Stainless and carbon steel fittings being used together 1

[Keith84]

I wasn't exactly sure which forum to post this in, so please move it to the appropriate one if necessary.

I'm a chemical engineer, and we're building a really small pilot plant right now. The piping is threaded and doesn't go above 1".

As usual, I've been called in on the tail of a project like this to fix problems, so I didn't get a chance to be involved in purchasing and most of the erection. The main reactor for this plant is a 304 SS vessel (my boss worries about some corrosion issues inside of it, plus we just had the vessel sitting around and it was the right size. However, a lot of the piping simply needs to be carbon steel. What I have now is an amalgamation of carbon pipe, a stainless reactor, and a mix of stainless and carbon threaded fittings (mostly bushings and valves). For the purposes of this pilot plant, this will be fine, though it annoys me.

My question however, is what can I expect out of these stainless fittings that have been threaded to carbon pipe. Are they contaminated with carbon and can no longer be used as stainless fittings if I need the properties of stainless? Is there some threshold where I might be okay to to use some of these fittings with in a totally stainless system in some cases but not others? Or are they basically the equivalent of expensive carbon steel fittings now? Luckily, they're small, but we have a lot of stainless valves, which probably weren't super cheap that I might like to reuse.

Probably a dumb side question, but is there any way to realistically decontaminate stainless after it's been "contaminated" with carbon steel, whether it's plate or forgings or whatever? Say someone cut a stainless pipe with the saw with a saw that's supposed to only be for carbon.

(As a side question, I'm always been told to separate your cutting and grinding tools when when working on carbon vs stainless to avoid the stainless contamination. We try to do this, but I've never understood how rigorous we need to be. If anyone has some tips or maybe a link to some information describing how to properly handle this, that would really help me out with out QC system.)

Sorry for the rambling, multifaceted question. I like to think I have good mechanical knowledge for a ChemE but, I'm still a ChemE, and I'm behind the gun on this plant which is going to require a lot of modifications. And I have a dentist's appointment I'm about to be late to. I'll appreciate whatever answers I can get. Thanks.

 

[racookpe1978]

Frankly, it depends. (Lots of "help " there isn't it?)

Seriously. Think about your problem. You have a pilot plant, using scrapped together parts that need to run for - what? - two months? Three shifts?

You have very small bore piping put together with scrap and left-over parts stainless and carbon steel parts. Corrosion won't be an issue (since your acceptance of the CS assures us (you!) that the process fluids won't burn through the CS pipes immediately nor will the CS pipes contaminate the process itself) over the short test part of the pilot plant ops. So, galvanic corrosion won't be an issue over the run-time of the plant either.

No, you will not be able to re-salvage the stainless fittings and re-use them in "real" SS applications later.

 

[Keith84]

"No, you will not be able to re-salvage the stainless fittings and re-use them in "real" SS applications later. "

That's pretty much what I figured. I really wasn't worried about the pilot plant itself, but I had hoped to maybe salvage some of these stainless valves for use in full scale plants later. Looks like that's not happening, so if we disassemble this plant, all those parts are going in the contaminated pile to be used in CS applications where no galvanic corrosion potential exists.

Just an additional curious question: I should probably treat stainless Swagelok fittings that have touched carbon the same way if everything needs to be stainless, right? Those could be easily replaced if corrosion appeared down the line, but I supposed they might transfer some of their contaminated carbon atoms into the stainless threads of the new stainless female fitting receiving them. It's just so easy to get tubing fittings mixed around that it's going to be a little more of a pain to keep everything separate.

 

[stanweld]

Stainless and carbon steels are often joined together at matrials spec breaks where corrosion or dissimilar expansion will not lead to premature failure. It is incumbent upon the designer to consider the service and apply the appropriate materials for the desired design life. For the full scale reactor, carbon steel clad with stainless steel may be appropriate.

 

[EdStainless]

If you tear this down and re-passivate the SS parts they will be fine.
Warm 20% nitric acid for an hour or so will dissolve any iron imbedded in the surfaces.
Rinse very well.

How important is it, do you like near the coast? Find a building, pedestrian bridge or piece of artwork that is made from stainless. Odds are you will be able to tell every place on it that they touched it with a steel tool while bending, forming, or installing it. There will be rust stains.
In a more aggressive environment these locations will pit and lead to failures.
Splatter from welding or grinding carbon steel near SS will do the same.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

The "clamped-on" valves (Swageloc) will be easier to clean, since there is a much smaller chance of the grit and CS debris getting actively embedded into the valve itself either interior or exterior of the valve.

A screwed-on valve physically forces the CS threads into the SStl valve. Welding or grinding with a CS-contaminated tool or grinding stone pushes that contamination and grit into the surface of the SStl so it is harder to clean off. But "casual" contact for a short time will pass little CS grit or debris into the valve, and what does get floated down from the CS pipe isn't getting forced into the SStl valve internals. Might be hard to get a primary nuclear or food-quality acceptance though. The clamping forces of the Swageloc fitting are minor by comparison.

 

[NickClark]

Like Ed said passivate the parts. Or pickle and passivate if its serious contamination. It gets done all of the time either as a clean up or as a precaution. I expect a local plating shop which services the aero or oil industry will be able to process in bulk. After treatment you could prove its effectiveness with a salt spray test if you can sacrifice one item. The plating shop may have the cabinet also.

ASTM A380 is a good starter for you

Nick

 

[moltenmetal]

If they're 150# CF8M stainless threaded fittings, they're not even worth pickling- just scrap them and buy new, if you are putting them into a service where carbon steel isn't an acceptable alternative from a corrosion resistance standpoint. You'll have a tough time getting the thread sealant paste off them completely anyway to do a proper pickle.

Stainless valves? If they're 3-piece valves, take the end caps off and have those passivated. If they're flanged, do nothing- no harm done.