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Why does stainless steel rust

Hi,

I am a Quality Engineer in the exhaust industry. I have a question on Stainless Steel Material 1.4713 ferritic grade.

In our manufacturing process we weld a sensor boss material 1.4713 to a cone material 1.4509 using a welding wire material 1.4502.This sensor boss is threaded on the inside. We than have to roll the thread with a tap,because thread size has shrinked because of the heat which has been induced while welding.This tap is made out of HSSE " High Speed Steel" and it's coated with titanium nitrate.The tap gets dipped into a tapping fluid main inhibitor of the tapping fluid is Trichloroethylene.We than gauge the thread with a thread plug gauge material TOOL Steel type not known.

This product get shipped from South Africa To Sweden by Air ETA ± 5 days.On arrival in Sweden the sensor thread has a brown powder residue on it.My customer claims it is rust.It seams to me as it is a discoloration process which occurrs during the welding process.My question is is this normal to get this reaction is it going to affect the product life drastically.I need someone to help me better understand this situation.

Omar Kariem
Exhaust Systems - Port Elizabeth, Eastern Cape, South Africa
2004

Hello Omar,

I wouldn't be surprised if the trichloroethylene causes your problem. You can see the same brown residue in vapour degreasers as we used these years ago. Try using another tapping fluid. If that's not working out feel, free to send me a sample so that we can have a look at it.

Good luck,

Remmelt Bosklopper
- Enschede, The Netherlands
2004

The brown residue is most likely contaminant left behind by the threading oil. These lubricants are not particularly clean with regard to oil-based contaminants, because they don't usually present a problem.

The problem is almost definitely not rust, especially if it presents a dusty surface that appears clean underneath.

Wipe the parts and see if they "re-rust", also change to a very clean threading lube to be sure it doesn't happen again.

Jeff Watson
- Pearland, Texas
2004

Stainless steel can lose its "stainless" properties due to the weld process. The welding should be done cold and slow. If it gets too hot, the alloys can separate out and you will have RUST at the weld interface.

Karl Hoyer
- Redding, California, USA
2005