A corrosion inhibitor should meet the following requirements :
1. It should be effective over a wide pH range.
2. It should react with the metallic substrate with the formation of products that are far less soluble tha the inhibitor.
3. Its solubility in the primer should neither be too slight ( in order to guarantee effectiveness ) nor too great, so that the inhibitor reservoir is not exhausted too quickly.
4. It should from a film at the primer/substrate interface which will enhance adhesionbut under no circumstances affect it.
5. It should be capable of reacting cationically as well as anionically.
There are many organic as well as inorganic corrosion inhibitors which meet above requirements. Inorganic inhibitors include those which act through oqidation, containing lead or chromium, lately also molybdates and tungstenates, and those which are non-oxidative e.g. variousphosphates and borates. Zinc dust also affords good protection against corrosion . Organic substances offer many possibilities. Zinc, calcium an sodium salts of aliphatic carboxylic acids also prevent corrosion whilst certain amine-based inhibitors can be used in a similiar manner, as well as amine salts, e.g. oleates. Finally, mention should be made of sulphonates and organic phosphorus compounds.