Galvanic corrosion

[Gary Peacock]

The Galvanic corrosion problem seems to be doing its rounds again with adverts springing up promoting various cures and tales of woe.

 

So lets do a little bit of online fact finding if you can spare a few moments to complete it.

[DHutch]

Do you believe Galvanic corrosion exists? Yes

 

Do you understand the basics of what it does? Yes

 

Do you know of a boat effected by it? No

 

 

But what about

"Do you beleave it is nessary to take proventative mesures against Galvanic corrosion"

- Not sure what i would answer for that.

 

Although im a great bleaver in the "if you unpug the shoreline you will fine" tactic!!

 

 

Daniel

[Yamanx]

Do you believe Galvanic corrosion exists? Yes the science is real under lab conditions. On narrowboats using the canal system, I'm not so sure.

 

Do you understand the basics of what it does? Yes.

 

Do you know of a boat effected by it? No, although the real answer I wanted to give was "I dont know" and the poll wouldnt let me leave the question unanswered.

 

My boat was very, very rusty indeed. It has never had any anodes fitted and in thirty years the base plate had practically rusted away in places. Is this proof? I dont know but I am fitting them now.

[dor]

I think the science is quite clear on the benefit of anodes. I can remember from about 40 years ago being taught in school how galvanising zinc not only protects the iron by the physical barrier of the zinc, but scratches in the zinc could also be protected by the chemical effect of the zinc, i.e. galvanic protection.

 

How far the effect of the anode reaches is another matter though, and perhaps we should be fitting anodes halfway down the hull as well.

 

However I think that the potential effects of a number of boats moored in a marina, all linked by a common shore line, could be much more significant.

 

I don't have a marina mooring and there are no landlines there so this aspect doesn't affect me, but if i was in that situation I would be looking very closely at some way of limiting the effect. I certainly wouldn't be leaving a shore line connected when it wasn't eing used.

Edited January 11, 2006 by dor

[DHutch]

Yeah, my grandad has often said that narrowboats should anodes half way down, or even tow half on longer boats,

- But in practice we only have two at each end like everon else.

- But the survayer liked them.

 

And as i said, we're never pluged in very much.

- I was living aboard with a fixed shoreline, i might look further into it..

 

Our boat is far better below the waterline than a lot of boats i see with that 6inches of rust at the waterline. (and a polished cabin... proritys...)

 

 

Daniel

[Bones]

 

However I think that the potential effects of a number of boats moored in a marina, all linked by a common shore line, could be much more significant.

 

why?

[John Orentas]

"I think the science is quite clear on the benefit of anodes"

 

I wouldn't say that, if fact I would say there is more woolly pseudo science talked about this than anything else. I have anodes fitted and yes they do make a difference, maximum 2ft radius around each of them.

 

"However I think that the potential effects of a number of boats moored in a marina, all linked by a common shore line, could be much more significant".

 

Why should it be much more significant, some people also say that mooring under a power line is "significant" too, and those tales about drilling rigs and pipelines they are true, I have seen film of them being fitted, they use that many that each is close to being in contact with the next.

[Gary Peacock]

why?

 

For a galvanic action to take place between dissimilar metals you need an electrical link (circuit) between them. Two or more boats in a marina without this circuit can't experience any galvanic action between one another.

 

Connect all the boats to shore via a land line and a circuit is provided between boats by the protective earth conductor of the shoreline. Galvanic action can now take place between the boats, it will be down to the content of the steel and your anodes whether your boat suffers the detrimental effects or your neighbours do.

 

Have a look HERE for a few diagrams.

[ChrisPy]

 

those tales about drilling rigs and pipelines they are true, I have seen film of them being fitted, they use that many that each is close to being in contact with the next.

are you referring to subsea or cross-country pipelines?

 

a properly coated cross-country pipeline, with some unknown defects in the coating, laid in soil with high conductivity (high potential for corrosion) will normally have anodes fitted at intervals of about one mile.

[Bones]

For a galvanic action to take place between dissimilar metals you need an electrical link (circuit) between them. Two or more boats in a marina without this circuit can't experience any galvanic action between one another.

 

Connect all the boats to shore via a land line and a circuit is provided between boats by the protective earth conductor of the shoreline. Galvanic action can now take place between the boats, it will be down to the content of the steel and your anodes whether your boat suffers the detrimental effects or your neighbours do.

 

Have a look HERE for a few diagrams.

 

 

what a fantastic explantion. I actually understand! Galvanic corrosion is something I need to learn about, my pretty little head seems to blank out when corrosion is mentioned! It didn't this time though...!

[pmms]

Sorry folks I have come to this thread a bit late, (I have to "borrow" a landline for browsing) there is a good (I think) bit on this on an american web site www.yachtsurvey.com. You need to get your head around the terminology Hot=live, Ground=neutral, Grounding=Earth. Look at the artical "corrosion in marinas"

 

 

See you soon

 

Paul Morris

 

NB Enchantress (on the Maccy for now)

[Amicus]

My only experience is on salty boats, does that count in the poll?

I know of a boat that sank, six months from new, s/s bolts in the stern tube rotted!

 

There was a photo on this forum a little while ago, a propellor showing the classic signs, pink metal.

[blackrose]

Very good description of galvanic action. I finally understand it (I think!)

 

Does anyone know how far away your neighbour's boat has to be from yours for galvanic corrosion to take place. My boat is steel and my neighbours (about 2 meters away) is iron which is higher on the Galvanic Table.

[blackrose]

Does anyone know how far away your neighbour's boat has to be from yours for galvanic corrosion to take place. My boat is steel and my neighbours (about 2 meters away) is iron, which is higher on the Galvanic Table.

[ChrisPy]

Very good description of galvanic action. I finally understand it (I think!)

 

Does anyone know how far away your neighbour's boat has to be from yours for galvanic corrosion to take place. My boat is steel and my neighbours (about 2 meters away) is iron which is higher on the Galvanic Table.

that's a 'how long is a piece of string' question. For example, if you did a scientific study you may find that the rate of corrosion is inversely proportional to the square of the separation distance. There is no absolute answer, depends on how much corrosion you can tolerate.

 

if the boats are separated by only 2m I would suggest anodes at about 4m intervals along the length of the boat.

[ChrisPy]

For a galvanic action to take place between dissimilar metals you need an electrical link (circuit) between them. Two or more boats in a marina without this circuit can't experience any galvanic action between one another.

 

Connect all the boats to shore via a land line and a circuit is provided between boats by the protective earth conductor of the shoreline. Galvanic action can now take place between the boats, it will be down to the content of the steel and your anodes whether your boat suffers the detrimental effects or your neighbours do.

 

and in theory if you fit either a galvanic isolator or a shoreline transformer there is no electrical continuity between the hull of your boat and the landline earth.

 

a galvanic isolator will only allow connection from the mains earth to the boat's hull if the earth fault current produces a voltage that will pass through the diodes (about 2V ??).

[blackrose]

Thanks Chris,

 

My boat came with a small galvanic isolator fitted from LB, but I'm not sure how well it works. I read somwhere that the best ones have indicators so that you can be sure it's working. Mine doesn't so I might change it for a better one.

 

My boat only has anodes bow and stern - the problem with trying to fit them along the lenght of the boat is that they would protrude outside of the hull profile and probably get knocked off. Do you have extra anodes fitted?

 

Mike

[jollyroger]

Just a few points to think about, galvanic corrosion is a lot worse in salt water than fresh water, you should use zinc in salt water and magnesium in fresh water. A lot of corrosion is caused by chemicals in the water, not by galvanic action, and lastly, to be effective an average narrowboat would need to have over 50 anodes fitted.

Comments more than welcome

[ChrisPy]

Do you have extra anodes fitted?

 

no. but my boat is only surrounded by air and gravel just yet.

 

others must have anodes fitted along the length - maybe a thin strip rather than a big lump??

[Liam]

We have the standard sized Anodes half way along the length of the hull, but they are in the profile of the rubbing strakes.. just.

 

A friend of mine has a John White tug style hull which is 52ft long. He has the usual two at the front, two at the back setup, but he also has one on either side which sit in a little recess so they're in the profile of the hull itself and not just the rubbing strakes.

[John Orentas]

I have just seen the 'Poll' for the first time, not very scientific I am afraid, questions 2 to 9 assumes a 'favourable' reply to the first.

 

If anyone did a simillar thing in a political election they would be guilty of ballot rigging.

[blackrose]

Does anyone know of a good make of galvanic isolator?

(preferably with some sort of indicator light so you know it's working)

[Gary Peacock]

The one we use can be found here http://www.canalboatbits.co.uk search for product code 8-40990 or browse through ELECTRICAL to ELECTRICAL ACCESORIES and you will find them there.

 

They seem to be a fairly good product but considering what is in them, this and all the alternatives are a bit pricey.

[DHutch]

The one we use can be found here http://www.canalboatbits.co.uk

Gary, that website for somreason doesnt seam to work me, it havnt in the past, and still doesnt.

 

 

Daniel

[Gary Peacock]

It seems to work in a strange way with a lot of frames I have had the occasional problem with it not refreshing too.

 

If anyone else can't see it let me know and I will investigate it a bit more.

 

In the mean time this is the same product HERE