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Problems of flushing after pickling


Q. Hello!
I'm responsible of flushing on a construction site in Russia and I've a problem that nobody can resolve. After three weeks of flushing of carbon steel lines I still have a lot of rust (particles of about 200 microns). Some people say that the pickling has not been done correctly, other say that it's a problem of passivation.

Here, you have the description of what we used:
- Type B: carbon steel piping (ASTM A106 B/API 5L Gr.x65);
- Hydrochloric acid: acid with mass content equal to 38% HCl + 62% H2O;
- Tri-Sodium-Phosphate: powdered Na3PO4
- sodium carbonate / washing soda [affil links] or sodium bicarbonate: powdered Na2CO3.
- Pickling chemical bath, type B: solution with mass content by volume equal to 50% (38%HCl + 62%H2O) + 50%H2O, i.e. 50% of hydrochloric acid + 50% of water with added corrosion retarding agent of amine group (type "AMDOR-IK-2") with concentration of 500-1000 g/m3;
- Flushing after pickling: flushing using soda solution and sodium phosphate for the purpose of neutralization of opposite in properties strong- acid pickling solution; solution with mass fraction by weight equal to 5% Na3PO4 and by weight 5% Na2CO3;
- Flushing: using 1% soda solution with corrosion retarding agent of 1% (type "FMT") based on demineralized water. - Corrosion retarding agent "AMDOR-IK-2": oily substance varying from light yellow to dark brown. The agent belongs to the moderate hazardous substances (hazard category II).

The product is manufactured on the basis of nitrogenated organic compounds, almost not water-soluble and does not form up a consistent emulsion.
- Corrosion retarding agent "FÌÒ": oily substance varying from light green to dark green, environmental-friendly (hazard category IV). The product is manufactured on the basis of deodorized vegetable oils and chlorophyll fat derivatives. It is virtually water- insoluble and it does not form up a consistent emulsion.

Waiting for your answer Thanks in advance.

Jerome Enjolras
gas and petrol company - Krasnodar, Krasnodar region, Russia

How old is this pipe? Was it heavily rusted inside?

When you pickled the pipe, did workers beat the outside of the pipe? On most industrial pipe pickling projects in the US, workers will beat the pipe with heavy hammers all along the pipe system so as to dislodge rust particles. I haven't pickled done that much mild steel pipe but this process seems rather simplistic. I have entertained the idea of a high pressure line mole spray to assist removal of rust particles through the mechanical action of the spray.

Did you keep cleaning until all iron was removed, based on titration? Cleaning specifications in the US typically call for a specific flow rate in the recirculation and subsequent rinse. The idea here is to get some mechanical cleaning action through the flow. I still feel a "sewer jet" or line mole would do better.

This is strictly addressing the mechanical aspects of your cleaning. I haven't really analyzed your chemical application yet. Others may have suggestions regarding formulations.

Good luck and keep us posted on your results.

Todd Turner
- Monroe, Louisiana


One possibility is that your acid may be too strong. Some amine type inhibitors lose their protective effect on free iron right around 50% Hydrochloric, with the result that the steel will start rusting again as soon as you rinse.
Perhaps on a sample of pipe, try the same concentration of amine in 20% acid, which should still be strong enough to dissolve any rust in a few minutes.

Craig Haseltine
- St Charles, Illinois USA

February 1, 2008

Could you let me know if you have been able to resolve your problem and if so how you solved it?

Frits Kronenburg
Consultancy - Sassenheim, Holland


200 microns is fairly thick to try and remove. HCl would eventually get it (helps if it's warm/hot), but may take a while.

It may be better to mechanically remove most of it (brush, blasting, etc) if at all possible. At that thickness, I would start here and see what your options are. You may still wish to finish it with the acid treatment.

Passivation is typically done for stainless steel (not carbon) as the chromium layer protects the base metal. Carbon steel does rust easily with humidity, rain, etc. If it's oiled, that'll help, or an inert gas kept in it, etc (any way to create a layer against oxygen).

Good luck!

Amanda Glass
- Springboro, Ohio, USA
July 7, 2011

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