Metal to metal contact for stainless steel 4

[eltooon]

Hi all, I just graduated and join a company on O&G industry. We deal with pipe for casing and tubing in a oil well.

Sometimes the pipes are made of stainless steel (13Cr, 22Cr, inconel etc). I was told that metal-to-metal contact for such material should be prevented as it will cause corrosion.

I don't understand how corrosion would start if stainless steels are in contact with or impact against each other. I'm also not sure whether this rule applies to all stainless steels or only certain grades.

Can somebody please help me clear the doubt, or recommend a reference book?

Thanks in advance.

 

[MikeHalloran]

Not corrosion; galling. Look that up.



Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

Unless they are worried about galvanic corrosion of the plain steel casing caused by contact of the more noble drill string.

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Plymouth Tube

 

[eltooon]

EdStainless:
To make it clearer, I was talking about m-t-m contact during manufacturing and handling. Galvanic corrosion is not a concern as pipes for a particular job are usually of same material.
We use plastic rings (bumper rings) or pipe cradles or wooden blocks to separate stainless pipes/products.
Another benefit of doing so is prevent them hitting against each other and cause damage. But that is not what I'm interested in.

 

[Vinch]

It is called contamination of stainless steel by free iron.
Chemical treatment exist to avoid any problem after fabrication, it is called passivation.

Look that up:

http://www.stainlessfoundry.com/FreeIron.asp

 

[eltooon]

Vinch,

Very helpful article.

One question:
will free iron contamination, if left untreated, damage the passive chromium oxide film at the surface, and accelerate metal corrosion?

 

[EdStainless]

sort of.
When the iron rusts it interferes with the formation of a coherent chrome oxide surface layer and it leaves a porous corrosion product. You now have the perfect site for initiation of crevice corrosion.
In many cases this happens in the field because of equipment and tools making contact. No advanced prep will offset this. All you can do is move to a more corrosion resistant alloy if it becomes a problem.

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

 

[blacksmith37]

The stuff in wells is not "pipe"; it is casing and tubing (except drill pipe is "pipe").
As Mike H says the potential problem with alloy casing and tubing is GALLING. I never saw a dissimilar metal problem in a well , not even with aluminum drill pipe ( which has steel connectors).

 

[eltooon]

Any idea why Cr material is easier to gall?

 

[blacksmith37]

Actually, the ordinary "carbon" steels will gall ; L-80 is especially bad. That is why somewhere in 5CT it has a requirement to treat the threaded surfaces with something like phosphate.
There might be some liturature but I suspect no absolute answer as to why Q & T steels in the lower strength ranges, like L-80, tend to gall.

 

[CdotS]

What is galling? It is just cold welding when metal comes in contact with another virgin metal. It is worse when you have similar metals. The mutual solid solubility determines the extent of galling - the higher the solubility limit the higher the tendency for cold welding. Stainless steel has Cr oxide when that thin oxide layer breaks down due to rubbing, the virgin metal underneath tends to weld together.

 

[clay87]

Galling also has to do with hardness differentials (or lack of) between the two metals. Or maybe I'm just using the term 'galling' in a different context??