How to stop piping corrosion

[sider]

Hi all,

I work in the modular construction industry, where we sometimes have a chilled water system incorporated into the design. Before construction, we treat the piping with a corrosion inhibitor which is water soluble.
There was a requirement by a customer to do the flushing of piping before shipping the modules to the site, however after the flushing, the water-soluble inhibitor was removed, and since the system was not immediately filled with water, the corrosion started to appear. I talked with a representative from one corrosion inhibitor company who suggested filling the system with inhibitors in a vapor state or filling the system with a mixture of water and inhibitor. How do you usually deal with this sort of matter?

 

[MJCronin]

I would remind the customer that Nature is indeed a Cruel teacher and that you reap what you sow ...

MJCronin
Sr. Process Engineer

 

[TugboatEng]

The customer needs to to flush with acid. Film forming inhibitors cannot protect dirty or corroded surfaces.

 

[EdStainless]

After flushing wasn't the system force dried?
Didn't the customer have this requirement also?
Yes, it will need to be acid cleaned before any inhibitor can help slow the pitting.
But once pits form the best you can hope for is to slow them down.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[sider]

No, there was no force drying.

I am not a corrosion expert, but the corrosion started to appear (surface undergoing coloration to orange at some places), but at that point, I didn't observe pittings.

I have read Lay-Up procedures with inert gas could be used to protect piping from corrosion after hydro test/flushing. Would such a procedure where the piping is drained and refilled with nitrogen on pressure higher than atmospheric pressure be sufficient, even though there would be some water inside? I assume the nitrogen would push the oxygen outside the system and the only oxygen present in the piping would be the one dissolved in the leftover water so there would be less corrosion?

 

[EdStainless]

Less but far from zero. You need to remove the water to keep pipe clean.
Typically you blow hot dry air for a while and then do the Nitrogen fill.
My guess is that if you cleaned some of the most rusted areas and looked with magnification you would see the start of pitting.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[sider]

Okay, I get the idea for pitting.

I see. I assume that hor air drying duration would depend on atmospheric conditions and probably could last long. Are there some common conditions that are required to say that the pipe is dry?

Thinking out loud, would it also make sense to circulate the air through installation and have some sort of desiccant dehumidifier which could take out the moisture from that circulated air?

 

[TugboatEng]

Heating the air lowers its relative humidity much the same as a desiccant would. If you can pump the air through fast enough that it is still hot as it leaves I see no need to do a closed loop with desiccant. After the liquid water has been removed the final dehumidification is done with nitrogen.

 

[MotorCity]

Was the customer advised of this possibility? If not, they should have been. If so, they presumably accepted the risk.

 

[EdStainless]

The other method that I have seen is to use warm air (and yes make sure that it exits warm).
Once the outlet RH stops dropping you can then look for ways to dry the inlet air (such as desiccants on the intake before heating).
Then finishing with Nitrogen until the exit dew point is low enough.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[sider]

Thanks for all the comments, would give it a try next time first with hot air and see what conditions could be met and if necessary apply additional proposals (desiccant/nitrogen)