Corrosion Inhibitor is used in the water in Hot Water ("Hydronic") Central Heating systems to protect ferrous components of the system (including steel radiators and cast-iron boiler heat exchangers) from corrosion caused by the action of oxygen dissolved in the water.
Corrosion inhibitors vary in price from about £5 to £15 (for a quantity intended to protect an average domestic installation). The chemistry of corrosion and corrosion-inhibition is, presumably, well known, so one would expect the various different makes of inhibitor to be pretty much equally effective. Unfortunately simple experiment suggests otherwise.
Corrosion Inhibitor test