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S&S Swan General - 040/12 CYGNUS Heat Exchanger
03 June 2008 - 21:29
Join Date: 31 July 2007
Posts: 86

040/12 CYGNUS Heat Exchanger
My boat has the Perkins 4-108 engine. I'm very happy with it: In all these years it has always performed well. It has a BOWMAN heat exchanger and I find corrosion in the header tank where the coolant/antifreeze is. The corrosion eats away the aluminum where the tube bundle rests at both ends of the tank. Inside the tube bundle is seawater, but on the outside is coolant. That should not promote corrosion. The oil cooler portion has no corrosion at all. The evidence shows that it is the coolant that causes corrosion of the aluminum. I have heard that owners replaced the header tank, but I can't get any reply from BOWMAN in England. I used a filled epoxy to re-establish the sealing surface damaged by corrosion. I pressure tested the assembly using air at 4 PSI. Would appreciate any comments by owners who have the same problem.

04 June 2008 - 15:32
Join Date: 30 January 2007
Posts: 447

I have a 4-108 too but, apparently, with a different heat exchanger setup from yours.
I am quite interested and would appreciate more precise specs of your system.

As I understand you have something like a "Combined heat exchanger, header tank and oil cooler" like the one enclosed that I found on the Bowman site.
My system has three separate units.

Besides that, I am surprised too that you have corrosion created by the coolant unless the glycol percentage was not sufficient or the solution was made with brackish water.
A fix can be made with epoxy filler as you did. You may even find special epoxy compounds with aluminum powder in them. Sometimes you have to machine the fixed part afterwards but if the seal is good, great! It will last forever.
Several times I fixed successfully corrosion damages of non-hot parts in this way (a good rule of thumb is: less than 50°C).

Daniel, 411/004 Luna Menguante

05 June 2008 - 05:20
Join Date: 31 July 2007
Posts: 86

Attached is a picture of this engine with the heat exchanger installed. I talked to an expert on galvanic corrosion. He told me that antifreeze does not prevent galvanic corrosion because it is a conductor of electricity. I made a test: In a small plastic container I put some antifreeze and inserted a strip of brass and a strip of aluminum separated by about 40 mm. Then I applied 12 volts from an automotive battery and measured the current between the two materials: I measured 90mA. Using just plain water instead of antifreeze, I measured 10mA. In the BOWMAN heat exchanger the tube bundle is copper alloy and the tank aluminum. Where the tube bundle rests against the tank there is galvanic corrosion. I am going to use plain water and add some water soluble oil, the same mixture I am using in my machine shop for cutting metal. In the winter I shall inspect again for corrosion. This forum is very valuable: I can talk to people like you who are very knowledgeable. Thank You!

05 June 2008 - 14:15
Join Date: 30 January 2007
Posts: 447

thank you, I take yours as a compliment to Matteo and me... :-)

Regarding your idea to use water and oil (in Italy it used to be called "old woman milk" for its color - not an elegant definition, I admit) it may not be safe: it is an emulsion, not a solution, and it will tend to separate.
I still believe that the conductivity comes mainly from impurities in the water and not from a possible ionization of glycol so I would suggest deionized water and the highest possible concentration of glycol.
But, alas! I am not a chemist neither an expert in galvanic corrosion...

I fully agree that the contact between copper and aluminum is particularly prone to galvanic corrosion but isn′t there an O-ring isolating and sealing the two parts?
Maybe the real problem comes from the O-ring of the wrong size or excessively squeezed so that it that allows electrical contact.
Moreover in my basically trustful attitude I would also tend to believe that the engineers at Bowman knew well what they did so the problem is unlikely to come from the original design.

This discussion is very interesting and I thank you for starting it; I′d like also to hear more and maybe different opinions.
In particular I wonder why the cooling systems of our two 4.108 are so different and whether one is better than the other.
Mine has the standard header tank, a cooling-water/saltwater exchanger on the rear, an engine-oil/saltwater exchanger on the port side and a transmission-oil/saltwater just above the transmission.

Daniel, 411/004 Luna Menguante

05 June 2008 - 21:24
Join Date: 31 July 2007
Posts: 86

I'm familiar with "Old woman's milk". I used it sixty years ago. It turns white when mixed with water. The liquid I'm using mixes also with water at a 50:1 ratio, the mixture remains clear and does not separate. The water does evaporate and leaves a waxy film that protects against all corrosion. But - of course - only testing can determine if it will work. I also have a heat exchanger installed on top of the transmission, just like you. There is no corrosion because it circulates ATF. I didn't mention it because it is not part of the problem. I'm not so sure about engineers always knowing what they are doing, being an engineer myself. Often there are marketing considerations that influence a design decision. In this case, fabricating the header tank out of copper-nickel would have been expensive compared to a sand-casting of aluminum. And the aluminum (aluminium to you) can be expected to last 15 years or so. Attached is a photograph of testing the conductivity of a 50% mixture of glycol antifreeze and di-ionized water. You can buy that here in auto part stores ready mixed. The power source is a 12 volt battery. The meter shows 50mA. Amazing isn't it? The aluminum does not touch the copper but is about 4 cm away. In my heat exchanger the tube bundle is a lot closer to the aluminum housing, but I'm not applying a voltage. As to proper choices of materials: I found that the nut that holds the heat exchanger together was made from brass. It eventually de-zincified and failed. I had a horrible mess in the boat because the engine kept pumping oil into the bilge. I made a new nut from silicon bronze, it has been in there since 2001 and shows no corrosion.

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