Stainless Steel Exhaust Elbow- 304 or 316?

[Zing]

With all due respect, I ordered 316 tubing for marine use. It should meet a standard suitable for marine use. My original drawings for my boat say 316 grade. Nothing more needed saying. The original s/s doesn't corrode. Why does modern 316 corrode?
Why does my friend on his new cat describe the s/s fittings that a huge boat manufacturer installed as junk? It appears that it was not just me that expected 316 grade stainless to be suitable for marine use

It may be the difference is the better one is electropolished and the rusty one is manually polished.

 

[Tranona]

The question is then pertinent : Is the 316 version a better buy than the 304 version? The second question is also pertinent - considering the similarity of the images is it likely these are not one and the same product (would you question? who would know? how would you tell?).

They may well be sourced from same foundry in China and and even use the same pattern, just using different spec material. Somewhat surprised by the difference in prices but of course the 316 version includes the cost of importing into the UK and UK taxes as well as being sold by a well respected distributor.

It is clear from this thread that there is a lot of misunderstanding on the one hand about both the properties of stainless and the processes for turning the metal into finished products and on the other the way it used in exhaust injection elbowss. exhaust bends are usually made of either cast iron or fabricated stainless steel, the latter often, like Beta and Yanmar 2 concentric tubes with the water injected into the space in between before it mixes with the gas. the Volvo type cast elbow injects the water direct into the flow of gas with a deflector to help it mix. Cast iron is fine for this application. As many have experienced the failure of the elbow is caused by the build up of deposits in the outlet, not corrosion of the cast iron, although often when trying to remove the deposits the casting is found to be corroded. Stainless is far more corrosion resistant than cast iron and 304 will be a big improvement - 316 even more resistant. What is unclear at this point is whether stainless will result in less coking over time, but given that the 304 version is pricewise competitive with the iron version it makes sense to buy it as there are no downsides.

The solution to increased life of Volvo elbows is to change the pattern of usage. my 2030 ran an average of 500 hours a year while being used as a charter boat under load for relatively long periods at a time and still had its original elbow. However not unusual to need to change the elbow on lightly used boats at less than 1000 hours, primarily because of the different hours of no- or light load running as a %age of total running time.

With the fabricated type elbows the main modes of failure are either the welds or perforation of the inner tube, although build up of deposits as shown earlier for the same reason as on the cast elbows is often why they need attention.

 

[geem]

It may be the difference is the better one is electropolished and the rusty one is manually polished.

I re-polished all my stainless steel using an angle grinder and impregnated mops. We had all deck fittings off when we had the boat resprayed last year so I did it all at home in the garage.
The new stainless tubes and cleats all came highly polished.
My manually polished 1980s s/s is perfect. I wish the new stuff was as good

 

[Daydream believer]

I have been on another forum comprising a number of engineers. Some have worked with SS in many guises.
One made the comment:-
"In my humble opinion chaps who complain about the quality of metal are on thin ice because the word quality is meaningless in this context.
Before drawing conclusions, you have to know the metal's specification. Otherwise off to a bad start by not comparing like with like. Not easy - there are a least 6 alloys with 316 in the title, and without looking them up, it's not known how close the specifications of the several alternatives to 316 are.
One possibility is the chap is comparing 316 and 316L: the difference is 0.05% Carbon, but that makes 316L more salt resistant and expensive to buy. 316L might be considered better quality by an owner focussed on corrosion, but an engineer will know that 316 is markedly stronger. On something like a yacht cleat, which matters most: persistent shininess or grubby strength? Alloys have many qualities, not just one.
Many reasons why metals perform more-or-less badly, but I think the idea that metal was generally better made in the past is unlikely."

Then another supplied a clue to the reason there is staining of the later 316 that Geem has when he said:-
"Another small point is that stainless steels need passivating to avoid rust spots when exposed to corrosive conditions. Pickling in sulfuric and or nitric acid is a good way to get rid of iron on the surface, which leads to rust spots.. "

Now if the first items were pickled & the later ones not pickled, then the difference is explained.

 

[Daverw]

Pickling is also being replaced by the use of an electrostatic tool that works like magic by wiping over the weld. Pickling with acid is nasty to do.
we had our arch made by a local company that do work supporting the local food factories were 316L is the norm to cope with at the stuff used in production and cleaning. Other stuff done with 316L from certified pipe suppliers, (We use 316L welded in our refrigeration pipework which is certified) gives me a useful source of off cuts ?

 

[geem]

I have been on another forum comprising a number of engineers. Some have worked with SS in many guises.
One made the comment:-
"In my humble opinion chaps who complain about the quality of metal are on thin ice because the word quality is meaningless in this context.
Before drawing conclusions, you have to know the metal's specification. Otherwise off to a bad start by not comparing like with like. Not easy - there are a least 6 alloys with 316 in the title, and without looking them up, it's not known how close the specifications of the several alternatives to 316 are.
One possibility is the chap is comparing 316 and 316L: the difference is 0.05% Carbon, but that makes 316L more salt resistant and expensive to buy. 316L might be considered better quality by an owner focussed on corrosion, but an engineer will know that 316 is markedly stronger. On something like a yacht cleat, which matters most: persistent shininess or grubby strength? Alloys have many qualities, not just one.
Many reasons why metals perform more-or-less badly, but I think the idea that metal was generally better made in the past is unlikely."

Then another supplied a clue to the reason there is staining of the later 316 that Geem has when he said:-
"Another small point is that stainless steels need passivating to avoid rust spots when exposed to corrosive conditions. Pickling in sulfuric and or nitric acid is a good way to get rid of iron on the surface, which leads to rust spots.. "

Now if the first items were pickled & the later ones not pickled, then the difference is explained.

The corrosion on the 316 tubing is not on a welded part. I had the end fittings welded on. I passivated the welds and polished them myself. Those bits are fine. It's the general quality of the tubes that seems suspect.
My pal was chief engineer for a heat treatment company. He had lots of dealings with Chinese manufacturing. They have in-house scientists as you can imagine. Their testing of metal suggested the Chinese were borderline on standards at best. His view was that the manufacturer may well not be meeting 316 spec to save money but still selling it as 316 if they can get away with it. Most s/s these days comes from China.
Regardless of the above. I have two grades of 316 (maybe). Old and new. Two different suppliers of s/s parts. They both rust like crazy compared to my 1980s original s/s.
I replaced the aluminium toerail on my boat last year. The toerail was fitted with 400 M8 slot head bolts. Every bolt came out easily. I fitted the new toerail with the old bolts as they were in perfect condition.
Unless every batch of stainless steel is tested you or I would never know the real spec.

 

[Daydream believer]

The corrosion on the 316 tubing is not on a welded part. I had the end fittings welded on. I passivated the welds and polished them myself. Those bits are fine. It's the general quality of the tubes that seems suspect.
My pal was chief engineer for a heat treatment company. He had lots of dealings with Chinese manufacturing. They have in-house scientists as you can imagine. Their testing of metal suggested the Chinese were borderline on standards at best. His view was that the manufacturer may well not be meeting 316 spec to save money but still selling it as 316 if they can get away with it. Most s/s these days comes from China.
Regardless of the above. I have two grades of 316 (maybe). Old and new. Two different suppliers of s/s parts. They both rust like crazy compared to my 1980s original s/s.
I replaced the aluminium toerail on my boat last year. The toerail was fitted with 400 M8 slot head bolts. Every bolt came out easily. I fitted the new toerail with the old bolts as they were in perfect condition.
Unless every batch of stainless steel is tested you or I would never know the real spec.

I do not recall mentioning welds in my last post.
Your last comment is noted, but I commented on that in #38 & it does not need repeating. It still does not make a material "junk" It is just not the one that you wanted.

 

[vyv_cox]

I suspect that some manufacturers were relatively generous with their content in the past, so an 18/8 might be a 19/9 in analysis. Today this almost certainly does not happen.

In alloy steel manufacture, where I was employed for a time, it was common for the steel of a higher alloy grade to be over-blown with oxygen, reducing the carbon and perhaps the chromium excessively. The batch would go for En8 with generous alloying elements.

 

[dankilb]

I agree. The old (1989) stuff seems better on our boat too. But I remember my grandfather (and engineer) making the self same complaint about his 1977 Westerly sometime in the 1990s!

 

[Neeves]

Ah...The Good Old days.

 

[Spirit (of Glenans)]

With all due respect, I ordered 316 tubing for marine use. It should meet a standard suitable for marine use. My original drawings for my boat say 316 grade. Nothing more needed saying. The original s/s doesn't corrode. Why does modern 316 corrode?
Why does my friend on his new cat describe the s/s fittings that a huge boat manufacturer installed as junk? It appears that it was not just me that expected 316 grade stainless to be suitable for marine use

Perhaps the material is 316, but the finish is inadequate for the purpose required?

 

[geem]

I do not recall mentioning welds in my last post.
Your last comment is noted, but I commented on that in #38 & it does not need repeating. It still does not make a material "junk" It is just not the one that you wanted.

If you buy 316 and it rusts, it's definitely not the one I wanted. 316 is marine grade. What else am I going to buy for cleats and tube if I don't buy 316? If new 316 rusts and old 316 doesn't, then in my view new 316 is junk

 

[Daverw]

I still think there is a lot of stuff being sold as 316 or 316L that is actually 304, this is often case with small fittings. Most of the 316L we use is not Chinese supplied and clearly marked with spec and batch etc, this is both tube and fittings,

 

[Portofino]

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.

I did say a myriad of other acids .
I did a bit of research into marine , note the “ marine “ bit of the chemicals produced by mixing seawater with the diesel exhaust gases .
Apparently a cake layer forms and all sorts of nasty acids develop under this layer with time .These eat even SSteel .Welds being the weak spots .

The happenings under the cake layer are different to just collecting samples of the product of the material expelled from the transom ……..something like that I recall .

 

[Neeves]

I did say a myriad of other acids .
I did a bit of research into marine , note the “ marine “ bit of the chemicals produced by mixing seawater with the diesel exhaust gases .
Apparently a cake layer forms and all sorts of nasty acids develop under this layer with time .These eat even SSteel .Welds being the weak spots .

The happenings under the cake layer are different to just collecting samples of the product of the material expelled from the transom ……..something like that I recall .

Interesting.

As you have researched this I wonder if you can advise which, nasty, acids are formed -

but

are not all acids nasty?

The blockage of an elbow can be removed with acid - which makes me wonder why (if there is so much, nasty, acid around) the blockage occurs in the first place?

Similarly if the issue is the blockage and/or the welds then a cast stainless elbow, no welds, would solve the corrosion problem and maybe we could harness your nasty acid formation to simply clean the elbow

Job Done.

Jonathan

 

[Tranona]

Similarly if the issue is the blockage and/or the welds then a cast stainless elbow, no welds, would solve the corrosion problem and maybe we could harness your nasty acid formation to simply clean the elbow

Job Done.

Jonathan

It is the design of the elbow that determines whether it can be cast or welded fabrication. The Volvo design is a very simple casting in cheap cast iron because it is essentially single (thick) walled. It actually lasts very well - the need for replacement (as I explained earlier) is not because the casting fails but because of the build up of deposits. only time will tell whether the stainless replacements have a longer life, but they are attractive as even the 316 version is cheaper than the Volvo item and "stainless" gives the aura of being better.

The fabricated types as used by many others including Beta and Yanmar are 2 concentric tubes welded to a thick solid plate that attaches to the engine and a welded spigot for the water injection. these tend not to suffer from build up of deposits, perhaps because the water does not mix with the gases until almost the end of the outer tube. The failure is either the weld of the tubes to the plate or perforation of the inner tube.

Not sure the concentric tube type could be cast. although it would eliminate the welds not sure there is a pressing demand for replacements because failure is not common except on the 1GM which is direct seawater cooled. The problem here seems to be the grade of stainless used in the original as the aftermarket 316 ones seems to last better.

 

[vyv_cox]

Interesting.

As you have researched this I wonder if you can advise which, nasty, acids are formed -

but

are not all acids nasty?

The blockage of an elbow can be removed with acid - which makes me wonder why (if there is so much, nasty, acid around) the blockage occurs in the first place?

Similarly if the issue is the blockage and/or the welds then a cast stainless elbow, no welds, would solve the corrosion problem and maybe we could harness your nasty acid formation to simply clean the elbow

Job Done.

Jonathan

Combustion of hydrocarbon fuels produces a few weak organic acids such as sulfonic, carbonic, nitronic. None of these has any effect on stainless steels. In high sulfur fuels, especially marine (ship) ones, sulfurous acid is generated that can be converted to sulfuric by contact with water. Low sulfur fuels have pretty much eliminated these inorganic acids.

The white deposit that forms in elbows and other water passages in engines is mostly magnesium carbonate, with minor parts of calcium and sodium carbonate and traces of a few other inorganic salts.

 

[geem]

Combustion of hydrocarbon fuels produces a few weak organic acids such as sulfonic, carbonic, nitronic. None of these has any effect on stainless steels. In high sulfur fuels, especially marine (ship) ones, sulfurous acid is generated that can be converted to sulfuric by contact with water. Low sulfur fuels have pretty much eliminated these inorganic acids.

The white deposit that forms in elbows and other water passages in engines is mostly magnesium carbonate, with minor parts of calcium and sodium carbonate and traces of a few other inorganic salts.

My old cast elbow on the generator used to carbon up in 100hrs to the point that it effected performance. Cleaning it out with a Dremel I always lost some wall thickness but I only cleaned out carbon. It seemed like the reduced wall thickness became carbon somehow. Lots of corrosion of the cast iron going on for sure.
In the last four years I haven't removed the new cast stainless one. There has been no generator loss of performance so I just leave it alone. One thing that was noticeable was how much larger the internal waterway is in the stainless elbow.

 

[vyv_cox]

My old cast elbow on the generator used to carbon up in 100hrs to the point that it effected performance. Cleaning it out with a Dremel I always lost some wall thickness but I only cleaned out carbon. It seemed like the reduced wall thickness became carbon somehow. Lots of corrosion of the cast iron going on for sure.
In the last four years I haven't removed the new cast stainless one. There has been no generator loss of performance so I just leave it alone. One thing that was noticeable was how much larger the internal waterway is in the stainless elbow.

Was it definitely only carbon? I have encountered carbon plus salts in engine waterway passages and in production gas compressors.it looked like carbon but there was a lot of salt mixed in.

 

[Portofino]

Combustion of hydrocarbon fuels produces a few weak organic acids such as sulfonic, carbonic, nitronic. None of these has any effect on stainless steels. In high sulfur fuels, especially marine (ship) ones, sulfurous acid is generated that can be converted to sulfuric by contact with water. Low sulfur fuels have pretty much eliminated these inorganic acids.

The white deposit that forms in elbows and other water passages in engines is mostly magnesium carbonate, with minor parts of calcium and sodium carbonate and traces of a few other inorganic salts.

Add in Marine .I thought i emphasised that .

Road diesel exhaust products from vehicles are totally different to spraying sea water into 550* C marine exhausts .
Lets no run away on a tangent of NONE marine .