SS 316L Pitting 2

[JohnJacob]

One of our devices is hydroformed from SS 316L. This device is heat treated at 1950F for 30-min and subsequently passivated. The devices are completely wrapped in SS foil during heat treatment in a vacuum oven. They show pitting during an aggressive coating operation. ESCA shows a presence of large amount of sulfur at the pit crest. One explanation presented to me is as follows:
“During heat treatment Mn from SS sublimes and gets deposited on the cooler sections of the oven walls and subsequently the Mn particles flake-off from the walls and deposit on the device and these particles disrupt the oxide film on the passivated device and cause pitting.” I find it hard to believe.
I believe that
1. During the hydroforming the sulfide inclusion become exposed on the surface
2. The temperature of the device during heat treatment since the foil covers it probably does not quite rise above 1500F and stays within the ‘sensitization’ range.
These two events probably are the reasons for pitting. Any comments?

 

[mcguire]

It is a certainty that 316L, when it pits, has the pits initiate at manganese sulfide inclusions. Recent research has shown that the sulfur in the steel combines with chrome from the steel at these inclusions and this causes local chromium depletion. This low chrome region around the inclusion is the start of the pitting. It keeps going if conditions are right.
You have not caused it by your heat treat nor is it sensitization. You can fix it by getting steel with very low sulfur, less than 0.002% isn't hard to find. Or you can passivate after the forming during which you ARE exposing these low chrome areas. Nitric acid will remove low chrome matrix and thereby take away the "fuel", i.e. the low chrome regions, which are required for pit initiation during your agressive coating which presumably has some chloride.

 

[JohnJacob]

Would you please suggest the grade and supplier for low sulfur (<0.002%) containing SS? It needs to be suitable for hydroforming. Thanks.

 

[mcguire]

The grade doesn't change. It is merely a stipulation you add to ASTM A-240 or whatever spec you're buying to. Lowering sulfur only helps forming. The only value of sulfur is in TIG welding. Allegheny-Ludlum for one makes it a practice to use extra low sulfur as their standard product, ususally less tahn 0.001%.

 

[JohnJacob]

Thank you very much. We just received test data; it shows that samples that did not receive any heat treatment and the ones that were heat treated in a belt-furnace (w/o foil covering) showed no evidence of pitting.
But the samples that were heat treated in the vacuum oven with the foil covering showed very high incidence of pitting.
Does this give credence to the 'manganese deposits' theory?

 

[mcguire]

I don't see much convincing about the Mn deposit theory. How are the ones annealed in the belt furnace, presumably in air, cleaned after annealing? This is more likely the distinction.
MnS inclusions on the surface will volatize in vacuum. This will leave a void with a low chroium surface, which probably accelerates pitting. You need to passivate to remove low-chrome regions. Passivating isn't for oxide removal; it's to restore surface chemistry.

 

[JohnJacob]

Thank you for your response. All samples were passivated. Our vendor tells me that they indeed use 316l with <0.002% sulfur. I wonder if it is the size of the MnS inclusions irrespective of low sulfur content in the bulk that is at the root cause?

 

[mcguire]

It would seem that the vacuum treatment rather than the base material is at fault. There is no perfect vacuum and whatever oxygen you do have will penetrate the stainless and cause internal oxidation since no protective film can be formed. This will give a surface de-chromiztion.
Treat an area with HNO3 to remove any low chrome layer and then see if this area resists pitting.

 

[TEV]

McGuire is leading you in the right direction. Are you sure of the passivation conditions used? I would suggest cleaning per ASTM A 380, followed by an aggressive nitric acid passivation process per ASTM A 967, treatment Nitric 4, described in section 6.1.1.4. This is an elevated temperature treatment in 50 percent nitric for a minimum of 30 minutes.