Galvanic Corrosion

[gfrazzle]

My diver recently informed me that I had a "hot" prop and it was being damaged by the galvanic corrosion even though I had zincs on the shaft and rudder.
I had a marine electrician check the entire system. His galvanic corrosion meter showed the reading for a bronze prop should be -.500 to -.700 , mine was -.225 which he said meant I had little or no zinc. I had my diver add another and the reading is now -.925. He thought that the installed zinc was not making proper contact with the shaft.
My questions are:
Can the excess zinc (-.925 vice -.700 ) cause a problem (overzinced)? For years I have installed a new zinc when the old one was about half corroded with no noticeable adverse affect
Is there a way to test the prop in the water to determine if it is safe to run? I have a video of the prop/ shaft (link) and the prop has a little bit of scalloping on the edges but not a lot. My old diver says it can be "pinged" with a hammer and if it rings it is safe to run. I really don't want to haul it if I can avoid it until I need to bottom paint but I would hate to have it throw a blade at full throttle!
Thanks for any input

 

[capta]

I'd have the diver remove the prop and I'd take (send?) it to a prop shop. I would never let just anyone "ping" my prop. It's unlikely you'd actually 'throw a blade', but operating an unbalanced prop can do damage to the stuffing box, the transmission and/or the strut, if you have one. A balanced prop is a very important part of your running gear.

 

[dlochner]

I'm wondering why this is a problem now. What's changed in the marina around your boat? New boats? Shore power changed? Do you have a galvanic isolator?

One telltale sign of problems on the prop is "pinking" where areas of the prop show a pink coloring due to the zinc leaching out of the prop. I didn't see any on the video.

Pulling the prop, cleaning it, and having it inspected by a well qualified shop might be prudent.

 

[SG]

My immediate reactions:

• The zincs look like they are working now.
• The prop looks pretty rough. Could this be old damage from another time? Did you leave the prop uncoated? (e.g., w/out PropSpeed, paint, etc. -- not that any of those should have been a problem, but the might have obscured that from being noticed.)
• Electrolysis protection in extreme environments is probably not so simple.
• Then I'd a zinc "grouper" that that is clearly connected to your boats ground.
• Do you have continuity through the prop to the engine? SOME couplings don't provide that -- they isolate the ground.

Something WAS or IS really working on your prop. I'd clearly I'd let the marina know RIGHT AWAY. I'd have them start looking. As Dave Lochner asks: What's your onboard electrical situation and equipment? It might be you. Clearly having a functioning galvanic isolator would be good.

 

[JamesG161]

I had my diver add another and the reading is now -.925

This "zinc meter" is a AgCl reference probe [ I have one]. That -.925 is full Zinc reference in ≈70°F and normal oceans salinity water. [Corrected]

That reading changes dramatically downward, with water salinity dropping.

Since I don't know the water condition changes between reading, hard to say if that varied.

I had the same issue as you, after my berthing area experienced a drop in salinity, following 2 weeks of steady rainfall.
I could not get a diver and the water was mirky.
We took a trip out into the clear water Gulf and before I dove the boat, I checked the "zinc meter" again.
Guess what?
It was -0.925 again. Dove and got camera shots of the Shaft, prop and Zinc. All were great!
_____
Over Zinc is never a problem on a fiberglass hull boat. Either there is Zinc protection or not. -.925 says just right.

____

reading for a bronze prop should be -.500 to -.700 , mine was -.225

One puzzlement is how the marine electrician isolated the bronze prop from the shaft Zinc. to measure -.225.

He could have clipped one reference directly to the Prop, is a possibility.

But it seems the diver is correct. Shaft Zinc is in proper contact now.
The number of Shaft Zinc only determines how long between changes, since the old Zinc corrode away slowly.

Jim...

 

[gfrazzle]

I have never coated/ painted the prop and have the bottom cleaned once a month. I do have a galvanic isolator and a battery isolator as well. The electrical system on my boat checks out fine. This problem just showed up and the damage occurred within a month between cleanings.
My understanding is that galvanic corrosion is a product of DC current so dock power should likely not cause it. I bought a corrosion tester so will check on a regular interval.
Having the prop pulled and inspected/ repaired or replaced by a prop shop sounds like the best solution.
Thanks to all for the inputs!

 

[dlochner]

My understanding is that galvanic corrosion is a product of DC current so dock power should likely not cause it. I bought a corrosion tester so will check on a regular interval.

Yes this is almost correct. DC is more destructive than AC, however, the kind of problem you're describing can involve the AC system.

DC current can travel between boats and shore installations via the AC ground wire. And AC and DC can travel on the same wire at the same time. When was the last time the galvanic isolator was tested? If you have a modern fail safe isolator it may have failed allowing DC current to travel along the AC ground wire.

Testing A Galvanic Isolator - Marine How To

A 30A Galvanic Isolator WARNING: The galvanic isolator used in this article does not meet the current safety standards. To meet the current safety standards you would need a "Fail Safe" unit that has been tested to the ABYC Fail

marinehowto.com

 

[Will Gilmore]

I don't have any experience with these issues, but in thinking about your problem in light of what has already been written here, I can see where a well placed zinc could, over time, lose contact with the shaft due to the zinc deterioration. No problems one day, the next day, problems.
Just thinking out loud.

-Will (Dragonfly)

 

[dlochner]

I don't have any experience with these issues, but in thinking about your problem in light of what has already been written here, I can see where a well placed zinc could, over time, lose contact with the shaft due to the zinc deterioration. No problems one day, the next day, problems.
Just thinking out loud.

-Will (Dragonfly)

In order for this to happen, water would have to get between the anode and the prop shaft, which would point to a poor installation. The anode degrades from the outside.

 

[JamesG161]

I bought a corrosion tester so will check on a regular interval.

Would you post the type tester you bought, please?
Jim...

PS: Check my chart for a bronze prop it should have been around -.260.

 

[rgranger]

I don't have any experience with these issues, but in thinking about your problem in light of what has already been written here, I can see where a well placed zinc could, over time, lose contact with the shaft due to the zinc deterioration. No problems one day, the next day, problems.
Just thinking out loud.

-Will (Dragonfly)

That is also what I was thinking. You may search and find that nothing is "wrong" with the marina and what may have happened is your zinc lost proper contact with your shaft... or was not properly installed the last time. ?????

 

[Crazy Dave Condon]

When this came up, I would look at other boats to see if possibly the shore power chords were defective, ask if any dock switches were tripping and so on. Then I would turning off everything in the boat plus unplugging any shore powers chords. Then I would suggest throwing over a sacrificial zinc to see if anything and go from there

 

[SG]

The shore power distribution system as well as adjacent boats’ issues can be issues. Those can be transient (non continuous, intermittent issues,) on only when some connects piece of equipment or a boat. It is complicated.

 

[Brian D]

In order for this to happen, water would have to get between the anode and the prop shaft, which would point to a poor installation. The anode degrades from the outside.

My thoughts also. The zinc was not installed properly. Is there a "correct" location for a zinc other than anywhere on the shaft? Since I use an outboard I have two zincs but the motor is not in the water except when running. In the video I thought I saw a chip in one of the blade's outer edge. Maybe it was the lighting.

 

[Will Gilmore]

Is there a "correct" location for a zinc other than anywhere on the shaft?

I was wondering the same thing, so I searched and found this propeller not anode.

-Will (Dragonfly)

 

[gfrazzle]

The tester I bought is a Seguard Pro:

Marine Digital Corrosion Tester - Seaguard International

The corrosion protection system on your boat should be checked at regular intervals. Failure to do so can lead to a costly and time consuming repair.

seaguardinternational.com

I have tested with dock power on/ off, and with each component breaker on/ off individually with no significant difference.

 

[dlochner]

I was wondering the same thing, so I searched and found this propeller not anode.

Some prop manufacturers want an anode on the shaft, some don't.

I have tested with dock power on/ off, and with each component breaker on/ off individually with no significant difference.

Turned off? or Completely disconnected. Even if the breakers are turned off at the power pedestal the ground wire remains connected to all the other boats on the line. Shore based breakers only break the hot wire, on board the double breaker breaks both the hot and neutral lines. The shore based neutral and ground are connected in the panel that supplies power to all the pedestals. Short story, there are lots of paths for the DC current to travel on in the AC system.

 

[Will Gilmore]

In order for this to happen, water would have to get between the anode and the prop shaft, which would point to a poor installation. The anode degrades from the outside.

Is this possible to do when installing in the water? Is there a contact grease or some way of purging the water trapped between the shaft and the anode when installing underwater? The surface of the few zincs I've seen didn't look smooth enough to assure a solid contact on a s.s. shaft.

-Will (Dragonfly)

 

[dlochner]

Is this possible to do when installing in the water? Is there a contact grease or some way of purging the water trapped between the shaft and the anode when installing underwater? The surface of the few zincs I've seen didn't look smooth enough to assure a solid contact on a s.s. shaft.

-Will (Dragonfly)

If properly sized and properly tightened on a clean shaft, there shouldn't be an issue. There is a lot of surface area, so even a few small imperfections shouldn't cause a significant problem. A dirty fouled shaft or one that is scaled over with marine crud will affect the connection.

The anode decays because it is part of a circuit. A dirty shaft or a loose connection should prevent the anode from decaying, no accelerate it.

 

[gfrazzle]

Some prop manufacturers want an anode on the shaft, some don't.



Turned off? or Completely disconnected. Even if the breakers are turned off at the power pedestal the ground wire remains connected to all the other boats on the line. Shore based breakers only break the hot wire, on board the double breaker breaks both the hot and neutral lines. The shore based neutral and ground are connected in the panel that supplies power to all the pedestals. Short story, there are lots of paths for the DC current to travel on in the AC system.

Disconnected at the boat end.