378 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



clearly understood, but is probably due to some peculiar quality 

 of the metal used. 



Corrosion is accelerated by impurities in the water, and 

 especially by the presence of decomposing organic matters or 

 of free acids. It is also accelerated by the contact of the iron 

 with any other metal which is electro-negative relatively to 

 the iron, or, in other words, has less affinity for oxygen. If two 

 portions of a mass of iron are in different conditions, so that 

 one has less affinity for oxygen than the other, the contact of the 

 former makes the latter oxidate more rapidly. In general, hard 

 and crystalline iron is less oxidable than ductile and fibrous iron. 1 



Mr. Kinipple remarks that, after a life of from thirty to 

 fifty years, structures depending upon cast iron exposed to the 

 rapidly oxidizing action of sea-water can only be looked upon 

 as of a comparatively temporary character, especially as regards 

 very light cast-iron pile structures. 2 



Experiments tend to show that steel immersed in sea-water 

 is more liable to corrosion than wrought iron. 3 



Experiments made by Mr. B. H. Thwaite, A.M.I.C.E., on the 

 action of oxidation, in order to ascertain the metallic life of iron 

 with a tolerable degree of exactitude, appear to show that a 

 bar of wrought iron 4 inches by 1 inch, subjected to the cor- 

 rosive atmospheric influences of a manufacturing city, would be 

 entirely corroded away in a little over a single century. 4 



The object of all methods of preservation is to protect the surface 

 from the oxidizing influence of the air or water. For ironwork 

 generally above water, good oxide of iron paints are perhaps the 

 best preservatives that can be applied. Mr. Matheson thinks that 

 for wrought iron oiling is a better protection than painting. 5 



At the Forth Bridge, two coats of oxide of iron paint were 

 applied over two coats of red lead. 6 



For under-water work, all castings should be subjected to 

 Dr. Angus Smith's process, which consists in heating the per- 

 fectly clean castings to a temperature of about 300 Fahr., and 

 at once immersing them in a bath of coal-tar and linseed oil also 

 heated to, and maintained at, a temperature of about 300 Fahr. ; 

 in this bath the castings should remain for about one hour. 

 " In place of heating the clean castings before immersion 



1 " Rankin's Civil Engineering," p. 514. 



1 "Lectures to R. E. Inst., from Engineering," vol. 1. p. 515. 



1 M.P.I.C.E., vol. Ixxxii. p. 292, 293. * Ibid., vol. Ixxiv. p. 215. 



* Ibid., vol. Ixv. p. 113. Engineering, vol. xlix. p. 273. 



