Stainless Steel Exhaust Elbow- 304 or 316?

[CJ13]

As a supplementary to Jonathan’s recent thread, both AliExpress and Parts4Engines offer cast St St Exhaust Elbows. The photos look identical, but AliEx is grade 304, while Parts4Engines is grade 316.
1. does the grade matter in this application?
2. Is it likely that these elbows are made in different grades of stainless?

110.47￡ |Ap03 861906 21190094 Stainless Steel 304 Exhaust Elbow For Volvo Penta Md2010 Md2020 Md2030 - Exhaust Headers - AliExpress

Volvo Penta D2-40 stainless steel exhaust outlet kit

 

[dankilb]

It's an often-debated question on here. Posters often claim that 316 is overkill, unless for fittings permanently below the waterline. There is a genuine question as to whether the elbow would survive the risk of clogging long enough for the stainless itself to corrode sufficiently to recoup the difference between the two grades.

Interestingly, the owner of our local boatyard completely disagrees. For him, it's 316 only. I wind him up about this and last time his response was along the lines of "...you owners want to put components into one of the most extreme environments on the planet and then you complain about the cost (etc.) of 316..."!

 

[vyv_cox]

A difficult one to answer! 316 would definitely have better resistance to pitting corrosion than 304 but the original in cast iron lasts for many years. You can almost buy 2 x 304 for the price of 1 x 316, so I think I would go for the 304.

 

[Neeves]

A difficult one to answer! 316 would definitely have better resistance to pitting corrosion than 304 but the original in cast iron lasts for many years. You can almost buy 2 x 304 for the price of 1 x 316, so I think I would go for the 304.

We are buying the Aliexpress 304 version. It will arrive in about 2-3 weeks. It will be installed soon after.

For those of you with patience and good memories - I'll let you know how we get on, in terms of any installation issues and more important - was it, long term, a good buy.

We actually found the original manufacturer of this item but the Aliexpress offer is through the retail arm of the manufacturer. I am sure the price is negotiable - if you buy 100 units - but we are sticking to just one (I hope we don't need 100!)

Jonathan

 

[Neeves]

But related

We will be installing a 304 elbow to the exhaust manifold - with a gasket between elbow and manifold. The studs are steel and I was going to use stainless nuts, greased.

Are there any issues of which I should be aware, primarily the nuts and the studs - and will the grease last anyway/

Jonathan

 

[geem]

With regard to 316 or 304 I would never use 304 on deck never mind below the waterline. Some recently purchased 316 cleats and tubing rust like it's going out of fashion. Can you imagine how bad 304 would be. If I could get none rusting s/s for on deck use like my original s/s fitting when the boat was manufactured I would be delighted

 

[Daydream believer]

Whilst 316 wins on most things such as strength & heat resistence it is the quailty of the welding that will make the difference.
I imagine that the welding system & filler used will be where the corrosion might occur. Distortion of the metal during welding could lead to failure at a later date. So it is not just the material, but the quality of construction that counts.
I would not discount 304 though, as corrosion in the engine bay should not be a major issue. It is not as if it is exposed to the elements. Will the heat of the engine not leave it dry after use? Possibly, subject to minor condensation . What is the rest of the engine like?
The fact that it is below the waterline is totally irrelevant. It is inside the boat, not outside. My paper charts are stored below the waterline & they all seem to have survived Ok. But if you have a potentially damp boat, 316 may be worth considering.

 

[Tranona]

With regard to 316 or 304 I would never use 304 on deck never mind below the waterline. Some recently purchased 316 cleats and tubing rust like it's going out of fashion. Can you imagine how bad 304 would be. If I could get none rusting s/s for on deck use like my original s/s fitting when the boat was manufactured I would be delighted

But this question is about exhaust elbows NOT deck fittings. Different problem and as Vyv points out it is replacing what is normally cast iron. Rust is not an issue here - the iron originals last for years and even then corrosion is not always the cause of failure. stainless is commonly used and the question is whether the potentially greater resistance to pitting of 316 over 304 is worth the extra cost.

Agree with the issue of quality of welding. The 316 stainless end plate on my Volvo muffler failed at the weld, a very common problem. Like wise the common point of failure of the original Yanmar 1GM stainless elbow is the welding of the internal tube to the outer. almost impossible to check the quality of the weld on the physical product.

 

[dankilb]

The 304 Aliexpress one discussed above looks cast, not welded, to me. Otherwise agree on the advantages of 316L for welding.

 

[Tranona]

Yes, both are cast so it is only the grade that is different.

Neeves raises the important issue of isolating the face of the elbow from the heat exchanger which is aluminium. The original gasket does not do that very well and there are plenty of examples of corrosion on the HE flange like here (on a D2 55, but basically the same design) forums.ybw.com/index.php?threads/heat-exchanger-repair.556326/ .

 

[Neeves]

I don't know how they fabricate the flange in the cast steel version, the original Volvo elbow - but the flange inside or underneath the spigot corrodes. When I receive the 304 version I will give it more than a cursory look. We too have had the end plates on the muffler, I'm guessing 316 stainless, failing - the stainless tube and the dome shaped end plate look fine - its the weld that has failed. The Aliexpress 304 stainless elbow looks cast to me but I've only seen the same pictures that everyone else has seen - how they have fabricated the internal flange - maybe I'll find out.

The Aliexpress elbow is supplied with a gasket - what it is made from, no idea - it does not matter, I can make my own gasket. I made my last gasket from what looked like a very robust card - but made from what looked like aluminium flake. It was very difficult to cut. It could have been mica.....??

But whether 316 is better than 304 is a bit academic as the option is for 304. So the question is not is 316 better than 304 but is 304 better than cast steel (or cast iron?). Interestingly the original cast version does not fail from corrosion - and it is soaked with hot seawater every time it is used - the failure mode is that it blocks (or in my case it fails because I clear the blockage with acid and its the cleaning process that eventually causes failure - not hot seawater.

Tranona How did you solve your muffler problem - did you have it rewelded, buy a new one or move to a different design like a Vetus version?

The elbow will be an interesting exercise as I under stand that the same elbow is used on the MD20xx, the D1 and D2 (?) series of engines and the aliexrpess versions are cheaper than a cast steel version, from Volvo (though inconvenient to source).

Jonathan

 

[vyv_cox]

When stainless steel welds fail in corrosion the cause is often sensitisation, the formation of carbides due to not using the low carbon L version of the alloy. This is available in 304 and 316.

However, the Yanmar GM elbow welds fail in fatigue due to differential expansion of the thick flange and thin internal pipe. This part does not become wet, upstream of the injection point.

 

[earlybird]

Thread drift;when I looked to purchase a small Beta marine diesel, they were supplied, as standard, with an exhaust elbow cast from aluminium alloy. Presumably they survive reasonably well.

 

[Spirit (of Glenans)]

The 304 Aliexpress one discussed above looks cast, not welded, to me. Otherwise agree on the advantages of 316L for welding.

The body is cast, but it is very likely that the interior architechture is composed of welded-in components

 

[Tranona]

Tranona How did you solve your muffler problem - did you have it rewelded, buy a new one or move to a different design like a Vetus version?

Jonathan

Bit of both. The first one I had fail was about 7 years old and I believe was brazed - was not there when it was done by my mechanic in Greece. The latest one failed after 3 years and was replaced in the belief that it was covered by the 5 year warranty. However Volvo rejected the claim as it only covered the engine and transmission. I had the old one repaired by welding a plate over the external face around the spigot. Gave it to the new owner as a present. Would be good if Volvo supplied the aft end plate as a spare.

The problem is easy to see. The outlet spigot comes through the plate at an extreme angle and the inaccessible lower section of the weld is under seawater most of the time, usually cold and stagnant. Perfect conditions for crevice corrosion.

Yes, you can use the Vetus WLOCKR 45 which is slightly longer and higher so needs a bit of tweaking of the hose and a different attachment to the panel . Roughly half the price of the Volvo one and potentially far more durable.

 

[geem]

But this question is about exhaust elbows NOT deck fittings. Different problem and as Vyv points out it is replacing what is normally cast iron. Rust is not an issue here - the iron originals last for years and even then corrosion is not always the cause of failure. stainless is commonly used and the question is whether the potentially greater resistance to pitting of 316 over 304 is worth the extra cost.

Agree with the issue of quality of welding. The 316 stainless end plate on my Volvo muffler failed at the weld, a very common problem. Like wise the common point of failure of the original Yanmar 1GM stainless elbow is the welding of the internal tube to the outer. almost impossible to check the quality of the weld on the physical product.

Before I retired I worked with 304 and 316 stainless steels quite a lot. An exhaust elbow is in a super aggressive environment. Given the importance of the elbow to keeping the engine running my view would be to fit the best material you can. In this case it would be 316. If you want to penny pinch, go for 304

 

[Portofino]

316 .
Exhaust internal environment is Uber aggressive .Move away from the mindset of seawater ( salt ) corrosion.It’s the myriad of nasty sulphuric acids and others that eventually undoes them , the product of 300- 500 *C gases and salt water mixing .
All sorts of metal eating chemicals under the cake layer that forms .

Talking about deck , rigging or other fittings is erroneous .Apples and pears .

 

[vyv_cox]

View attachment 137741
316 .
Exhaust internal environment is Uber aggressive .Move away from the mindset of seawater ( salt ) corrosion.It’s the myriad of nasty sulphuric acids and others that eventually undoes them , the product of 300- 500 *C gases and salt water mixing .
All sorts of metal eating chemicals under the cake layer that forms .

Talking about deck , rigging or other fittings is erroneous .Apples and pears .

Where is the sulfuric acid coming from? Current diesel has a tiny sulfur content. Prior to this sulfamic acid, a weak acid, was present in exhaust gases but not sulfuric or sulfurous.

 

[geem]

View attachment 137741
316 .
Exhaust internal environment is Uber aggressive .Move away from the mindset of seawater ( salt ) corrosion.It’s the myriad of nasty sulphuric acids and others that eventually undoes them , the product of 300- 500 *C gases and salt water mixing .
All sorts of metal eating chemicals under the cake layer that forms .

Talking about deck , rigging or other fittings is erroneous .Apples and pears .

I was pointing out that 316 can't even stay rust free on deck so 304 has no chance in a more aggressive environment. I probably didn't word things right

 

[Tranona]

I was pointing out that 316 can't even stay rust free on deck so 304 has no chance in a more aggressive environment. I probably didn't word things right

The "rust" stains you get from the strip and bar stainless used in deck fittings is very different from the corrosion that might affect cast stainless on an exhaust elbow. While the hot exhaust gases and hot seawater that go though the elbow are potentially corrosive, it is not corrosion of the metal that cause problems but coking up with unburned hydrocarbons mixed with salt deposits. As already stated the resistance to pitting of 304 is only marginally better than 316 and both of course are substantially better than the cast iron if the original. The question was whether the 316 version was worth twice the money of 304.