Electricians? Stray current- Conclusion

A silver silver chloride half cell develops a -0.199 voltage when immersed in seawater. Zinc develops -0.76 volts when it is immersed in seawater. Both the zinc and the silver stuff function individually as half bridge.

Typically they are shown with each metal in a separate glass containers connected with a salt bridge. Now on a boat the zinc is in teh water along with the half cell. So placing a silver silver half cell or for that matter or other metals (but different voltages) and then measuring the voltage between them.........in this case it would be between the attached silver silver chloride half cell and the boat's bonding or any place that is attached to the bonding system that is also sharing the salt water.

Now lets get to measuring zinc! The voltage between the zinc and the half cell will be zinc's -0.76 volts and the half cell's -0.199 volts. Voltage measured is |0.199| + |0.76| = |1.059| volts. In my above post I simply took the first three digits, 1.05 volts.

This should not be complicated. Take the reading, record them. Check the self voltages for the various metals your boat has in the water. Add the half cell's voltage to those of the concerned metals. As long as the readings are LESS positive than the sum of the half cell's voltage plus that of the concerned metals, you should be OK.

Now this even gets easier if you instead use the tables that show the metal voltages that were read using YOUR particular type of half cell. Your readings then just need to be less positive than those shown in the tables. And for the questions about over protection..... take the effort to read the attached paragraph or even better, check out the URL

The paragraph below PERTAINING TO OVER PROTECTION is copied from Crawley Advisor

http://crowleysyachtyard.blogspot.com/2013/05/marine-corrosion-part-2.html

"You may think that over protected is ok, but unfortunately, being over protected has its own set of problems. Under water coatings can be literarily blown off the bottom by gas bubbles forming on the surface of the metal. Also, an alkali solution can form on aluminum which will eat into the metal. Neither of these issues are good and their symptoms can look like galvanic corrosion. The reason we use precise alloys for our reference cell and a volt meter that is accurate measuring 10s of millivolts is so that we can know what is going on with the boat, not make guesses, nor assume that the dock chat about corrosion is correct."
 
Thanks for the explanation. I understand the overprotected problem and think I found my leak. I'll do some more testing tomorrow just to verify consistent readings.

I am getting a reading of -1.00 on just the zinks but will verify that as well. Now, you bring up an interesting point... I'm assuming that the chart that came with the half cell is taking into account the -1.00 (or -1.05) reading produced by the cell and zinks when it says my safe range on a fiberglass boat with aluminum outdrive is -.850 to -1.100. Now, if I also have to add my base reading to that, I will be overprotected. But the kit does not say to do that. What do you think?

I bought this one...
http://boatzincs.com/corrosion-reference-electrode-specs.html
 
quote:
Originally posted by GilligansIsland

Thanks for the explanation. I understand the overprotected problem and think I found my leak. I'll do some more testing tomorrow just to verify consistent readings.

I am getting a reading of -1.00 on just the zinks but will verify that as well. Now, you bring up an interesting point... I'm assuming that the chart that came with the half cell is taking into account the -1.00 (or -1.05) reading produced by the cell and zinks when it says my safe range on a fiberglass boat with aluminum outdrive is -.850 to -1.100. Now, if I also have to add my base reading to that, I will be overprotected. But the kit does not say to do that. What do you think?

I bought this one...
http://boatzincs.com/corrosion-reference-electrode-specs.html

Click to expand...


Could it be that you have too many zincs? Before I looked into galvanic corrosion I followed the herd, added zincs every where. Two each on each prop shaft, two on each rudder, two on each SS tab plus a diver's plate. What a waste of money!

Today I use ONLY ONE ANODE and it is NOT ZINC! I use a diver's plate aluminum anode mounted the transom AND THAT IS IT! I did install my self made shaft brushes to replace useless shaft anodes that seem to last about a month before falling off.

Now one is foolish to do what I did without taking the effort to measure the bonding voltages on the critical metals immersed in salt water. For that job I was lucky to find a silver silver chloride half cell, new-- never used on EBay delivered for $40 that was a part of some company's expensive equipment for taking commercial measurements.

All my measurements are in the -0.75v or so range which is just fine. The beauty of aluminum alloy (Mil-A-24779) is that it is about 1/2 the cost of zinc, has a slightly higher self voltage, -1.1v, lasts much longer than zinc because it has a higher electron capability and finally it can be used in either fresh or salt water. You should think about aluminum!
 
I don't think I'm over zinked. I have the prop and transom zinc plus one on each tab. That's how my last one was and it was fine.

I took some more readings today and I'm within the recommended levels according to the chart. I will look into aluminum next year also.

Has anyone ever taken all the anodes off and then taken the readings? I may do that when I have time one day to see exactly what my zincs are doing.
 
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