Why bonding ?

[versine]

Hello,

I had some questions about bonding (maybe stupid questions ?) on a GRP sailboat.

As the GRP is insulating, it seems to me that interconnecting the metallic mass in contact with water is not required : True or untrue ?

in case of bounding, there is a risk of galvanic corrosion, so I do add zincs in the bonding ciruit : true or untrue ?

Why connect the negative pole of the battery ? It is possible that stop the risk of electrolytic corrosion in case of bad isolation of the positive conductor, via a fuse ?

Thank you by advance


Jacques

 

[Tranona]

You are just making it all too confusing!

Electrolysis occurs when two dissimilar metals are close to each other in an electrolyte (seawater). This can be obviated by putting a lower (on the galvanic scale) metal in the circuit - usually Zinc. Connected (bonded) to the two components you want to protect. The Zinc wastes, leaving the other metals untouched.

For more information go to the MG Duff (makers of anodes) website.

 

[vyv_cox]

Bonding of seacocks is regarded as unnecessary and possibly harmful in most cases.
Connection of a P-bracket to an external anode that can 'see' the P-bracket may be justified but I prefer to epoxy coat the P-bracket.
Engines cannot be protected by external anodes.
For these purposes I would leave the batteries well out of it, although there could be other considerations.
Stern gear is best protected by a shaft anode if possible, but a brush and external anode is used where this cannot be done.

 

[john_morris_uk]

Electrolysis occurs when two dissimilar metals are close to each other in an electrolyte (seawater).

I don't wish to add to the confusion, but the dissimilar metals need to be connected (electrically) for electrolytic action to take place.
Just to have them near each other doesn't necessarily make for electrolytic action. If there is no external source of electric then its Galvanic corrosion - if there is an external soruce of electric then its described as Electrolytic corrosion.

 

[Tranona]

I don't wish to add to the confusion, but the dissimilar metals need to be connected (electrically)
Just to have them near each other doesn't necessarily make for electrolytic action.

No, they just both have to be in the electrolyte. That is what creates the "battery" - one acting as a cathode and the other as an anode. The connection is required to the zinc so that it acts as an anode and wastes rather than your nice expensive prop or shaft.

 

[VicS]

You are just making it all too confusing!

Electrolysis occurs when two dissimilar metals are close to each other in an electrolyte (seawater). This can be obviated by putting a lower (on the galvanic scale) metal in the circuit - usually Zinc. Connected (bonded) to the two components you want to protect. The Zinc wastes, leaving the other metals untouched.

For more information go to the MG Duff (makers of anodes) website.

The two dissimilar metals in the electrolyte must also be connected together electrically for any corrosion to occur. Just near each other is no problem. There must be a complete circuit or no current will flow and no corrosion will occur.

This is normally called "galvanic corrosion" The term "electrolysis" or "electrolytic corrosion" implies that the corrosion process is driven by "stray currents" from an externally applied EMF.

Decent explanation of it all , as said, on MG Duffs website http://www.mgduff.co.uk/leisure-craft/cathodic-protection.html


BUT Versine, what problems do you have, in the way of underwater corrosion, that you need to solve.

Dont go bonding things and fitting anodes all over the place if you have no problem... you could create problems you do not have.

Do you have any items "bonded" at present? If so what?
Do you have any anodes at present? If so what are they intended to protect? How arethey connected to the items they are protecting.?

Do you have a shore power installation?


.

 

[William_H]

Galvanic corrosion

Yes you do have have an electrical connection between dissimilar metals to get galvanic corrosion.
I think it was Mr Galvan who invented the first chemical cell where 2 dissimilar metals are in a conductive fluid. A voltage appears between the two metals. A current flows if a connection is made. this current can be used to light a light bulb. The metal actually migrates from the less noble metal to the more noble metal with the current flow. No current flow no metal migration. (corrosion). From this it follows that if another electrical potential/current is applied to the 2 metals in a way which encourages more current flow we get more metal migration. If we apply voltage/current in the opposite direction we can stop the migration or even reverse it. This is the basis of electroplating of one metal onto another.

Problems come with boats when a bronze prop is fitted to a stainless steel shaft. If you could insulate one from the other then no galvanic corrosion however this is not practical. So we lure the corrosion away from our bronze prop with a zinc anode connected to the shaft and prop.(or other stern gear)
Bonding is only necessary when the anode is separate from the metal being protected. We must electrically connect the anode to the protected metal.
Hence it is not necessary to worry about galvanic corrosion of a sea cock if it is insulated from all other metals. (the sea water does not count here.)

Regarding connection of battery negative to the metal parts in the water. This is undesirable but usually inevitable as our engines and particularly electrics come from cars and trucks where the metal body is used as negative conductor to engine block is connected to negative. While this is not normally a problem it can be. ie cause corrosion of electrics if they get into salt water. Some starters alternators etc are made with insulated negative to help these problems. good luck olewill

 

[VicS]

I think it was Mr Galvan who invented the first chemical cell where 2 dissimilar metals are in a conductive fluid.

You are thinking of the Italian physiologist Luigi Galvani (1737-89) Professor of anatomy at Bologna University and famous for his experiments in "animal electricity"

It was in fact the Italian physicist Alessandro Volta (1745-1827) who invented the voltaic, or galvanic, pile.

 

[prv]

The two dissimilar metals in the electrolyte must also be connected together electrically for any corrosion to occur. Just near each other is no problem.

I've been wondering about this. When we got her, KS had the anode connected to both the engine and the rudder stock. After the overhaul I reconnected it to the engine, but couldn't immediately reattach it to the rudder stock due to the very poor way it had been fitted. I've been meaning to do that, but perhaps I don't need to?

Why does the engine need bonding but not the rudder fittings? Dissimilar metals between prop (bronze, I think) and shaft (stainless)?

The rudder fittings and their bolts are stainless (A4 for the bolts, not sure about the fittings). They're quite close to the prop. But not being connected to it, are they safe?

I know a common answer with galvanic stuff is "do you have a problem that needs fixing?" The trouble is that, being my first season of ownership, I have no way of knowing (do I?) if things are corroding below the water. October/November (probably when we'll pull her out) seems a long way away. A previous owner fitted the bonding to the rudder (at least, I hope Crabbers wouldn't do such a shoddy job) but did he do it in response to a problem or because he didn't know what he was doing? I have no way of telling.

Cheers for any advice.

Pete

 

[fireball]

Easiest way to check would be to stick a mask and snorkle on and have a look .. or dry out and check ...

 

[john_morris_uk]

I've been wondering about this. When we got her, KS had the anode connected to both the engine and the rudder stock. After the overhaul I reconnected it to the engine, but couldn't immediately reattach it to the rudder stock due to the very poor way it had been fitted. I've been meaning to do that, but perhaps I don't need to?

Why does the engine need bonding but not the rudder fittings? Dissimilar metals between prop (bronze, I think) and shaft (stainless)?

The rudder fittings and their bolts are stainless (A4 for the bolts, not sure about the fittings). They're quite close to the prop. But not being connected to it, are they safe?

I know a common answer with galvanic stuff is "do you have a problem that needs fixing?" The trouble is that, being my first season of ownership, I have no way of knowing (do I?) if things are corroding below the water. October/November (probably when we'll pull her out) seems a long way away. A previous owner fitted the bonding to the rudder (at least, I hope Crabbers wouldn't do such a shoddy job) but did he do it in response to a problem or because he didn't know what he was doing? I have no way of telling.

Cheers for any advice.

Pete

I would be thinking of drying her out on a tide and having a look. Pick a nice day and it will give you a chance to give the bottom a quick scrub and tidy up. Polish the prop etc etc. You can have a look at the underwater metals and anodes and make your mind up as to what is happening.

 

[VicS]

If all the rudder fittings and fastenings are 316 stainless steel then all you really have to do is be on guard against is crevice corrosion although MGDuff do in fact recommend bonding the rudder fittihgs to the main anode(s).
Make sure that the fittings are bedded on a sealant and that all fastenings are well sealed in so that no crevices exist where crevice corrosion can occur.

The usual reason for bonding an anode to the engine is to provide protection to the prop and shaft.

The anode must be situated reasonably close to and within "line of sight" of the prop. It should be bonded to the engine with 4mm² PVC Insulated Multi-Stranded Single Core Copper Cable or larger. If you have a flexible coupling in the shaft then you will have to bridge that to ensure a good low resistance electrical connection all the way from anode to prop.

The question is how good is the circuit through the bearings etc?

An "electro eliminator" is MG Duffs solution by making connection directly to the shaft immediately inboard of the stern gland using brushes in contact with the shaft.

The alternative to a hull anode connected via the engine and shaft or via an electro eliminator is of course a shaft anode if there is room to fit one. It should not be fitted close up to the prop. There is a recommended minimum spacing somewhere on MG Duff's Website IIRC

Having said that a bronze ( or manganese bronze ) prop on a stainless steel shaft should not normally require the protection of an anode. Thousands are of course fitted as a precautionary measure and it is all good business for the anode manufacturers.

 

[prv]

Hi Vic - thanks for the answer but I'm not sure it leaves me any the wiser! The whole subject seems more than a little self-contradictory and black-magic.

Things I can say for sure are that I think the rudder bolts are well bedded, there are no non-metallic joints in the prop shaft, and I think the bonding cable to the engine is bigger than 4mm^2. There's no room for a shaft anode; the prop is in an old-fashioned aperture between sternpost and rudder.

I still don't know whether I should go to the effort of connecting the rudder post to the anode.

Pete

 

[VicS]

I still don't know whether I should go to the effort of connecting the rudder post to the anode

. i would have said not, especially if you feel that it was not connected originally. However a previous owner may have found some reason for doing so.
I don't see that there would be anything to lose by connecting it even if there was no gain.

Maybe worth posting the question on the Cornish Crabbers forum just in case it is advisable to correct some known problem.

 

[versine]

Thank you for yours answers.

It seems to me now there are 2 problems ... or a horrible dilemn !

If I assume that a boat had no AC circuit :

Problem of galvanic corrosion :
no bonding => no corrosion (because hull isolated)
bonding => create a "pile effect" => corrosion => zinc anodes required

Problem of electrolytical corrosion (fault in positive conductor)
no bonding => risk of corrosion
bonding (with negative bonded) => the main fuse blow => stop of corrosive action

Is this abstract correct ?


But with a AC circuit, it seems better to "all bonding", following the rules of Europe or ABYC.

Thank you again


Jacques

 

[prv]

I don't see that there would be anything to lose by connecting it even if there was no gain.

In theory no, except that the current implementation is a loose wire emerging from the deck (originally through a plain untreated hole in a cored deck, causing comprehensive rotting of the core ) and flying over to the tiller head where it was crimped into a ring terminal round the securing bolt. Ugly, liable to catch on things, and slowly failing due to fatigue as the tiller moves.

The cable ended up being cut while mending the deck, but the anode end is difficult or impossible to get to now the engine's back in. I was planning to join new cable onto the old by soldering, and bring it through a deck gland, but all in all the whole thing is fairly unsatisfactory. The connection has to be on deck because the rudder stock passes through a sealed GRP tube from hull to deck.

Maybe worth posting the question on the Cornish Crabbers forum just in case it is advisable to correct some known problem.

I'm not aware of a functioning Crabbers forum. There is some moribund thing at www.cornishcrabbers.org but it only seems to get a couple of posts a year, mostly pleas cast into the void with very few answers coming back.

Pete

 

[VicS]

It seems to me now there are 2 problems ... or a horrible dilemn

Wherever possible avoid using metals and alloys below the waterline which are not resistant to seawater corrosion. Where unavoidable protect with sacrificial anodes.

Avoid contact between, or bonding of, dissimilar metals. Where unavoidable protect with sacrificial anodes.

Avoid connecting non ferrous items to the same anodes as as ferrous items.

At all cost avoid any situation that could lead to electrolysis by correctly designing and maintaining circuitry. Sacrificial anodes are unlikely to be an effective solution. Currents may well be high enough to cause immense damage but not high enough to blow the fuses.


Once an AC power supply is brought on board electrical safety considerations dictate that the DC negative, except in the case of fully isolated DC systems, and the boats internal earthing system where it exits are connected to the shorepower earth. Galvanic corrosion that might result from this should be preventable by installing a galvanic isolator.

 

[William_H]

You are thinking of the Italian physiologist Luigi Galvani (1737-89) Professor of anatomy at Bologna University and famous for his experiments in "animal electricity"

It was in fact the Italian physicist Alessandro Volta (1745-1827) who invented the voltaic, or galvanic, pile.

Thanks Vic clearly mine was a wild guess. So why is it called galvanic corrosion or a galvanic cell?
No don't answer that completely pointless question. olewill