Electrolyisis in SS exhaust?

[David M]

I have over 10,000 hours on a set of stainless steel exhaust elbows with no corrosion. Stainless is not necessarily a bad material for this application. I wish I had an answer for you. My best guess is that it relates to the type of stainless used.

[boatpoker]

Originally Posted by David M

I have over 10,000 hours on a set of stainless steel exhaust elbows with no corrosion. Stainless is not necessarily a bad material for this application. I wish I had an answer for you. My best guess is that it relates to the type of stainless used.

I agree ... along with improper/poor welds.

[Andrew Troup]

Originally Posted by boatpoker

Please show me where anyone said they were the same thing.

Sigh ... you called my assertion that they were not the same thing, "incorrect"

And your comment about the galvanic table disproving electrolytic corrosion makes no sense, unless all electrolytic corrosion is considered to emanate from a galvanic mismatch.

Which implies that they are the same process, rather than one being a subset of the other

[boatpoker]

Originally Posted by Andrew Troup

Sigh ... you called my assertion that they were not the same thing, "incorrect"

And your comment about the galvanic table disproving electrolytic corrosion makes no sense, unless all electrolytic corrosion is considered to emanate from a galvanic mismatch.

Which implies that they are the same process, rather than one being a subset of the other

I apologize for my inability to make you understand. I suggest you do as I did and take the ABYC Corrosion Analysis course and Captain David Rifkins course or at leasr buy Mr Colliers book.

[sfnz]

salt water + diesel exhaust gas = an acidic mix that chews the crap out of 316, sulphuric from memory, the stainless to use for exhaust bends and mixers is grade 2RK65, there is another but it escapes me just at present, black iron is 2nd best after these. 2RK 65 typically comes in flat sheet form and needs fabricating into bends sweeps etc but it will last a lifetime if welded with yep.........2RK65 filler wire, it's actually a very easy material to tig weld

[transmitterdan]

This is what we have on our Yanmar. It looks custom fabricated. Has about 1,600 hours on it with no problems. I took it off last year and inspected it. There was just a thin layer of black soot on the inside surface. Looks brand new other than that. I agree that shiny 316 stainless steel is not a good idea in an exhaust system. It looks nice and doesn't last long. That's a great concept for the seller but not so much for the boat owner.

[bglad]

I second the notion quoted below. Carbide precipation has destroyed the ability of the stainless to resist the corrosive exhaust. I think you can prove it if it is without damage in areas away from the welds...

You may also see a form of crevice corrosion on SS elbows if they pool water in their cavities. The puddle of water will turn anoxic causing the stainless to lose its protective oxide coating and it will begin to freely corrode. No current stray or otherwise needed.

Originally Posted by Reefmagnet

Looks like it could have been caused by carbide precipitation in the heat affected zone adjacent to the weld bead. This happens during welding of stainless steels if the metal is kept between certain ranges of elevated temperatures for too long.

[Lepke]

I think Andrew Troup is right on. I was a certified welder. It looks like impurities in the weld. A lot of bolt on OEM engine parts are contracted out to the cheapest bidder. You could have 2 different alloys of stainless welded with a 3rd alloy filler.
Lepke

[Cheechako]

Originally Posted by sfnz

salt water + diesel exhaust gas = an acidic mix that chews the crap out of 316, sulphuric from memory, the stainless to use for exhaust bends and mixers is grade 2RK65, there is another but it escapes me just at present, black iron is 2nd best after these. 2RK 65 typically comes in flat sheet form and needs fabricating into bends sweeps etc but it will last a lifetime if welded with yep.........2RK65 filler wire, it's actually a very easy material to tig weld

Yeah, there are a number of more exotic things that can be used, A whole list of heat resistant SS cousins, Hastelloy, Inconels etc..... unfortunately builders of boats or engines etc are just not into doing things to last anymore. Remember when shafts and tanks were Monel? They last forever. I've had new SS fuel tanks with holes in them (no where near a weld) in less than 7 years. Why anyone would build shafts, tanks, chainplates out of 300 series SS I have no idea other than they want to save $100 building a $500,000 boat!

[transmitterdan]

Unfortunately Monel is a lot more than a $100 bill above the price for stainless. Every $ saved in material cost translates to between $3-$5 selling price. So a few hundred $ here and there adds up quick. Monel is not that easy to machine either. Many (most?) semi-custom builders are gone due to material and labor costs relative to production models. The answer to your quandary is simple, they want to stay in business.

[leftbrainstuff]

Originally Posted by Reefmagnet

Looks like it could have been caused by carbide precipitation in the heat affected zone adjacent to the weld bead. This happens during welding of stainless steels if the metal is kept between certain ranges of elevated temperatures for too long.

It's hard to see from the pics but reefmagnet's suggestion makes sense. What happens with carbide precipitation is the weld transition area (think of a thin band of heat affected area between the weld and the parent metal) becomes a different material composition than the weld or parent metal.

For poor quality and overheated welds you have effectively 3 dissimilar materials and therefore poor corrosion resistance.

The other suggestion to solution heat treat after welding is valid but only if a suitable composirion of weld filler was used. Weld material selection often needs to consider the effects of precipitation.

It's probably simpler to replace the component as others have suggested.

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