×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Black oxide in SS steam pipe

Black oxide in SS steam pipe

Black oxide in SS steam pipe

(OP)
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

RE: Black oxide in SS steam pipe

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Black oxide in SS steam pipe

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.

RE: Black oxide in SS steam pipe

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Black oxide in SS steam pipe

(OP)
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)?

RE: Black oxide in SS steam pipe

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.

RE: Black oxide in SS steam pipe

(OP)
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.

RE: Black oxide in SS steam pipe

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources