Effects of cleaning agents on surgical steel

[FrankenBerry]

I'm a manufacturing engineer in a medical device company. I actually have my B.S. in Materials Science Engineering... but it's been a while. Can someone please lend a helping hand here? We have a handful of 455 SS parts which go through a cleaning/passivation line containing Potassium Hydroxyde (to "clean&quot and another tank of Nitric Acid (to passivate) the product. There are additional rinse tanks (water) along the way as well. A great deal of these particular products are becoming discolored and the laser engraving is being washed away. The discoloration ranges from greyish/blackish to almost orange in color. Obviously, the orange might indicate rust, but this is supposed to be surgical steel. Can you offer any hints/tips as to why this is happening. The department responsible for the cleaning keeps sending the parts back to my area claiming it's our fault. Could something be inherently wrong with our cleaning process? Any recommendations????

 

[unclesyd]

I don't have my Carpenter data on 455, but as I remember we didn't passivate it with HNO3. Your cleaning system is too aggressive in that it removes the etch. It would take more information to really help.

It sounds out like the HNO3 is to strong, contaminated, or the residence time is to long. To passivate, if necessary is a quick dip and rinse. If your problem is a "smut" from the KOH bath then 20% HNO3 @ RT dip will do the trick.

What are you trying to remove as caustic cleaners don't require any post treatment? Our 455 parts were cleaned in a NaOH based solution and just water rinsed.

Check out the Carpenter website:

http://www.cartech.com

Even though there is a lot data onsite also contact the technical people for assistance. At one time they were very good a helping with problems such as yours.

Also the supplier of the cleaning compounds should be able to help, though I doubt very many have heard of 455.

Keep your KOH bath refreshed and at the proper temperature.
Depending on the soil and amount it has a finite life.

Watch your rinse water, dragout or carry over from the KOH solution could be a problem.

PS
It can't be all bad as 455 is used on golf clubs.

 

[kenvlach]

My experience and thoughts are similar to those of unclesyd. Have these parts undergone final heat treatment, are they free of oxide, what is the point of your cleaning? Is the discoloration general, or only where laser engraved?

I have done passivation of Custom 455 for an ISO 9000 medical company. Used QQ-P-35C, Type II solution (20-25 vol. % nitric acid, 2.5 wt.% sodium dichromate). As the parts were delivered in sealed containers after PH in a vacuum furnace, no pretreatment was necessary. The customer required new passivation solution per job, and lots of rinsing in 18 Meg-ohm water.

Be sure your solutions are clean; I haven’t seen it occur in oxidizing acids, but in hydrochloric or sulfuric solutions, dissolved Cu will sometimes plate out on steels by displacing the less noble Fe surface atoms.

When pre-passivation cleaning is necessary, it should be per ASTM A380. Haven’t used a KOH-type cleaner, but expect it could cause copper hydroxide smut on Custom 455 (1.5-2.5% Cu). Possibly other hydroxides, too, whereas a very high pH, NaOH-type cleaner will dissolve many metal hydroxides stable at pH 10.

Blasting with air pressurized-DI water is a good method of removing smut (use filtered air) from PH SS. E.g., that sometimes left when nitric-fluoride pickling is used to remove heat treat oxidation.

Re preserving the laser engraving. Either shorten the passivation time or switch to a suitable citric acid passivation solution. For the latter, contact Lee Kremer of Stellar Solutions

http://www.stellarsolutions.net/

 

[tomwalz]

All the above plus Rinse water? Clean, deionized?

Post on

http://www.finishing.com

Try Ted Mooney - electroplating