Looking for advice re Zinc Anodes, to bond or not to bond.

[Jcorstorphine]

I am looking to fit a Zinc anode to my 40 year old GRP motor sailer as I noticed a bit of corrosion on my stainless steel rudder when boat was lifted out for the winter. I have no idea why this has suddenly started and it may be an inherent flaw in the plate. The boat has been in a Marina on the Clyde for last 7 years but before that was on Loch Lomond for unknown period. My pontoon in the Marina is some distance from the larger motor yachts so I do not think it is a locality problem to third party stray currents.

The full inventory of metallic items immersed is as follows.

Stainless steel rudder (Spec not known 40 years old)
Bronze propeller (unknown SH off Moody fitted 4 years ago)
Bronze shaft (Stuart)
Bronze engine seacock (Stuart)
Bronze stern tube (Stuart)
DZR Toilet Seacocks.
A4 stainless steel bilge keel bolts (flanged GRP bilge keels)

At the moment there is no bonding whatsoever, the engine is indirectly cooled, the drive to the prop shaft is via a "Carden Shaft" with two Hardy Spicer couplings so there will be some electrical continuity between the engine and the prop shaft.

I am proposing to fit a sandwich anode to either side of the Rudder but am not at all sure if I should bond the rudder stock to the engine and seacocks.

Can anyone advise, thanks.

 

[zambant]

If you just want to protect the rudder then there is no need to bond to anything else BUT have you considered using aluminium anodes? Is the Clyde salt water or is there a chance it could be brackish? If so than definitely fit ali anodes.
Fair wins
John

 

[PaulRainbow]

Anything that is electrically isolated cannot be effected by galvanic corrosion. So if your rudder and rudder stock are not connected to anything other than the GRP hull the corrosion cannot be galvanic and fitting an annode will be pointless. Likewise the seacocks, isolated they cannot suffer galvanic corrosion, add them to a full bonding circuit and there then exists the possibility that they could (although technically they shouldn't as the annode should protect them).

 

[Jcorstorphine]

If you just want to protect the rudder then there is no need to bond to anything else BUT have you considered using aluminium anodes? Is the Clyde salt water or is there a chance it could be brackish? If so than definitely fit ali anodes.
Fair wins
John

Yes, the Clyde is salt and the marina is open to rise and fall of ~3m tide so plenty of salt water flushing through the pontoons.

 

[MM5AHO]

I have no bonding at all.
Volvo MD2030 engine, has bronze intake valve for cooling. There's a stainless propshaft a bronze prop, various bronze seacocks, stainless rudder shaft, none bonded together.
There is insulation between engine and gearbox - a standard VP thing of that day I think
It's taken 24 years to slightly corrode the prop nut, but rest is 42 years without visible corrosion.
And that's mostly also in the Clyde.

 

[Tranona]

Any corrosion on stainless is likely to be crevice corrosion so an anode wont help. However you should check any fastenings, brackets, bearings etc to check that they are all the same grade stainless. If not replace them. If that is not practical then an anode on the plate might help. Does not matter if it is zinc, magnesium or aluminium as the choice depends on the water, not on the metals you are trying to protect.

 

[vyv_cox]

Anything that is electrically isolated cannot be effected by galvanic corrosion. So if your rudder and rudder stock are not connected to anything other than the GRP hull the corrosion cannot be galvanic and fitting an annode will be pointless.

Sorry, not true. An anode will protect any metal more noble than itself. Steel offshore structures are plastered with aluminium or zinc anodes, nothing to do with galvanic couples.

 

[PaulRainbow]

Sorry, not true. An anode will protect any metal more noble than itself. Steel offshore structures are plastered with aluminium or zinc anodes, nothing to do with galvanic couples.

So you can get galvanic corrosion without an electrical circuit ?

 

[VicS]

So you can get galvanic corrosion without an electrical circuit ?

Galvanic corrosion is by definition corrosion between two dissimilar metals or alloys immersed in the same electrolyte and in contact or electrically connected to each other.

Not all corrosion involves two different metals or alloys but in seawater almost invariably proceeds by an electrochemical mechanism between cathodic and anodic sites on the same metal or alloy. Corrosion of iron and steel, dezincification of brass and crevice corrosion are examples.

The corrosion can be prevented by cathodic protection, except perhaps crevice corrosion, using impressed current or sacrificial anodes, which makes the entire surface cathodic with respect to the anodes.

 

[PaulRainbow]

Galvanic corrosion is by definition corrosion between two dissimilar metals or alloys immersed in the same electrolyte and in contact or electrically connected to each other..

So that's a no then.

 

[VicS]

So that's a no then.

It is a no by virtue of the definition of galvanic, or bimetallic, corrosion

It is also a no to your suggestion in an earlier post that a single metal or alloy isolated from any other and suffering from uniform, or general, corrosion cannot be be protected by an anode .

 

[PaulRainbow]

It is also a no to your suggestion in an earlier post that a single metal or alloy isolated from any other and suffering from uniform, or general, corrosion cannot be be protected by an anode .

What type of corrosion on a stainless rudder (electrically isolated) on a yacht is an anode going to protect ?

It's not a steel oil rig leg or an underwater pipeline, or even a steel boat hull.

 

[vyv_cox]

What type of corrosion on a stainless rudder (electrically isolated) on a yacht is an anode going to protect ?

It's not a steel oil rig leg or an underwater pipeline, or even a steel boat hull.

You seem to be assuming that 'stainless steel' is immune to corrosion in seawater. As many will testify, this is not the case. 304 underwater will corrode badly, mainly due to pitting, and 400 series will corrode generally. In each case an anode will help to some extent. Many people use an anode to reduce corrosion of stainless steel, for example rudder pintles.

A single metal, as stated, will be cathodically protected by an anode. In a galvanic couple the more anodic metal will corrode at the expense of the more noble one. If a third, even more anodic metal is introduced to the couple it will corrode while protecting both of the metals in the original couple. The mechanism is exactly the same in both cases.