Metals to rest color change at high temperature. 2

[TugboatEng]

My employer came to me with an interesting problem. He wants to have polished exhaust stacks on our tugboats but doesn't want to do the maintenance required. I proposed electro-polishing the stacks for maximum corrosion resistance however, that does nothing to prevent discoloration from heat. Are there other non-ferrous alloys that can resist discoloration at temperatures around 500°F and handle severe salt exposure? I jokingly threw out gr2 titanium but I don't know much about it. The pipe is 8ft x 18" diameter 10ga and I'm assuming the budget needs to stay below the $10k mark for two of them.

 

[SwinnyGG]

My suggestion would be to look at coatings.

Ceramic coatings typically used on automotive headers, firearm parts, etc are very corrosion resistant and 500F isn't out of range either.

These coatings come in colors aplenty, including clear, and in some cases the coating itself can be polished.

 

How much is your employer willing to pay for bling that stays shiny in a hostile environment?
The somewhat cheap solution is a sheet metal shroud of chrome plated steel, which appears to be what some transport trucks have. Or just plate the stack itself.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Does it really only reach 500F? And it discolors?
304 SS should only turn a light yellow at this temp. But then 304 will not tolerate the salt air.
There are higher alloy SS grades (AL-6XN) That will easily tolerate both temp and salt.
And the alloy is strong enough that you could also think of making them a bit lighter.


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P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

It turns a reddish brown. Exhaust temp out of the engine is around 1000°F but these pipes are about 20 feet upstream and after a muffler. I haven't taken a surface temperature measurement yet. I can say it's cool enough that a bunch of silicone strips I added to support the top of the muffler appear to be holding up fine.

I may grab a small piece of titanium, polish it, and attach it to the stack does a couple of months and see how it holds up.

 

[EdStainless]

Be careful if you decide to go with Ti. It has a very low modulus (50% of steel) so they will flex a lot
What are they made from now?

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P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

They are currently type 316 stainless. We have some in the fleet that I believe have a coating and I feel it's performance has been outstanding and hate to depart from it. The owner wants the polished look which none of the coatings (that I know of) offer.

 

[EdStainless]

The reason for the reddish brown on 316 is that after it tints it is corroding.
The heat tint significantly reduces corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

I priced out 10ga gr2 titanium. It's less than $3k per stack in material and will be fairly easy to form into 18" pipe. I might go that route. I can get it electropolished locally for a very reasonable price as well.[ignore][/ignore]

 

[EdStainless]

Caution,
1. you may need additional internal bracing due to it being more flexible than steel
2. look at Ti welding done by the guys that you hire, there should be no color along the welds at all (without cleaning afterwards)
If you get contamination (air) in the welds they will be very brittle.
And it will not weld to any other metal. All attachments will need to be mechanical.

What would a sheet of AL-6XN cost you? (call Rolled Alloys) You could likely go to 12ga in this material.
It is easier to weld correctly and polishes very nicely.

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P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

The owner wants the polished look which none of the coatings (that I know of) offer.

https://www.cerakotehightemp.com/finishes/W-400Q/cerakote-glacier-chrome/

Polished parts can also be covered with clear Cerakote. Such as: