least noble bit of metal will start to dissolve. If this is your engine, then you are in trouble…
It is all a bit more complicated than that of course, and electrical currents added to the mix can cause Electrolysis. This can be minimized by always ensuring all electrical systems, including wiring, switches and all electrical equipment are installed correctly and are maintained in good working order. Always isolate all batteries before leaving the vessel as this also helps to increase the life span of the batteries. Batteries should be kept clean and dry and this includes the battery posts, casing and the battery box as dirt can act as a pathway to facilitate a current leakage, which may result in a stray current.
Other boats and and improper shore power wiring can cause galvanic currents as well, and this can cause your anodes to dissolve faster. You can get galvanic isolators to prevent this.
Polluted water, warm water temperatures and stray electrical currents can also cause increased corrosion.
Zinc anodes should be used in salt water. Magnesium anodes are best in fresh water only – if used in salt water they will dissolve rapidly. Aluminium anodes are best for brackish water – i.e. if you boat mostly in river deltas.
The other issue is with copper based paints – i.e. in antifouling, causing extensive galvanic corrosion of other parts of your boat – like your aluminium sail drive. Wildwood has a sail drive and we do not paint the normal copper based paint on the sail drive for this reason. We have to use a different kind of paint on the sail drive – that doesn’t work as well, but is still better than having your sail drive eaten away!
Make sure you use marine grade fasteners – yes they are more expensive, but yes apparently there is a reason for that. Always use bronze, monel or 316 stainless steel to avoid them from corroding away.
Signs of electrolysis, galvanic action and corrosion include: paint blistering, a white powdery substance on the exposed metal, then it starts pitting.
So there you go! Your juicy bit of information for the day, and another topic ticked off my study list. Have I got it right? Let me know if you have any other words of wisdom on this fascinating subject to share.
13 thoughts on “All About Anodes…”
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Very informative! I know a bit about anodes, a girl friend asked me what they were for, so I did a bit of research. Yesterday we went to buy two tiny anodes for our fridge! It has a keel cooler on the outside of the hull and an anode screws into the middle, about the size of a large coin.
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Oh wow I didn’t realise fridges have them too! Must have a look at mine!
Only if they are keel cooled, I think.
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The heat exchanger for our fridge is inside the galley sink drain – and includes an anode “doughnut” that surrounds the whole thru-hull! Very easy to inspect, at least.
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I had nightmares about this when I brought my steel yacht, and someone said it has previous issues with electrolysis, so much stripped back an external section to inspect and did find evidence of corrosion with areas the size of a 10c coin all over, I was told the previous owner patched it up too, however it was all over nothing because it was 6mm steel and .5mm of corrosion still left plenty of strength, but when you’re investing $50k on a yacht its as serious as leaky house syndrome and often painted over, after all the fuss the only hull corrosion was under a leaky toilet, it cost $500 to cut out a plate and have a new one, the old plate was stronger then any section of a fiberglass yacht anyway so I stopped worrying (until I found a crack in the skeg, thats another story), if buying steel or aluminium then this is serious stuff to check out, good artical Vicki 🙂
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Eeek! Yes leaky house syndrome alright. A bit of rain on your head is one thing, but sinking is another… thanks for the heads up Dudley!
wow. just wow, very technical! You are very smart! 🙂
Good Article. I know that it is implied, but has to be said – the anode will only protect items that it is in direct ELECTRICAL contact with. Sometimes there with be a stray item that is insulated, or not on a direct electrical path to an anode, and will suffer from corrosion as a result.
Anodes work much better when placed on a clean unpainted (conducting) surface, or else you are relying on the electrical contact through the bolt which may reduce over time – just when you need it the most! The anode also works best when placed in direct contact with the electrolyte = salt water, this makes the “battery”.
This also means that the anode should not be painted over!
A similar subject is “Crevice Corrosion”. This can be a problem with submerged fastenings – your mooring shackle? Stainless steel needs contact with oxygen to replace its protective oxide film. A nut on a bolt is already a bad situation that can be easily made even worst by coating the threads with grease which seals the oxygen out!
I am an Engineer and used to work in Marine related industry – A more qualified Metallurgist might like to comment/correct/add to this?
Thanks heaps for your comment and very good advice. I didn’t realize re the grease issue either 🙂
Based on my understanding of the galvanic series chart, we can do without anodes entirely. All we have to do is build our engines and other critical metal components out of gold, silver, and platinum. Let me run the numbers on that. . .
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I’d like to see that! 😉
I just change my anode yesterday! It was no easy venture!
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