Galvanic isolators

[woc]

My marina appears to have higher than normal stray current depleting my saildrives anodes very quickly. I installed a galvanic isolator that is supposed to prevent this from happening.
My questions are.
1- How effective are they
2- can you measure current flow at the dock and if so how

I would like to be able to run a test with the galvanic isolator connected and disconnected and “ see” if there is a difference.
Possible?

[Nicholson58]

One of your dock neighbors has a leak. If you are connected to the shore power your anodes are protecting the other boats. We avoid this by having an isolation transformer in our shore power line.

[Andreas W]

I used a galvanic isolation transformer. Mine was Mastervolt, because the rest of my system was.

[CharlieJ]

@WOC
Here is a very general primer on stray current corrosion:

A galvanic isolator, by design, will block galvanic current up to a galvainc potential of about 1.4VDC. A galvanic isolator cannot block stray current as stray current is produced by battery potential.

Stray (electrolytic, not "electrolysis") current is caused by an inadvertent B+ connection to an underwater metal component which becomes the anode. The cathode is connected to the B- of the same DC source and is protected from corrosion. Both the anode and cathode are immersed in the same electrolyte to create the stray current cell. With both anode and cathode in the same electrolyte, metallic ions leave the surface of the anode, which deteriorates, and migrate towards the cathode which is protected.

Aluminum sail drives are a special case because aluminum cannot be protected to a Ag/AgCl reference cell potential >1100mVDC. To go above this limit will "overprotect" the aluminum sail drive and the caustic water created on the surface of the saildrive will badly deteriorate it to the point of failure. I suggest you search the web for "aluminum sail drive stray current corrosion" as Yanmar, Volvo and others have references available.

Some corrosion factoids:
1. Galvanic corrosion is a very slow process; durations of months and years.
2. Stray current corrosion is a very fast and very destructive type of corrosion; durations of hours and days to see the effects are not uncommon.
> A sail drive can completely disintegrate in a few days.
> A 3" SS shaft looks like it has been eaten by worms in < 1 week.
3. In my experience, it is extremely rare for another boat to cause stray current corrosion. The victim boat is almost always the cause of the stray current that is destroying its underwater components.

Note: There is one galvanic isolator on the market that advertises that it blocks 2.2VDC but since the standard galvanic series shows magnesium (used in freshwater) with a galvanic potential of -1630mVDC referenced to a AgAgCl cell, this added range will do nothing to protect against stray current corrosion.

There is a lot of bad info available regarding this subject. I recommend that you engage the services of an ABYC Certified Corrosion Tech to help you determine and correct the problem on your boat.

[HankOnthewater]

Such a galv.isolator is in fact only 4 diodes, often encased in a large aluminium heatsink.
Here is a diagram copied from the net:

Sometimes there are fancy LEDs, but more for show, not to make it work better.
here is a link that put things simply:
https://www.bcet.co.uk/galvanic-isol...nic-corrosion/

Yes, an isolator transformer is still king in this world.
But, galvanic isolator with diodes can be effective
Yes, one can measure current (in series with the diodes), and the fancy isolators may have LED indicators for such current, but..... if there is a wiring short then many (ie 30+ amps will flow) and destroy the delicate and small amp current meter if permanently connected.
Note: the approx 1.5 Volt that the diodes block, stops the galvanic current, so one should not measure any current. If one want to measure current, one should measure the current flow in parallel with the diodes, then in series. and then one knows how much current the diodes stop/block.
Tell me if the above does not make sense.

ON edit: I see a much better explanation is given by Charlie above

[CharlieJ]

@WOC
Here are is a well written reference to help you:
http://www.jboats.com/images/stories..._Corrosion.pdf

And a photo of a saildrive to scare you:
https://www.boatzincs.com/saildrive_..._corroded.html

[YANDINA]

Originally Posted by CharlieJ

@WOC
Here is a very general primer on stray current corrosion:

A galvanic isolator, by design, will block galvanic current up to a galvainc potential of about 1.4VDC. A galvanic isolator cannot block stray current as stray current is produced by battery potential.

Stray (electrolytic, not "electrolysis") current is caused by an inadvertent B+ connection to an underwater metal component which becomes the anode. The cathode is connected to the B- of the same DC source and is protected from corrosion. Both the anode and cathode are immersed in the same electrolyte to create the stray current cell. With both anode and cathode in the same electrolyte, metallic ions leave the surface of the anode, which deteriorates, and migrate towards the cathode which is protected.

Aluminum sail drives are a special case because aluminum cannot be protected to a Ag/AgCl reference cell potential >1100mVDC. To go above this limit will "overprotect" the aluminum sail drive and the caustic water created on the surface of the saildrive will badly deteriorate it to the point of failure. I suggest you search the web for "aluminum sail drive stray current corrosion" as Yanmar, Volvo and others have references available.

Some corrosion factoids:
1. Galvanic corrosion is a very slow process; durations of months and years.
2. Stray current corrosion is a very fast and very destructive type of corrosion; durations of hours and days to see the effects are not uncommon.
> A sail drive can completely disintegrate in a few days.
> A 3" SS shaft looks like it has been eaten by worms in < 1 week.
3. In my experience, it is extremely rare for another boat to cause stray current corrosion. The victim boat is almost always the cause of the stray current that is destroying its underwater components.

Note: There is one galvanic isolator on the market that advertises that it blocks 2.2VDC but since the standard galvanic series shows magnesium (used in freshwater) with a galvanic potential of -1630mVDC referenced to a AgAgCl cell, this added range will do nothing to protect against stray current corrosion.

There is a lot of bad info available regarding this subject. I recommend that you engage the services of an ABYC Certified Corrosion Tech to help you determine and correct the problem on your boat.

There is more to the picture than the DC voltage you are blocking. All boats on shore power also have AC leakage. Unless there is a faulty appliance it is usually negligible however as the boat gets older and salt deposits accumulate the AC to ground leakage increases.

AC is usually ignored with regard to electrolytic corrosion because there is no DC net current involved because the positive current is immediately reversed each cycle by the negative current and the net is zero.

HOWEVER that AC voltage is added to any DC voltage being blocked by your Galvanic Isolator. If the net voltage to the water due to underwater metal is 0.6 volts, for example, then when added to 1 volt of AC leakage the diode isolator voltage is exceeded every half cycle resulting in a net DC current.

With the natural potential in salt water as high as 1100mVDC for some Aluminium alloys a 1.1 volt isolator can lose nearly 50% of its protection when the DC and AC are in the same direction so Aluminium boats should use a 2.2 volt isolator.

It is for this reason that the hull of Aluminium boats is isolated (as much as possible) from any connections to the shore power connection so that its voltage can float and the connection for current flow is broken.

[woc]

Originally Posted by Nicholson58

One of your dock neighbors has a leak. If you are connected to the shore power your anodes are protecting the other boats. We avoid this by having an isolation transformer in our shore power line.

Interesting, how does it work? Any brand model info?

[woc]

Originally Posted by YANDINA

There is more to the picture than the DC voltage you are blocking. All boats on shore power also have AC leakage. Unless there is a faulty appliance it is usually negligible however as the boat gets older and salt deposits accumulate the AC to ground leakage increases.



AC is usually ignored with regard to electrolytic corrosion because there is no DC net current involved because the positive current is immediately reversed each cycle by the negative current and the net is zero.



HOWEVER that AC voltage is added to any DC voltage being blocked by your Galvanic Isolator. If the net voltage to the water due to underwater metal is 0.6 volts, for example, then when added to 1 volt of AC leakage the diode isolator voltage is exceeded every half cycle resulting in a net DC current.



With the natural potential in salt water as high as 1100mVDC for some Aluminium alloys a 1.1 volt isolator can lose nearly 50% of its protection when the DC and AC are in the same direction so Aluminium boats should use a 2.2 volt isolator.



It is for this reason that the hull of Aluminium boats is isolated (as much as possible) from any connections to the shore power connection so that its voltage can float and the connection for current flow is broken.

Sounds very complicated and above my head. It appears even a common electrician will not be able to determine if I have stray current and or AC leakage. In addition to the galvanic isolator I am presently hanging a large zinc anode plate over the side with a heavy battery cable connected to the engine in the hope it will protect the saildrives.

[CharlieJ]

@WOC
You are correct, most marine electricians do not have the training and equipment to perform a leakage current survey.

Apparently you did not read #4 & #6 as anodes are used to combat galvanic corrosion and have no effect on stray current corrosion. Hanging supplementary anodes will do nothing to solve your problem.

Repeating: I strongly recommend that you engage the services of a qualified corrosion tech to perform a corrosion survey and determine the cause of your problem.

As a final note that adds more complication; although rare, aluminum is susceptible to stray AC electrolytic corrosion.

My bet is still that your boat is causing the damage observed.

[woc]

Many thanks for your in depth explanations and clarification. I will follow your advice and search for a qualified corrosion tech to help determine the source of the problem and effective solution

[slug]

Originally Posted by woc

Many thanks for your in depth explanations and clarification. I will follow your advice and search for a qualified corrosion tech to help determine the source of the problem and effective solution

Antifoul containing copper oxide , on your hull , attack’s any wet aluminum

Typically rudder bearings and sail drives

Yanmar warranty prohibits the use of copper oxide with thier sail drives