What is the Best Method of Passivation?

Have you ever noticed that many day-to-day things seem to get divided into two opposing camps?  Things like asking what’s the best computer quickly become the PC versus Mac debate.  Phones? iPhone versus Android.  So, passivation?  Yup, nitric versus citric.

There are real advantages to each nitric and citric passivation, but that is a topic for other FAQ’s because once you pick one you still have several choices to make.  Choices like: which standard do I use? which method do I choose?

There are two primary standards that address passivation. The SAE Aerospace “Passivation of Corrosion Resistant Steels” (AMS2700) and the ASTM “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts” (A 967).

Each allows for citric or nitric and each allows for several options within each citric and nitric.  So what’s the difference? Each standard is copyrighted (and protected) and has to be purchased from the publishing agency, so getting into a lot of detail is not allowed. But here’s a short version of the differences in the various methods:

Concentration of the specific acid – each method varies based on how concentrated the citric or nitric acid is – and in the case of nitric if the nitric acid is used alone or with sodium dichromate.

Temperature of the bath – again, each method specifies a temperature range that must be maintained for the duration of the passivation bath.

Time – the length of time a part must remain within the passivation solution is spelled out as well.

What seems complicated becomes really simple.  The standards spell out the concentration, the temperature and the time.   Each method simply varies one or more of those three items.

 

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