The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Having a problem with rust inside of tubing.

August 3, 2008

After plating we go thru a trivalent chromate sealer. we are picking up surface rust on the inside of the tubing. Any one have any ideas about how to help fix.

Parts are clean before plating
Parts are getting Alkaline Zinc coating on outside
Parts are double rinsed after plating
Parts go thru a Trivalent Chromate
Then rinse
Parts go thru a sealer made up at 20% (recommended)
Then dried

Please help!

Vern Schrimsher
supervisor of plating shop - Memphis, Tennessee

Hi, Vern. The short answer, but I think you already know it, is that you don't plate the inside of the tubes, the chromate does not react with the bare steel, and the sealer does impart sufficient corrosion resistance to the raw steel.

The long answer is you need to come up with a process that doesn't cause the problem. A water soluble wax or lacquer may help a bit. Plugging the tubes would help. You might talk to your supplier about whether a one-step iron phosphate can help without damaging the outside surface.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 4, 2008

---

The bare steel, on the inside of the tube is activated by the preplate process just like the outside and may be flash rusting before you plate. Also, most trivalent chromates contain chlorides, nitrates and/or fluorides in addition to other chemical compounds. All of these are highly corrosive to unplated steel and will cause flash rusting of the steel possibly before the sealer. Further, the sealer offers limited protection against rusting especially if there is a rinsing problem within the tube. By this I mean that if the tube does not allow for the flow of rinse water through it, you may be trapping corrosive chemistry inside it and the somewhat alkaline sealer may be getting neutralized and not creating a protective film on the inside of the part.

I would suggest you talk to your vendor about the use of a rust preventive compound in your final rinse and also look at the racking of the tubes to improve solution movement inside them.

Gene Packman
process supplier - Great Neck, New York
August 6, 2008

---

Vern Schrimsher

Auxiliary anodes will help.A simple Mild steel rod or wire will do. but you may need to fit a rubber insulation to avoid short circuit
Regards

T.K. Mohan
plating process supplier - Mumbai, India
August 7, 2008