What are the criteria underlying the choice of corrosion inhibitors?
A corrosion inhibitor should meet the following requirements :
- It should be effective over a wide pH range.
- It should react with the metallic substrate with the formation of products that are far less soluble tha the inhibitor.
- 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.
- It should from a film at the primer/substrate interface which will enhance adhesionbut under no circumstances affect it.
- It should be capable of reacting cationically as well as anionically.
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.