SS 316L Pitting
SS 316L Pitting
(OP)
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?





RE: SS 316L Pitting
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.
RE: SS 316L Pitting
RE: SS 316L Pitting
RE: SS 316L 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?
RE: SS 316L Pitting
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.
RE: SS 316L Pitting
RE: SS 316L Pitting
Treat an area with HNO3 to remove any low chrome layer and then see if this area resists pitting.
RE: SS 316L Pitting