At first, thanks everybody for your help. I think I will try to put nitrogen in exchanger when stream will stop.
Stream and corrosion are in tubes. The type of attack is not like pitting. If you want I can send you a picture of this corrosion.
I have a problem with a exchanger stream/air. It's rapidly corroded (changed all 4 years) by O2 or Cl-, I don't know.
Tubes are in carbon steel and I would like to know if we can use 316L to avoid corrosion, with this composition :
*Cl- = 10 to 15 mg/l
*pH = 8
*143°C
*3.5 bar
Help me please.