Ideas to stop corrosion in aluminum heat exchanger?

[fourlyons]

I got no response last time I posted this so I'm going to try again.

I recently bought a older, used Nanni 3.75he, it is a 21hp Kubota block.

The heat exchanger was leaking antifreeze slowly from below the exit cap for the tube stack. I discovered an epoxy repair, that has failed, to an area of corrosion in the aluminum just inside the bore where the exit end of the tube stack is capped. The corrosion is inside the freshwater chamber, again in the aluminum, so it shouldn't be due to saltwater. The tube stack is a copper alloy, but it is, in theory, isolated from the aluminum by the o ring and paint. Plus, there is no corrosion at the other end of the tube stack where it sits in an aluminum bore.

I have ground out all of the corrosion, cleaned it, epoxied, and trued the bore. It no longer leaks.

My question is, why did it corrode, and how do I keep it from corroding in the future?

Is it possible it's as simple as the anti corrosion additives in the antifreeze went away because it wasn't changed often enough?

Should I add a zinc to the freshwater chamber, I didn't think antifreeze is an electrolyte, so one shouldn't be needed, right?

Thanks

[ItDepends]

Lots of possibilities... Maybe it was run with just plain water instead of antifreeze for example. Or stray current issues, or the saltwater side zinc wasn't changed when it needed to be (very common!).

Zincs on the fresh water side are not needed, and will not fix an issue here.

If at all possible, I'd replace the heat exchanger. Epoxy CAN work for a long time, but it is by no means a permenant repair. It will leak again. Witness the first epoxy repair that failed for you.

[Bowdrie]

You mentioned corrosion at the outlet end of the bore but not at the inlet end.
Without knowing the alloy that was used, my first guess would be impingement attack on the Aluminum.
In prior time might salt water gotten into the fresh water/anti-freeze side of the circuit?
Or am I misunderstanding the scenario?
An exchanger that is all copper alloys is to be preferred.

[Fishspearit]

Several things that could cause that corrosion. First, aluminum is amphoteric, meaning it can react to both both acids and alkaline substances. pH of the coolant going high or low can damage the aluminum. Also coolant IS an electrolyte, just not as good of one as salt water. Those end caps are a common corrosion point. I learned the hard way years ago myself that the heat exchanger end caps need to be removed, cleaned, o-ring inspected/replaced at least every other year.

I agree with ItDepends, your repair will likely fail as the repair before it did.

I would not add a zinc where one was not designed to be installed.

[fourlyons]

Thank you for the responses. I do plan on replacement, but I don't have time to get one before a planned trip. A Nanni exchanger for this engine is very hard to find, especially in the US. So I think I'm going to fit a Beta Marine exchanger, they sell one for this block, but I will have to change some hose's etc, and that will take more time.

[kiviana]

I d'ont think is easy to replace a nanni heat exchanger for diferent one except Nanni original.
Going back to your question: The cap heat exchanger you mention is loosing , has a O-ring that should be cheked regularly and replaced quite often ( 5/6 years ? ) otherwise a leak starts at that part and corrosion afect the shell of the heat exchanger that is aluminium, and time mekes the corrision worst. No way to stop the leak from outside Once that happens the wise solution is to buy a new heat excahanger from nanni, ( contact the nearest dealer or factory they are woe¡rld wide) t'hats an expensive solution , but is the only one I know ( we sail with a Nanni engine for 22 years now and got the same problem )

[Dougtiff]

AS already stated, buy a all copper exchanger and rehose, reposition, it doesn't need to be on the engine if it doesn't fit.

[Rumrace]

You can add Mercathode or similar device to redirect current chasing molecules of aluminum away. Make sure the water intake has no bottom paint on it. Bottom paint with tin or copper turns the painted aluminium into a sacrificial anode

[Lepke]

Your heat exchanger is probably acting as a zinc, protecting other metals and sacrificing itself.
I've added many zincs to small engines that stopped corrosion problems and didn't lead to other problems. Zincs are omitted on many small yacht engines for manufacturing cost considerations.
Or you can have a custom HE made from a more survivable metal.

[Rumrace]

Stainless steal sink brass drain zinc nut.
Electrical toggle switch with brass rubber coated nut outside zinc inside.
Anodes are not exclusive to boats just more urgently needed. Salt is a marvellous conductor and we should be producing electricity from it passing through not leaving a trail of aluminum oxide behind.
Use em if you got em.

[Rjt]

A cheaper solution to replacing the aluminium housing, is to have it welded and machined. Had mine done twice now. Spare parts pricing on Volvo’s is horrible. Welding is cheap.

[Rumrace]

I’m surprised more parts are not made of plastics or at least isolators.
BMW starting making radiators tanks and necks. In the factory class it’s fine but the don’t survive M series. The plastic was short lived. The factory switch to a new plastic. ASA PEEK and other plastics sure make a lot more sense than aluminum. They are stronger than cast aluminum.
I should try printing one. I use a lot of ASA but think PEEK would be better.