magnesium sacrificial anode getting eaten like candy

Howdy,
Since this NG looks like it has a bunch of folks much smarter than I, I'll
ask my question here.
I've got a neighbor with a wood fired boiler that has been eating
sacrificial anodes (magnesium) at a rate that is over 10x faster than other
identical boilers in other locations from the same manufacturer. The
manufacturer has exhausted all of his ideas and I was called in to see if I
could help. I found a simple low impedance loop which was passing about
1/4amp of 60Hz AC and we fixed that (broke the loop) and now it's gone but
the anodes are still getting eaten at the same incredible rate. The boiler
tank is steel (I believe). These anodes are somewhat expensive if he has to
replace 3 of them every 2 or 3 months. He's had the water tested and it's
found to have a higher conductivity than normal but not something that
concerned the folks that tested it.
What would cause a setup like this to lose so much magnesium off of the rods
so quickly? I'm no chemist and not really up on the process but I'm at a
loss. The only thing that he has done that is different from other
installations is that he made a nice "faceplate" out of aluminum and bolted
it onto the front of the boiler. Makes kind of a nice facade but is this
"dis-similar metal" that is bolted on the outside of the boiler what is
causing all the problems? If it was inside, I might think more about it but
I don't see how it would have an affect from the outside.
Thanks for any advice or feedback!
Jim
Reply to
Jim
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I'd take the faceplate off this installation & put it on one of the others & see if the behavior transfers.
regards Bob
Reply to
Bob K 207
Jim:
The anodes being used up rapidly are telling you that you have a serious corrosion problem that needs to be taken care of before you have the larger expenses of leaks and repairs. The water conditions may have been different. Possibly you have lots more oxygen than normal, but it's hard to tell.
You might want to look at a Corps of Engineers Public Works Technical Bulletin titled "Boiler Water Treatment: Lessons Learned"
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Pittsburgh Pete
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Reply to
Pittsburgh Pete
Thanks Bob. He's going to take the Aluminum off at least and see what the rods look like after 2 weeks.
Reply to
Jim
Thanks for the link Pete! I'll take a look and see what I can learn from their experiences. I DO believe he's got a serious problem but where it's coming from & why has got me stumped. He did have some leaks and he's already fixed them once.
Nice disclaimer!
Jim
Reply to
Jim
--
It may not be the boiler consuming the electrodes, but something else, to which the boiler is simply acting as a conductor. Without a doubt the boiler is well-grounded, and it may be difficult to un-ground it, altho it can be done, by using plastic plumbing as a conductive break, altho you have to be careful on the hot-water side. I would imagine this would include electrical ground wires as well. Once the boiler is conductively isolated, observe the rate of Mg consumption. Alternatively, high consumption implies a voltage, and you could measure the voltage between the Mg and the boiler, and between the Mg and *other* contact points in the area. If you get similar readings (from a DVM), that would also suggest a culprit elsewhere. You could test this voltage with the boiler full as well as drained, which might provide a clue. If there is any extraneous DC voltage being applied somewhere, this could be consuming the Mg as well, perhaps a grounded rectifier or diode or something. AC loops are unlikely to be the culprit, by the definition of AC! As you found. ---------------------------- Mr. P.V.'d formerly Droll Troll
Reply to
Proctologically Violated©®
IIRC, The Magnesium is slightly more positive than the Aluminum. They are both reactive materials, Mg being 12 and Al being 13. Since the two metals are used, there are currents being generated by the dissimilar metals. You actually have three or more metals with the iron or steel vessel.
Iron, Fe - lies between them on the chemical chart and is less reactive (by a long shot) from Aluminum. So looking at the Electrochemical Potential Series :
Vo/V values : Iron = -0.04 Magnesium = -2.37 Aluminum = -1.66
Cast iron = -0.18
So normally there is -2.33 difference between the Mg and Iron and the ionic metal migrates from the Magnesium to the Iron, but goes into solution.
When the Al is added, the difference is -1.62 to the Iron (Al goes into solution or salts), and -0.71 between Aluminum and the Magnesium.
So in this case, Mg is giving to the Iron and giving to the Al. Al is giving also.
This information comes from the "Handbook of Physics" [ AIP press ] ISBN 0-387-95269-1 and can be gotten from
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where I got mine.
Martin
Jim wrote:
Reply to
Martin H. Eastburn

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