Rouging in 316L? 1

[MNgman]

I have a biotech system where we use 316L SS fittings in a off gas line. The line carries air that is at 37 degree C and is saturated with water vapor. As the air travels along the system the water vapor condenses... as expected. What I didn't expect was the rusting and pitting of some of my fittings.

Has anyone else had a similar experience?

The research that I have done so far has lead me down the road of rouging, or impurities in the metal from which the fitting was constructed.

So far the problem has been small and few but I would like to head off any more potential problems.

Any advice?

Is there a way to prevent rouging?


Any advise?

Is there a way to prevent rouging?

Thanks

 

[EdStainless]

How frimly attached in the rouge? Can you wipe it out? Can you find locations with active corrosion (pitti g under hte rouge)?

Is the gas filtered? Are you sure that there is no particulate in the gas? What is really in the gas, is it from high purity water?
How was the system fabricated? Welded or compression fittings? How was it cleaned prior to srevice? Was it passivated?
Was the tubing bought with any control on the interior surface (roughness or cleaning)?

My hunch: not pure water; compression fittings; no cleaning.
There is a ton of info on rouging. It is almost never the tubing itself, but connections, valves, pumps or fabrication welds.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[MNgman]

The rouge is firmly attached. It won't wipe out. In one case there is pitting. One on the fittings was seize onto another.

The gas is filted. The gas is off gas from a oxygen transfer membrane. Bacially a hydrophobic filter with water on one side and gas on the other. The gas supplied to the membrane is not filtered but is passed through a 40 micron screen. The liquid vapor in the line either has to come from ambient room air or through the oxygen transfer membrane. Therefore all of the liquid is filter. It should be similar to a distilation process.

The fittings are actually barb fittings. Our system only opperates at 4 PSI.

The fittings were cleaned with alcohol prior to service.

I do not know if the fittings were passivated or not... I did not passivate them.

Since the fittings are barb type, there is a change in ID similar to a compression fitting. I would assume that the condensate could puddle in this area.

How would you recommend cleaning these fittings prior to use?

 

[EdStainless]

You could nitric acid passivate them in a fume hood. It would be simple.
What is the hose material?
Is there corrosion where the tube meets the barb?
Can you see any way to minimize the pooling and let the system drain?

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[MNgman]

The hose material is Tygon... PVC.
The corrosion is where the tubing meets the barb... minimally. The corrosion is more pronounced inside the barb fitting.
This fitting was oriented vertical to allow the system to drain. Inaddition there is always air flow through the fitting. The air flow is a mixture of air and CO2. Ranging from 20% CO2 to straight air. There is always a minimum of 120 mL/min flow though the system.

I am not sure what else I can do.

If I do passivate it in a fume hood... wouldn't I have the same problem if it is caused by rouging. As I understand it rouging can't be stopped because the absence of ions in the high purity water leaches out the chromium and nickel leaving you with ferric material. Is this correct?

 

[EdStainless]

Not exactly. Roughing is the selective disolution of Fe when the passive film breaks down. Passivation helps clean the surface (especially removing imbedded iron) and helps build a passive layer.
The CO2 will lower your pH, that isn't good. The tubing may also be a source of Cl ions, not good either.

The inside of the fitting may be worse because of torn and smeared metal from the machining. A pickle followed by a passivation may be a better choice. Hot nitric HF pickles well, rinse and then passivate in straight nitric or citric.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[stanweld]

In addition to the comments from EdStainless, have you verified that the fittings are indeed 316L and that they had been solution treated. Mixes of 316 and 304 SS fittings often occur. In one large construction project that I was directly involves over 12% of the fittings were mixed. Just because the fittings are marked 316L and are supported with CMTRs is no gaurantee.

 

[MikeHalloran]

OR, you could substitute PVC fittings.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA