Revelations about zinc electrodes.

Yes, it was a long shot - My brother installs/repairs MRIs for Siemens, is all. I thought you may have crossed paths but I seriously doubt it now!
Motecuhzoma Xocoyotzin (that's Nahuatl, or Aztec, language) Most Mexicans I have heard pronounce his name "Moctezuma" - Maybe I just havn't listened closely. Most Nahuatl doesn't exactly flow off the lips of non-Mexico City peoples. My wife smiles every time she hears a waitress pronounce "chipotle", a smoked jalapeño-like cuaresmeño, "chipolte", a knot on the head ("would you like a knot on your head salsa?"). Kinda like German technical manuals...

Carry on with the zincs, sorry.

 

Xocoyotzin could be a medication for a nasty disease Mark.
I definitely prefer German manuals!

 

http://www.archive.org/stream/diegalvanischek00ohmgoog#page/n16/mode/2up

Viel Glück!

 

Sacrificial anodes, what a wonderful concept.

I also use them successfully on my automobile.

-Tom

 

The freeze plugs in your engine are sacrificial anodes no?

 

Not really unless you use cheap antifreeze, then they'll be sacrificed and it's usually the one where you have to remove half the engine to get at whereby the nuckles become sacrificed along with one's entry into heaven. Possibly whats happening to me right now having to go with measuring tape,flashlight and artic gear to confirm a few dimensions of my hulls lines, not working on paper,somehow I missed a station. Geo

A yacht is not defined by the vessel but by the care and love of her owner

 

That is an intersting application. Tell us more about it.
Bert
P.S. CDK, if you allow us.

 

No, thank you Hoyt, that is just as terrible.

 

Not much to tell. Drill and tap into the frame and screw one or two in.

I don't bother on the engine.

Most applicable where salt is used on the roadway through the winter.

-Tom

 

How much zinc do you need?

I've done some tests with 2 sq.ft. of steel plate and various sizes of zinc in seawater 3 ft apart, measuring both voltage and current.

The voltage is always the same (700 millivolts or 0.7 V) but the current is almost proportional to the surface area of the zinc. As little as 2 sq. inch generates approx. 15 milliamps.
Because the rate at which the zinc is sacrificed also is proportional to current, using large electrodes to protect a small amount of steel is just a waste of money: the zinc will just dissolve faster. Only using more compact, thick electrodes can prolong their life.

A relatively simple way to optimize protection is the circuit below.
The electrode is electrically insulated from the hull by using through hull bolts with nylon sleeves and collars. A suitable terminal is placed under the bolt head inside the hull.
The rheostat can now be set to a current of 10 milliamps regardless of the dimensions of the electrode. There will be adequate protection with minimal zinc loss and the gauge will show a decrease in current when it is time to replace the electrode. It will also signal paint damage by a significant current increase.

 

That is really clever! Thanks

 

The protection current density, Js, of all the individual surfaces, Si, yeilds the total protection current required, Is, where

Is = Sum (sigma) Js.Si

put another way, the number of anodes required, n:

n= Sum (sigma) [Js.Si]/Jmax

(Since n.Imax = Js.Si)

Then you can obtain the required amount of mass, m, of anodes.

m(zn) = 0.337S(m^2)

based upon a 2 year surface life.

For example, (from a text book example) a steel ship with a below-waterline surface area of 4500m^2 with a Js=15 mAm^-2, the current requirement is 67.5A, from the above Eqn. This gives 1517kg of zinc for a 2 year life. if the anode weighs 15.8kg the number is 96 with a current output of 0.92A.

Thus, surface area plays a major role and is directly proportional.

 

That is quite a lot of zinc John, half the weight of my boat.
Does the text book specify a paint system? For an epoxy painted hull I think 15 mA/m^2 is rather high.

 

Brilliant CDK!

I think you get a gold star AND move to the front of the class.

(For those of you who like to read into these things, there is no sarcasm expressed or implied in my compliment.)

-Tom

 

You allow for a current density within a range of 10 to 40 ma/m2. Ocean going vessels need a lot less than coastal vessels. We use 20ma/m2 for inshore commercial.

For Johns ship above I would have specified 1.6 tonnes of zinc ! I use 730 Ahrs/kg for the zinc andodes manufactured here.