Black oxide in SS steam pipe

[BLogsdon]

First post on the Forums...
During an upgrade at our pharmaceutical plant, we cut open some Clean Steam piping to weld in a tee for future expansion. I assumed the internals would be polished stainless like the Water for Injection (WFI) systems. No, it has a matte black oxide layer that is not tightly adhered. I can easily pick some up by wiping a finger across it.

It's 316L stainless tubing. As best I can figure, it's a magnetite oxide layer. Is this common? What is the mechanism? I'm sure it was polished at one point. The Clean Steam system is not required to be passivated like the WFI systems are, but I'm not sure what to think about this.
Thanks
Ben

 

[EdStainless]

Two possibilities, either it formed in place.
Depending on steam temp and quality that is highly possible.
Or it is Fe transported from someplace else that settled and oxidized here.
Either way I don't see it hurting anything.
If you need clean clean steam then put a filter at point of use.

By the way, this is similar to rouge in WFI systems, it doesn't actually hurt anything.
You are better off leaving it alone.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

That probably has to be from iron transport somewhere in your process rather than service exposure to steam. I have seen many stainless steel boiler tubes exposed to superheated steam (highest quality) conditions and have not seen what you are observing.

 

[EdStainless]

Superheated steam won't do it, but wet steam will.
But I believe that most of it is from transport as well.
Given that at least some of it can be wiped off that tends to favor the transported Fe theory.

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P.E. Metallurgy, Plymouth Tube

 

[BLogsdon]

Thank you for the replies. The conditions are saturated throughout the entire system, so that corroborates what Ed suggested about the conditions in which this would form. I'm under the "if it ain't broke, don't fix it" school, so I have no intention of suggesting a passivization of that system. What a nightmare that would be! We do have 0.2 um filters at point-of-use locations. This particular leg fed humidification in a cleanroom AHU.

For transport, would that mean that there is some non-stainless component upstream somewhere, or is this an oxide that stainless forms under particular conditions (such as rouging)?

 

[ornerynorsk]

Iron content in local water supply? Here in SW Minnesota, we have localized areas of high iron in the water, requiring extensive treatment prior to many kinds of usage. Just a thought.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[BLogsdon]

I doubt iron in the water would do it. Our municipal water is surface water and does not have a particularly high iron content. The water is treated by a RO system and a evaporated by pure water still to <1.0 micro Siemens conductivity before it is evaporated again for the Clean Steam system.

 

[mrfailure]

This sounds unusual to me. Even in high temperature systems I am much more likely to see hematite carried over from upstream (magnetite tends to be much more adherent). My suggestion is for you to collect this scale and have it evaluated using EDS in an SEM for semi-quantitative compositional analysis. This would be easy, cheap, and give you piece of mind if you find this is only oxidized steel.