CHAPTER XXVIL 

 PAINTS AND PAINTING. 



Corrosion of Steel. If iron or steel is left exposed to the atmos- 

 phere it unites with oxygen and water to form rust. Where the metal 

 is further exposed to the action of corrosive gases the rate of rusting 

 is accelerated, but the action is similar to that of ordinary rusting. Rust 

 is a hydrated oxide of iron, and forms a porous coating on the surface 

 of the metal that acts as a carrier of oxygen and moisture, thus pro- 

 moting the action of corrosion. If nothing is done to prevent or retard 

 the corrosion of the iron and steel used in metal structures, the metal 

 rapidly rusts away and the structure is short lived. Wrought iron is 

 affected by corrosion more than cast iron, and steel is affected more 

 than wrought iron. 



The corrosion of iron and steel may be prevented or retarded by 

 covering it with a coating that is not affected by the corroding agents. 

 This is very effectually accomplished by galvanizing; but on account 

 of the cost it is impracticable to use the process for coating anything but 

 sheet steel and small pieces of structural steel. The most common 

 methods of protecting iron and steel are by means of a coating of paint, 

 or by imbedding it in concrete. 



PAINT. The paints in use for protecting structural steel may 

 be divided into oil paints, tar paints, asphalt paints, varnishes, lacquers, 

 and enamel paints. The last two mentioned are too expensive for 

 use on a large scale and will not be considered. 



OIL PAINTS. An oil paint consists of a drying oil or varnish 

 and a pigment, thoroughly mixed together to form a workable mixture. 

 "A good paint is one that is readily applied, has good covering powers, 



