Cleaning before passivating

[danh33]

I'm planning to clean my new tanks with a hot water rinse followed by a caustic cycle and another rinse. I won't be able to passivate until a week later when my acid arrives. Any problem with this delay after a caustic wash? I heard somewhere that you should passivate as quickly as possible after your first caustic wash otherwise you leave the stainless steel "unprotected". Any truth to that?

[dick murton]

Ideally you would passivate immediately afterwards, but as far as I know, there is no problem leaving the tanks / pipes degreased, clean and dry. Most stainless from reputable suppliers will be degreased and effectively passivated anyway. Any welds should have been pickled / passivated immediately on completion of welding if they are accessible, and if not, e.g. in a long pipe run, they should have been welded under inert gas. So all in all, passivation as a separate step should not be required, and the big companies that I have worked with never do this as a separate process. Having said all that, passivation of new kit will not do any harm, and may save problems later.

There have been plenty of discussions about passivation procedures - nitric acid or tartaric acid (the latter much to my surprise when I first heard of it - it is not commonly used in the UK at least). The main thing is to avoid halides in acidic, unbuffered solutions.

[aeschill]

The biggest question is the source of the equipment. Some suppliers provide better prepared stainless than others. If you bought Chinese I would not trust as much as most other suppliers that the stainless will not be damaged by running caustic and letting it dry. I run a small brewery that uses equipment purchased from a Chinese supplier and we have run into many issues, including having rust spots in some areas upon delivery, so I have developed a somewhat paranoid approach when it comes to the condition of stainless and its treatment. If your supplier has already treated weld points than you should be good to go if you'd like to run caustic, then run passivation acid later, just make sure you rinse the tanks to remove any dust that may have accumulated before running the acid.

[wailingguitar]

Use Bru R Eze, Cell R Mastr or PBW and you will be fine