Passivation Tanks

Q. We have provided a passivation system constructed of 304 SS to a customer. They are passivating to Method 1, Type 2 of AMS 2700. The dichromate portion of the liquor appears to be darkening the process tank’s wetted surfaces only, as a non-dichromate liquor does not alter the tank’s surfaces. Are we damaging the tank? Will the tank wear out prematurely? Will it adversely affect the passivation of the customer’s parts? —J.K.

A. The method and type of passivating described in AMS 2700 is a combination of nitric and sodium dichromate passivation (20-25 percent nitric acid with 2-3 percent sodium dichromate, operated at 120-130°F for a minimum of 20 minutes). Typically, that combination of chemicals is used for passivating stainless steels with lower levels of chromium. In the case of austenitic stainless steels that typically have 18 percent or more chromium, the nitric acid is generally used alone (if referring to the AMS 2700, reference Method 1, Type 6, 25-45 percent nitric acid near room temperature for 30 minutes minimum). The darkening you are seeing is likely just an oxide that is forming on the stainless steel. If you are passivating a lower-chromium stainless steel, you may be depleting the solution of sodium dichromate, but other than that, the solution should reach an equilibrium and likely not take up any additional sodium dichromate.