Well dang, if you can get a factory cast 316 elbow, why the F would you have one welded up??! It is rarely cheaper to get a custom part, but it's never a better deal...
Rust Never Sleeps.
- Thread starter Muttondressedaslamb
- Start date
For those of you with Yanmar engines, please note that this is a Catalina with an exhaust nipple in the pipe between the manifold and the muffler. It is completely different than the Yanmar exhaust "elbow". The OP did call it an exhaust elbow 'cept it ain't. 
Could it be that the rust is from water seeping from the exhaust raw water hose?
Could it be that the rust is from water seeping from the exhaust raw water hose?
Not sure of you pipe size , however if mine went I would replace it with 316 ss pipe from Mcmaster carr . It's difficult to get the Y for water Injection . In that case you buy a 316 Y strainer and gut it , you now have the proper Y angle and no welds and because all the pieces thread together ,they are all replaceable. Also you can buy the ss fitting hose barb to thread into Y strainer ,it can also be bushed .
The corrosion (rust) is from iron on the surface of the stainless. Could be the welder used a brush that had been used on carbon steel or used the wrong rod. To stop the rust, you need to remove the rust and trace iron. Brush with a STAINLESS steel brush and try removing the balance with naval jelly followed by more brushing.
Over the years it seems that the Yanmar Exhaust elbow has been a plague on boats so powered.
This causes me to wonder. Is this a design issue from the Yanmar engineers, or is it something else. That is probably an issue for another thread.
I suspect the exhaust elbow used in this "rust" question was fabricated using 304 Stainless. This is a more malleable and a less expensive stainless steel. It's use in this case is questionable (my opinion).
Stainless steels gain their properties by the mixing of certain alloys into the steel displacing the Iron and Carbon content. The addition of chromium in 304 stainless gives the material it's non corrosive properties.
This question dragged me down the extensive hole of (welding 101 what material to use).
In the term of properties of the steel, 304 has an issue when being welded. It is "Sensitization". Fancy word... what does that mean?
My understanding, the temperatures used in welding are in the 800 to 1600°F range. Turning the chromium in the metal boundaries of the weld join to Chromium carbide then subjecting this join to a continuous flow of heat and salt water is bound to be fertile ground for rust formation.
Sure the idea of a welding shop doing a budget job replacing the damaged exhaust elbow is extremely tempting for a boat owner.
Perhaps this is why the parts being sold (at premium cost) for Yanmar engines appear to be molded material not welded material.
It would also appear to support why the parts that are welded out of 316L stainless hold up longer when used as exhaust elbows. The make up of 316L stainless includes another element Molybdenum. This contributes to the 'reduced' corrosion properties of 316L stainless steel. Welding is a lesser problem. If you take the time to explore the linked "Stainless Steel Welding Guide" you will find ideas on how to achieve a more corrosion resistant welded exhaust elbow.
This causes me to wonder. Is this a design issue from the Yanmar engineers, or is it something else. That is probably an issue for another thread.
I suspect the exhaust elbow used in this "rust" question was fabricated using 304 Stainless. This is a more malleable and a less expensive stainless steel. It's use in this case is questionable (my opinion).
Stainless steels gain their properties by the mixing of certain alloys into the steel displacing the Iron and Carbon content. The addition of chromium in 304 stainless gives the material it's non corrosive properties.
This question dragged me down the extensive hole of (welding 101 what material to use).
In the term of properties of the steel, 304 has an issue when being welded. It is "Sensitization". Fancy word... what does that mean?
"Sensitization leads to intergranular corrosion in the heat affected zone as shown in Figure 1. Sensitization is caused by chromium carbide formation and precipitation at grain boundaries in the heat affected zone when heated in the 800 to 1600°F (427 to 871°C) temperature range. Since most carbon is found near grain boundaries, chromium carbide formation removes some chromium from solution near the grain boundaries, thereby reducing the corrosion resistance of these local areas."
My understanding, the temperatures used in welding are in the 800 to 1600°F range. Turning the chromium in the metal boundaries of the weld join to Chromium carbide then subjecting this join to a continuous flow of heat and salt water is bound to be fertile ground for rust formation.
Sure the idea of a welding shop doing a budget job replacing the damaged exhaust elbow is extremely tempting for a boat owner.
Perhaps this is why the parts being sold (at premium cost) for Yanmar engines appear to be molded material not welded material.
It would also appear to support why the parts that are welded out of 316L stainless hold up longer when used as exhaust elbows. The make up of 316L stainless includes another element Molybdenum. This contributes to the 'reduced' corrosion properties of 316L stainless steel. Welding is a lesser problem. If you take the time to explore the linked "Stainless Steel Welding Guide" you will find ideas on how to achieve a more corrosion resistant welded exhaust elbow.
BTW, the engine on this vessel was a Universal M25XPB which is basically a marinized Kubota farm tractor engine
Thanks for the heads up on that one. While the engine is running, I keep the companionway, as well as the bow hatch open, allowing constant air circulation. Little if any carbon monoxide should be present. For whatever its worth, if you and your significant other are having a candlelight dinner on Valentines Day, you are taking in a bit of this gas
@Muttondressedaslamb Sorry about the dispersion of your Universal engine, suggesting it was a Yanmar.
This does not change the message that a 304 stainless welded exhaust elbow is a problem for all Diesel engines using salt water to cool the exhaust.
This does not change the message that a 304 stainless welded exhaust elbow is a problem for all Diesel engines using salt water to cool the exhaust.
Should I make a stink about it with the yard that made the piece, get the motorcycle fabricator to re-weld it, or cover the joint with extreme heat JB weld epoxy putty?@Muttondressedaslamb Sorry about the dispersion of your Universal engine, suggesting it was a Yanmar.
This does not change the message that a 304 stainless welded exhaust elbow is a problem for all Diesel engines using salt water to cool the exhaust.
What makes 316L desireable for welding is the "L" which signifies low carbon. Less carbon at the grain boundaries (and elsewhere) means that less chromium is sucked out of the alloy during welding and remains in the elemental form to act as a detterant to corrosion.
I can remember one lecture in which the prof. wrote on the board:
and stated "and that's where the chromium goes. Any questions ?"
As chromium carbide is being formed during the welding process, the carbon knocks out a lot of chromium.
I wouldn't bet the farm on any one of those ideas. The original mixing elbow is just wrong right out of the gate
.If it were me, and you're handy with a hack saw and grinder, and wanted something that would last for a long, long, time, I go about purchasing the parts in 316L SS and then fabricating them to the required dimensions. Get the motorcycle fab. to do the welding. OR if the motorcycle fab. is so inclined, get him to do the cutting and grinding. But it should be YOU that purchases the 316L to ensure you have the right material from the start.
Since I found that link I posted earlier about the Westerbeke nipple assembly, I've been thinking a lot about this. My experience (I've replaced three of them, I know these things
), I often think that you may be over reacting to what you're seeing. That's Option 1. Keep an eye out on it, don't slather anything else on it because if it was my boat, I'd definitely WANT to see what's there and keep monitoring it, rather than hiding it. I am fortunate: I can open the door under my head sink and see it clearly. Always a b*tch to work on, though, right?I don't think making a stink of it will help them to help you. Psychologically, it's backwards, you want help, not alienation. A re-weld seems it could only make it worse, perhaps even weaker. Covering it may be counterproductive, see Option 2.
Option 2 - You think it's leaking, and want to seal it was best you can. While JBWeld is OK stuff, I have found Devcon to be far superior, a tad more expensive. I posted a photo of the Devcon I bought, a package of two tubes epoxy mix, 50/50. Easy stuff, stronger and good for slathering/sealing, as well as seating. Do an advanced search with my name and Devcon, you'll see it. ~$14 CAN.
Option 3 - Do option 2, then wrap in muffler tape. You won't be able to see it, but that's where they ALL crack and go to die, so anything to strengthen it wouldn't hurt, but only if you feel it needs it. Wouldn't hurt if you do, and it's where it needs more support.
Option 4 - Given your situation, I recognize you've replaced it already. Other than the confusion of the Yanmar "apologists"
, now that you're good at it, this may be the best time to get one of those prefabbed nipples or any of the other suggested methods. I've seen all kinds. One of the funniest ones was a pipe T with a nipple screwed into another fitting on the T! Pretty cool solution. I doesn't have to come in at an angle, the water is under pressure anyway.Whatever you choose to do, good luck. I wrote a tech note in a Mainsheet a few years ago when I both replaced my muffler and the exhaust riser. If you'd like the article, send me a pm, but I think you already know how to do it!
Fair winds.
" It doesn't have to come in on an angle" I totally disagree . Spraying it straight in thru a Tee it hits the back of the pipe and sprays up and down , this could send water into the engine. Sending the water down under pressure in a wye creates a negative suck vacuum ( Venturi) on the exhaust side of the engine helping exhaust to move and mix with the water. That's why it was made and engineered that way.
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