Volvo MD2020 (and others) heat exchanger corrosion

[mikebz]

Investigating a salt water leak from my MD2020D (about a teaspoon of water soon after startup, and no more after that) I found it was wet around the exhaust elbow (S/S fabricated item from Keypart) and the lower RHS stud nut was very rusty. Removed the elbow and found the weld between the tube and the flange had cracked. That was minor compared to this:



After removing the studs and brushing off the loose bits:



The yellow circle approximates to where the hole in the gasket would be.



The notch is where I've attacked it with a file to find some solid metal. I wonder if the corrosion has been exacerbated by having a S/S elbow.

I think this is may be salvageable by milling around 7-8mm off and making a 7-8mm spacer to go between the housing and the elbow. If the spacer were aluminium and had a slot to match that in the housing rather than a circular hole then that would give some protection to the face of the housing, and the spacer could be considered sacrificial in and replaced when it has corroded sufficiently.

However... despite that fact that the engine was running fine I find that turning it over by hand there is good compression on #1, some compression on #2 (hissing of escaping air), and nothing discernable on #3. I've been told that #3 is prone to this being so close to the exhaust, the valves can rust. So next job is head off to have a look at the valves and valve seats.

Having trawled the internet I find similar issues with the heat exchanger housing on other Volvo engines (D2 55, MD2030) especially on the Bavaria owners forum.

So next time you check your exhaust elbow also check the state of the heat exchanger face that it mates to.

[Zanshin]

Wow - that's more corrosion than I've ever seen before!

[mikebz]

Not as bad as this one:

[Zanshin]

I'm going to have nightmares tonight !

[SY Racer]

Stainless steel elbows are a bad idea in combination with aluminium engine blocks. The electrochemical potential series of aluminum vs stainless (316) ist horrible compared to aluminum vs. cast iron, resulting in a greater risk of galvanic corrosion.  Basically you sacrifice the aluminum heat exchanger in favor of the stainless exhaust elbow.

See structx.com/Material_Properties_001.html for reference.

cu

[mikebz]

I know from decades of dinghy sailing experience that stainless fittings attached to an ali spar result in corrosion around the bolt/rivet holes.  Fabricated stainless elbows are an even worse idea because the weld fails due to heat cycling.  Parts4Engines sell a composite gasket to isolate the housing from the elbow so maybe a cast stainless elbow which is isolated would be OK.

The "not as bad as this one" picture I posted above was from a D2-55 with a cast iron elbow, and there are plenty more similar examples around the internet.

[SY Racer]

Isolating the different metals is common practice in shipbuilding with aluminum hulls. Every stainless bolt going through aluminum must be isolated with a plastic bushing and plastic washer, to prevent corrosion. And yes, I remember well the issues with rivet holes in Laser booms  .

[eoin2020]

Yeah unfortunately galvanic corrosion can be catastrophic and you probably know already but hasn't been mentioned above.  But the sacrificial anode within the heat exchanger is vital to help mitigate against failure. Unfortunately, not everyone knows there's one fitted there.

[SY Racer]

Unfortunately neither the Volvo Penta MD2020 nor the MD2030 have an anode in the heat exchanger. Volvo Penta categorized the HE as corrosion resistant. Perhaps the affected heat exchangers didn't get the memo.

[Trevor]

Thanks to all for contributing to this really interesting post. I've learnt a lot reading this post and become more aware (that means concerned ) about my own heat exchanger. I fear what condition it will be in when I am finally forced to confront it.