Passivate before or after lap

[falcon4]

We are having a discussions as whether it is better to passivate before or after performing final Lap / Polish of a metal seat. Material 300 series CRES, min 8 finish.

we perform the lap in house but the part is machined and passivated prior to our reciept.

Is there any spec that stipulates this order?
Certainly using iron lap tools leads me to believe the Passivation would occur post lap.

 

[unclesyd]

We always passivate with HNO3 (20% @ RT) after lapping SS. The majority of our components are made from 316 SS though all other stainless steels are treated the same.
If it was my choice I would eliminate the initial passivation step.

 

[israelkk]

I assume you are referring to a valve metal to metal seat. If this is the case and the seat is by nature will mechanically deform through its work I would prefer your current process because the passivated layer will anyway damage by the valve operation.

The passivation process after lapping will add a process that can harm the lapped seat if is not very carefully done and will add one more inspection stage after the passivation.

300 Series are by nature stainless steel and naturally create a thin passivated layer. Be sure that at the lapping process you do not embed metal particles that can corrode.

 

[EdStainless]

I vote for passivate after lap. This will assure a clean surface, remove any miro pices of imbedded iron, and help assure that there will not be any corrosion of the seat before it goes into service.

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

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