Cathodic Protection

[Lee1]

I am currently refitting a Contessa 32. It has Blakes seacocks which are all in good condition, however the bolts through the skin fittings were in very poor condition and have all been drawn and replaced. These bolts were previosly bonded to the anode. Should this be the case? or should the anode be bonded to the engine mounts / shaft only?

Any assistance would be very welcome

 

[cliff]

engine block / shaft only
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[Lee1]

Any idea why the skin fittings shouldn't be bonded?

 

[VicS]

Welcome to the forum.

It seems Cliff is in one word answer mode, maybe you are interupting his lunch time drink!

Bronze or even DZR brass should not need cathodic protection in normal circumstances but if you detect any problem then they can be protected.

It is imortant that anodes are resonably close to the item(s) they are to protect and within "line of sight" although that does not have to be taken too literally. They must also be connected electrically to the items and it is important that it is a good low resistance connection directly between the two.

I would suggest that the bolts were of the wrong material. Any idea what they were made of? Bronze would be best, stainless is best avoided below the water line but if it is used it MUST be A4 (316) and they should be well sealed to prevent sea water getting to the shanks and causing crevice corrosion.

This sort of subject comes up regularly and has been discussed fairly recently. Do a search or two and you should find plenty to read.

 

[Tomkat17]

On my Contessa 32 only the engine seacock and the galley sink have a bonded wire which I assume is attached to the engine as the anode is wired to the shaft and engine only. The forward seacocks for the toilet and sink have no wiring.

 

[VicMallows]

I too am rather puzzled by Cliff's answer on this one. In fact none of my seacocks are bonded, and all are still perfect (23yrs) ........ but I can't see any reason NOT to bond them?

Edit: should have said skin-fittings, not sea-cocks.

Vic

 

[VicS]

[ QUOTE ]
but I can't see any reason NOT to bond them

[/ QUOTE ] If you have anodes protecting vulnerable ferrous bits, eg steel rudder or bilge plates and you then connect bronze or other copper alloy fittings to the same system then you do have a situation in which if the geography was wrong the copper alloy fittings could increase the corossion of the steel bits and of course accelerate the wasteage of the anode....maybe. The small surface area of the non ferrous fittings vs the large surface area of the ferrous bits would mean that the corrosion of the latter would still be pretty small though

MG Duff do say you should not connect ferrous and non-ferrous fittings to the same anode which basically means connecting the non-ferrous fittings to their own anode(s) and not interconnecting with any ferrous fittings / anode systems.

However if you have a shore power installation the wisdom is that all metal parts of the boat (and the 12volt negative should be connected to the shore earth). Maybe non-ferrous fittings (and their anodes) should be excepted from that.

I still feel that bronze and DZR brass fittings should not need cathodic protection in most normal situations and I am quite certain there are thousands of anodes fitted to boats quite unnecessarily. It's jolly good business for the manufacturers though.

The thirty year old Westerly that I sail regularly has no anodes and it still has all the original skin fittings, propellor and shaft.

 

[gjeffery]

Not clear to me why any (insulated) through hull component should require cathodic protection, if the seawater does not complete a circuit with a dissimilar electrode.

Simplistically, one would not expect electrolytic corrosion of an connected anode.

 

[VicS]

[ QUOTE ]
Not clear to me why any (insulated) through hull component should require cathodic protection,

[/ QUOTE ] All metals and alloys corrode to a greater or lesser extent (precious metals such as gold and platinum excepted perhaps). You surely cannot have failed to notice that a bit of steel left outside rusts without being connected to a "dissimilar" metal. In some cases eg aluminium or chromium the formation of an impervious oxide film effectively stops the process once the film is formed and that is also what gives stainless steels their corrosion resistance. The oxide layer (rust) than forms on ordinary carbon steels does not offer this protection so it contiues to corrode.

The situation is of course quite complex with alloys as in some cases it is one of the alloying elements that forms the protective oxide film as in the case of stainless steels but in other cases the alloying elements are the cause of the problems as in the case of zincification of brasses.

Those that are protected by oxide films are dependent on this film remaining to protect the metal. High and turbulent flow rates can remove this film and cause quite rapid corrosion (sometimes called corrosion erosion) Stainless steel suffers from crevice corrosion when parts are not exposed to continuous stream of oxygenated water. However in this case it is possble to identify anodic areas, where the oxgen level is depleted, and cathodic areas, where it remains high, which lead to corrosion occuring by an electrolytic mechanism much as it does in the case of dissimilar metals being in contact.

Maybe not specifically relevant to subject of the forum thread these two lovely pictures from a very old Admiralty publication on boiler corrosion do illustrate the electrolytic nature of corrosion that does not involve dissimilar metals.

Here endeth the latest chemistry lesson and I hope some of it will be of interest.

 

[VicMallows]

Full Marks!

Had you been my Chem teacher, I might well have gone for Chem/Physics/Maths rather that Physics/Double Maths. Chem back then ( in Wales) seemed to be dominated with learning the periodic table by rote; and mixing solutions in test tubes. (though I did learn a lot about gas flames and multi-spigot gas taps !)

Vic (M)

 

[VicS]

[ QUOTE ]
Had you been my Chem teacher

[/ QUOTE ] No chance. I looked at that possibility when I retired early. It did not take very long to decide not to go down that route and I was looking at a boy's Grammar School which was the second best one in an area where there were three.

 

[gjeffery]

Many thanks for that excellent explanation. Is it possible to generalize the extent to which corrosion affecting a single component might be reduced by bonding with a remote sacrificial anode?

Thinking on the bonding of all through hull fittings in boats with shore power connections. I suspect the original purpose may have included a need for equipotential bonding for safety reasons.

 

[VicS]

[ QUOTE ]
Is it possible to generalize the extent to which corrosion affecting a single component might be reduced by bonding with a remote sacrificial anode?


[/ QUOTE ] I'm not quite sure what you have in the back of your mind. Cathodic protection with sacrificial anodes can be quite effective even to the point of eliminating corrosion totally but as I said earlier the anode must be reasonably close, the closer the better but I would not like to say what would be the maximum distance you could use, and also "be able to see" the item it is protecting although anodes can "see" round gentle curves etc. The anode size will depend upon the area to be protected . The larger the area the larger the suface area of anode that will be needed but it eventually becomes more effective to split the anode up into two or more smaller ones located so that all parts are reasonably close to one of them.
[ QUOTE ]
through hull fittings in boats with shore power connections

[/ QUOTE ] Once you start taking about shore power connections and earthing of the boats systems to it all sorts of arguments start to surface beginning with those who do not believe in having an earth connection on board at all! However I think there is now increasing acceptance of the official line that eveything on board should be earthed. I cannot see though how a few unearthed skin fittings pose a threat any more than your unearthed anchor chain!

 

[gjeffery]

What I was getting at was that where a component exhibits both anodic and cathodic areas in very close proximoty, is the resulting corrosion greatly reduced by a remote anode?

I think I may try this one with a bright "tin can" and my old shaft anode in a bucket of brine!

As for equipotential bonding as a safety requirement, on reflection I agree with you.