A system for the addition of a corrosion inhibitor to an engine cooling system wherein a container in the coolant system holds the corrosion inhibitor in a solid package which is withheld from contact with the coolant solution by a spring under load. The inhibitor package is counterbalanced by a pair of opposed springs with one spring being subject to corrosion when the corrosive effect of the coolant increases; corrosion of the spring allowing the inhibitor package to be urged into contact with the solution to provide treatment thereof.