CHROME EXTRACTED FROM STAINLESS STEEL

[Yak2002]

WE ARE RIDING PLASTIC PARTS ON 304/304L S.S. RODS AND PLATES. WE DISCOVERED A BLACK RESIDUE ON THE PLASTIC (ERTALYTE) PARTS. THIS WAS IDENTIFIED AS CHROME TURNED BLACK BY CHLORIDE (MOST LIKELY THE CUTTING COOLANT). IS CHROME LEAVING THE S.S. NORMAL FOR 304? SHOULD WE BE USING A DIFFERENT S.S., OR SHOULD WE TREAT/COAT THE PARTS? THANKS FOR ANY HELP.

 

[mcguire]

The oxide formed on 304 is half iron and half chromium. If the abrasive effect is sufficient to remove the oxide, which it apparently is, this is exactly what one would expect. Fretting produces a similar residue.
It is a little surprising the plastic is hard enough to do this. Is there some third party abrasive around?

 

[Yak2002]

NO OTHER ABRASIVES USED/FOUND, THE S.S. PARTS ARE POLISHED TO A #4 FINISH. THIS HAPPENED WITH OUR DELRIN AND ARTALYTE PARTS. ERTALYTE HAS A TEFLON LUBRICANT IN IT, SO WE WERE VERY SURPRISED TO GET THIS. HAVE YOU DEALT WITH MANY COATINGS/PLATINGS TO RECOMEND ONE. THERE IS A THIN DENSITY CHROME (78Rc) THAT I'M LOOKING AT, OR WOULD NICKELIZING OR HARD COATING BE BETTER?

 

[mcguire]

#4 finish may be the problem. It is microscopically very jagged and work-hardened. Fragments can dislodge from the surface. Can you get a 2B or 2D finish or some other smooth finish?

 

[CalvinKelly]

This is just a guess as I have no experience with plastic corroding stainless. But, 304 does not well resist corrosion from chlorides. Molybdenum is the alloying addition to stainless that improves resistance to chloride corrosive attack.
The most commonly available grade that is a notch above 304 is 316L, which containes 2%Mo & costs a bit more.
Next above that would be the duplex stainless (magnetic) 2205 (S32205/S31803)with 3%Mo, about 40% more expensive than 316L. 2205 is also somewhat more wear resistant that either 304 or 316L.
A grade probably far more than you need would be AL-6XN (UNS number N08367, ASTM B 688 plate, B 691 bar) with about 6.2%Mo.
In theory there are a zillion alloys you might consider, but the above grades can actually be purchased in small quantities from steel distributors.

James Kelly

http://www.rolledalloys.com