Galvanic corrosion, opinion please

[[2574]]

Dear Galvanic Corrosion cognoscenti,

Please have a look at these images: https://www.dropbox.com/sh/vt8sbxlq8ryenk5/ZkIu5qUrRl

Specifically at the area of bare metal on the lower bearing holder on the skeg, I am wondering if this is caused by galvanic corrosion.

The prop anode is completely shot and there are no other anodes below the waterline. I am thinking that a pear anode in the area of the skeg which is bonded to the propshaft/rudder post might be a good idea.

Live aboard boat, 240v connected 24/7.

Erudite opinion much appreciated.

Rob

 

[VicS]

Dear Galvanic Corrosion cognoscenti,

Please have a look at these images: https://www.dropbox.com/sh/vt8sbxlq8ryenk5/ZkIu5qUrRl

Specifically at the area of bare metal on the lower bearing holder on the skeg, I am wondering if this is caused by galvanic corrosion.

The prop anode is completely shot and there are no other anodes below the waterline. I am thinking that a pear anode in the area of the skeg which is bonded to the propshaft/rudder post might be a good idea.

Live aboard boat, 240v connected 24/7.

Erudite opinion much appreciated.

Rob

First questions, as your shorepower is normally connected, must be do you have a galvanic isolator and if so is it "working" and are the corroded parts bonded to the earth.

Apart from that what metals or alloys are we looking at and what other metals/ alloys are electrically connected. Galvanic corrosion require two "dissimilar" metals to be connected to each other and to be immersed in the same electrolyte.

Rapid loss of the prop anode may or may not be relevant. You'd expect a smallish chunk of zinc connected to the relatively large large surface area of a copper alloy prop to be consumed fairly quickly anyway.

 

[vyv_cox]

A bit difficult to make a definite statement without knowing the metals involved. I assume the corroded part is an aluminium alloy? In photo 2 the corrosion is highly localised and confined only to that part, suggesting that it is galvanic in nature. Some confusion in my mind though because photo 1 seems to show corrosion on the bearing cap also but photo 2 seems to show that it has machining marks and no corrosion. If the cap is also corroded, and aluminium, it would suggest that the corrosion is simply general, and the result of poor materials selection.

Without knowing the composition of the parts I could not be certain but I would think that a zinc anode would help, assuming the composition of the bearing housing to be somewhat similar to a sterndrive leg. I don't think the prop anode loss is significant unless there is bonding between the rudder bearings and the engine/prop shaft.

 

[KellysEye]

We had a round anode each side of the rudder which protected the prop and and steel rudder, as said if you don't have one fit a galvanic isolator.

 

[savageseadog]

I would suggest earth leakage currents from your mains supply. Use a galvanic isolator or better still an isolating transformer. Is the rudder still sound?

 

[vyv_cox]

I would suggest earth leakage currents from your mains supply. Use a galvanic isolator or better still an isolating transformer. Is the rudder still sound?

Difficult to understand how earth leakage could arrive at bearing housings attached to a skeg, presumably isolated from everything else. Hence my query about bonding.

 

[savageseadog]

Difficult to understand how earth leakage could arrive at bearing housings attached to a skeg, presumably isolated from everything else. Hence my query about bonding.

Just and observation based on the severity of the corrosion.

 

[VicS]

I would suggest earth leakage currents from your mains supply. Use a galvanic isolator or better still an isolating transformer. Is the rudder still sound?

If leakage currents from the main supply or even from the vessels 12 (24/) volt DC system are to blame a GI will not help as it will only block current from very low voltage sources such as galvanic or dissimilar metal sources.
Electrolysis caused by higher voltage sources would be severe and a serious matter!

 

[[2574]]

First questions, as your shorepower is normally connected, must be do you have a galvanic isolator and if so is it "working" and are the corroded parts bonded to the earth.

Apart from that what metals or alloys are we looking at and what other metals/ alloys are electrically connected. Galvanic corrosion require two "dissimilar" metals to be connected to each other and to be immersed in the same electrolyte.

Rapid loss of the prop anode may or may not be relevant. You'd expect a smallish chunk of zinc connected to the relatively large large surface area of a copper alloy prop to be consumed fairly quickly anyway.

I'm afraid I can only partly answer your questions. Yes, there is a galvanic isolator fitted, professionally fitted, and as far as we are aware correctly fitted.

As regards what metals, I have no idea. The boat is manufactured by Malo, a respected builder, so hopefully it has been properly built.

Rob

 

[[2574]]

A bit difficult to make a definite statement without knowing the metals involved. I assume the corroded part is an aluminium alloy? In photo 2 the corrosion is highly localised and confined only to that part, suggesting that it is galvanic in nature. Some confusion in my mind though because photo 1 seems to show corrosion on the bearing cap also but photo 2 seems to show that it has machining marks and no corrosion. If the cap is also corroded, and aluminium, it would suggest that the corrosion is simply general, and the result of poor materials selection.

Without knowing the composition of the parts I could not be certain but I would think that a zinc anode would help, assuming the composition of the bearing housing to be somewhat similar to a sterndrive leg. I don't think the prop anode loss is significant unless there is bonding between the rudder bearings and the engine/prop shaft.

Vyv,

Thanks. I am not aware that the rudder is bonded to the propshaft and that souds an unlikely scenario. If a zinc anode were fitted do you think it best be attached on the skeg in isolation or would it be better bolted through hull with bonding to the rudder post?

Rob

 

[[2574]]

Is the rudder still sound?

Yes, the damage is superficial at present.

 

[vyv_cox]

Vyv,

Thanks. I am not aware that the rudder is bonded to the propshaft and that souds an unlikely scenario. If a zinc anode were fitted do you think it best be attached on the skeg in isolation or would it be better bolted through hull with bonding to the rudder post?

Rob

I would fit it as close as possible to the items being protected. The bearing housing bolts would probably be the easiest and best mounting position. It's difficult to know what is going on, if the rudder bearings are bronze, quite likely, then the bearing housing (aluminium?) is protecting them and the shaft. One would have thought that Malo would know better than that.

 

[Schuss39]

Rob,

Your post has prompted me to check over the earth wiring on my Nc39. There is an earth wire from the anode fitting (under aft cabin bunk) direct to the shaft. This is fitted onto the shaft where the hydraulic steering is fitted to the shaft. About 15 inches above where the shaft enters the hull. You may have to lie on your back with head and shoulders into the small aft locker.

When I got my boat I had to clean up the accumulated muck off the anode terminals but not had any problems since. I also fitted a galvanic isolator about 3 years ago.

 

[sailinglegend420]

Hi Rob.

Just a suggestion but the wider shot suggests you may have hit something which has caused the damage!!!!! I know you get very worried about your lecky, but it might not be that - the pictures aren't very clear.

If you do have Galvanic Isolators you should check them once a year as the diodes can fail - see Nigel Calder.

 

[boatmike]

I was waiting for someone to say "isolating transformer" Even the manufacturers admit that the diode types are of limited value. My own background is shipbuilding and I had never seen the diode type until I started looking at cheapie kit for yottie use. I am sure they are certainly better than nothing but isolating transformers are the proper solution if there is any serious AC current usage. As far as the comment regarding dissimilar metals is concerned, this is also true in theory but what people miss is that they are often parked against a pontoon that has gert great steel piles holding it in place....... When I worked for Vosper Thornycroft in Southampton it was well known that any vessel moored against the "iron wharf" in the Itchen would "fizz" so violently that you could sit on the quayside on a warm summers day watching the bubbles rise!

 

[vyv_cox]

When I worked for Vosper Thornycroft in Southampton it was well known that any vessel moored against the "iron wharf" in the Itchen would "fizz" so violently that you could sit on the quayside on a warm summers day watching the bubbles rise!

I would love to know the mechanism for this oft-quoted phenomenon. Just putting two different metals in seawater doesn't create a battery, they have to be in electrical contact. Same as 'stray currents', another common phrase. Do you know the science behind these?

 

[boatmike]

I would love to know the mechanism for this oft-quoted phenomenon. Just putting two different metals in seawater doesn't create a battery, they have to be in electrical contact. Same as 'stray currents', another common phrase. Do you know the science behind these?

Quite correct of course but the connection is difficult to avoid especially when working aboard a steel or aluminium ship using welding gear etc. Basically the "earth" of the shore supply is connected to (say) the pile of a marina pontoon or in this case the quay by virtue of the shore earth being grounded somewhere and the seabed being part of the same ground. In the case of the old "iron wharf" the problem was far worse because over years the structure had corroded and dumped iron oxide into the silt beneath the ship which often grounded at low tide. Most marinas are serviced by supplies grounded nearby and with a plastic boat it is possible to ensure that the shore earth is isolated from seawater by using a GI of some kind that only makes connection if a dangerously high current flows due to a dead short. Very often it is possible to protect yourself by mooring midway between piles and letting your neighbours fizz instead! But it would be antisocial of me to suggest such a thing.........

 

[Jabs]

I need some help here.

I am concerned about galvanic corrosion and asked about it at the Boat Show.

What I found was that every single person has a different view.

I talked with Coppercoat, Vetus, Volvo, Mc Duff, Mastervolt, bruntons and others.

I got two diametrically opposing views at Volvo and every possibility on this earth from the others.

So, where to go?

My conclusion is to take every possible precaution and to doubt every solution. Minimise every possible risk.

I moor in a marina, with shore pore most of the time.

I have piles close and a piled wall fifty feet away.

I have a galvanic isolator but can't find a way to test it. An isolating transformer seems perfect.

Any comments from the gathered experts?.

Thanks

Tony

 

[[2574]]

Hi Rob.

Just a suggestion but the wider shot suggests you may have hit something which has caused the damage!!!!! I know you get very worried about your lecky, but it might not be that - the pictures aren't very clear.

If you do have Galvanic Isolators you should check them once a year as the diodes can fail - see Nigel Calder.

Matt,

Thanks for that though it's not actually my boat we're talking about here, it's a friend's Malo 39. Interesting thought that it might be collision damage though having seen it close up I'm pretty sure there hasn't been an impact from anything. The area affected has ridges as though the material either side has been eroded which made me suspect galvanic corrosion. I'll certainly have a read of Calder for testing a GI.

Rob

 

[[2574]]

Rob,

Your post has prompted me to check over the earth wiring on my Nc39. There is an earth wire from the anode fitting (under aft cabin bunk) direct to the shaft. This is fitted onto the shaft where the hydraulic steering is fitted to the shaft. About 15 inches above where the shaft enters the hull. You may have to lie on your back with head and shoulders into the small aft locker.

When I got my boat I had to clean up the accumulated muck off the anode terminals but not had any problems since. I also fitted a galvanic isolator about 3 years ago.

Hi Paul,

It's actually a mate's Malo that has the issue, not my N39. I have seen the connection on the quadrant that you mention and suspected it was part of the galvanic bonding, good to know, thanks!

Rob