Carbonmonoxide / chromium plating resistance

[Onno]

Dear all

In our laboratory for earth sciences we in installed a carbonmonoxide gasline. Tube materials used are : copper (app. 10 m) and stainless steel ( app. 2 m) . At both ends of the gasline chromium plated reducer valve were installed with the 10 m copper line in between. #1 valve reduces from bottle pressure (150 bar) to about 5 bars. At the end of the line #2 valve reduces to about 0.5 bar. Line length is app. 10 m. We found out that the chromium plating is chemically attacked i.e. nearly completely eaten away from the underlyig metal surface. Apparently a chemical waste product is produced which is poisonous to men. At the moment it is summer holiday and most of our staff is on holiday. So I posted this thread. Any comments and tips most welcome. Greetings from Onno

 

[Goahead]

Carbon monoxide, although poisonous, is not known as a corrosive gas. Valve base metal is not indicated (is it brass?). What you describe, plating detachment and removal, is not necessarily caused by corrosion. Could it be the result of inadequate plating procedures?

http://www.welding-advisers.com/

 

[EdStainless]

Is the gas pure CO? I suspect that it isn't. How were the lines cleaned when the were intalled? Any dryers in the system?
Are your valves actual regualtors, or just valves?
A little bit of CO2 and some moisture and you will get some corrosion.
Knowing how toxic CO is I would do the entire system in stainless, no copper, no brass, no plated parts.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

One possibility is attack of the nickel underplating by anhydrous carbon monoxide at 50-60^oC, the so-called Mond reaction:

Ni(s) + CO(g)--> Ni(CO)₄(g), nickel carbonyl

The thermal decomposition of the poisonous nickel carbonyl at 180-200^oC produces high-purity nickel. I posted some info in an earlier thread
Inconel 600 corrosion
thread338-42194

Note: A reducing atmosphere, absent water & oxygen, is required to avoid formation of a protective, passive surface oxide. From the description, the nickel underplating was through pores in the chromium plating, while the chromium itself remained passive.

 

[kenvlach]

Last sentence should read: From the description, the nickel underplating was attacked through pores in the chromium plating, while the chromium itself remained passive.

 

[Onno]

I have had another look at the reducer valves we use. They are regulators using a diaphragm of unknown material. However it's body and fittings is chromium plated brass. What I know of Chromium plating: it needs an intermediate layer between bulk material and Chrome layer e.g. Nickel.

Kenvlach
suggested an attack mechanism (Mond reaction), with CO attcking thorugh the pores of the chromium plating. Over longer periods at roomtemperature (our system) the reaction seems to have an effect also. This seems to correspond with the conditions of the reducer valce chromium plated components.

EdStainless
Your recommendation to built the supply system using all metal parts out of safety is very valid. Our system is all metal from the very beginning. Furthermore we have a CO alarm installed which is tested regularly.

It would seem that using stainless steel tubing is possible because the chromiumoxide layer at it's surface is passive protecting the bulk material.

Thank you all for helping out on this problem.
Onno

 

[rorschach]

Onno, EdStainless did not say metal, he said stainless steel, BIG difference.

 

[Onno]

Dear Rorschach

I am aware of the difference between "metal" (in all it's generality) and stainless steel. In this case I understand that the Mond reaction takes Nickel. In stainless steel this reaction is not occurring because of a pasivated surface, chromiumoxide layer, preventing this reaction. Stainless steel is a possible tube material to transport CO. Copper might also be used. So , copper and stainless steel are under consideration.

With "all metal" I mean : a transport system built out of copper or stainless steel that uses the same metal as tube fittings, valves etc. I would prefer no O-rings.

Thank you mr. Rorschach