Sacrificial Plate - Caution/Question

My zinc anodes on my outboards are not conductive. Last week I found out that there is an open path from the outboard ground wire to the zinc anode on the cavitation plate. Then probing further, I found that the there was conductivity between the lower unit and the ground wire, but that something was electrically isolating the anode from the ground conection. When I replaced the anode there was conductivity.

This weekrnd I tested the old anode (see photo) and found that it, standing by itself, was non-conductive. I could take ohmmeter probes and touch two different spots on the anode, and read an open circuit. I did the same test on another outboard with identical results. When I chiseled down to the shiny zinc on one of the anodes, and touched the probes to the scratched surfaces, there was conductivity.

In other words, in fresh water, the zinc anode builds up a non-conductive covering that, in my opinion, renders the anode totally ineffective as a sacrificial plate. Only when the anode is ground down showing the shiny metal does the sacrificial plate become conductive and have any value. I would assume the same occurs in salt water, but maybe not.

I recall an earlier discussion about sacrificial anodes, that I either never understood or agreed with, that there should be multiple sacrificial plates. Perhaps the reason is that the plates do react with the surrounding elements and develope a non-conductive surface covering.

I would be interested in any comments. Otherwise, make sure you check the conductivity of your sacrificial plates before launching this spring to protect your motor's lower unit from electrolysis.

Well, I have never tested the conductivity of my zinc on the outboard....so I have no idea if the self-shielding of zinc actually happens in saltwater......but i don't think so (see below).
But, I thought I read somewhere that one should use magnesium in freshwater (not zinc). Magnesium is lower on the galvanix scale than zinc, and maybe that makes sense;

<a little chemistry tangent here>
Zincs protect other metals by being giving their atoms to move the electric charge to negate a positive charge somewhere else. In saltwater, the ions (think Na and Cl, among other salts components) carry the positive charge around from one piece of metal to another. The "resistance" of the water to carry this charge is inversely proportional to the concentration of the ions;
So in freshwater, where there are fewer ions, one needs the voltage potential to be higher (relative to saltwater) to make the current flow, and thus protect the good metal parts.

It may be that in freshwater, the "resistance" of the water part of the circuit is too high for the voltage potential to overcome. Thus, instead of the zinc giving up it's atoms and revealing a fresh new surface of zinc, it just covers over.

Try a magnesium anode, I would bet a beer that with its higher voltage potential, it would not self-shield, but instead loose Mg atoms to reveal fresh new ones below (the chemical version of scraping the top layer off to regain conductivity)

So....this was all just an educated guess....I have a good chemistry book at my desk at work....I will check on monday.

Your idea is most interesting and perhaps correct. The "skin" is a rather substantial layer and not easy to scrape off. I have also wondered if it could be a calcium build up - but I don't know why, particularly since Lake Michigan, I believe, is soft water.

I do have the old anode (the one in the picture) so can run tests. Inherent with using the ohmmeter, I had a 1 1/2 volt (or something like that) across it and there was no current flow. I would think that for electrolysis that would be a good bit. Shoul I try 6 volts or 12 volts. Would that prove anyting?

If I were to try to etch the skin off, that might tell something. Would a solution of Chlorox work, or would that blow up? What could I use to dissolve the skin.

Comments from the salt water sailors might also be definitive. Thanks for your comments and help.

The latest observation in this drama of the non-conducting zinc plate:

This AM I noticed that the OB lower unit is pitted. The pitting has gone right through the TRI-X anti fouling paint, the primer, and the new motor finish. That does tell me that the sacrificial plated does not work, because there is no reason, at least as I see it, for these pock marks to develop, other than electrolysis occasioned by a non-functioning zinc anode. I would imagine that if I had done nothing about it, one of these years soon, I would have had to buy a new lower water unit. Hopefully the lower unit would not fail while on a trip, but then it could. Insideous stuff, a malfunctioning sacrifical plate can be.

Prof mrgnstm's theorem sounds intriquing, his argument persuasive.

Certainly sounds like it's worth a try. And as the light went on, I wondered if your local OB shops have those 'mags' - and are they also aware of this problem?

Getting information from Yamaha, or a Yamaha dealer is most problematic. The communication largely stops when the purchase check has been conveyed to the dealer. Their concept of communications with their clients tracks the letters policy of "The Onion, America's Finest News Source" in their letter policy:

"The Onion neither publishes nor accepts letters from its readers. It is The Onion's editorial policy that the readers shall have no voice whatsoever and that The Onion shall be solely a one-way conduit of information. The editorial page is reserved for the exclusive use of the newspaper staff to advance whatever opinion or agenda it sees fit, or, in certain cases, for paid advertorials by the business community."

(The Onion, as it is known to afficianados, is a satiric paper of great note)

Anyway, such is Yamaha's policy with regard to their client communications. They have even fallen behind Microsoft in this regard. Other than that it is a good motor.

However, the research does continue. But in the meantime, I have to, in addition to replacing the anode (already done), re finish the parts of the lower unit where it is obvious that the finish has popped off and the metal become directly exposed to the water and corroded electrically.

Thanks to the nudging, I went to my Yamaha's owner's manual and then to the Yamaha site. The anode issue is in the owner's manual under MAINTENANCE:

"Inspecting and replacing anodes:
Yamaha outboard motors are protected from corrosion by sacrificial anode(s). Inspect the external anodes periodically. Remove scales from the surfaces of the anode(s). Consult a Yamaha dealer for replacement or external anode(s)."

I saw nothing about a magnesium anode but I will inquire further.

That said, it is clear that this is a common KNOWN issue with outboards and should be part of everyone's regular check out. The ramifications are not that the anode will disappear, as I had assumed (although that might happen as well), but that the anode becomes non-conductive. Taking the scale/skin off the anode, or replacing it must be part of the periodic check up.

Incidentally, in the Johnson manual they say "Galvonic corrosion may occur in fresh water or salt water. However, salt water usage will accelerate corrosion......Replace the anodes before they are completely eroded or corrosion to motor will increase."

Research continues....

A few websites with info:

http://www.westmarine.com/webapp/wcs...roductId=72340

http://www.diynetwork.com/diy/bo_mai...277519,00.html

http://amos.shop.com/amos/cc/pcd/909...6977/ccsyn/260

Note that I don't personally vouch for the correctness of all these, but at least one of them seems to be a source for Mg anodes for yamaha's.

I am really suprised that the local boating stores in the midwest don't sell Mg anodes exclusively, since they have no saltwater.

anyway, later.....

Yeah, it does sound like Magnesium is the deal in frsh water. Maybe I'll have to pound on Yamaha's chest to get an answer from them. It only requires a 200 mv differential, whereas zinc requires 900mv. Fresh water just doesn't have the conductivity. What really bothered me was that aluminum, as in the material used for the OB lower unit, is also suitable as a sacrifical anode. That is really dangerous for us OB people. It is not a comforting thought that my OB could be the sacrifricial anode for other things on the boat.

But, in the meantime, I'll have to use the Yamaha zinc, and adhere to other web advice. First and foremost is that checking/replacing the zinc anode is done at least annually. It is now on my annual check up list. But now comes what appears to me to be counter intuitive. I had always considered zinc anode is ready for replacement when it is largely gone. But no. It is replaced when it is only 50% gone (there is still a lot left). Mine was well under 50% in volume, although the horizontal area covered was perhaps 80-90%. The fact that it is badly pitted means that it has, at one time, been working. The reduced volume means that it is shot and, regardless of the metal used, inadequate to arrest electrolysis.

However, returning to my original issue, Yamaha and Interlux all recommend that the aluminum metal lower unit parts be covered by the right kind of paint to prevent electrolysis. So I am doing that too. Why take a chance, particularly considering the aluminum lower housing is of aluminum, a material suitable to act as a sacrificial anode -