The authoritative public forum
  for Metal Finishing since 1989
-----

High Temperature 316L Stainless Steel Deterioration Problem

June 6, 2008

I am writing in regards to a problem we are having with one of our stainless steel applications in our radiator manufacturing facility. We are having problems with our 316L pallets corroding in one of our new braze furnaces.

The furnace in question is a carbon muffle case braze furnace. The furnace has a nitrogen atmosphere and oxygen levels are being kept to a level lower than most of our other furnaces. However the pallets still appear to be rusting, and deteriorating much more rapidly than any other furnace especially at the weld areas.

When the furnace was brand new the pallets showed no signs of rust or deterioration for 7 months. Now when new pallets are introduced to the furnace they are showing signs of oxidation after a few weeks.

A few things have changed from brand new condition until now. When the furnace was new the pallets would leave the nitrogen atmosphere at 290 degrees Fahrenheit, now they are exiting at 385 ˜ 550 degrees Fahrenheit due to cooling heat exchanger problem.

I have been told that the oxidation temperature for 316L 800 ˜ 900 degrees Fahrenheit. We are still coming out well below this temperature so should be seeing no oxidation.

With the presence of the carbon dust from the muffle, as well as the presence of hydrofluoric gas from the brazing of powder flux could we be seeing oxidation at 385 ˜ 550 degrees Fahrenheit even though it is below the critical temperature?

One other point of interest is the mesh belt the pallets are riding on. It is also made of 316L stainless steel; however it shows no signs of corrosion. It goes through the same furnace environment and is older than the pallets however there is no flux buildup on belt and it exits the nitrogen atmosphere at about 80 degrees Fahrenheit.

Are there any other concerns or recommendations you would have for our application?

Thank you for your help with this problem and if you need any further information please let me know.

Ben Carpenter
Process Engineer - Battle Creek, Michigan, USA

---

Step 1 is to immediately get the cooling heat exchanger repaired and functional.
At 550 F, 316 will oxidize, (but not much) but after numerous cycles, it would not shock me to see pitting, especially in the welded areas.

I am not positive, but I think that your fluoride is not as hydrofluoric acid, but in nascent and possibly ionic FLUORINE or fluoride form. This is an extremely active material and at 285 F, it will be incredibly aggressive. Since it should be lighter than air, it would not be a shock to see the lower material to be unaffected.

There is a possibility that the hot fluoride is acting as a catalyst for the oxygen in the air.
Fluoride is more active than chloride and 316SS does not like chlorides.

Carbon dust is never a good idea on the inside of a furnace. Can you improve your housekeeping?

James Watts
- Navarre, Florida
June 9, 2008

---

June 11, 2008

Thanks for the reponse. I'll try to answer the questions you brought up. We are definitely cycling the pallets frequently they were orginally scheduled for about 1200 passes through the furnace in a year, but when they started deteriorating we had to start pulling them out for repairs or disgarding them. Right now they will go through 1200 passes in about 6 months.

Thank you for the information about the Fluorine. I will look into that further, and if there is any more information that could be provided it would be appreciated.

The heat exchanger is currently under repair so hopefully we will be able to drop the exit temperature shortly. However, I am not sure how much we can do regarding the carbon dust because we can only enter the furnace every 6 months for cleaning.

Ben Carpenter
- Battle Creek, MI, USA