Anodes- are they worth the money????

[WotEver]

16 minutes ago, blackrose said:

Anodes work galvanically but they don't protect the boat against galvanic corrosion. Only a galvanic isolator or isolation transformer can do that. Or just unplugging the boat from the the mains.

Confusing, innit? I would say that...

Anodes protect against Galvanic Corrosion. 

GIs or ITs protect against Stray Current Corrosion.

It's not a Galvanic current if it's an introduced voltage...

[WotEver]

4 minutes ago, Wanderer Vagabond said:

... any suggestions as to reasons for the differential erosion?

The rear ones might be protecting the prop. As to the front ones, do you have a bow thruster...?

[Catnip King]

In my opinion best having them at bow and stern as those are the most complicated areas to replace steel. 

[jddevel]

8 hours ago, WotEver said:

The rear ones might be protecting the prop

Stupid question I`m sure but with modern technology is it possible to use a type of plastic to make a prop. Even reinforced ones. Narrow boats move very slowly so perhaps they could cope with the stresses and vibration? I must say that having watched a very sophisticated 3D printer in operation ( not the inexpensive ones on EBay) in New Zealand a couple of years ago they could certainly be manufactured very cheaply in comparison. I did say some might think this silly.

[Chewbacka]

8 hours ago, Wanderer Vagabond said:

Having just had the boat out of the water for it's second blacking since I purchased it, it was interesting seeing the difference in the erosion of the anodes. The boat is fitted with 8 anodes/ 4 on each side. The first time I had it blacked the anodes were all OK, this time both bow anodes had been eaten away and both stern anodes had been eaten away (all 4 now replaced) the 4 remaining one's along the sides of the hull are still in much the same condition that they were last time the boat was blacked. We continuously cruise and very  rarely connect up the shore line, any suggestions as to reasons for the differential erosion?

One can only speculate, but anodes only have a limited range and so the ones at the back do have the propeller to look after.  Apart from that it may simply be related to 'paint' damage on the hull, the more exposed steel there is the more the adjacent anodes will 'corrode' away.

[Murflynn]

8 hours ago, WotEver said:

The rear ones might be protecting the prop. As to the front ones, do you have a bow thruster...?

the rear ones will be protecting the steel hull from galvanic corrosion set up with the bronze prop which is a more noble metal; the prop is not protected, but it is half of the galvanic cell.

[Murflynn]

36 minutes ago, jddevel said:

Stupid question I`m sure but with modern technology is it possible to use a type of plastic to make a prop. Even reinforced ones. Narrow boats move very slowly so perhaps they could cope with the stresses and vibration? I must say that having watched a very sophisticated 3D printer in operation ( not the inexpensive ones on EBay) in New Zealand a couple of years ago they could certainly be manufactured very cheaply in comparison. I did say some might think this silly.

even if you had a plastic prop, there will be other metals (like the stainless steel prop shaft) that will result in galvanic corrosion.

 

plastic props are not difficult to mould - you don't need a 3D printer; small outboards have been using plastic (like nylon) for decades.

Edited May 14, 2017 by Murflynn

[mross]

Plastic props and supermarket trollies together do not go.......................

But, you can get metal props plastic coated or hard polymer coated.

Edited May 14, 2017 by mross

[Martin Megson]

9 hours ago, blackrose said:

No, you're confusing two different things. Anodes work galvanically but they don't protect the boat against galvanic corrosion. Only a galvanic isolator or isolation transformer can do that. Or just unplugging the boat from the the mains.

It's not the first time I've been confused by electricery.  Thanks for clarifying. 

[Alan de Enfield]

11 hours ago, Dr Bob said:

Galvanic erosion we see is normally a shore power circuit - ie it doesnt seem to happen if you are not on shore power

 

[WotEver]

11 hours ago, Dr Bob said:

Galvanic erosion we see is normally a shore power circuit...

Corrosion please. 

[Neil2]

Isn't there a theory that the pitting that occurs in submerged steel is actually galvanic corrosion, caused by "impurities" within the steel?   

[WotEver]

1 minute ago, Neil2 said:

Isn't there a theory that the pitting that occurs in submerged steel is actually galvanic corrosion, caused by "impurities" within the steel?   

That is one of the many theories, yes. The fact that it's in deep pits rather than evenly spread would support the theory. 

[b0atman]

You say Liverpool do you mean in the docks if so your Anodes should be showing signs of being eaten due to the salt water in the docks .

 

[Alan de Enfield]

3 minutes ago, b0atman said:

You say Liverpool do you mean in the docks if so your Anodes should be showing signs of being eaten due to the salt water in the docks .

 

That's a good point - if it has been in Salt water it should have had Zinc anodes, if it is now in fresh water the Zinc anodes will not be as efficient (or 'corrode' away as quickly) as MG Anodes.

Vice-Versa, if it has MG anodes in sea water they should 'corrode' away in a matter of weeks

[Iain_S]

16 hours ago, Captain Pegg said:

(snip)

Actually rust is a coating that stops rust. Anything that stops oxygen getting to pristine steel will do.

JP

Err, no. Most oxides of iron are permeable to air and water, so the rusting continues until that nice shiny sheet is a pile of brown dust.

Compare with aluminium and magnesium (both more active metals than iron), where the oxide is air and water proof.

[Chewbacka]

23 minutes ago, Iain_S said:

Err, no. Most oxides of iron are permeable to air and water, so the rusting continues until that nice shiny sheet is a pile of brown dust.

Compare with aluminium and magnesium (both more active metals than iron), where the oxide is air and water proof.

The problem with rust is it's volume is about 3 times greater than iron, so as it forms it expands and cracks and flakes so exposing more iron.  This is why iron keeps on rusting, it also creates huge pressure when steel rusts inside concrete, and so causes cracking and spalling.  

[Captain Pegg]

2 hours ago, Iain_S said:

Err, no. Most oxides of iron are permeable to air and water, so the rusting continues until that nice shiny sheet is a pile of brown dust.

Compare with aluminium and magnesium (both more active metals than iron), where the oxide is air and water proof.

Stops wasn't the right word. In my defence the bit you quoted wasn't the main gist of my thread.

Bright metal oxidises very quickly but the rate of rusting thereafter is very slow. Untreated steel doesn't turn into a pile of brown dust in any timescale that should bother anyone unless there are catalysts present.

The point is that corrosion of steel has a lot more to do with the environment than it does with the basic properties of the steel itself. It is also easily mitigated with coating which was the question posed to which I responded. The problem in industrial usage is keeping the coatings in place.

JP

[system 4-50]

Its not the oxides that are the problem, its the hydroxides. But I'm just nit-pitting.