Galvanic reaction around bronze skin fittings

[jenevers]

How come the steel around bronze skin fittings doesn’t gradually corrode...............or does it?

[Tony Brooks]

Are we talking about traditional Bronze (copper & tin) or so called aluminium bronze that in my view is more like brass. If the latter then I would expect the aluminium content to provide protection while getting eaten away, much like the zinc in conventional brass when it gets de-zinced.

[DHutch]

I expect sometimes it does cause issues, and my even attack the fitting not the hull as above.

However with a good thumb of PU sealant around it and paint over the top your unlikely to have any issues. 

Certainly however they are not suitable for use under the water on a narrowboat.

 

Daniel

[Alan de Enfield]

1 hour ago, jenevers said:

How come the steel around bronze skin fittings doesn’t gradually corrode...............or does it?

Mild steel is further 'down' the table than Bronze so it will be affected before the bronze fittings are.

 

Edited December 7, 2017 by Alan de Enfield

[mross]

In the galvanic series, some bronzes and steels can overlap.  Corrosion needs the water to conduct and the water you are floating in may have very low conductivity.  As dutchy says, if you can electrically separate the two metals with washers or sealant, corrosion is prevented.

https://www.nordhavn.com/resources/tech/images/Galvanic_Series.pdf

Edited December 7, 2017 by mross
added link

[jenevers]

38 minutes ago, Tony Brooks said:

Are we talking about traditional Bronze (copper & tin) or so called aluminium bronze that in my view is more like brass. If the latter then I would expect the aluminium content to provide protection while getting eaten away, much like the zinc in conventional brass when it gets de-zinced.

Whatever bronze was being used in the 1930s.

[Alan de Enfield]

And, it doesn't necessarily matter if you have an isolation transformer or not (depending on your wiring methods) -

If you have an aluminium drive leg, and your neighbour has a bronze prop - you are in trouble, (or any number of other permutations)

 

 

Edited December 7, 2017 by Alan de Enfield

[Tony Brooks]

1 hour ago, jenevers said:

Whatever bronze was being used in the 1930s.

That would be proper bronze then but I think you have the answers

[jenevers]

So has anyone ever heard of a boat sinking because the steel surrounding a bronze (underwater) skin fitting has corroded away?

[mross]

It's very unlikely in fresh water.  Any leak would start as a weep or drip and would take months/years to sink a boat.

[Alan de Enfield]

Another benefit for GRP

I have 11 underwater skin fittings, my concern is purely de-zincifiation but hopefully the Zinc anodes will be the priority anodic protection

[Tony Brooks]

For others' information. Anodes on a GRP boat will not protect skin fittings unless the skin fittings are electrically bolned to the anode. Usually done by interior cables fixed to each skin fitting and the anode.

[David Mack]

3 hours ago, Alan de Enfield said:

And, it doesn't necessarily matter if you have an isolation transformer or not (depending on your wiring methods) -

If you have an aluminium drive leg, and your neighbour has a bronze prop - you are in trouble, (or any number of other permutations)

 

 

Surely the whole point of having an isolation transformer or galvanic isolator on your mains supply is so that you break the earth connection (while maintaining electrical safety) and don't set up a circuit like the one illustrated.

[Alan de Enfield]

1 hour ago, Tony Brooks said:

For others' information. Anodes on a GRP boat will not protect skin fittings unless the skin fittings are electrically bolned to the anode. Usually done by interior cables fixed to each skin fitting and the anode.

Bonding of sea-cocks

 

Then bonding of propeller shafts is also important.

The electro eliminator has been designed to stop electrostatic radio and depth sound interference created by shaft rotation and when used in conjunction with a good cathodic protection bonding system it also aids in preventing unnecessary electrolysis.

Edited December 7, 2017 by Alan de Enfield

[DHutch]

4 hours ago, David Mack said:

Surely the whole point of having an isolation transformer or galvanic isolator on your mains supply is so that you break the earth connection (while maintaining electrical safety) and don't set up a circuit like the one illustrated.

I thought that.

6 hours ago, jenevers said:

So has anyone ever heard of a boat sinking because the steel surrounding a bronze (underwater) skin fitting has corroded away?

Doubt it. Such fittings should be above the waterline anyway as they are not mechanically wrong enough for underwater fittings on a narrowboat.

 

Daniel 

[Alan de Enfield]

33 minutes ago, DHutch said:

Such fittings should be above the waterline anyway as they are not mechanically wrong enough for underwater fittings on a narrowboat.

Don't forget some NBs are 'raw water' cooled, or have heat exchangers, and MUST have external water inlets below the waterline.

Presumably the fittings are in fact 'mechanically strong enough' or surely we would hear about sinkings due to sea-cock failures.

[DHutch]

13 hours ago, Alan de Enfield said:

Don't forget some NBs are 'raw water' cooled, or have heat exchangers, and MUST have external water inlets below the waterline.

Presumably the fittings are in fact 'mechanically strong enough' or surely we would hear about sinkings due to sea-cock failures.

Some boats must indeed have fittings under the waterline, Emilyanne is in effect raw water cooler and also has a boiler blow down under the waterline.

However neither of these fittings are brass screw together type fittings, which I maintain are in round figures not suitable for use underwater on a narrowboat, given the environment they operate, hull shape, and the ease of doing something so much better.

I am sure there are such fittings used underwater on narrowboats, and if placed in a relatively protected location within say the swim area, but I wouldn't recommend it. 

 

Daniel

[mross]

If you have a leaking seacock, you can change it while afloat with this device.  You could also seal a leaking through hull fitting in some circumstances.  

 

[rusty69]

9 minutes ago, mross said:

If you have a leaking seacock, you can change it while afloat with this device.  You could also seal a leaking through hull fitting in some circumstances.  

 

Cool, I'm gonna ask Mrs Rusty to get me one for xmas. Usually I just wait for the tide to go out

[mross]

Could also be useful for cleaning the speed log if you lose your dummy.

[rusty69]

9 minutes ago, mross said:

Could also be useful for cleaning the speed log if you lose your dummy.

I never usually clean my logs, just flush em, after all, you can't polish a turd......unless its fossilized.

Edited December 8, 2017 by rusty69

[David Mack]

21 hours ago, DHutch said:

 

Doubt it. Such fittings should be above the waterline anyway as they are not mechanically wrong enough for underwater fittings on a narrowboat.

We'll just about every working motor narrow boat had underwater skin fittings since they were all originally fitted with direct water cooled engines. And many still are.

[Alan de Enfield]

6 hours ago, DHutch said:

However neither of these fittings are brass screw together type fittings, which I maintain are in round figures not suitable for use underwater on a narrowboat, given the environment they operate, hull shape, and the ease of doing something so much better.

Indeed - I don't know of anyone who would use brass - 'proper' (better) fittings are Bronze.

[blackrose]

One well known chandler (UBC) often recites a mantra about not using dissimilar metals together - despite selling all types of brass fittings for narrowboats! Like many people, while she has some understanding about the relative nobility of metals, she's completely forgotten the basic principal that an electrolyte is required to form a galvanic circuit and what she's saying only apples to dissimilar metals below the waterline. 

[Iain_S]

1 hour ago, blackrose said:

One well known chandler (UBC) often recites a mantra about not using dissimilar metals together - despite selling all types of brass fittings for narrowboats! Like many people, while she has some understanding about the relative nobility of metals, she's completely forgotten the basic principal that an electrolyte is required to form a galvanic circuit and what she's saying only apples to dissimilar metals below the waterline. 

I wish someone had told my Land Rover that