Detection of rust formed under paint films

Corrosion is an evil that has plagued man since before the iron age. Three ways of controlling corrosion are: 1. alloy the metal to make it chemically inert to corrosion, .2. change the surroundings to a noncorrosive atmosphere, and 3. isolate the metal from the corrosive atmosphere, as for instance by the application of a number of coats of paint that has been found to be the most economical method. At present, there is no reliable method of testing the effectiveness of a paint film that does not require the corrosion products to destroy the paint film and show themselves on the surface. This takes a very long time when good paints are used. Both the salt fog exposure and the field test require the corrosion products to destroy the paint film. Various tests have been used to try to find some correlation between the anti-corrosive properties of a paint and another physical property such as: elongation- tensile strength, water-, ion-, and oxygen-permeabilities, electrical resistance, dielectric strength, and Impact resistance. None of these seem to have a direct correlation to the corrosion preventive properties of different paints. With both conducting and non-conducting metallic compounds being used as pigments, a wide variety of electrical properties can be expected. Since the vehicles include oils, resins, lacquers, emulsions, etc., and all kinds of blends, a wide variety of permeabilities and other physical properties can result. There are a few paints within each group of vehicles and pigments that seem to be fairly good at controlling corrosion. A quick laboratory test method that would put all the paints, regardless of their pigment or vehicle class, on an equal footing with regard to their corrosion inhibiting properties is badly needed and it was the purpose of this investigation to try to develop such a method --Introduction, pages 1-2