154 PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 



liquid but is thrown out of solution as rust by some chemical agent inherent 

 in neither the metal nor liquid, the dissolved parts do not raise the potential 

 of the solution and the dissolving goes on to the extinction of the metal. The 

 same results will be attained if the liquid is changed as fast as the metal dis- 

 solves into it, such as a plate on the side of a ship in a large body of water. 



The diffusion of the dissolved parts of any substance in a liquid is 

 believed to be due to the fact that the potential of the liquid is raised at the 

 dissolving point and that the tendency of all parts of any liquid body at 

 different potentials is to equalize or come to a common potential. This in 

 itself tends to keep the potential of the liquid at the dissolving point down 

 until the potential of the whole body of the liquid is brought to the same as 

 that of the dissolving substance. That is, the liquid becomes a homogeneous 

 solution throughout its whole mass by diffusion of the dissolved parts into 

 the solvent. 



If a metal has all of the points in its wetted surface at the same poten- 

 tial, and that potential is higher than that of the liquid by which wetted, 

 it will corrode or dissolve at all of its points and general corrosion is said 

 to exist. If the potentials of some of the points are higher and others lower 

 than that of the liquid in which it is immersed, it will corrode only at the 

 points where its potential is higher than that of the liquid, and local corro- 

 sion is said to exist with pitting of the metal as a result. 



The potentials of pure metals and liquids vary with their temperatures 

 and with the way in which they are stressed, and those of impure metals and 

 solutions vary in the same way, and also with the way in which the ingre- 

 dients vary with the temperatures. That is, an alloy would not have the 

 same potential that any one of its ingredients would have at the same tem- 

 perature, but one common to that of all of the ingredients. Also, the poten- 

 tial of a solution at any temperature would not be the same as that of 

 either the liquid or dissolved substance at that temperature, but at some 

 one potential common to them both. In other words, the theory of poten- 

 tials is assumed to be similar to the theory of heat. If two bodies, solid, 

 liquid, or gaseous, at different temperatures are brought near each other 

 they assume a common temperature in proportion to the amounts of heat 

 each contains, and it is easily conceivable that the same objects at different 

 electrical potentials would come to some common potential in proportion 

 to some inherent qualities of the two bodies in question. 



If two metals are connected by a metal conductor and they are immersed 

 in a liquid, and if one of them is at a potential higher than the liquid and the 

 other at a lower one than the liquid, the higher one dissolves into the liquid 

 and raises its potential, the liquid discharges its potential to the one lower 



