Need help on anodes, corrosion, stray currents, etc.

I noticed some pretty bad corrosion on my two Frigoboat Danfoss compressors and the related lines that are keeled cooled. They are behind a corner and I haven’t looked at them in awhile, and I was pretty shocked how bad they were rusting. The copper lines going to the keel cooler were also pretty heavily corroded.

There are two zincs on the keel cooler for these compressors. My diver that comes once per month is supposed to be replacing the zincs, but when I talked to the actual diver doing the work recently he said he can’t see anything down there and is doing everything by feel, so now I wonder if all my zincs have been depleted for awhile. All the zincs were replace last year at my last haul out, and the boat was in fresh water for 3 months of that year and the Bahamas for a month. I should have looked at the zincs in the Bahamas I realize now. I have been in this marina for about 8 months.

I’m scheduled for a haul out tomorrow morning so I can check all the zincs and just do a basic inspection, and in the between time I started doing some testing with my silver cell and multimeter. I put the electrode for the cell to the negative on my multimeter and tested some things with the positive electrode. Here are the DC voltage readings to some things I tested.

Bonding system -232mv
Copper wires running from the compressors to the keel cooler -232mv
Seacocks -232mv
A transducer for my Garmin system -125mv [emoji15]

According to this table, the readings should be in these ranges. I’m not familiar with galvanic corrosion or much concerning electrical so maybe I’m thinking about this wrong, but if it’s less negative (higher reading) don’t I have a problem? Also, how do I measure if there is a stray current at my marina? There are some pretty crappy steel hull boats a few slips away from me so I wonder about that. How do I test for that?

I found my Nigel Calder book and did some more testing and reading.

So it appears that the zincs connected to my bonding system are producing -232 mv of “protection” for other the other metals that are underwater— stainless, bronze, whatever the prop is made out of, etc. According to Calder’s table with the galvanic series of different metals, each metal needs about 200 mv more than its stated value or it will start depleting. For bronze with a -260-290 mv value that’s means it needs a metal nearby of at least -460 to -490mv to be stable. So my -232 reading on the bonding system is not getting it done, so I assume the zincs are depleted (or not making contact with the bonding system maybe).

To experiment I bought a big 4lb zinc, hung it over the side and connected it to the bonding system. That immediately raised the reading at the bonding system to -730 mv from the previous -232mv.

So, did this lack of zinc protection actually cause my steel compressors and copper lines to corrode? Copper lines have a value of -300 to -500 mv value and need -200 mv below that to prevent corrosion, so my bonding system was way below that. Steel is even less noble. So I guess it is a possible since that system is connected underwater and is contact with more noble metals. Could it be that my expensive refrigeration system became the zinc?

Anyway, someone please chime in to tell me if I’m looking at this correctly or not. I may have it all ass backwards.

I do think you need more anodes to get you into the acceptable range. I am not sure that the lack of anodes would cause the rusting on the outside of the A/Cs. It would probably be causing your underwater metals to be eroding away. The exterior rusting on your A/C is probably being caused by high moisture or a salt water spray from maybe the shaft logs.

Comodave said:

I do think you need more anodes to get you into the acceptable range. I am not sure that the lack of anodes would cause the rusting on the outside of the A/Cs. It would probably be causing your underwater metals to be eroding away. The exterior rusting on your A/C is probably being caused by high moisture or a salt water spray from maybe the shaft logs.

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The compressors are located in the generator “garage” along with the batteries and some other electrical equipment. None of that seems to be corroding, so it’s a mystery. There is a vent to the outside so the gen can get air, so I guess maybe it gets too humid in there. It does get hot in that room when the gen is running so maybe it’s condensing the salty moisture in the air?

I’m hauling out tomorrow to replace the zincs and check some stuff. Hope I still have a propellor left!

Looks more like surface corrosion from humid and salty air. I don't think a bonding systems does much if anything to prevent that type of corrosion. The only thing I can think if to prevent it would be to spray the equipment every once in a while with a protective coating like boeshield. We have some equipment in our engine room, such as the tank for our air compressor, which looks much the same. Not pretty but seems to be working fine.

Carolena said:

Looks more like surface corrosion from humid and salty air. I don't think a bonding systems does much if anything to prevent that type of corrosion. The only thing I can think if to prevent it would be to spray the equipment every once in a while with a protective coating like boeshield. We have some equipment in our engine room, such as the tank for our air compressor, which looks much the same. Not pretty but seems to be working fine.

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Well it’s damn embarrassing that I let them get that bad. I looked the compressors up and each one are like $1000 each. Sheesh.

Any of these look similar? Its just a simple google search using danfoss compressor.
https://www.google.com/search?q=Dan...gc3bAhVwqlkKHfEQBXkQ_AUICigB&biw=1708&bih=789

Yeah that’s a possibility, just getting the compressor. I was looking at the whole setup. https://www.coastalclimatecontrol.c...ooled-K50F-with-manual-speed-board/p/95699136

These frigoboat systems are nice and efficient and all that, but if they fail I’m not sure I’ll spend $2000 plus to replace them.

I don't see anywhere in your postings about your Frigoboat system not functioning correctly, only that it suffers from "corrosion" and looks unattractive. BTW, that "corrosion" looks like salt water rusting; best fix whatever that leak is before your electrical connects get shorted. If your Frigoboat system is still functioning correctly, no need to change any components.

Anna

It’s all still working. I cleaned the exterior of the system off and sprayed some rust/corrosion inhibitor on it and the terminals.