Adventures in heat exchanger corrosion

[Sailuke]

I recently bought a Catalina 27 with a 1986 Universal M-18 diesel inboard engine. while inspection the engine, i noticed the original 12" by 2" heat exchanger had succumbed to corrosion. the end cap, that is, the end that is not meant to be removable by screw with a gasket, had had its solder corroded and had fallen off completely. Its still intact, and not that far corroded, and i plan on replacing it with a newer 13" by 3" heat exchanger anyways. Buy for the sake of science i want to re solder the end back on. I suspect the material used to cast the exchanger is bronze, but im not sure. I know to sand and cleat the contact points and flux them, but what type of solder should i use? Would standard silver solder be ok in the marine conditions? Keep in mind that the solder joint will come in direct contact with salt water because of the curculation, but the zinc anode is less than an inch away from it. Any advice?

[Stumble]

It really depends on the alloy they used initially, but the biggest problem I see is galvanic corrosion. Most copper alloys fall around -.3 on the galvanic chart, where silver is -.1. This is far enough apart that they will react in salt water to cause problems, add in the high temprature of a heat exchanger and it's a foregone problem.

50/50 lead/Tin solder however is also -.3 and would be my choice from a galvanic standpoint. Though I know nothing about its structural suitability.

[Sailuke]

Originally Posted by Stumble

It really depends on the alloy they used initially, but the biggest problem I see is galvanic corrosion. Most copper alloys fall around -.3 on the galvanic chart, where silver is -.1. This is far enough apart that they will react in salt water to cause problems, add in the high temprature of a heat exchanger and it's a foregone problem.

50/50 lead/Tin solder however is also -.3 and would be my choice from a galvanic standpoint. Though I know nothing about its structural suitability.

hey thanks. I never even thought of galvanic corrosion. I live by the sea so i think ill try the lead/tin solder and leave it in a bucket of salt water and see if it corrodes. It does sound like itll work better than silver solder because silver solder is so soft so i would think it would decay faster but i really dont know.

[Cheechako]

Ask a radiator shop? If one still exists in your area.

[RaymondR]

Silver solder is much stronger than 50/50 but also requires a lot more heat which is a problem with large brass items.

[OS2Dude]

If it is going to be subject to any heat at all, I don't think the 50/50 will hold. I did Live Steam Garden Trains (Accucraft 1:20.3) for a while, and one of my saddest days was the day I fired up an engine I had spent the week detailing with soldered parts, only to have them all slide off (A few even melted) as the engine heated up...

Our Catalina 30 had a similar experience: A PO just covered the end in JB Weld and called it a day. BTW: Our engine (Universal M-25) runs MUCH cooler with the 3" exchanger.

[topmast]

It's been a few years but I always used a silver solder rod called Everdure .Easier than brazing,much better than solder.I used a soft oxy/acyt flame. Ask a local instaler of refrigeration .

[Nicholson58]

Originally Posted by Sailuke

I recently bought a Catalina 27 with a 1986 Universal M-18 diesel inboard engine. while inspection the engine, i noticed the original 12" by 2" heat exchanger had succumbed to corrosion. the end cap, that is, the end that is not meant to be removable by screw with a gasket, had had its solder corroded and had fallen off completely. Its still intact, and not that far corroded, and i plan on replacing it with a newer 13" by 3" heat exchanger anyways. Buy for the sake of science i want to re solder the end back on. I suspect the material used to cast the exchanger is bronze, but im not sure. I know to sand and cleat the contact points and flux them, but what type of solder should i use? Would standard silver solder be ok in the marine conditions? Keep in mind that the solder joint will come in direct contact with salt water because of the curculation, but the zinc anode is less than an inch away from it. Any advice?

There are specialty shops that re-build these. They will clean out, replace tubes, re-solder & pressure test. Ours are 3-1/2 diameter X 20 inches. Cost was about 60 bucks. I would have to be desperate to do this myself & I fix everything.

[Cheechako]

Originally Posted by Nicholson58

There are specialty shops that re-build these. They will clean out, replace tubes, re-solder & pressure test. Ours are 3-1/2 diameter X 20 inches. Cost was about 60 bucks. I would have to be desperate to do this myself & I fix everything.