Ok a couple of point that need addressing.
Yes you could "over passivate". Passivation is really just using an acid strong enough to remove Iron (Fe) from the surface, but not the Chromium and Nickel, you are left with a Cr/Ni rich surface that better resists corrosion. By using some Phosphoric acid you form a surface of Cr/Ni Phosphate that is really tough and even more resistant than the metals alone.
Passivation is really important especially where the metal has been stressed by either working (bending and shaping) or heated as in welding or direct fired. Over time the surface becomes less protected and benefits from re-passivating but every treatment will remove some metal, do it too often and you have no metal... so once or twice a year is pretty common.
The other reason for using an Acid cleaner (usually the same or similar to passivating chemicals except for concentration and temperature) is to remove Beer Stone, which is a build up of mostly Calcium Oxalate. Beer stone is sort of rough and porous and provides a great home for bacteria.
In the beer cellar using Caustic isn't really enough on its own, we are are looking to remove different soils than those in the brewhouse (on big systems pumping hot caustic into a fermenter that hasn't been emptied of CO2 can be catastrophic vacuum imploding the tank), in most system Caustic followed by Acid cleaners are preferred, but not at passivating strengths.
Personally I doubt that Star San or similar is really strong enough to passivate stainless, there isn't enough Phosphoric acid and no Nitric which is really the workhorse of passivation. Even a little suspicious of its ability to tackle a heavy beer stone buildup. Probably OK for routine sanitation and perhaps to prevent growth of beer stone.
Don't pump high foam cleaners through a spray ball.
Good reasons to make sure you have enough Calcium in your water to precipitate the bulk of the Oxalate earlier in the process (over say 100ppm at end of boil).
On CIP pumps - sorry, for a spray ball to work well you need pretty massive flows (often the problem is getting the return out of the bottom of the tank fast enough - pissy little 1/2" plumbing isn't big enough), just had a quick look on eBay and found this one in a couple of seconds
Stainless Steel Food Grade Centrifugal Pump Sanitary Beverage Wine Pump 3T/h
750W Stainless Steel Centrifugal Water Pump Food Grade 120L/min WINE JUICE MILK
3T/H being 3,000L/h the second one being around 7,200L/h
As above hot Caustic and Acids are bloody dangerous - I still have a scar from hot caustic, been there for over 30 years and not going away any time soon, so be very careful.
Little things like hoses coming off barbs, seals that don't, fine mist from spray getting in your eyes...
Thinking ahead, reading labels and taking care is cheap and fairly painless.
Citric acid isn't really up to the job, to work it needs very specific concentrations, some other chemicals added and a lot of time, not things most homebrewers will find easy to setup and use effectively.
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