Stainless Steel Pipe bearing on Carbon Steel Support 1

[aswierski]

I have a 12" SCH. 10 Stainless Steel Pipe bearing on a carbon steel structural member. My question is this: Do I need some kind of wear pad between the two? I know I've heard something about this in the past, but I can't seem to remember what the rule is (nor can I find any literature on it). Additionally, If I were to guide this stainless steel pipe with a galvanized U-bolt, do I need a pad of some sort? Thanks in advance.

 

[jmiles]

I would want a wear pad there, however without knowing the motion of the pipe tis hard to tell if you really need one.

But look at it this way, your asking the question so somethign is tellign you its needed, the pipe is large diameter and thin wall, so it does not have alot of extra meat, and a 12" pipe filled with fluid, if that moves its a fair bit of frictional force to start eroding the pipe.

You might also want to consider the bi-metallic corrosion issue as well.

 

[aswierski]

JMiles,

Thanks for the response. I'm not too concerned with the friction due to pipe movement (the contents are simply water at room temperature, so no impact loads are anticipated). The Bi-metallic corrosion is what I'm most concerned with. Is this something you can elaborate on a bit? Or point me in the direction of some decent literature that explains it? Thanks.

- Adam

 

[rconner]

A web search with perhaps the keywords 'stainless steel sensitization' may also be of some interest.

 

[SJones]

Sensitisation is from heat treatment or welding not from a pipe contacting a pipe support. PTFE is a useful material in these applications. A web search with 'pipe supports +PTFE' might net you more luck.

http://www.fluorocarbon.co.uk/products_ptfe_slide_bearings_fluoroglide_pipe.html

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[rmw]

You haven't told us how much the SS pipe moves against the steel support if it moves at all. I'd want to know that before trying to reason out an answer.

rmw

 

[MikeHalloran]

It's probable that the stainless pipe will not be bearing on carbon steel, but on paint that someone paid money to apply to the carbon steel to protect it.

You need a pad.

Depending on the environment, you might want to go farther and also prevent crevice corrosion with I-Rods and Nu-Bolts:

http://www.stoprust.com/i-rod-pipe-supports.htm

Mike Halloran
Pembroke Pines, FL, USA

 

[strider6]

You can have also problem of SSC on Stainless Steel if the temperature is higher enough ( approx 50-60°C) due to chloride accumulation in pipe support.

maybe also these paper can be helpful:

http://www.hse.gov.uk/offshore/corrosion.pdf

http://www.hse.gov.uk/offshore/infosheets/is7-2007.pdf

S

Corrosion Prevention & Corrosion Control

 

[jmiles]

aswierski,

Bi-metallic corrosion is a concern whenever you have two dissimilar metals in contact, by their nature one of the metals will be more prone to lsoe electrons and the other will be more prone to take them, thats whalf your corrosion circuit there, the rest of it will come from a medium to transfer the electrons.

Now on the external part of the pipe contacting the supports your not as likely to encounter a situation witha high rate of corrosion unless its very humid or the pipe and support will be getting wet frequently. as for how much corrosion there will be.... well im sure some will argue with me on this, but the bottom line about corrosion is you can only use rules of thumb, the science is there to calculate it out exactly, but no one has any way of knowing what the EXACT conditions are, or wants to take the time to do the calculations.

So all that being said i would suggest you put a wear pad betweent he pipe and support, like a PTFE material or some other inery material, ie a plastic not a metal.

But if your in a fairly dry environment and for some reason you want to avoid spending the few dollars on some wear pads i dont think you have to be overly concerned about corrosion, but here im just guessing as i dont know the environment.

 

[aswierski]

Very valuable posts guys. The piping and support we've been discussing is inside a water purification building, so there's definitely a lot of potential to get wet. The carbon steel pipe support elements will either be hot-dipped galvanized or painted. I'm going to simplify things and just go with some wear pads between the two and stop worrying about it. I probably should have just done that in the first place, but at least now I know the method behind the mayhem.

 

[jmiles]

Thats probably a good idea, as well even galvanized to stainless you will set up a corrosion cell, and now that i check my Oilfield metallurgy and corrosion book, putting stainless steel in contact with galvanized steel is actually going to make for a worse corrosion cell than stainless stell and carbon steel. (That is to say there will be more corrosion with the galvanized), so yeah the wear pad is likely a very good idea.

If your ever curious about 2 metals setting up a corrosion cell a simple way to judge si to look up a Galvanic series, most of these tables will at the very least list the metals in order from active to passive, basically the further apart the metals are on the table the more corrosion there will be, and it is the more active metal that will corrode, the passive metal will wather the extra electrons.

 

[rconner]

Steve, I believe years ago I heard explanations or excuses (and maybe even in some published work) that carbon picked up from contact by carbon steel tools etc. (wire brushes, chipping hammers, or ???? etc.) with higher some carbon content than the sometimes by design VERY low content stainless steel or weld sort of seeded/caused the eventual corrosion failures in the normally rather thin welded stainless steel pipe. Are you in effect saying such arguments may be specious?

 

[SJones]

No - not specious; I think you just used the wrong term. Sensitisation is a microstructural change in the stainless steel: the formation of carbides at high temperature; welding or heat treatment principally. The corrosion phenomenon you have now clarified is simply 'iron pick up' whereby particles of carbon steel get embedded in the surface of the stainless steel. These particles will corrode, since they don't have a passive film, thereby creating pits with a nasty corrosion product solution which is now aggressive to the underlying stainless steel and 'kicks off' corrosion in that material. The carbon content of the embedding steel is generally irrelevant.

So, a galvanised U-bolt will also need to be shielded from the stainless since the zinc will be removed at some point (accelerated by the galvanic effects discussed above). Generally, this is achieved with PTFE sheet. Some people use rubber but it ages and falls apart.

The above is also a reason why you will find specifications demanding strict segregation of work areas and tools in fabrication shops dealing with carbon and stainless steels. Even grinding dust can initiate pitting in stainless steel.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[rconner]

Thanks for further explanation (it may be kind of tall order to isolate something from "iron" and chlorides in many environments, once one learns all these proper terms)