Mild Steel and HaOH?!?! 1

[Biggy]

I'm about to spec up and install a pipe run for 24% NaOH at Ambient Temp. I'm fairly sure that I don't need to stress relieve it. My main question is on passivating prior to use.

I was always under the impression that caustic lines need passivating prior to start up. (i.e fill with caustic and leave it, then drain it off)

Can anyone tell me what this does and how long I should leave the caustic in before draining.


Thanks in advance

Biggy

 

[rustbuster1]

I wonder why this "passivating" notion exists. Carbon steel in 24% NaOH at RT normally will spontaneously form a passive film on the surface, so letting it sit and then draining it will not change the nature of that film. I'll wait to hear from others but I'm from Missouri on this - it sounds like folk lore to me.

 

[Biggy]

I'm lead to believe that the caustic becomes coloured thus one can have process problems down stream.

You mention that mild steel and 24% caustic should be ok, are there concentrations where it is a problem?

Biggy

 

[rustbuster1]

Flushing with caustic solutions is a standard cleaning operation and would also remove any rust from the surface, so using it to avoid the higher Fe content of the first batch of caustic makes good sense. Also the process of forming a passive film normally involves an excursion around the Flade potential nose between the active and passive conditions and this would occur at first exposure and contribute very low "additional" Fe content to the caustic. Carbon steel can be used for NaOH concentrations up to 50% and at temperature up to 60C without any concern. Above 75C the steel can experience stress corrosion cracking.