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Ted Mooney, P.E. RET
Pine Beach, NJ

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Allowable Iron Content in Passivation Bath

We have a nitric passivation process that needs some control and parameters. The concentration and temperature required by ASTM ASTM A380 and ASTM A967 are what we have set our standards to be. However there is a bit of confusion on the amount of allowable dissolved iron content in the passivation bath. I have read in the ASTM A380 section A2.6 that the concentration of iron should not exceed 2 weight %. I have also read suggestions of limiting this to 250 ppm max iron.

In converting 2% weight to ppm I am getting 20000 ppm iron content! If this is to be true then that is about 2.75 lb of iron in a 15 gal 20% nitric solution (about 9 cubic inch piece of iron). This seems to be a lot. If I were take the 200 ppm as the standard it would be about .03 lb or 0.5 cubic inch of iron! I would like to trust the ASTM in this situation but I am having trouble with the conversions to make it make sense.

So the question. What is the ppm allowable concentration of iron for a maximum of 2 weight %? It seems to be obvious but it is not matching suggested practices.

1% (by wt) iron should not be a problem.

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