The authoritative public forum
  for Metal Finishing since 1989
-----

Dissolving Silicon Carbide Scale

We have discovered that a whitish thick scale has developed on the polypropylene walls of our 50:50 nitric acid tank. Further investigation using an SEM lead to the idea that the solid scale is silicon carbide. My question is how can we safely remove the scale. I've tried acids with no avail, the only thing that works is a dichloromethane product we have, but that poses health concerns.

Thanks,

Derrick Kasunic
Chemical Process Engineer - Winnipeg, Manitoba, Canada
2004

---

Long-term nitric acid exposure deteriorates PP, creating a somewhat chalky, oxidized surface which can be mechanically shaved off. Your coating must be quite hard to be considered SiC. It is thermodynamically more likely that Si in an oxidizing environment should form silica or a silicate. I rather doubt that enough Si could be dissolved by a nitric acid solution to form a thick glassy layer within the lifetime of the PP tank. Is it possible that the tank contained a nitric-fluoride solution? What is this solution used for? I have seen hard, white aluminum fluoride deposits from nitric-fluoride solutions used to etch aluminum castings. Also, leaving the scale may protect the PP.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2004