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Hydraulic tubing finish corrodes in ammonium nitrate service

August 11, 2015

Q. Dear Sir / Madam,
Good afternoon,
I have read a number of the threads on this site and believe that you may be able to help me.
I work for a company that uses Hydraulic Steel Tubing within the equipment they produce.
We have a customer who has a piece of equipment which they are using in an environment that processes Ammonium Nitrate.
The tubing is Chromium 6 Free tubing and the finish on the tubing and hydraulic fittings has been badly affected after only a few weeks.
Could you please give me some advice as to the best way of protecting the tubing & fittings from this corrosive substance.
Any help would be greatly appreciated.

Paul Workman
- Cambridgeshire, England

A. Hi Paul. Ammonium nitrate is used as a stripper for cadmium plating, so cadmium plating would not hold up at all. You haven't actually told us what the finish is that is being attacked by the ammonium nitrate, but if people are promoting it as "Chromium 6 Free", I would guess that it is zinc plated with a trivalent chromate conversion coating. Zinc plating is for normal atmospheric conditions; it is not for chemical exposure, and is dissolved both by acids and alkalis.

The tubing probably ought actually to be a 300-series stainless steel for this exposure, but electroless nickel plating should hold up, and high quality nickel-chrome plating might.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2015

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