plating, anodizing, & finishing Q&As since 1989
Pickling problem for carbon steel pipes
January 5, 2009
We are a company working in field of hydraulic pipes pickling & flushing.
Now, we are engaged in a huge plant including C.S hydraulic pipes which must me pickled and flushed due to a Re number greater than 10,000! Now here are my questions.
1)After pickling by citric acid and passivation process a lot of yellow spots can be seen inside the pipe. first time when I saw inside the pipe I was shocked .it seems to be some corroded points which not removed during pickling, but surprisingly, it can be removed easily just by rubbing finger or a piece of cloth.when you remove yellow spots,passivated surface is visible.What happened during degreasing,pickling and passivation ?
2) When you can remove rusty colour spots just by rubbing finger, you can be sure this will removed during flushing. But after flushing by a Re more than 10,000 nothing happened. just yellow spots changed to black sticky thin layer! Again can be removed by slight rubbing. Please let me know your opinion .
- Tehran, IRAN
January 20, 2009
What passivation process did you use? Citric acid is a good passivator of stainless but on carbon steel it will only clean and etch, it cannot passivate.
Stellar Solutions, Inc.
January 28, 2009
here is our procedure for passivation just after pickling is carried out.
1)citric acid by concentration of 0.5%
2)adding ammonia to increase pH up to 9
3)after one hour adding sodium nitrite by concentration of 0.5%
4)circulating for 5 hours at 50 °C.
Please let me know if you suggest a better way for passivation.
- Tehran, Iran