Nitrite is an excellent ...

Nitrite is an excellent product for closed systems but does require high levels of nitrite in the order of 500-1,0000 ppm.  It is usually buffered to ensure that it is at its optimum pH within the system of 9.5.  Yellow metal corrosion inhibitors will need to be added to protect copper.  Lower levels of nitrite may be used when combined with materials such as molybdate.  Nitrite should not be used in the food industry unless it is totally separate from any processing.  Certain bacteria will utilise the Nitrate and grow producing nitrate, which is not a good corrosion inhibitor.  So monitoring the closed systems is vital.  Once bacterial growth starts in a closed system, it is very difficult to control.  Dosing biocides may have an impact, but very often it returns.  The selection of biocide is difficult because at the pH involved, many non-oxidising biocides are inactivated over a period of time reducing their effectiveness.  Some of the bacteria which break it down are resistant to a number of biocides, while in other cases the biocide breaks down immediately  killing the bacteria. Any which are left will grow again.  The other issue is that they may have a hazardous classification of toxic and if it leaks into a water course it is a pollutant.  After saying all of this, nitrite is still a very useful corrosion inhibitor.