As stated previously, check that no foreign debris has wound up inside the vessels. I have found square inch pieces of scrouge pad in a couple. In one strange case, we had rinsed, cleaned with caustic, then passivated a tank to find a foreign piece of metal (soft iron I figure) at the bottom of the unitank, which during passivation, created a noticeable pit around it (electrolysis)! The whole bottom assembly had to be replaced because of this. There can also be many little pieces of metal that have found there way into the lauter tun and remain stuck between the wire. DO NOT ASSUME the manufacturer has removed every little bit of dirt/grim/metal shavings/etc. And some of this stuff can be introduced during site construction.
Anyways, after visual inspection rinse everything well and be sure no metal has accumulated at the bottom of vessels, especially below the lauter screens. Remove and clean all gaskets from manways and clean under their position. Replace the gaskets. Clean every vessel and all piping with an appropriate detergent at normal concentrations at 60-75 deg C for twenty to thirty minutes. Rinse well.
For passivation I add nitric acid to water (before heating the water) at a concentration of 2-3% (pay attention to the concentration of the nitric you're using) using around 200-300 liters of water. You can use many different acids (phosphoric/nitric blend, citric acid...), I just have always used nitric. Just ensure proper concentrations of different acids-don't have the literature in front of me. Heat the solution to 70-80 deg C. Say a prayer. Make sure you've got all you gear on (boots, glasses, parka, etc.), and start passivating the tanks. I run a 45 minute cycle through the CIP of each vessel, making sure it's at least 60 deg C. If you have multiple heated vessels (mash mixer, HLT, kettle), space them between the cellar tanks so you can heat the solution back to temperature during the passivation of those vessels. Be mindful to run the solution through all of your process piping also. If you have copper cladding, be careful not to let any dripping solution come into contact with it. Do not rinse after passivation. Remove all fittings so that the tanks are as open as possible, and drain any dead legs in your piping. Let everything sit undisturbed for around 24 hours.
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