Dead Saildrive; electrolysis? but why isn't the anode more damaged?

[Plevier]

It's a Volvo MD2030 with 130S drive. It's been in Holland for the last two years, I sailed (motored a bit as well) from the south through to the Isselmeer via the canal network last year . I'm wondering now if freshwater is the problem, although I thought that there was very little freshwater in those lakes now.
What I have had for the last couple of years is a discharge of about 0.2A showing on my battery meter even when everything is switched off with the main switch. I think I tracked this down (but never been able to fix it) to the engine circuit - I have house & engine circuits. I had a new fuel tank fitted and the guys fitted a fuel gauge which they linked to the engine fusebox (on the engine itself) and I noticed the light stayed on (hence the 0.2A discharge I guessed). This seemed to be overcome by turning the engine key which made the starter motor solenoid clonk, and hey presto the discharge and fuel gauge light went off. Potential problem? But this has been for three years now and I haven't seen any corrosion before this time.

For comparison I have just bought a boat with 130S that has spent its whole 5 year life on the IJsselmeer, which is definitely fresh water. It still had the original zinc anode on, almost as new but it had developed a yellow coating which I assume passivated it!
The leg itself had been epoxied but a small patch had flaked off adjacent to the anode and the casting was superficially corroded.
I would not think your problem is freshwater related, much more likely electrical.

 

[ashanta]

You are right about the paint being an important protective coating and that its should be maintained

You are also right about folding props being protected by attached anodes

But you are wrong in suggesting that the sail drive leg anode can offer any protection to a prop without its own anodes simply by being in close proximity.

Sacrificial cathodic protection is dependent upon there being a good electrical connection between the anode and what it is protecting. This does not exist because the copper alloy prop will be electrically insulated from the shaft.
If the prop were not insulated from the shaft the sail drive leg would suffer from galvanic corrosion due to its proximity to the prop and an electrical connection between the two.

The electrical insulation of prop from the shaft prevents galvanic corrosion of the SD by the prop and also of the SD anode.

It was my thought that this is what has occurred. ie "the sail drive leg would suffer from galvanic corrosion due to its proximity to the prop " due to either the anode failure or the failure to protect the leg with adequate paint and different metals used. (Stainless steel drain plug and aluminium socket) I believe the SD has been sacrificial to the CB Prop.
"But you are wrong in suggesting that the sail drive leg anode can offer any protection to a prop without its own anodes simply by being in close proximity." Many years ago we would suspend zinc anodes over the side and under the hull to help the protection of the prop purely by proximity. Were we wrong in doing this?

 

[nigelmercier]

... Many years ago we would suspend zinc anodes over the side and under the hull to help the protection of the prop purely by proximity. Were we wrong in doing this?

If they were not connected to the thing they were protecting, then yes. Why do you think hanging anodes have a wire.

 

[Tranona]

It was my thought that this is what has occurred. ie "the sail drive leg would suffer from galvanic corrosion due to its proximity to the prop " due to either the anode failure or the failure to protect the leg with adequate paint and different metals used.

Can only echo what others have said - your "thoughts" are misplaced. Proximity does not lead to galvanic action, it is physical contact allowing current to pass, and the prop is not electrically connected to the housing as it is isolated by a rubber bush. The prop will have its own anodes to protect the materials (usually stainless pins and bronze castings) in its construction. The saildrive housing is protected by the anode and it seems this is where the failure has occurred. The answer may lie in the type of anode or in a poor connection between the anode carrier and the housing, but impossible to say just looking at photos.

 

[vyv_cox]

Difficult to assess the propeller from the photographs but I would not scrap it on the basis that it does not 'ring'. Do folding props ring anyway? I have no experience with them. As has been clarified by several knowledgeable posters, in this case any corrosion of the prop is entirely separate from the problem with the saildrive and I suspect that the boatyard staff may be putting 2 + 2 = 5. You should assess it by abrading the metal back to its natural colour, which will reveal the depth of any dezincification.

 

[ashanta]

Can only echo what others have said - your "thoughts" are misplaced. Proximity does not lead to galvanic action, it is physical contact allowing current to pass, and the prop is not electrically connected to the housing as it is isolated by a rubber bush. The prop will have its own anodes to protect the materials (usually stainless pins and bronze castings) in its construction. The saildrive housing is protected by the anode and it seems this is where the failure has occurred. The answer may lie in the type of anode or in a poor connection between the anode carrier and the housing, but impossible to say just looking at photos.

This is my understanding : "Galvanic corrosion is caused by the existence of a galvanic cell - essentially two metals submersed in an electrolyte - that results in an attack on one metal at the expense the other. For a galvanic cell to form, two electrochemically different metals must exist within a localized electrolytic environment.
This type of corrosion is often witnessed in marine environments due to salt water's effectiveness as an electrolyte. Different metals submersed in close proximity in salt water will form an electrolytic cell, resulting in galvanic corrosion"
My 3 blade VP Copper Bronze folding prop is not isolated by any rubber bush and neither is any 2 or 3 blade CB props that I have had in the past? Anodes do not need to be connected another metal to protect it. If it is that's good but not essential. Being as close as possible also works. If the saildrive anode is the correct one it should give protection to the prop that is behind it, regardless of what you feel is the primary use of this anode. It will give protection to the propeller.

 

[nigelmercier]

This is my understanding : "Galvanic corrosion is caused by the existence of a galvanic cell - essentially two metals submersed in an electrolyte - that results in an attack on one metal at the expense the other. For a galvanic cell to form, two electrochemically different metals must exist within a localized electrolytic environment.
...

Yes, but just as batteries don't go flat in the box, it doesn't happen unless the circuit is completed.

 

[ashanta]

Can only echo what others have said - your "thoughts" are misplaced. Proximity does not lead to galvanic action, it is physical contact allowing current to pass, and the prop is not electrically connected to the housing as it is isolated by a rubber bush. The prop will have its own anodes to protect the materials (usually stainless pins and bronze castings) in its construction. The saildrive housing is protected by the anode and it seems this is where the failure has occurred. The answer may lie in the type of anode or in a poor connection between the anode carrier and the housing, but impossible to say just looking at photos.

This is the point I am making and it's from Volvo Penta : "Volvo Penta folding propellers are designed to be electrically isolated from the propeller shaft
of the sail drive, and they have their own anodes. These anodes protect the propeller, and do not place a load on the sail drive anode. Sometimes owners may install other brands of folding propellers on their sail drive. Not all other propellers are electrically isolated, or they may not have their own anodes. If this is the case, the propeller will place a load on the sail drive anode and may cause more rapid loss of the anode."

This the point I was trying to make. I cannot see the propeller very well in pics but it does look like its pitted. The Prop does not look as if it has it's own anode and it is a big lump of CB. The anode has not eroded much at all which suggest that its the wrong type or something is stopping its effectiveness. The protective coating on the SD appears to be lost and my view as mentioned earlier is that the SD is suuferring at the expense of the CB Prop.

 

[VicS]

This is my understanding : "Galvanic corrosion is caused by the existence of a galvanic cell - essentially two metals submersed in an electrolyte - that results in an attack on one metal at the expense the other. For a galvanic cell to form, two electrochemically different metals must exist within a localized electrolytic environment.
This type of corrosion is often witnessed in marine environments due to salt water's effectiveness as an electrolyte. Different metals submersed in close proximity in salt water will form an electrolytic cell, resulting in galvanic corrosion"
My 3 blade VP Copper Bronze folding prop is not isolated by any rubber bush and neither is any 2 or 3 blade CB props that I have had in the past? Anodes do not need to be connected another metal to protect it. If it is that's good but not essential. Being as close as possible also works. If the saildrive anode is the correct one it should give protection to the prop that is behind it, regardless of what you feel is the primary use of this anode. It will give protection to the propeller.

This is the point I am making and it's from Volvo Penta : "Volvo Penta folding propellers are designed to be electrically isolated from the propeller shaft
of the sail drive, and they have their own anodes. These anodes protect the propeller, and do not place a load on the sail drive anode. Sometimes owners may install other brands of folding propellers on their sail drive. Not all other propellers are electrically isolated, or they may not have their own anodes. If this is the case, the propeller will place a load on the sail drive anode and may cause more rapid loss of the anode."

This the point I was trying to make. I cannot see the propeller very well in pics but it does look like its pitted. The Prop does not look as if it has it's own anode and it is a big lump of CB. The anode has not eroded much at all which suggest that its the wrong type or something is stopping its effectiveness. The protective coating on the SD appears to be lost and my view as mentioned earlier is that the SD is suuferring at the expense of the CB Prop.

Sorry you are wrong . You misunderstand the theory of sacrificial cathodic protection.

The anode MUST be electrically connected to the item it is to protect. Being in close proximity is not enough

There must be a complete electrical circuit. The electrolyte forms one half of that circuit. Bonding or direct contact forms the other half.

There is a good article on the National Physical Laboratory website on galvanic corrosion and cathodic protection. I suggest you find it and read it.


http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf

 

[ashanta]

Sorry you are wrong . You misunderstand the theory of sacrificial cathodic protection.

The anode MUST be electrically connected to the item it is to protect. Being in close proximity is not enough

There must be a complete electrical circuit. The electrolyte forms one half of that circuit. Bonding or direct contact forms the other half.

There is a good article on the National Physical Laboratory website on galvanic corrosion and cathodic protection. I suggest you find it and read it.


http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf

So the written word from Volvo Penta regarding the SD anode and a CB prop that doesn't have its own anode is wrong? As they state, not me and as indicated in my last post :

"Volvo Penta folding propellers are designed to be electrically isolated from the propeller shaft
of the sail drive, and they have their own anodes. These anodes protect the propeller, and do not place a load on the sail drive anode. Sometimes owners may install other brands of folding propellers on their sail drive. Not all other propellers are electrically isolated, or they may not have their own anodes. If this is the case, the propeller will place a load on the sail drive anode and may cause more rapid loss of the anode."
The last two sentences are the point I have tried to make. It is only in recent years that they have added anodes to their folding props. There are many CB props out there being protected by the SD anodes or other anodes in close proximity because they do not have their own .

Is this statement by Volvo Penta as quoted here above Wrong? Is the practice of many over the years wrong? I haven't had a problem over the years. It has worked OK for me as my equipment has been of an older generation.

 

[Salty John]

All they are saying is that if the prop is not isolated from the SD and doesn't have its own anode it will place a load on the SD anode to which it is electrically connected by virtue of it not being isolated. They could have worded it better. Google will provide you with a plethora of information on cathodic protection all of which will confirm that an anode must be electrically connected to the item it's protecting to be effective.

 

[VicS]

So the written word from Volvo Penta regarding the SD anode and a CB prop that doesn't have its own anode is wrong? As they state, not me and as indicated in my last post :

"Volvo Penta folding propellers are designed to be electrically isolated from the propeller shaft
of the sail drive, and they have their own anodes. These anodes protect the propeller, and do not place a load on the sail drive anode. Sometimes owners may install other brands of folding propellers on their sail drive. Not all other propellers are electrically isolated, or they may not have their own anodes. If this is the case, the propeller will place a load on the sail drive anode and may cause more rapid loss of the anode."
The last two sentences are the point I have tried to make. It is only in recent years that they have added anodes to their folding props. There are many CB props out there being protected by the SD anodes or other anodes in close proximity because they do not have their own .

Is this statement by Volvo Penta as quoted here above Wrong? Is the practice of many over the years wrong? I haven't had a problem over the years. It has worked OK for me as my equipment has been of an older generation.

Not it is not wrong but you are misinterpreting it as SJ explains above

 

[bigwow]

Previous VP folding props, without the anodes were a nightmare. The splined inner used to corrode itself onto the shaft and the the folding cogs destroy themselves, I know this by the six props on the shelf in my workshop that taunt me every-time I go in there. The SD anode did nothing to stop this. Since VP fitted anodes to their folding props I have had no problems with prop corrosion.

 

[Tranona]

Previous VP folding props, without the anodes were a nightmare. The splined inner used to corrode itself onto the shaft and the the folding cogs destroy themselves, I know this by the six props on the shelf in my workshop that taunt me every-time I go in there. The SD anode did nothing to stop this. Since VP fitted anodes to their folding props I have had no problems with prop corrosion.

That was the poor quality of the alloy used for the prop as much as anything. The corrosion is the dissimilar metals in the alloy and the alloy against the pivot pins. The anodes reduce this substantially, but as many owners will testify they often have a very short life because of their small size. Other folding props made of different alloys suffer less and some, like the 2 blade folding FlexOFold on my boat does not have an anode at all and does not corrode.

 

[ashanta]

All they are saying is that if the prop is not isolated from the SD and doesn't have its own anode it will place a load on the SD anode to which it is electrically connected by virtue of it not being isolated. They could have worded it better. Google will provide you with a plethora of information on cathodic protection all of which will confirm that an anode must be electrically connected to the item it's protecting to be effective.

Thanks Vic & John, I am more knowledgeable now! I appreciate you expertise Pity for the owner as his SD looks BER!

 

[Twister_Ken]

View attachment 42850

If that's a Flexofold prop (looks like it might be) there should be two 'button' anodes attached to it. Number 11 in this pic…

 

[Plevier]

If that's a Flexofold prop (looks like it might be) there should be two 'button' anodes attached to it. Number 11 in this pic…

My 2 blade flexofold doesn't have the button anodes and no screw holes to fit them.
Darglow say (that flexofold say) that thery aren't necessary and were only introduced to shut up customers who thought they should have anodes!
My prop has lasted 5 years so far without them and looks fine but that was in fresh water.
I'm waiting anxiously to see the effect of a year in the sea.

 

[LostinFrance]

This is the point I am making and it's from Volvo Penta : "Volvo Penta folding propellers are designed to be electrically isolated from the propeller shaft
of the sail drive, and they have their own anodes. These anodes protect the propeller, and do not place a load on the sail drive anode. Sometimes owners may install other brands of folding propellers on their sail drive. Not all other propellers are electrically isolated, or they may not have their own anodes. If this is the case, the propeller will place a load on the sail drive anode and may cause more rapid loss of the anode."

This the point I was trying to make. I cannot see the propeller very well in pics but it does look like its pitted. The Prop does not look as if it has it's own anode and it is a big lump of CB. The anode has not eroded much at all which suggest that its the wrong type or something is stopping its effectiveness. The protective coating on the SD appears to be lost and my view as mentioned earlier is that the SD is suuferring at the expense of the CB Prop.

Unfortunately I never thought to rub away the mess on the SD leg, but I can assure everybody that one year ago the paint was intact; it is possible that I knocked off some of the paint from inside the indent where the drain screw is fitted, and there is obvious corrosion there.

It IS a flexifold prop and as the previous poster says, there are no anodes for this. The prop didn't look any different than any other year at first sight, any pitting in the photos are more the result of a lazy rub down last year, the craters are where there is less antifouling.

I'm still bemused as to why the anode is looking so good.

No one has commented on one of my earlier theories ie:
When I craned in last year I fouled the prop with a dirty great sling from the crane. It stopped the engine dead, and I wonder if any resultant stress on the drive casing surrounding the bearing housing (exactly where the most damage seems to be) caused fissures in the alloy and cracked the paint.The bearing housing itself is not damaged, but it may have stressed the body of the leg?

 

[vyv_cox]

No one has commented on one of my earlier theories ie:
When I craned in last year I fouled the prop with a dirty great sling from the crane. It stopped the engine dead, and I wonder if any resultant stress on the drive casing surrounding the bearing housing (exactly where the most damage seems to be) caused fissures in the alloy and cracked the paint.The bearing housing itself is not damaged, but it may have stressed the body of the leg?

It is more likely that aliens have invaded it and turned it into an allotment for the chemical that drives their interplanetary craft

A couple of months ago in another thread, VicS found that the zinc anodes supplied by MGDuff contain some indium that makes a surprising difference to the potential in seawater, i.e. considerably more anodic. In the case of a saildrive leg the material is quite anodic anyway, and some grades of zinc anode may actually be more cathodic than the thing they are trying to protect. I believe that is what happened here. The leg is more anodic than the anode and has done its best to protect it.

 

[VicS]

A couple of months ago in another thread, VicS found that the zinc anodes supplied by MGDuff contain some indium.

It's aluminium anodes that contain indium it makes them very slightly more anodic than zinc compared with other aluminium alloys which are more cathodic