THE PRESERVATION OF METALS USED IN MARINE 



CONSTRUCTION. 



By Lieutenant Commander Frank Lyon, U. S. N. 



[Read at the twentieth general meeting of the Society of Naval Architects and Marine Engineers, held in 



New York, November 21 and 22, 191 2.] 



In taking the above title for this paper the writer intends to apply it 

 to vessels only under the two subdivisions : — 

 The metal of the hull. 



The metals of the engines, boilers, and machinery accessories. 

 The causes of the wasting away of such metals are three : — 



1. Corrosion. 



2. Abrasion. 



3. Erosion. 



Corrosion is the most serious of the three, is ever present, and is the most 

 difficult to combat. 



Metals are weakened by all of the three causes and also by the con- 

 tinued variation of stresses in them. These variations tend to open up and 

 emphasize the inherent defects in the metals due to segregation of the impuri- 

 ties in them and to their different physical characteristics which are induced 

 or produced by the different temperatures at which they are worked during 

 stages up to and including the finished article. 



Corrosion, the chemical decomposition of metal, is due to the differ- 

 ences in electric potential between the metal and the liquid or moisture that 

 wets its surface. It is produced only when the metal is wet or moistened by 

 some liquid of lower potential than metal itself. Metals that are absolutely 

 dry do not corrode, and wet metals corrode only on their wetted surfaces. 



In this paper it is assumed that there is some definite, absolute zero of 

 electrical potential similar to the absolute zero of heat, and that all bodies 

 and liquids have some definite potential at each temperature. When a metal 

 at a certain potential is immersed in a liquid of lower potential the metal 

 dissolves with the liquid and raises its potential; if the metal remains in solu- 

 tion in the liquid until the potential of the liquid solution is the same as that 

 of metal, the solution is then saturated and no fiurther dissolving takes place 

 until the conditions of temperature or liquid solution are changed. If the 

 potential of the liquid is the higher, the liquid exerts an electrical pressure on 

 the metal but does not cause it to dissolve. If the metal can dissolve in the 



