sacrificial anodes

[Lynda]

advice please, we are still designing our narrow boat (she will be a 70ft cruiser stern liveaboard due to launch May 06), we are being advised to fit sacrifcial anodes during the build - we would like to know others opinions on these additions - do we/don't we? do they work? are they worth the money? do they have to be renewed at each haul out? your opinions would be appreciated

 

Lynda

[Alastair]

Anodes *do* work - but only for a smallish area adjacent to the anodes.

 

So you have a choice; fit them to target areas (rudder, prop), or fit lots and lots.

 

Most damage is on the waterline, so you want them near that.

 

 

They only need renewing when about 30% remains. how long that is depends on your boat and local conditions.

 

Get the bolt-on type, they are more easily replaced

[DHutch]

1 Do they work?

2 are they worth the money?

3 do they have to be renewed at each haul out?

 

1 - yes, but its debatable how well.

2 - Again, very debateable, even if you assume they work, how much is protecion for you hull worth?

3 - no, they only need replacing when there geting low (small) which is nomaly only every 5/6years (thes how oten ours need doing anyeay)

 

 

 

Dainel

[NB Willawaw]

It might be questionable whether they work on not, but I've never seen a boat come out of the water that didn't have pitted anodes (apart from one where they painted them). So they must do something ?

 

I don't think you can afford to not fit them - they're not that expensive an addition to a new boat anyway.

 

If I were you, I would also get a galvanic isolator fitted if you are going to have shore mains. These fit inside the boat on the mains input circuit.

I keep hearing about pitting and strange marks on the hull under the waterline, that are attributed to galvanic action.

Again, not worth not having (IMHO).

 

Mark

 

NB

[Clive A]

Hi,

I've got an iron hull, and have been told that I'd need to use different metals for anodes, but most iron hulls don't have them fitted because the amount of normal corrossion is minimal compared to steel.

Can anyone confirm this for me?

Cheers, Clive

[John Orentas]

Clive.

 

I wouldn't bother at all, the amount of corrosion you get on on iron hull is minimal, just paint it now and then, but even that is mainly for cosmetic reasons.

[Clive A]

Cheers John, thats one less expense to worry about!

[Amicus]

I bin finking, I know I shouldn’t but somebody has to do it, about this corrosion and pitting malarkey.

I was wondering if it is because steel is an alloy, iron ain’t, being an alloy means it’s made of a bit of this, a bit of that, and whatever.

Now, this mixture is going to have a granularity, albeit on a fine scale (hopefully), like molecular, but not atomic, spect its crystalline. This means that we got two (or more) chunks of material of different nobility adjacent, do it not? Add water, boom boom, we got a cell, a nano cell. Could this be the cause of pitting?

 

 

 

 

Praps we should ask Cap’n Neal

Edited April 19, 2005 by Amicus

[David Schweizer]

Hi,

I've got an iron hull, and have been told that I'd need to use different metals for anodes, but most iron hulls don't have them fitted because the amount of normal corrossion is minimal compared to steel.

Can anyone confirm this for me?

Cheers, Clive

<{POST_SNAPBACK}>

I agree with John, I used to help crew a composite iron hulled working boat in the 1960's. It was aproaching 35 years old then and had no significant pitting in the hull, (and obviously no anodes) We also never bothered to black under the water line.

Edited April 19, 2005 by David Schweizer

[Gary Stacey]

I bin finking, I know I shouldn’t but somebody has to do it, about this corrosion and pitting malarkey.

I was wondering if it is because steel is an alloy, iron ain’t, being an alloy means it’s made of a bit of this, a bit of that, and whatever.

Now, this mixture is going to have a granularity, albeit on a fine scale (hopefully), like molecular, but not atomic, spect its crystalline. This means that we got two (or more) chunks of material of different nobility adjacent, do it not? Add water, boom boom, we got a cell, a nano cell. Could this be the cause of pitting?

Praps we should ask Cap’n Neal

<{POST_SNAPBACK}>

 

 

Correct - look at most decent web sites that deal with corrosion and one form is detailed to occur in this fashion. Galvanic action occurs on the small as well as large scale.

 

Gary

[Alastair]

With regard to your iron hull - the iron will be ok, but remember that the prop shaft is almost certainly steel. Fit a shaft anode, and it needs to be magnesium.

[mayalld]

advice please, we are still designing our narrow boat (she will be  a 70ft cruiser stern liveaboard due to launch May 06), we are being advised to fit sacrifcial anodes during the build - we would like to know others opinions on these additions - do we/don't we? do they work? are they worth the money? do they have to be renewed at each haul out?  your opinions would be appreciated

 

Lynda

<{POST_SNAPBACK}>

 

Opinion - fit them

 

Do they work - Yes.

Some suggest that they only protect the area of hull immediately adjacent.

 

This is only partially true. As the anode works, it deposits metalic zinc at any corrosion spot within six feet of the anode.

 

So initially the protection is limited.

 

HOWEVER, as they work, each spot where the zinc has been deposited becomes a mini-anode itself, and the zinc migrates to other corosion spots further away (whilst the original spot gets replenished from the main anode). It's a kind of cascade effect.

 

Provided you keep the anodes renewed, it stays in equilibrium, and they will protect the whole boat.

 

Are they worth the money - Yes! The cost in comparison to the cost of rectifying hull corrosion is insignificant.

 

Do they have to be renewed at each haul-out - Depends how often you haul out. You renew them when they are over 50% wasted (in fact you usually fit another set and leave the old ones to completely waste)

 

Oh, and as another poster suggested, fit a galvanic isolator if you have shore power. They are another thing that it is false economy to leave out.

[Lynda]

Opinion - fit them

 

Do they work - Yes.

Some suggest that they only protect the area of hull immediately adjacent.

 

This is only partially true. As the anode works, it deposits metalic zinc at any corrosion spot within six feet of the anode.

 

So initially the protection is limited.

 

HOWEVER, as they work, each spot where the zinc has been deposited becomes a mini-anode itself, and the zinc migrates to other corosion spots further away (whilst the original spot gets replenished from the main anode). It's a kind of cascade effect.

 

Provided you keep the anodes renewed, it stays in equilibrium, and they will protect the whole boat.

 

Are they worth the money - Yes! The cost in comparison to the cost of rectifying hull corrosion is insignificant.

 

Do they have to be renewed at each haul-out - Depends how often you haul out. You renew them when they are over 50% wasted (in fact you usually fit another set and leave the old ones to completely waste)

 

Oh, and as another poster suggested, fit a galvanic isolator if you have shore power. They are another thing that it is false economy to leave out.

<{POST_SNAPBACK}>

[Lynda]

thank you all for your advice/opinions you've been very helpful - we will have anodes!! and I know where to come for the next decision our builder requires

 

Lynda

[Alastair]

erm, unless there is something new in the world - the anodes don't deposit anything - they converted into an oxide, so zinc becomes zinc oxide, magnesium magnesium oxide, etc.

 

Also, to work in fresh water, you should really be using magnesium anodes. Aluminium will work in some (oxygen rich) water, but not canals.

[David Schweizer]

erm, unless there is something new in the world - the anodes don't deposit anything - they converted into an oxide, so zinc becomes zinc oxide, magnesium magnesium oxide, etc.

 

<{POST_SNAPBACK}>

I'm glad someone has pointed that out. you can only plate with an anode and a cathode, and you cannot remove metal from a piece of metal and deposit it onto another piece of metal unless they are separated in an electrolyte and each piece, is connected to opposite poles of a DC electrical supply. Well that's how we used to do it.

 

There is a well known Canal magazine which some years ago started this story ofhow after pressure washing, you could see the spots where the zinc had been deposited onto the small cavities in the hull, and claimed this be evidence that the anodes were doing their job. It was the builder of my boat (a qualified engineer) who suggested that if you wait until the next day to start the blacking, all those "zinc plated spots" will have gone brown with a thin surface coaing of rust, All you were looking at was bright steel cleaned of all its rust by a high pressure jet. Try it, I think you will find that he had a point.

[John Orentas]

I remember the article, it expanded the "theory" to how mooring under power lines can also take metal off your boat (they never did explain how that worked). Since then we have had all the pseudo science about one boat getting bigger at the expense of the one moored alongside. Now of course we have have the shore line business.

 

Anyone who has been in an electroplating plant will know that things are not that easy, small incidental electrical currents don't do the job, they use units like massive battery chargers the size of a car and with special electrolytes. A lot of power is needed.

 

Sacrificial anodes do work, especially in seawater but you need lots and lots of them.

 

Oh dear, who started this again!

[Big John]

Sacrificial anodes do work, ... but you need lots and lots of them.

<{POST_SNAPBACK}>

SO how do you fit lots and lots of them to a narrowboat without them getting knocked off in locks and shallow moorings?

[John Orentas]

"SO how do you fit lots and lots of them to a narrowboat without them getting knocked off in locks and shallow moorings?"

 

Well you don't, just fit two at each end to let future buyers know that you are looking after the boat and keep it well painted, that is as much as you can do.

Edited April 26, 2005 by John Orentas

[Jon]

To put this thread in context, how much is an annode likely to cost?

 

Jon

[David]

Depends on size but for a 70 Ft you would fit the biggest about 3Kg and £40 ish each.

And just to put this argument into context, 1989 to 1991 I worked on the Conway Estuary crossing which was a submerged steel tube tunnel i.e we welded the sections of tunnel up on dry land then sank them into a trench in the river bed and joined them up.

Each 12meter high side section had a sacrificial annode every 6 meters apart along it's length and ever 3meters apart on it's height.

The annodes in the bottom and second row were 1.5m long and 100mm square, the top two rows were the same length but only 40mm square. They were all welded on and the back face was coated with the two pack epoxy the tunnel was grit blasted and coated with. I know I welded on several hundred myself. Of course they were salt water zinc but if they don't do anything companys spend millions fitting them on any submerge steelwork fresh water or salt

david

[jollyroger]

Anodes are a big con trick, of coarse the anodes corrode on a boat, it's caused by the aciditty of the water.

One more thing, If an anode costs about £10/Kg, that is £10000 a Tonne, todays price for zinc is £665 a Tonne, who wants you to buy anodes?

[DHutch]

If an anode costs £10000 a Tonne, todays price for zinc is £665 a Tonne

 

Cant you just buy lump of zinc then?

- Or a long strip to put all round the boat?

 

 

Daniel

[ChrisPy]

Anodes are a big con trick, of coarse the anodes corrode on a boat, it's caused by the aciditty of the water.

 

<{POST_SNAPBACK}>

............. what on earth are you talking about?

 

Anodes are used in any conditions where steel may corrode in a wet environment. We fit them on pipelines, platforms & foundations.

 

They work. i.e. they eliminate corrosion of the steel structure. Period.

 

Who introduced acidity into the equation? Dissolving and corrosion are not the same thing.

[John Orentas]

"They work. i.e. they eliminate corrosion of the steel structure. Period."

 

Don't attribute the wisdom of the sphere's to the people who build pipelines and the like, they may well privately take the same view as many boat owners;

 

"Well they don't cost much, and they can't do much harm".

 

Anyway if there are no other metallic structures close to the pipeline that would blow out of the water some of the funny theories about other peoples boats pinching your atoms.

 

I wouldn't be too scornful of chemical attack this can be demonstrated in the laboratory. The action of sacrificial anodes cannot.