224 PAINT TECHNOLOGY AND TESTS 



carry quite a percentage of water, without the admixture being 

 apparent to the eye. In addition to this, careful experiments 

 have proved very conclusively that linseed oil films, even 

 after they have oxidized and hardened, have the power to a 

 certain extent of absorbing water from the atmosphere. It is, 

 therefore, safe to say that no linseed oil film in a paint coating 

 is dry all the time. As a matter of fact, there is abundant evi- 

 dence to show that in rainy weather, and, in fact, when the 

 humidity in the air is high, paint films have absorbed water. 

 As the sun comes out and warms the paint coating, and the 

 humidity content of the atmosphere falls, this water to a large 

 extent evaporates out of the film, only to be taken up again when 

 the weather conditions change. This action may be likened 

 to a breathing of the paint film, that is to say, an indrawing of 

 water under humid conditions, followed by an exhaling of water 

 under dry conditions. With these facts in mind, it must be 

 apparent that pigments laid out in intimate contact with the 

 surface of steel are subjected at all times either more or less to 

 the reactions produced by water contact. Furthermore, as it 

 is a property of water to become saturated with the gases of the 

 atmosphere, such as oxygen, carbonic and sulphurous acids, 

 and other impurities, there is present in a protective paint film 

 at all times the elements necessary to carry on the corrosive 

 process and reactions. 



An outline of Cushman's original research work, upon which 

 has been based the classification of pigments as inhibitors, 

 stimulators, and inerts, is clearly presented in his report * as 

 Chairman of Committee U of the American Society for Testing 

 Materials, of which the following is an excerpt : 



" Three years ago the suggestion was made in a paper pre- 

 sented before the Tenth Annual Meeting of this Society that the 

 various types of substances used as pigments in protective coat- 

 ings might exert a stimulative or an inhibitive action on the rate 

 and tendency to corrosion of the underlying metal. It was 

 further suggested on a theoretical ground that slightly soluble 

 chromates should exert a protective action when employed as 

 pigments by maintaining the surface of the iron in a passive 

 condition in case water and oxygen penetrated the paint film. 

 In view also of the well-known fact that alkalies inhibit while 



1 Page 73, 1910 Proceedings of the American Society for Testing Materials. 



