Anodes?

[alldodge]

Re: Anodes?

I disagree, You are describing stray current corrosion not Galvanic corrosion.

Galvanic corrosion : Corrosion that occurs at the anode of a galvanic cell.

You may remember high school chemistry class where a battery was created by connecting two dissimilar metals with wire and immersing the whole contraption in water thereby activating magnetic fields and starting an electrochemical process causing current to flow.

On a boat with bronze, aluminum, galvanized and stainless steel that are connected with grounding wires and immersed in the lake...... you accomplish the same thing. The more noble metal is the "cathode", the less noble, the "anode". In this process the less noble metal gives up electrons to the more noble thus weakening the metal, otherwise known as "galvanic corrosion".

The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes or "zinc's" in good condition and never paint them.

A vessel suffering from galvanic corrosion is usually the source of it's own problem, although two vessel's linked by shore power grounds can create a galvanic cell between two very close boats.

2. Stray Current corrosion : Corrosion that results from an electrical source causing a metal in contact with an electrolyte (water) to become anodic with respect to some other metal in the same electrolyte.

In simple terms a wire touches something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge sending current into the water, causing one metal to give up electrons and corrode. Again any vessel suffering from this type of corrosion is likely the master of it's own disaster but the culprit could also be a neighboring vessel. This type of corrosion can can eat metals at an alarming rate. I know of one 42' motoryacht that lost both shafts, both rudders and both propellers in a space of less than two weeks.

Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.

Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods attempting to combat corrosion.

Salt water is generally regarded as a more serious breeding ground for marine corrosion as the salt makes it more conductive however, polluted fresh water can be even more conductive with the right contaminants.

Your welcome to your opinion but need to do a bit more research and my statement was made in the most simplistic of terms.

Most the time the boat being damaged by galvanic corrosion is caused by other boats and electrical connections (marina) around them. If your boat is maintained and well grounded, stray electrical currents from other boats, shore power connections will be drawn to yours. Hooking up more items which draw power only makes you more susceptible. If you were not connected to shore power there would be no galvanic corrosion because you would not be grounded. There are two ways to stop or diminish these stray currents. Your boat needs to produce sufficient apposing current to make your boat less appealing, so to speak. Galvanic isolators should be installed to isolate your boat from external ground. A GI is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize stray direct current / DC from flowing into your vessel. While blocking stray DC current it also has to allow for the passage of AC fault current. Dissimilar metals cause corrosion as well, aluminum casings and stainless steel props. A stainless steel prop makes a boat more attractive to stray currents then aluminum prop.

 

[bruceb58]

Re: Anodes?

Dodge, galvanic corrosion is just caused by dissimilar metals. Having a stainless prop does not attract currents from other boats. Stainless steel adds to the galvanic problem within that particular stern drive ,compared to having an aluminum prop, itself and has nothing to do with other boats.

 

[lg260ss]

Re: Anodes?

Dodge, galvanic corrosion is just caused by dissimilar metals. Having a stainless prop does not attract currents from other boats. Stainless steel adds to the galvanic problem within that particular stern drive ,compared to having an aluminum prop, itself and has nothing to do with other boats.

This ^^^. The dissimilar metals themselves with NO additional current creates the magnetic field that causes galvanic corrosion. This is not an opinion, it's fact.

 

[alldodge]

Re: Anodes?

Dodge, galvanic corrosion is just caused by dissimilar metals. Having a stainless prop does not attract currents from other boats. Stainless steel adds to the galvanic problem within that particular stern drive ,compared to having an aluminum prop, itself and has nothing to do with other boats.

Agree with the definition but the issue is the dissimilar metals being in an electrolyte solution. An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents such as water. This includes most soluble salts, acids, and bases. Water fresh and salt has different impurities which can increase dissimilar metals to react. The SS prop makes the boat a bigger target for stray currents due to it's index of .60 compared to aluminum of .95, and a good ground makes it all the better.

We are calling it galvanic but not calling it galvanic it two metals are discussed and no electrolyte is mentioned. A boat sits in a solution, just not a very good one and we need sacrificial anodes to protect against the problem.

This puts me into remembering the heavy discussion of exhaust risers and elbows. They are called elbows but we keep calling them risers.

 

[Chris1956]

Re: Anodes?

Gee, finally some convergence of thought.

Current can come from other unusual sources within a boat as well. Once I had a nick in my positive battery cable insulation. The cable sat in the bilge touching some bilge water, which is common enough. Overnight the current from the battery travelled thru the bilge water, thru a metal fitting into the saltwater. In the morning, the battery was dead and the marine-grade battery cable was reduced to blue paste. My anodes took a pounding as well.

 

[bruceb58]

Re: Anodes?

Good explanation:

Types of Marine Corrosion - BoatTECH - BoatUS

 

[alldodge]

Re: Anodes?

:thumb:

 

[Home Cookin']

Re: Anodes?

first, none of this really matters to the OP who leaves his outboard overboard for less than 5 hours, and has the annode on the motor to boot. OP has nothing to worry about.

But for others" in 55 years of boating, all saltwater, I have never once used shore power but I sure have dealt with corrosion. And at my home, there is no power and are no other boats nearby (150 yards).

A major factor is antifouling paint, with all its metals sitting in salt water, making a battery unto itself.

Then there the "leaky current." Once salt water boats get some age to them, if there's a battery on board, there's power around. A battery switch helps isolate this immensely.

You can't help but have metal to water contact even in a fiberglass boat. The tip of most lower units touches the water when the motor is tilted up. Trim tabs, bilge plug, through hulls, ski steps, depth finder--all immersed.

I once left my ignition key on overnight: fiberglass boat, no shore power anywhere for miles. Old OB motor. The next day the motor was coated with "salt" chrystals even above the water.

Maybe you won't get corrosion with no shore power nearby if your boat's electrical system is perfect; that state of perfection does not exist in the real boating world.


PS another source of stray power is underwater power cables, either defective ones at the marina or larger general municipal lines buried nearby.

 

[UncleWillie]

Re: Anodes?

Bruce, That statement is too simplistic. Boats are grounded unless they are all fiberglass and all metal is out of the water. Even a brass bilge plug, and some bilge water can ground them. Not many are not grounded when tied to the dock...

A boat floating in the water is not grounded. It is floating. Physically, Metaphorically and Electrically Floating!
In a microsecond, it will float to the same potential charge level as the water, at which time there becomes no difference in potential.
No potential difference produces no electrical activity.

This is the same situation as the bird sitting on a high voltage wire.
You can describe it as being grounded to the wire if you must, but without a difference in potential, there will be no electrical activity.

 

[Bullrider]

Re: Anodes?

Please excuse my newnes and possible naive ways but my understanding of anodic obliteration will make a colliodid suspension of your metalic parts from all friction ei; wind,water movement, salt, and anything that excites electrons to a grounded mass/liquid. I might be just confused. I am very interested in all this discussion. My 19 year old boat has only been in fresh water and the "zincs" are in very good shape but have shown good operation even in just fresh water.
Jason

 

[Chris1956]

Re: Anodes?

Willie, Birds do not have batteries in their beaks, or metal feathers. Boats have batteries in their bilges, copper paint on their bottoms, SS bolts and other parts, and aluminum gearcases and outdrives. Different situation

 

[UncleWillie]

Re: Anodes?

Willie, Birds do not have batteries in their beaks, or metal feathers. Boats have batteries in their bilges, copper paint on their bottoms, SS bolts and other parts, and aluminum gear cases and outdrives. Different situation

Birds and all animals have biological batteries operating their hearts and muscles, and conductive salt fluids flowing throughout their bodies. The similarities are closer than the differences.

You could attach a cable from a 37,000v power line (AC or DC), and hang either terminal of an auto battery to the cable floating in the air, the battery would behave exactly the same as if it were sitting on a rubber mat on a wooden table in your garage.
Without a conducting path to the 37,000 volt reference point (Ground) the potential between the battery terminals remains at 12 volts.

Ground is a relative reference point, it is not an absolute reference. We use the potential of the mass of the earth as our local reference for ground as a convenient point because everyone has easy access to it.

The earth most likely has a charge in the millions of volts on it due to the influence of of the Local solar winds.
The question that statement should have raised was, "Millions of Volts Relative to what?" A very good, and Valid question!
We have not defined the reference ground point of the universe. So the statement is only valid in the academic sense.
Still the evidence of the Northern lights indicate that field potentials in the 10s to 100s of thousands of volts range exist between the earth and the beyond.

If you could run a wire from the Earth to the Moon you would certainly find a huge potential difference.
But astronauts and their spacecraft, like the bird on the wire, just referenced themselves to Lunar Ground without ever knowing the difference. (Pun!)

 

[Chris1956]

Re: Anodes?

Willie, Boats sitting in the water are grounded. The bird is not. Different situation, no?

Run a cable from one of the birds legs to the ground below. Now let him sit on the 22KVAC power line. Instant fricassee.

 

[UncleWillie]

Re: Anodes?

Willie, Boats sitting in the water are grounded. The bird is not. Different situation, no?
Run a cable from one of the birds legs to the ground below. Now let him sit on the 22KVAC power line. Instant fricassee.

This is a difficult concept. The water can be ground if you want to use it as the reference. A valid reference if you are at sea.
In Port we prefer to use the Earth's, and your shore power's ground potential as ground.

Consider one boat attached to shore power and leaking current into the marinas water.
Consider another boat (Aluminum) floating nearby with no attachment to shore.

A person sitting on the shore and sticking his foot into to water risks an electric shock or worse.
A person sitting in the floating boat, reaching into the water is not at risk.

An inspector comes along to investigate the shock complaint.

He attaches One lead of a voltmeter to ground post driven deep into the shore (Earth Ground.)
He places the other lead of the meter in the water next to the boat leaking electrical current into the water.
The meter indicates that the voltage(Potential) of the water is at 35 volts. Hence the shocking experience.
He also measures the potential from the floating boat to the shore and measures the same 35 volts.
The free floating boat and the water are at the same potential. With No potential difference, No shock. The bird on the wire.

Ground the Free floating boat to shore power, or the bird on the wire, Now you have a problem!

 

[bruceb58]

Re: Anodes?

Chris, draw a picture of a boat floating in the water next to a boat that has a leaky AC shore power leaking into the water. Then draw current lines that are supposedly flowing into this floating boat and then out again. Sorry...can't happen.

 

[phillyg]

Re: Anodes?

No anodes needed for your trailered boat. The little rudder looking thing is in fact an anode, but it's primary purpose is to adjust unwanted steering veer.

 

[achris]

Re: Anodes?

If you have 2 (or more) different metals in saltwater and you don't one to disappear, fit anodes that are less noble than the least noble of the metals you wish to keep. Galvanic corrosion occurs the instant dis-similar metals are in seawater. 5 hours is plenty enough to see the effects. And it makes ZERO difference if the boat/metals are moving or not. On the up side, all outboards (and sterndrives) come from the factory with anodes. Just keep those clean and replace them when they are down to about 30% and you shouldn't have any problems. That little fin you're referring to may not be an anode. On many John/Rudes the anode is a block either screwed to the side of the drive-shaft housing or a block installed in a specially cast location above the anti-cavitation plate.

I've read all of the posts here and my head is spinning. You guys are talking about 2 different effects. One effect (and the one asked by the OP) is galvanic corrosion, 2 (or more) different metals in an electrolyte (in this case, seawater). The other is stray current corrosion caused by a faulty installation from a shore power connection.

Chris........
(I used to be a 3.4U subsea inspection engineer, specializing in galvanic corrosion and underwater welding)

 

[Bullrider]

Re: Anodes?

I used to teach about corrosion at the Boeing Company. I really respect what you guys are saying! There is a whole lot I don't know and it is fascinating!
I have an antennae(copper wire) suspended way high. It comes down to some components I have soldered up. Then it is grounded well. I measure around 12-18 volts at around .5 to.8 amps of current. (FREE PPOWER) This shows to me the constant flow of energy we live in.
I don't want to hijack this thread so I do like knowing my trailered boat is fine and my zincs will last a long time.
Cheers

 

[UncleWillie]

Re: Anodes?

I've read all of the posts here and my head is spinning. You guys are talking about 2 different effects. One effect (and the one asked by the OP) is galvanic corrosion, 2 (or more) different metals in an electrolyte (in this case, seawater). The other is stray current corrosion caused by a faulty installation from a shore power connection...

Yes we are!

The other effect is the Galvanic effect.
This is stickily chemical in nature. Essentially, it is a battery.

Consider the Nickle-Cadmium Battery; composed of a Positive Nickle Anode and a Negative Cadmium Cathode separated by a paste of Potassium Hydroxide.
As long as there is no current path between the Anode and the Cathode, the battery produces a voltage, but no current, and can be stored for long periods without effect.
Connect a conducting device (Lamp?) across the battery and the battery will produce a current and the Cadmium plate is converted to Cadmium Hydroxide in the process.

Now let's consider our Aluminum boat floating freely in the water.
It is a virtual battery consisting of various metals all separated by the water which contains various dissolved minerals that form the electrolyte for our battery.
The bad part of this picture is that all the metal parts are electrically connected together placing a short circuit on our Battery/Boat.
Let's simplify the example and only consider a stainless steel propeller and the Aluminum of the boat hull and motor housing.
The Steel Propeller will become the Positive Anode and the Aluminum will assume the Negative Cathode position.

The Water can vary from very low activity freshly melted snow, to highly active salt water of the Ocean, or even worse, the Dead Sea.

Remembering that our Battery/Boat is shorted out; The current flows between the Aluminum and the Steel.
This electrochemical reaction reduces the Aluminum Cathode to various Aluminum salts that dissolve in the water and wash away.
If it is is allowed to go on long enough, the Aluminum is eventually destroyed as it all dissolves.

To counter this effect a third metal is attached to our battery/boat.
A piece of Zinc or even more active piece of Magnesium will now assume the role of the Negative Cathode.
Now the Aluminum is more positive than the Zinc and becomes the Positive Anode along with the Steel, of our Battery/Boat.

The Zinc/Magnesium plate now become the sacrificial Negative Cathode of our Battery/Boat.
And slowly dissolves, much to our delight, as it is much more easily replaced than the hull or the motor.

If you followed all this you will have noticed that your Zinc Anodes are really Zinc Cathodes.
This is what happens when engineering hands the system over to the marketing department. :facepalm:

At least Mercury Marine correctly markets their active corrosion prevention system as a "MerCathode". :joyous:

 

[Ned L]

Re: Anodes?

Thank you IG260, the first correct response. Yes, galvanic corrosion & stray current corrosion are two different things.