Anodes, test cell, fresh water & hull potential.

[SycloneDriver]

I have recently taken an interest in anodes and underwater corrosion. In all the time I've had boats I've never replaced an anode, crazy right! My boat lives in a fresh water lake and I've never had a corrosion problem that I'm aware of. I've ask others at my marina if they know of anyone having corrosion issues and the answers were all "no." A few years ago I realized how easy it is to make a wiring error and cause a potential issue, in my case I installed an automatic bilge pump switch with non waterproof crimp connectors. I did fix that later. What really got me thinking was the fact that my dock neighbor converted his boat from diesel to electric. He has a 48 volt setup, that much voltage could cause damage fast if something was wrong.

I bought a hookah last year and I've been inspecting and cleaning the hull, prop and anode. It's not an easy job. The water is very merky making inspection difficult. The prop anode is probably the original from 2008. I cannot see any significant corrosion other than one rust spot on the iron keel. I decided to take it a step further and ordered some new aluminium anodes and test cell. It's too cold now to enter the water so the anode will have to wait until next summer. Last weekend I dropped the test cell in the water and began taking measurements. The good news is that my boat tests at the same hull potential with the shore power cable connected or not. But, the hull potential is only -0.180 volts. The instructions that came with the test cell indicated that the voltage will be lower in fresh water, but it didn't specify an acceptable range. A normal voltage in salt water would be closer to -0.900 volts.

Any input would be appreciated.

 

[Tally Ho]

I don’t know that it makes a difference, but I use magnesium anodes on my boat in fresh water.

I think aluminum is also a recommended material.

Do you keep your boat in the water all year long….in Oklahoma?

Greg

 

[dlochner]

Do you know for certain that you do not have zinc anodes? In your research you have probably learned that zinc anodes form a crust which electrically isolates the anode so it does not degrade and doesn't work.

Same for the other boats in the marina, do they have zinc anodes or aluminum or magnesium? In marinas around me, many more boats have zinc anodes than have the proper magnesium or aluminum ones.

Freshwater readings are hard to find. I suspect the reason is the question is seldom asked and there are varying degrees of freshness in fresh water. The conductivity of "fresh water" could vary depending on the minerals dissolved in the water, salt from winter highway maintenance, acid rain, and some other pollution source. A total dissolved solids meter (Amazon Link) may give you some insight. The purer the water, i.e, fewer dissolved solids, is the less conductive it would be and reading would be closer to 0 v.

Finally, email Bob Olson at Boatzincs.com and ask him. He is really knowledgeable and helpful. l've had several good conversations with him.

 

[SycloneDriver]

I don’t know that it makes a difference, but I use magnesium anodes on my boat in fresh water.

I think aluminum is also a recommended material.

Do you keep your boat in the water all year long….in Oklahoma?

Greg

I got Al anodes because I figured they have to be better than 30 year old zinc.

My boat stays in the water year round as do most non trailer-able boats in Oklahoma.

 

[SycloneDriver]

Dave,

I don't know for sure what anode material is on the boat. I suspect it is the original, probably zinc. It does form a crust that I clean off with a wire brush.
I've seen a total dissolved solids meter used before but I didn't know what it was called. I'll look into that. Thanks

 

[jssailem]

BoatZincs sells a corrosion reference electrode (A portable silver/silver-chloride electrode for performing corrosion potential
surveys on yachts and boats.)

saves you diving into unknown or dangerous waters. You might ask Bob (mentioned by @dlochner in the above post about it.

 

[JamesG161]

Make your own fresh water calibrations for your AgCl probe.

Get 5 gallon of your Lake water and use in a work space at home.

Items needed...
1) Lowes 2.5 gallon plastic bucket
2) Several known piece of metal
3) Temperature of test water
4) DVM

Distance from submerged test metal to probe not critical.

Avoid contamination of the test water with your salt laden hand.
Jim...

 

[dlochner]

Make your own fresh water calibrations for your AgCl probe.

Get 5 gallon of your Lake water and use in a work space at home.

Items needed...
1) Lowes 2.5 gallon plastic bucket
2) Several known piece of metal
3) Temperature of test water
4) DVM

Distance from test metal to probe not critical.

Avoid contamination of the test water with your salt laden hand.
Jim...

Care to elaborate a bit on the process.

 

[JamesG161]

Care to elaborate a bit on the process.

Sure, if PO is interested.

 

[dlochner]

Sure, if PO is interested.

The rest of us don't count?

 

[JamesG161]

If there is a large interest in this procedure of Fresh Water Lakes and normal marine metals found on a boat...

I will put a simplified procedure in this Forum.

Sailboat Owner's Guide to Corrosion

And yes Dave, you and others do count.
Jim...

 

[JamesG161]

Mini version...
Set up is same as if on your boat.

Bucket of water is the Lake Water analog.

1) submerge test metal in water
2) submerge AgCl probe in same water
3) you can use a Stainless Steel Alligator testing wire clamps. One end on test metal , other end on DVM probe

Why did i suggest this step?

You will get tired of holding the probe tip on the submerged test metal. It may take 5+ minute for a stable DVM reading.

4) Record Water Temps , metal type and DC Voltage
5) repeat with various KNOWN metals

Jim...

 

[SycloneDriver]

BoatZincs sells a corrosion reference electrode (A portable silver/silver-chloride electrode for performing corrosion potential
surveys on yachts and boats.)

saves you diving into unknown or dangerous waters. You might ask Bob (mentioned by @dlochner in the above post about it.

That's exactly what my test cell is. Got it from BoatZincs. Maybe test cell isn't the correct term?

 

[jssailem]

You can use the unit in the above reference establishing set up that @JamesG161 describes.

Once you get the reference voltages you can take the unit (the CRE) to your boat. Drop the electrode in the water near the metal pieces you are testing. Hook up your DVM and with one lead to the CRE electrode the other lead goes to the metal inside your boat. You stay warm and dry (i.e. the shaft). Depending on your shaft metal (stainless) the voltage shown should appear on the DVM and match our test reference sample of stainless in lake watter. If you see the same value on the DVM then you know there is an electric corrosive current passing the electrons of the shaft into the water (electrolyte) - NOT GOOD.

Assuming one of the test metals in your bucket reference setup is zinc, when you touch your shaft you should see zinc voltage on the meter. GOOD NEWS.
The zinc is sacrificing electrons to protect your shaft.

 

[JamesG161]

Maybe test cell isn't the correct term?

Yes it is !!

The AgCl probe has been calibrated for sea water at a certain temperature.

To use it in fresh water is ok, but the calibrations for Fresh Water is rarely published.

Thus my Mini-Procedure to calibrate it, is in my post #12 above.

By the way..

I can help you better by a phone call. Send me a PM is you want more info.

I am a Boomer Sooner too.

Oklahoma Jim...

PS: Also a Chemical Engineer = knowledge of Galvanic Corrosion thus my Link to SBO corrosion above.

 

[SycloneDriver]

I goofed off a little today. I dropped my test cell in the water in approximately the same location as before. I measure -205 mV. Then for kicks I attached a long wire to a beer can and dropped it in the water. I wrapped the other end of the wire around the steering wheel. I know the wheel is bonded to the motor. The voltage then tested about -245 mV. It slowly crept up to -350 mV. Then I replaced the beer can with a new Al shaft anode. The voltage then measure -600 mV but slowly dropped to about -500 mV. I now have a new temporary hanging anode made out of my favorite beer can, Goose Island IPA.

I found it interesting that in the course of an hour or two the Al shaft anode developed a white power on it that looked like aluminum oxide.

I also tested a neighbors boat. It measured -140 mV.

 

[JamesG161]

Aluminum in a beer can maybe coated.

Aluminum Anodes are an alloy.

Time to get a Steady reading is dependent on...

1) Temperature of water
2) Flow of water around the anode.
3) The Surface Area of Anode exposed to the electron flow.
______

the Al shaft anode developed a white power on it that looked like aluminum oxide.

This your Anode in galvanic action.

If it was completely Coated during your test, not a good sign.

Hopefully you shaft has a Al anode on it now.

Jim...

 

[SycloneDriver]

Jim,

The new shaft anode, it's really a prop nut anode, will have to wait until next summer. The water is too cold. Speaking of cold, Tuesday night is supposed to drop into the low 20s. It's time to winterize a few things.

The anode that I hung on the wire only had a little oxide on it, but it was more than I was expecting in such a short time. I think I'll keep my beer can anode in the water as a long term experiment. I think I'll order a shaft collar anode to go with the prop nut anode.

I've been helping several other boat owners lately and I suspect that I will be replacing their anodes next year.

 

[SycloneDriver]

I finally replaced my prop anode today, that was a tough job. It's in another thread.

Hull potential measured 320 mV. It's in fresh water BTW.

 

[jssailem]

Hull potential measured 320 mV. It's in fresh water BTW.

I’m unclear what you are measuring. The hull is fiberglass so the hull potential should be zero.

The metal fittings and parts that are submerged are what I measure. When I make contact to the various metal parts I look for the reading to be equal to the anode I am using. In my case zinc in salt water. In your case magnesium or aluminum based on the fresh water lake.

When I see the number approximate steel or bronze I get very concerned.