New Exhaust Elbow - Cast Iron or Stainless?

[Compass790]

Originally Posted by Cliffhanger

I would avoid stainless steel for one simple reason, galvanic corrosion. Alumiuium and carbon steel are close to each other on the nobility chart but the stainless steel is far more noble. So, yes, it won't corrode but it will encourage the rest of the engine corrode instead.

https://www.jlconline.com/how-to/ext...vanic-metals_o

The exhaust elbow will be bolted to either a cast iron or aluminium exhaust header (often with heat exchanger housing). Then add in a nicely heated Hydrogen Chloride solution as a electrolyte (i.e. salt water) and you have created the perfect conditions for galvanic corrosion.

So, your exhaust elbow might still look shiny and new in 5 years time (on the outside at least) but so what if you have to replace the entire heat exchanger instead $$$$$$$$$$$$$$$$$$$$$$$

In the OP's case he has a cast-iron ( mostly) raw water cooled engine with no heat exchanger so that warning doesnt apply. Have been running ss exhaust on our cast-iron ( mostly ) raw water cooled yanmar of a close era to OP's for about 6 years IIRC & no corrosion issues & no increase in anode consumption. Not looking for an argument as I'm no galvanic corrosion expert but just relaying experience.
I certainly would not ever use our style of Yanmar cast iron elbows even if I could afford one. Guess they'd be fine if you replaced every 4 years as recommended by Yanmar.

[ftamburrano]

Cast iron rusts quite quickly. Often, the hot water ducts get progressively plugged by growing deposits, rust etc. I recently changed the original Volvo-Penta steel part with a stainless steel one. I could not note any change in anything (noise, water flow at the exit...). I know it will last longer than the original, which had 31 years of life ... but was partly plugged.
Stainless steel is the perfect replacement for the original, but it should cost much more. Make sure the SS one is an exact copy of the cast iron one (in that case, you won't probably need any other during the lifespan of your engine); otherwise, if there is a change in design, exert your critical mind to judge whether the new design is better (allows good or better flow) than the old one. In any case, I would bet SS is anyway superior, by far. No doubt il will be a successful replacement. As in my case, so far.

[ftamburrano]

Originally Posted by Cliffhanger

I would avoid stainless steel for one simple reason, galvanic corrosion. Alumiuium and carbon steel are close to each other on the nobility chart but the stainless steel is far more noble. So, yes, it won't corrode but it will encourage the rest of the engine corrode instead.

https://www.jlconline.com/how-to/ext...vanic-metals_o

The exhaust elbow will be bolted to either a cast iron or aluminium exhaust header (often with heat exchanger housing). Then add in a nicely heated Hydrogen Chloride solution as a electrolyte (i.e. salt water) and you have created the perfect conditions for galvanic corrosion.

So, your exhaust elbow might still look shiny and new in 5 years time (on the outside at least) but so what if you have to replace the entire heat exchanger instead $$$$$$$$$$$$$$$$$$$$$$$

I do not see any possibility for Hydrogen chloride (HCl) to be present. The heated solution contains Sodium Chloride (NaCl), not HCl ! It would be industrially nice to be able to produce HCl just heating a salt (NaCl) solution!

[const0022]

Originally Posted by Homer Shannon

I was working in the engine compartment of my boat today and was surprised by the amount of rust on the back of the engine and evidence of water everywhere. A closer inspection turned up a cracked exhaust elbow.

The engine is a Yanmar SB12. I replaced the elbow in 2005 - though it seems like I just did it. I guess sixteen years is a reasonable lifetime, even if I do only put a hundred hours or so on the engine each season. I paid $120 for the cast iron elbow I purchased in 2005 from Torrensen Marine. TOAD, which evolved out of Torrensen, wants $450 for the same part today.

It seems that stainless steel elbows are also available and at a much better price - $200 from exhaustelbow.com. Can anyone comment on the stainless elbows? I would suspect that they are less impacted by corrosion than cast iron. Are they noisier or hotter? Are there other issues with them that I should be aware of?

There is an excellent way to extend the life of an exhaust elbow which I used successfully on my Bukh. Paint the inside with several coats of epoxy paint.
I had the boat engine for 27 years and only replaced one exhaust elbow, then replaced with painted one (cast iron) I taped up one end, rolled the paint around inside, poured out excess and left to dry. No further problems.

[StoneCrab]

I'd like to see a picture of the installation and if the crack. I suspect that the cracking is being caused by some stress put on it.

Do you have any flex joints in the system or is the motor vibration being transferred to the exhaust through this elbow?

[Darjee]

Hi to you all!
I have replaced my exhaust pipe on a Volvo 2030C by a cast 316SS equivalent, supplied by parts4engine.com in UK. I'm very satisfied. The cost was less than half the price quoted by volvo penta France for a cast iron spare. These people manufacture for various engines including Yanmar. The quality is very good, they are very professional, and they speak english! But this was before the brexit. Do not hesitate to choose cast 316 SS.

[Frankly]

"There is an excellent way to extend the life of an exhaust elbow which I used successfully on my Bukh. Paint the inside with several coats of epoxy paint."

A novel and practical alternative approach to the problem.

On the GM series Yanmars the mixing elbow is attached to an exhaust adaptor which is bolted to the heat exchanger. The water from the heat exchanger passes through a rubber hose (with siphon break) to a port on the side of the mixing elbow where it is injected into the hot exhaust stream. That is enough galvanic isolation that just don't think the SS corrosion applies in this case, but time will tell.


Frankly

[SOLAR SUPPORT]

Cast iron elbows are not reliable since they get rusted very quickly. I replaced my VP D1-30's original iron cast elbow with the one I produce by myself using 316L welded pipes. Sea water coolant splash back to aluminium exhaust manifold since the original iron cast elbow is short. I made the SS 316L elbow to myself 50 mm longer. I am using it for three years. No clogging, no rust. Engine's volumetric performance is very good. No black&white smoke, high rpm always.

Go with SS and don't listen reports telling it will cause galvanic corrosion. If you have a siphone attachment installed between the seawater coolant pump and the heat exchanger you don't need to wory about the galvanic corrosion in the heat exchanger, because all sea water coolant in the heat exchanger and exhaust elbow are drained in to the muffler. When you stop your engine the air taken through this siphone in to the sea water coolant pipe helps draing all of the sea water coolant going to the heat exchanger. No electrolite, no galvanic corrosion. 👍

[Sailor 99]

Originally Posted by BillKny

This is not at all true for SOME of the smaller Yanmar engines.

In 16 years you would be lucky if you ONLY need a half dozen exhaust elbows. They are considered routine maintenance parts, are called out as such in the maintenance schedule by Yanmar, and it really is true. A couple hundred hours, and they are toast.

I'll leave it to others to debate if this is a design flaw or not...

When the exhaust elbow goes I have made a new one out of pipe and fittings from home depot. 3 90s and a tee and a couple of nipples. The only question is can you attach it to the flange at the exhaust manifold or can you make something up. This will last for years and costs $20. Use galvanize fittings if you can

If I recall the yanmar has an nipple and thread

[Dieseldude]

Originally Posted by Homer Shannon

I was working in the engine compartment of my boat today and was surprised by the amount of rust on the back of the engine and evidence of water everywhere. A closer inspection turned up a cracked exhaust elbow.

The engine is a Yanmar SB12. I replaced the elbow in 2005 - though it seems like I just did it. I guess sixteen years is a reasonable lifetime, even if I do only put a hundred hours or so on the engine each season. I paid $120 for the cast iron elbow I purchased in 2005 from Torrensen Marine. TOAD, which evolved out of Torrensen, wants $450 for the same part today.

It seems that stainless steel elbows are also available and at a much better price - $200 from exhaustelbow.com. Can anyone comment on the stainless elbows? I would suspect that they are less impacted by corrosion than cast iron. Are they noisier or hotter? Are there other issues with them that I should be aware of?

Selecting stainless alloys of the best composition involves a bit of technical research, but with good info sources, is not difficult. Just found a great link that concurs with "Metal Corrosion in Boats" By Nigel Warren, copyright 1980 ISBN 0 540 07397 0.


https://www.gerrmarine.com/Articles/MarineMetalsPt1.pdf



Welded stainless marine fittings require much scrutiny, and full assurance that correct materials and methods were used in their fabrication is required. Chromium depletion occurs near the weld seam in common alloys, leading to corrosion.


A weakness of stainless steel is oxygen depletion in the water that they are subject to. Components in systems that have reduced or no circulation lose the oxide layer that forms their protection from corrosion. Systems shut down and laid up for extended periods will corrode unless drained, fresh water flushed, and drained of flush water. Joints between stainless steel and other alloys will be subject to galvanic corrosion. I have seen a cast iron sea strainer housing that was connected to stainless steel pipes reduced to the carbon component of the iron to a depth of several millimeters, making it useless. The surface went chalky soft.



Galvanic corrosion can of course be reduced by fitment of cathodic protection. Zinc anodes must be fitted at intelligently decided points. Welding in pipe nipples to fit threaded anodes might only compound the problem due to chromium depletion next to the weld. Common anode plugs are made of brass into which the zinc rod is pressed or threaded. This introduces a third alloy into the chemistry. Will the zinc mitigate this? Maybe someone can offer their experience with brass anode plugs in stainless steel. Stainless steel plugs would be a better choice.


I'm sure that you be able use this general information as a starting point to help decide on your specific application.

[walt41]

Read the Yanmar manual regarding running engine at high RPMs prior to shutdown. This will drive water from the exhaust line thereby reducing corrosion and extending elbow life.

[Kerryf]

I got one made by Deangelo Exhausts in Ft Lauderdale. Its made from AL-6XN (UNS designation N08367) superaustenitic stainless steel alloy.

Its a type of weldable stainless steel that consist of an alloy of nickel (24%), chromium (22%) and molybdenum (6.3%) with other trace elements such as nitrogen.

Its the only material to make a wet exhaust from.

[saillr]

My Island Packet 40 is 25 years old. The factory elbow rotted through at the water injection point. I replaced with a bronze elbow from a third party Mfr. Don't remember who. Later I discovered that Yanmar offered a bronze elbow of their own. Mine is now 20 years old and going strong.
SS is subject to crevice corrosion if it finds a place to lie. Bronze does nor have that problem.

[Compass790]

Originally Posted by saillr

My Island Packet 40 is 25 years old. The factory elbow rotted through at the water injection point. I replaced with a bronze elbow from a third party Mfr. Don't remember who. Later I discovered that Yanmar offered a bronze elbow of their own. Mine is now 20 years old and going strong.
SS is subject to crevice corrosion if it finds a place to lie. Bronze does nor have that problem.

Yes no argument good bronze is the way to go but finding one will be OP's problem.

[captchetco]

A point seldom noted. 300 series SS does not do well when subject to very High heat and pulses or repeated hammering. This is just what an exhaust system is subject to. If it is not subject to load, and always has water going thru with the exhaust, it should not be a problem, but be aware it can fail catastrophically if in the wrong environment. I found out when I replaced a bronze riser pipe with 316 SS it failed after 18 months. Replaced with cast iron which has lasted 22 years. About the same level of use.