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Old 03-22-2014, 01:29 PM   #1
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Question about corrosion?????

I have a 671 Detroit with fresh water cooling that is developing a white powder around the brass threads of the zinc plugs and around the rubber gaskets on either end of the heat exchanger. I changed out the antifreeze and carefully mixed up another batch of 50/50 but still see the problem. Any ideas?
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Old 03-22-2014, 03:38 PM   #2
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Could be a very small leak. You can see white on my zinc and also on the coolant side connections on top. You cant see it but the other end of the HX has a white ring at the gasket.
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Old 03-22-2014, 03:59 PM   #3
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Hi Quicksilver.

Regarding the "white powder" on both the zinc plugs, and the end caps of the heat exchanger, it's hard to say what it might be, other than either a product of corrosion between the parts and the CU-NI heat exchanger, or salt precipitation from the raw salt water circulating on the cold side of the heat exchanger. As the raw water absorbs heat from the fresh water on the hot side of the heat exchanger, the salt tends to evaporate out and settle after shutdown almost like a high-tide mark on the beach inside the heat exchanger. In some cases, it can form a bridge clear across the intake. This might be showing up as your "white powder".

I would therefore recommend removing the inspection covers on both ends of the heat exchanger, and determining the health of the exchanger itself. You may find this salt precipitate, products of the zinc erosion, other "stuff" possibly blocking the heat exchanger tubes. If so, the best fix is to remove and have all the heat exchangers on the engine professionally cleaned and pressure tested.

An interim do-it-yourself fix can be accomplished by first manually "rodding-out" the tubes with a small flexible probe. Next, rig a 500 gph bilge pump in the bottom of a bucket, plumbed to pump phosphoric acid into the raw water cooling system just downstream from the raw water cooling pump (remove the zincs first!), through the raw water system, and back to the bucket as it exits the engine enroute to the exhaust system.

Let this thing percolate, boil, and bubble for several hours, and much of the "stuff" that lives in the raw water cooling system will end up in the bucket. Use a new gasket on both ends of the heat exchanger after re-assembly, re-attach your raw water cooling hoses, fire the engine for a few minutes, and you should be good to go, for a while at least.

Sorry to be so wordy. Hope this info helps.

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Old 03-22-2014, 05:12 PM   #4
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You sure they are zinc ? On a recent visit to a chandlery I saw boxes of anodes and most of them were not identified but some were marked "made in China". I found zinc, magnesium and aluminum anodes all mixed in together.
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Old 03-22-2014, 06:32 PM   #5
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A little "weepage" through the NPT threads will cause corrosion product/minerals to show up at threads. Not a big deal. Take the plug out and clean threads and put a little sealant on threads. That will seal it up. A NPT tap will clean the internal threads, a wire wheel to clean the plug.

End caps on HX, same thing: minor leaks cause corrosion product to show up. Take apart, clean mating surfaces, put together with new gasket.

Changing coolant has no effect on this, that is on the fresh water side. Zincs are on the sea water side.
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Old 03-22-2014, 07:04 PM   #6
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Thank you for the great ideas. I will get to work on it.
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Old 03-22-2014, 08:25 PM   #7
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Quote:
Originally Posted by Ski in NC View Post
A little "weepage" through the NPT threads will cause corrosion product/minerals to show up at threads. Not a big deal. Take the plug out and clean threads and put a little sealant on threads. That will seal it up. A NPT tap will clean the internal threads, a wire wheel to clean the plug.

End caps on HX, same thing: minor leaks cause corrosion product to show up. Take apart, clean mating surfaces, put together with new gasket.

Changing coolant has no effect on this, that is on the fresh water side. Zincs are on the sea water side.
What kind of sealant do you recommend for the zinc plugs?

Usually none is recommended but as long as you get continuity I don't see it as a big deal either.
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Old 03-22-2014, 08:42 PM   #8
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I use Rectorseal #5, but Ace TFE paste or even permatex#2 works well. Permatex is more likely to make a mess, though. A quick check with a digital voltmeter- one probe on plug head, one on HX, if zero that verifies sealant has not insulated the threads. Don't check with ohm setting, use meter in mVdc range. I've never seen one insulate, so I have quit checking.
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Old 03-22-2014, 10:11 PM   #9
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Quote:
Originally Posted by Ski in NC View Post
A little "weepage" through the NPT threads will cause corrosion product/minerals to show up at threads. Not a big deal. Take the plug out and clean threads and put a little sealant on threads. That will seal it up. ...
I was told that for electrical continuity, it's best not to seal the threads with dope or tape. Doing so may isolate the zinc from the rest of the boat, defeating the effectiveness of the sacrificial anode.

Have you or others heard differently?

(edit: just saw your reply above.)
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Old 03-22-2014, 11:05 PM   #10
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Greetings,
Dielectric grease?
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Old 03-23-2014, 01:26 AM   #11
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I have used sealers on zinc threads for years. I now use Rectorseal #5 but have used other stuff.
I wondered about conductivity but a quick check with the meter over many time put that to rest.
The white powder is no more. It shows up as a result of tiny sea water weepage past the threads or gaskets as pointed out. When the water dries the salts are left as a crust.
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Old 03-23-2014, 07:32 AM   #12
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I'm starting to think the no sealant/tape is just another urban myth that's passed along by pseudo-experts (meaning book and magazine article writers).

I have wondered for years about as long as there's great continuity (and it has to be near zero for the zinc to work) what should it matter???

I used to wonder about Dielectric grease until after much research it was generally agreed from sources I thought were trustworthy that said after tightening most connections the dielectric grease would not prevent metal to metal contact but would be squeezed out to become a corrosion barrier.

So why not zinc plugs?

But I still don't use it as the little weep around the zincs and other saltwater components doesn't reall bother me as it has no effect on performance or longevity if the engine is regularly serviced and wiped.
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Old 03-23-2014, 09:47 AM   #13
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OK, let's do a little test. It doesn't take long for this stuff to reoccur after cleaning. There is one plug on each side of the HX and the two end caps. I will try a Dielectric on one plug, Rectorseal #5 on the other plug, Permatex on one end cap, and nothing on the other end cap. It may take a couple weeks but I will post the results. Thank you for all the info.
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Old 03-23-2014, 11:49 AM   #14
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Whatever you choose to put on a pipe thread, when seated the threads are metal to metal. The sealant fills voids and minor "valleys".
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