Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

N285, N284, N925 Passivation

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Our Customer request to passivate above the material. We are not sure can this material be passivated using nitric acid (22% V/V) and sodium Dichromate (3%). The density of the makeup is about 1117 . Problem face are blackish and surface being attack with pitted holes.
Question: 1. Is this passivation not suitable for this material?
Question: 2. Is the parameter not right and what are the limitations if these processes are workable?
Question: 3. If the material is heat treated, can it be done also?
Can anyone give any advice on this issue?
Thanks,

Hello Arthur!

I'm assuming that the N925, N284, and N285 are steel grades whose compositions you've given above. Of these, only N284 is appropriate for nitric acid passivation- the other two alloys will be attacked. Further, the free-machining N284 may also have problems with the 22v/o + sodium dichromate. In our company we have switched to a type VIII nitric solution, i.e. a bath composition 45 to 55% by volume HNO3, a bath temperature 120 to 130 °F (49 to 54 °C), and an immersion time of 30 minutes.

Good luck!

Passivate means the formation of a thin, protective, and very adherent oxide film over the surface of certain metals. Once this film is formed, the oxidation stops. It cannot be further oxidized. You can only passivate certain metals or alloys that have a passive element inside. Of the above mentioned, only Cr and Ni are passive.