Bizzare Question with Stainless

[MATE45]

So i have a bizzare question/ problem with some 316 stainless steel. I have two parts that connect together, than came from the same heat number. the larger part is corroded and is also harder (33HRC compared to its counterpart at 12 HRC) The smaller part however has become magnetized. The area that this part has come from is the only area we see difficulty with. We have had this occur before and to me it looks like CO2 corrosion occuring ( speaking of which does anyone have some good links for CO2 corrosion examples), but only on the bigger part. Ive done some hardness tests on what we have in stock and the smaller part is harder in some cases compared to the bigger part. Correct me if I am wrong, but doesnt stainless steel have a passive layer on the outside, which should reform if it is worn away? Is it possible since one part has been magnetized that a galvanic cell has been created. Any thoughts?

 

[EdStainless]

Well, let me start by saying that heat numbers are nearly useless. Master coil numbers are much better for tracking since each coil could have different thermomechanical histories.

What kind of system in this?
First please re-check the hardness in RB.
The harder piece could be that way for a couple of reasons. It is probably work hardening, and there may or may not be any strain induced martensite. If there is none, then it would not play a role in corrosion.

I would focus more on the surface condition of the parts. Their roughness, cleanliness, and passivation. Yes, in normal oxygen containing systems stainless will naturally re-passivate. There are some things will interfere with this process.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[MATE45]

the part is from a well pumping unit. The HRB measurements came out to 108 HRB and 92 HRB. The part that is corroded is also showing signs of wear, almost as if someone took a brush and scratched and grooved the surface. The smaller part is in perfectly fine condition, its jus the large part that looks bad. You would think that the same material would behave the same way when connected in the same well. As for why the part fractured, corrosion pit, created stress raiser, applied cyclic stresses, propogation of crack.

 

[MATE45]

heres another question that might help elimintate something: is it possible for CO2 to attack 316 stainless. If it is not possible maybe what i am seeing is cavitation.

 

[EdStainless]

Yes, I have seen CO2 grooving in stainless, but I tend to doubt it.
First of all, even the 92 RB sounds high for 316.
Secondly since the part broke it must of had a lot of load on it.
Fatigue? I tend to think of flow related issues.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[MATE45]

the part has three fracture lines breaking away from one of the pitted areas. Its possible that this part saw 2.27 million cylces. Possible crevice corrosion. the stainless doesnt repassivate itself after some wear and then the small pitted areas become the anode, and the part that looks fine is the cathode. I checked the MTRs for the and it said 189 Brinell. 316 is subjectable to H2S corrosion when the part is over 23 HRC or so, however no H2S in this well. Thanks for the info.