looking for same ideas...
In the process of replacing an OEM exhaust mixing elbow on a yanmar genset. This is the 3rd replacement and the dealer admits that they only last about 5 years. I'm fed up with the $270 crap they sell so I am building one I hope I wont have to deal with again. The existing junk is a thin steel nipple into the cast iron mixing elbow. The nipple sheared completely off while the engine was running.
I'm making the new one with 1.5" NPT pipe fittings.
Considered bronze and I cant find any data to suggest its not good for this. Groco tells me their silicone bronze melts at 1800F so I think it will be fine for the dry nipple connection at the manifold. Never checked the temp there but I don't think its even close to that.
Stainless steel has a problem with pitting in an exhaust system and I have seen it with a previous thin SS setup on a genset. I will have no welds which is where the first problems seem to start. The Beta Engine guys I chatted with at the show said they use SS and never have any issues.
My idea so far is to go with 316 SS pipe fittings. 316 is not quite as strong as 302/304 but has better corrosion resistance. Since it's a single cylinder engine
I want a strong part that can handle the vibration. I found schedule 80 316ss nipples along with Hi pressure 316ss elbows. The elbows are rated for 3000psi steam. (not a typo). They have a 1/2" wall thickness and the schedule 80 pipe is about 1/4" wall thickness. I also sourced a 316ss hose barb for the 1.5" exhaust hose connection.
The thick-walled elbow may be overkill but will be good to drill and tap in a SS fitting for the water injection on the downside. The stuff isn't cheap but still less $$ than the OEM crap.
I may be wrong but I don't think dissimilar metals is a concern in the dry part of the exhaust (manifold to pipe nipple). None of it is immersed in liquid and any moisture is quickly dried by hot exhaust. None of my previous problems have been galvanic corrosion but rather hot saltwater/exhaust gas corrosion.
I replaced the aluminum elbow on my 62hp Perkins diesel only because it was corroding at the hose barb end where the wet exhaust was in contact. The dry upstream section was fine after 15 years.
I notice that lab testing of corrosion resistance on materials always seems to include hot saltwater spray.
OEM stuff for small diesels always seems to be cast iron and galvanized/black iron but the high-end big diesels now are touting SS exhaust systems. (I think all the premium car makers also use SS for exhaust)
happy for any input/experiences/opinions from all of you ....