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

What Passivation Process is recommended for 3156 SS

We're currently going though a process of changing suppliers on a axle shaft used on a pool cleaner. Our dwg. calls for the shaft to be passivated by does not specify what specific process to use. I'm debating between my purchasing department and engineering rather the passivation process used by the supplier should be included on our dwg. The Engineer in charge of the project thinks this is not necessary since this is just a cleaning process.

Reading over a bunch of articles I learned passivation is not only a cleaning process which make me concern about rather the supplier is using the optimal (correct) passivation process for the application.

The shaft is made out of 316 SS and our supplier is recommending to use QQ-P-35, Type VI to passivate the parts.

Is this passivation spec recommended for the application, thinking the part is expose to chlorine environments and temp up to 85°F

Yes.
QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil]Type VI is recommended or you could use ASTM A967 and Citric Passivate.
Both types if done properly will "passivate" the stainless.

Passivation will improve the resistance to the chlorine-containing environment. The less resistant Fe & Ni atoms are selectively leached away, creating a transparent chromium oxide layer about 20 Angstroms thick. Of importance, any iron (carbon steel) contamination from the mfg. process which would otherwise rust is removed.

For properly selecting the passivation process, see QQ-P-35C or ASTM A967. Type VI (Nitric 2 in A967) is suitable for 316 SS. Consider the heated citric acid passivation solutions described in A967 and available as CitriSurf products from Stellar Solutions [a finishing.com supporting advertiser].

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