[Samuel Johnson]

Hi All, I use 70% Nitric acid in a rusting process to black steel tubes, the tubes are coated with a formula that promotes rusting and the parts go in a tank of hot water to convert the rust black,  for the past couple of years the finish has become poor the parts do not rust as aggressively as they used to and when boiled not all the rust converts a lot of it seems to end up in the tank, the tanks that used to last for several years now only last months as they rust out very quickly, the rust on the parts is also removed a lot easier than it used to be,  I wondered if something had changed in the making of Nitric acid ? being a strong oxidizing agent I guessed that this chemical would be what bonded the finish to the surface, I have noticed that the acid does not fume like it used to when opened ( even with new stock ) could anyone shed some light on this please, thank you in advance.

[Borek]

Way too many unknown factors to give a reasonable answer IMHO.

[Shashbaugh]

           I'm a rookie here too...

           Questions:  Is your final product black iron oxide?
                           Did you use to order fuming Nitric acid? 
                           Have you changed suppliers or type of black pipe previously supplied?
                           Are your current tanks from the same supplier and same type of material previously supplied?

                These questions may help the experts to guide you to an answer...