"Built-in corrosion inhibition" - Does it work? How?
The plastic contains Volatile Corrosion Inhibitor (VCI) molecules that are adsorbed onto the metallic surface, protecting the metallic surface by forming a film. The Zerust FAQ claims that the inhibitor “passivate the electron flow between the anodic and cathodic areas on metal surfaces and interrupt the electro-chemical corrosion process.” 

Zerust products for storage are advertised as being effective for five years, but I would expect them to be effective for a much shorter time in a toolbox that is frequently opened and used. You may find numbers in the specs for their military products.