Metal finishing Q&As since 1989

Degreasing and passivating a purified water system

I am wishing to degrease and passivate a 316L stainless steel pipe line for purified water. Could you advise me of what % of Nitric Acid I would require, also what chemical would I need for a degreaser I have been advised Sodium Dichromate; if so to what % ratio. Finally do I need to rinse with deionised water and to what PF level.

Regards

You need to purchase ASTM A967 and ASTM A380 specs. Your chemical selection will depend on a few factors and these documents will walk you through it. Sodium dichromate should not be used to degrease. We routinely use 1-2% sodium hydroxide at 120-140F and get great results. The dichromate can be used near the end of the passivation process to promote the formation of the oxide layer. You will need very good water for rinsing. If you have an RO/DI system to produce enough water, use that. Proper rinsing is critical! My opinion may be biased (my company does this type of work) but you might want to contract with a firm in your area to carry out the passivation for you.

Why is nitric acid to be used; can we use sulfuric acid?

Hi, Prashant.

You cannot use sulfuric acid because the spec does not permit it. You may, however, find that citric acid is acceptable for the passivation step. Good luck.

Regards,

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