Galvanic corrosion theoretical question

[fontmell]

Hi I am sure someone here has good understanding of the theoretical question below based on conversation with a sailboat owner with sail drive asking neighbours not to leave boats permanently plugged into mains

if a boat is Moored in a marina not connected to shore power and the battery switches to off. Can the anodes still be attacked/depleted. If is the likely source another boat?
Thanks in advance

 

[noelex]

if a boat is Moored in a marina not connected to shore power and the battery switches to off. Can the anodes still be attacked/depleted.

Absolutely.

Disimimar metals in an electorate (seawater) will generate errosion.

If you are experiencing rapid and unusual erosion of the anodes the most common cause is stray current corrosion.
Most (not all) stray current problems result from the boats DC system.

Battery switches do not typically disconnect the negative supply.

If is the likely source another boat?

Very unlikely given your situation.

 

[Supertramp]

Hi I am sure someone here has good understanding of the theoretical question below based on conversation with a sailboat owner with sail drive asking neighbours not to leave boats permanently plugged into mains

if a boat is Moored in a marina not connected to shore power and the battery switches to off. Can the anodes still be attacked/depleted. If is the likely source another boat?
Thanks in advance

I think there are 2 parts to answering this.

The anodes on a boat protect the other (less anodic) metals. The process will happen whenever the boat is in the water and the anode metal is lost instead of important bits of boat metal in the water. All good and you want this to happen.

In a marina and when connected to shore power all boats are in effect connected. Any boat with badly worn or no anodes will "use" the anodes of neighbouring boats to protect itself, wearing them out faster. The problem worsens if there are electrical faults anywhere causing stray currents down the earth wires.

So anodes will erode naturally even with no shore power and batteries disconnected as they protect propellor, seacocks etc. But the erosion may be worse with shore power left connected as you protect your own and other people's metal.

A galvanic isolator on the shore power earth wire will help.

 

[Tranona]

Absolutely.

Disimimar metals in an electorate (seawater) will generate errosion.

If you are experiencing rapid and unusual erosion of the anodes the most common cause is stray current corrosion.
Most (not all) stray current problems result from the boats DC system.

Battery switches do not typically disconnect the negative supply.

That is not correct. There is no connection between the DC system and the saildrive - it is specifically isolated. the two metals (aluminium and stainless steel in this case are in the electrolyte (seawater) and the action between the two occurs whether the DC system is on or off - the zinc wastes being lower on the galvanic scale erodes rather than the aluminium it is bolted (electrically connected) to.

 

[noelex]

That is not correct. There is no connection between the DC system and the saildrive - it is specifically isolated. the two metals (aluminium and stainless steel in this case are in the electrolyte (seawater) and the action between the two occurs whether the DC system is on or off - the zinc wastes being lower on the galvanic scale erodes rather than the aluminium it is bolted (electrically connected) to.

Agreed. There does not have to be a connection between the DC system and saildrive for errosion of the anode to occur. Saildrives have a scary mix of dissimilar metals all immersed in an electrolyte .

However, stray current corrosion is a common cause of rapid depletion of anodes. This mechanism should be considered whenever there is excessive and premature anode loss. While most salidrives should be isolated from DC system this isolation can fail.

The engine is normally grounded (connected to the negative supply). Electrically isolating the saildrive that is mechanically connected to engine is not easy over the long term. This isolation does sometimes fail.

 

[Daydream believer]

deleted

 

[vyv_cox]

As I understand them, Volvo saildrives are isolated from the engine and the remainder of the boat. Yanmar saildrives are not. I am not aware of other makes although I would not be surprised if there were some

 

[VicS]

ITYWF VP sail drives have been isolated since the "B" version of the MD 2010/2020 etc series engines.. An exception is when fitted to MD22 series engines
VP2001 etc series engines, early MD 2010 series and MD22 engines had isolated DC systems instead when fitted to saildrives.

Some forum members may remember that Nigel Mercier (RIP) lost the entire sail drive leg ( fitted to an MD22) to electrolysis. Three factors combined caused this.

He dis not normally switch off the battery isolators ( I don't remember why),​

Although the engine block was isolated from the DC system the DC negative was earthed to the keel.​

There was an electrical fault which made the engine block and saildrive 12 volts positive​

A circuit from sail drive , through the water to the keel made short work of the sail drive

 

[fontmell]

Hi. Many thanks all above. So in conclusion my boat neighbour could be right in assuming the nearby plugged in boats could be causing his anode rapid wear. Interesting I will now in plug when not on the boat
Thanks again

 

[vyv_cox]

Hi. Many thanks all above. So in conclusion my boat neighbour could be right in assuming the nearby plugged in boats could be causing his anode rapid wear. Interesting I will now in plug when not on the boat
Thanks again

There is plenty of information about this. If your mains lead is attached to the pontoon, even if switched off, the earth wire will be connected to the pontoon and possibly to other boats by the same means. Your anode could then be protecting the pontoon and other boats. If your lead is disconnected the situation is exactly the same as when you are a mile out to sea.

 

[superheat6k]

If my boat neighbour asked me to unplug I would politely (of course) decline his invitation.

Too many assume simply fitting a GI is the end to all problems of electrolytic and galvanic corrosion. Whereas it will largely resolve the latter, not ever the former.

In my experience the commonest culprit for electrolytic leaks is the flappy style bilge pump switch. Permanently fed with 12 VDC + and invariable permanently immersed in salty water, which in turn is in contact with metal underwater metalwork, often via the bonding circuit. If the bonding circuit itself has corroded connections then the resistances caused can set up voltage differences along the circuit, and that's it, an electrolytic corrosion nightmare begins. With luck it is the zinc that light up and take the hit, but any underwater metal on the circuit can be badly affected.

However, checking if your bilge pump circuits have a problem is a very simply check ...

1 Turn everything off except the auto bilge pump circuits, as though the boat is being left on its berth unmanned.

2 Turn off the charger and disconnect the + VE feed to the batteries.

3 Disconnect the battery + terminal connection.

4 Measure the voltage between the + terminal and the loose connection.

5 If it is not showing 0 volts start hunting as you have an electrolytic leak somewhere.

I do this as an annual check as part of my routine maintenance.

AC leaks into the water are the very worse, as well as being potentially lethal, they will eat zincs and other metal at a truly alarming rate.

But the key is an attitude of diligence for your own boat, so even if a neighbour is illuminating the water right next to your boat, you can still fully protect your valuable asset.

So, if it is properly isolated within the neighbour's own boat his sail drive leg should simply not be affected, thus the responsibility here is mostly if not all his.

 

[vyv_cox]

If my boat neighbour asked me to unplug I would politely (of course) decline his invitation.

Too many assume simply fitting a GI is the end to all problems of electrolytic and galvanic corrosion. Whereas it will largely resolve the latter, not ever the former.

In my experience the commonest culprit for electrolytic leaks is the flappy style bilge pump switch. Permanently fed with 12 VDC + and invariable permanently immersed in salty water, which in turn is in contact with metal underwater metalwork, often via the bonding circuit. If the bonding circuit itself has corroded connections then the resistances caused can set up voltage differences along the circuit, and that's it, an electrolytic corrosion nightmare begins. With luck it is the zinc that light up and take the hit, but any underwater metal on the circuit can be badly affected.

However, checking if your bilge pump circuits have a problem is a very simply check ...

1 Turn everything off except the auto bilge pump circuits, as though the boat is being left on its berth unmanned.

2 Turn off the charger and disconnect the + VE feed to the batteries.

3 Disconnect the battery + terminal connection.

4 Measure the voltage between the + terminal and the loose connection.

5 If it is not showing 0 volts start hunting as you have an electrolytic leak somewhere.

I do this as an annual check as part of my routine maintenance.

AC leaks into the water are the very worse, as well as being potentially lethal, they will eat zincs and other metal at a truly alarming rate.

But the key is an attitude of diligence for your own boat, so even if a neighbour is illuminating the water right next to your boat, you can still fully protect your valuable asset.

So, if it is properly isolated within the neighbour's own boat his sail drive leg should simply not be affected, thus the responsibility here is mostly if not all his.

I agree but I have a question. I do not understand how any current 'leaking' into the water, whether AC or DC, can do anything. The sea must all be at earth potential, so how can current pass through it other than between its own supply and return? I know that Nigel Calder describes the danger to swimmers of AC in the water but I really do not understand it.

 

[VicS]

I agree but I have a question. I do not understand how any current 'leaking' into the water, whether AC or DC, can do anything. The sea must all be at earth potential, so how can current pass through it other than between its own supply and return? I know that Nigel Calder describes the danger to swimmers of AC in the water but I really do not understand it.

Calder explains it fairly well.
He explains that a voltage gradient over 2 volts/foot can be dangerous. He explains why it's more dangerous in fresh and brackish water than in salt water.
He tells us that 2 conditions must exist. Namely an AC fault on board that puts AC into the water and a discontinuity in the earth bonding or shorepower earth connection.
What he fails to mention , I think, is that an AC leakage into the water should trip the rcd or elcb.

 

[superheat6k]

I agree but I have a question. I do not understand how any current 'leaking' into the water, whether AC or DC, can do anything. The sea must all be at earth potential, so how can current pass through it other than between its own supply and return? I know that Nigel Calder describes the danger to swimmers of AC in the water but I really do not understand it.

The current leak creates an anode i.e. the metal become s more positive, and this overrides the galvanic scale, which means lost metal wherever that anode forms.

The mis-understanding with AC is that in theory the ions (atoms with a missing electron) jump off on one ~+VE cycle of the AC current , and back on with the next -VE cycle, but in reality that just does not happen, because at an atomic level one atom (or ion) leaving one place is relatively speaking the equivalent of a thousand miles from where it might return. Add water flow and the Ion might be entirely lost.