I know the term anode is the compliment to cathode. It is the source of electrons flowing towards the cathode in the reverse direction of conventional current. I assume that's why it degrades. It gives up its electrons more easily.
-Will (Dragonfly)
Galvanic Corrosion
- Thread starter gfrazzle
- Start date
-Will (Dragonfly)
That's correct. However, if it is just sitting there is it not part of the circuit. It needs to be connected the cathode to complete the circuit and to allow the electrons to flow. The AC ground wire serves this purpose in a marina. On the boat itself, it is the bonding between the various underwater metal bits that completes the circuit.
Think of a battery, the + ad - terminals just sit there and no electrons flow, until the two poles are connected then the electrons flow. It is the same concept with the AC ground wire. If the AC ground wire is disconnected, then the electrons can't flow. That's why Galvanic Isolators and transformers are used. They break the circuit for DC current while allowing AC current to flow.
Correct.... it is a sacrificial anode. In order for the iron in the SS shaft to oxidize, it must give up an electron to something in the water.. it will eventually be oxygen that receives the electron. However the zinc hangs onto it's electrons less strongly than does iron so... as the electron is lost by iron, it is quickly replaced by an electron from the zinc anode (actually each zinc can lose two electrons) eventually producing ZnCl2 which is water soluble and drifts away. So the zinc anode slowly dissolves.
Thanks for the feed back!
If you bought the Professional version, then you have the AgCl reference probe.
If you have the Standard version, then you have a Zinc Reference probe,
I am sure you are using the correct Reference guide for the probe you bought.
But... the voltage numbers from your first post, were from a AgCl reference probe,
The Voltages are different, Zinc vs AgCl [Silver Chloride] probes. [Corrected]
______
Galvanic Corrosion is a Direct Current [DC] phenomena. Not Alternating Current [AC]
However, nearby boats can rectify AC--> DC unknowingly, and seek out your properly grounded boat.
Also at DC currents can be imposed on the AC green ground wire, by other boats too. Thus the Galvanic Isolator to block those DC currents.
Sometimes a cheap version of a Galvanic Isolator does not really block them, at a low DC Currents.
_____
To monitor you propeller, you must clip the Common Probe line directly to your propeller, other wise it will read Zinc for your shaft Zinc.
If you lose your shaft Zinc, then the Bronze propeller is now the sacrificial anode.
Still to me, the shaft Zinc lost contact thus, the low reading.
The Diver fixed your problem.
Jim...
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If the Shaft Zinc is properly tightened by the Diver then in this picture, the Copper bead is smashed and does the electric bonding.
There is sea water still in the thin space between the Anode [LOL not Anoids] and shaft.
I got the pict from a YouTube where the guy was saying ANOIDS.
So...
It is the Surface area of the exposed Zinc that sacrifices. Inside and outside.
Jim...
There is sea water still in the thin space between the Anode [LOL not Anoids] and shaft.
I got the pict from a YouTube where the guy was saying ANOIDS.
So...
It is the Surface area of the exposed Zinc that sacrifices. Inside and outside.
Jim...
I don't know anything about the probe you are using but most electrochemical probes use the silver-silver chloride reference electrode (Ag/AgCl). Are you sure it is AgCl3? The Ag/AgCl reference electrode is very common however the +3 oxidation state of silver is very rare and unstable. Gold +3 (AuCl3) is more common but silver +3 is not so much.
you are Correct AgCl = Silver Chloride.
In my late night stupidity , I typed Aluminum Chloride AlCl3 I will correct, thanks.
Jim...
Here is the Silver Chloride reference voltage potentials versus many metals used in marine service.
The lower the water temperature in that range shown in Blue, the lower the voltage potential.
Please note Manganese Bronze potential range is -0.270 to -0.340 Volts... My Propeller material.
Also if we all had Titanium props and shafts...
Look Ma, no stinking Zincs
Jim...
The lower the water temperature in that range shown in Blue, the lower the voltage potential.
Please note Manganese Bronze potential range is -0.270 to -0.340 Volts... My Propeller material.
Also if we all had Titanium props and shafts...
Look Ma, no stinking Zincs
Jim...
I love the fact that you are aware that the probe is temperature dependent. Just more evidence that sailors are more engaged thinkers...So many "experts" in chemical analysis are ignorant to that fact. I teach a course on Instrumental Analysis and one of my pet peeves is the fact that many textbooks are ignorant to the fact that pH probes are temperature dependent and pH neutral has a different number depending upon the temperature. Neutral pH is only 7 at room temperature. (it has more to do with the equilibrium constant of water than the equilibrium dynamics of the reference electrode but....) Anyway, I like to show my students a quote from an Environmental Chemistry textbook where the author states that the great lakes are not as affected by acid rain because of the buffering effects of the limestone bedrock .... and the evidence he cites is a pH slightly below 7. Well ....at the bottom of the lake, where the pH was measured and where the temperature hovers in the high 30's, pH neutral is slightly below 7... doh. So the real question I would ask that environmental chemist is why is the water at a neutral pH when it is in contact with a base (limestone) that should produce a slightly basic pH? Another example I like to cite is from a biochemistry textbook where they state the pH of blood is slightly acidic because it has a pH slightly below 7. Guess what? Blood is not at room temperature. doh!
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I love how multiple posters are suggesting an improperly installed anode as the source of the problem but nobody mentions the fact that as an anode depletes from the outside in, the fasteners can become loose and the anode lose good connection with the shaft.
My tester is the Pro model= Ag/AgCl probe (reference cell).
Lots of good info folks, thanks to all!
Lots of good info folks, thanks to all!
Well one did.
_____
For the predominant part, but there is still sea water between the bulk Zinc and shaft too!! It will deplete around the copper bead too.
Jim...
PS: I said Diver tightened it back up.
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Also salinity and water movement.
Why water movement?
If water moving, it is like a "lightning strike" the salt water will start to conduct "ion exchange" quickly.
I was missing shaft Zincs and was using a drop Zinc to protect my boat until Diver could show up. Cold water not my thing.
I unclipped my drop Zinc and timed the the drop in voltage until it read my Propeller - 0.268 volts.
It took 98 minutes from -.970 to -0.268 steady.
But when diver put on the first of 2 shaft Zinc it went to -.970 in two minutes. [proved good contact too!]
Why?
Diver was mixing up the water around the boat.
Jim...
PS: Master of Science Chemical Engineer.
Nice!Also salinity and water movement.
Why water movement?
If water moving, it is like a "lightning strike" the salt water will start to conduct "ion exchange" quickly.
I was missing shaft Zincs and was using a drop Zinc to protect my boat until Diver could show up. Cold water not my thing.
I unclipped my drop Zinc and timed the the drop in voltage until it read my Propeller - 0.268 volts.
It took 98 minutes from -.970 to -0.268 steady.
But when diver put on the first of 2 shaft Zinc it went to -.970 in two minutes. [proved good contact too!]
Why?
Diver was mixing up the water around the boat.
Jim...
PS: Master of Science Chemical Engineer.
James, I'd appreciate your expertise on this a little.Also salinity and water movement.
Why water movement?
If water moving, it is like a "lightning strike" the salt water will start to conduct "ion exchange" quickly.
I was missing shaft Zincs and was using a drop Zinc to protect my boat until Diver could show up. Cold water not my thing.
I unclipped my drop Zinc and timed the the drop in voltage until it read my Propeller - 0.268 volts.
It took 98 minutes from -.970 to -0.268 steady.
But when diver put on the first of 2 shaft Zinc it went to -.970 in two minutes. [proved good contact too!]
Why?
Diver was mixing up the water around the boat.
Jim...
PS: Master of Science Chemical Engineer.
That "moving water" thing is going to be a bit of a bug-a-boo for me. How much "moving water" are we talking about here and does it invalidate the data or just make it more difficult to interpret?
I ask this because we are in an area with a 8 to 9 foot tidal range and some pretty stiff currents. So if the water has to be still for this, then the only time to do it will be at slack water, which does not last too long here. I am pretty sure I could not get to all the fittings I want to check during one slack water.
From your short example, it looks like the diver moving around (not still water) just made the corrosion reference respond faster but did not cause a reading that was unacceptable? Am I correct on this?
Also, we have a fairly high concentration of phosphates in the ground immediately below where our boat is moored and the local "knowledge" is that it accelerates zinc wasting. I guess if the zincs weren't ther it would accelerate galvanic corrosion of the item the zinc was there to protect too?
A penny for your thoughts? Pm me and I'll give you my phone number if you think a response it too complicated for a forum chat. Thanks in advance
Yes.
A moving current does NOT stop or accelerate an electrical path to complete the Galvanic circuit.
But...
It can interfere or add variability to the AgCl reference cell reading. Thus, from the chart, a Range of values.
Although this is a educated guess on my part...
Stagnant water or just plain mirky sediment water has competitive "crap" that impedes electron flow.
Note: The Diver's video was not in Caribbean clear water.
Galvanic corrosion is not a speedy chemical reaction in the first place. Slow and steady like the turtle in the race.
____
Example: Your fiberglass hull boat is anchored in a current in 86°F Caribbean Water, you are sipping an adult beverage and your boat crew is checking your Zinc Protection with your AgCl reference probe.
Crew: Captain sir, your reading is -1.060 volts [somewhat with a panicked voice]
You, the Captain, glance at your Galvanic Chart. [range -0.980 to -1.030 for Zinc]
Captain: Good work, but no problem, bring me another Margarita, please!
But if...
Crew: Captain Sir, your reading is -0.340
Captain: Ok the spare Zinc and tightening hex wrench is below, Dive and fix the problem. Then another Margarita, please.
Jim...
The non typical sea water salinity is the key here. The more the dissolved minerals the better [faster] the electron flow.
I would have to check the Galvanic half cell references to see if phosphates are stronger [ greater potential] than Sodium or Calcium [typical sea water], but your experience says... Yes.
____
A simple view is Galvanic Corrosion is just a Battery. Just like Lead-Acid, you can get electron flow from the two "half cells".
Jim...
PS: The two cells with the tester... one is AgCl and the other is say Zinc. The circuit uses sea water to conduct.
The tester measures the voltage difference. Like checking your Lead Acid battery terminals.
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To bore you a bit more, I found some research, by the US Navy, on Galvanic corrosion.
Taken from their report...
"In General, the rate of attack is increased with (a) larger concentrations of dissolved Oxygen, (b) Higher sea water velocities, and (c) raising the temperature."
Hmmmm? Maybe it was the Diver's exhaust air in the water, not velocity.
Jim...
Taken from their report...
"In General, the rate of attack is increased with (a) larger concentrations of dissolved Oxygen, (b) Higher sea water velocities, and (c) raising the temperature."
Hmmmm? Maybe it was the Diver's exhaust air in the water, not velocity.
Jim...
Thanks so much for the education. I'll be going to the boat soon and will try out my new toy ( Ag/AgCl Corrosion Reference Electrode).
So, to make sure I understand, it would be better to do it around slack water but it doesn't have to be totally slack to get meaningful information. As long as the readings are in the range things should be okay. If its near the limits (high and low) and there is a good current running, then try again on that component when there is no or very little current. We do have some sediment and the phosphate ground so I should reasonably expect to be at the higher readings from that.
Is that a correct interpretation of the info you provided?
Phosphate is being use in batteries now. LiFePO4 type. So they may act like the Cathode on a battery.
So the phosphates would draw Electrons from the Zinc, therefore making the Zinc sacrifice more than normal.
But they should not interfere with your testing.
I have this Probe...
Corrosion Reference Electrode: Corrosion Quiz
That comes with a proprietary manual on testing sequences. I called them and asked for a PDF file of their manual.
The owner said NOPE, since he is a certified ABYC electrician. So he is selling his knowledge too.
Here is what I suggest.
1) Record your testing volts and sea water temperature [I keep mine in a notebook for comparison ]
2) Drop the AgCl probe at the ≈ depth of your shaft Zinc and close to it.
3) Then test the shaft and wait till your meter stabilizes. [I have a alligator clip on a long testing wire to clamp on testing points] Next time on boat I will take a picture of my testing clamps.
4) If you have Zinc protection it should be close to the range on that chart above.
5) Disconnect shore power, see if the meter changes. No significant changes then you are not getting anything from the AC green wire.
Then proceeded to test any component on your boat, step by step.
One example noted that no problem until he turned on his radar. The radar transmission was not the problem but the radar supply circuit was bleeding.
I freaked out the first time I tested each of my bronze thru hull valves. I got -.470 V , [not right] until I realized it was my Lead-Antimony Keel I was reading.
My probe bought me a lot of...
Peace of Mind
Jim...
So the phosphates would draw Electrons from the Zinc, therefore making the Zinc sacrifice more than normal.
But they should not interfere with your testing.
I have this Probe...
Corrosion Reference Electrode: Corrosion Quiz
That comes with a proprietary manual on testing sequences. I called them and asked for a PDF file of their manual.
The owner said NOPE, since he is a certified ABYC electrician. So he is selling his knowledge too.
Here is what I suggest.
1) Record your testing volts and sea water temperature [I keep mine in a notebook for comparison ]
2) Drop the AgCl probe at the ≈ depth of your shaft Zinc and close to it.
3) Then test the shaft and wait till your meter stabilizes. [I have a alligator clip on a long testing wire to clamp on testing points] Next time on boat I will take a picture of my testing clamps.
4) If you have Zinc protection it should be close to the range on that chart above.
5) Disconnect shore power, see if the meter changes. No significant changes then you are not getting anything from the AC green wire.
Then proceeded to test any component on your boat, step by step.
One example noted that no problem until he turned on his radar. The radar transmission was not the problem but the radar supply circuit was bleeding.
I freaked out the first time I tested each of my bronze thru hull valves. I got -.470 V , [not right] until I realized it was my Lead-Antimony Keel I was reading.
My probe bought me a lot of...
Peace of Mind
Jim...