Stainless steel discoloration

[CoffeeMan]

I'm just wondering...

I've got some .012" thin test coupons of AMS 5510 (321 s/s) which will be spot welded. There is a dull gray color on the surface of the coupon, which when ground off reveals the shiny metal underneath. What is this dull gray coating? And is there any way to remove it besides grinding/sanding/rubbing/etc. (i.e. by mechanical means) ??

Thanks in advance.
-CoffeeMan

 

[unclesyd]

Is it interfering with welding or is it aesthetic?
What specification did you purchase the strip under?
Can you see if is very adherent film, loose or is part of the surface of the material?

If it’s the material surface grinding or pickling is the only way to remove it. If it's some film cleaning might help.

Normally a strip or sheet that this thickness will be on the shinny side. You would normally see this finish(grey) on thicker material.

I can see your problem as spot welding this thin a material is a chore.

 

[kenvlach]

It’s the annealing oxide. As 321 SS work hardens readily, annealing is needed after every couple of rolling passes.

Perhaps the best way to remove is with Simichrome Polish (for very light film) or BlueAway (can handle heavier oxides) from

http://www.competitionchemicals.com/

unclesyd recommended both products in another post, and Simichrome is used by electropolishers for fine finishing touch-up (rack marks, etc.). It leaves a protective film, though, so degrease with acetone before welding. You should be able to find it in an automotive or motorcycle shop.

You could use an inhibited acid (much cheaper if you should also have to dip large parts). See the acid inhibitors & supplier links in
Alkaline base descaler to replace HCL
thread330-78679

 

[mcguire]

I find it odd in the exttreme that annealing oxide would be left on thin, cold-rolled strip. which almost always is pickled on line after annealing. It could ahppen, but it would be real malpractice. There could be a more reasonable cause.