Passivation Problems with 17-4 SS

[IndianaEd]

Has anyone experienced a coloration between this and improper heat treat? I am a quality/process engineer running a passivation line for medical devices. If we get product (17-4 H900) back from our heat treat vendor that is slightly discolored, and we don’t properly finish it thru polishing or bead blasting, we get dark grey or black discoloration. (I posted this in another forum before I found this one.)

 

[lgearhart]

Tuthill and Avery wrote an article titled "Heat Tints on Stainless Steel Can Cause Corrosion Problems", which can be found at the Nickel Institute, as their Publication 14050. So yes, this does happen. The discoloration effectively takes some of the chromium away from its corrosion protection duties. We use an inhibited hydrochloric acid commercial cleaner ("Everite") to remove heat tint after non-optimum heat treat cycles.

 

[IndianaEd]

Thanks for the info, I'll check it out.

 

[unclesyd]

Hydrochloric Acid isn't recommended for use cleaning 17/4PH SS.
17/4 that has been mistreated during heat treatment can be a little hard to clean. The best way to eliminate cleaning problems is to correct the heat treatment problems.

The recommended cleaning procedure is a Nitric/Hydrofluoric bath, normally 12% HNO3 / 2% HF v/v @ RT. The bath can go up to 20% HNO3 / 3% HF v/v @ RT if the scale is stubborn.
If a quick dip in a HNO3/HF bath doesn’t do the job you will have to go with an oxidizing pretreatment.

 

[EdStainless]

Thanks Syd,
Please don't use HCl. Nitric and Nitric/HF are the way to go. Some people passivate in Citric or Phosphoric, but they are less effective.
When you have oxide from the heat treatment the passivation will not remove it.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[israelkk]

As I mentioned in your second post on the "stainless steel for engineers" forum

You must remove the heat tint before passivation. Even with good vacuum heat treatment oven you may get a slightly discolored surface. So, the best is to use mechanical cleaning process such as fine glass bead blasting. However, beware of excess metal removal which may affect the part dimensions.

Using a chemical etching process to remove the heat tint may result in hydrogen embrittlement and will surely remove too much metal which can be as much as 0.05mm from the surface. The chemical removal process is not accurate and it is difficult to control the metal removal thickness.

Your best bet is to rough machine the part accurate dimensions (leave enough material to remove after heat treatment) then heat treat it to H900. Now use fine mesh glass beads to mechanically clean the part and then machine/grind the accurate dimensions of the part to final dimensions. This way you will have a clean part ready for passivation.