Rust on Passivated 416 Stainless Steel

Q. We are trying to passivate 416 stainless to QQ-P-35, Type II.  Parts look good coming out of the tanks but we are not getting good results after humidity testing.  Some parts show no signs of rust/stains while others are covered with spots.  Parts are all machined from the same stock and processed through passivation at the same time. –S.A.

A. You are using the correct solution for passivating 416 stainless steel (20-25% nitric, 2-2.5% sodium dichromate, 120-130°F for 20 minutes).  This grade of stainless steel has elevated sulfur to aid in machining.  As a result, it is possible to get sulfur inclusions at or near the surface of the part that can be removed in the passivating process, leading to a small pit or discontinuity at that location.  The pit can retain some of the passivating acid that may not be fully removed in the rinsing steps.

Carpenter Technology proposed a solution for this alloy and other free-machining grades intended to have inclusions that may be removed in passivation.  They suggested a 30-30-30 answer to this problem.  The first step is 30 minutes in a 5% sodium hydroxide solution at 160-180°F followed by a rinse.  After this is the passivation process, also for a suggested 30 minutes, followed by a rinse.  The process is finished by repeating the 5% sodium hydroxide at 160-180°F for 30 minutes, then rinsing.  The first alkaline step may remove some of the inclusions while the final alkaline step will be effective at neutralizing any passivating acid retained in the pits.  This should promote a much more uniform passivation and the end result with your 416 stainless steel parts will be much better.