1 62 PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 



4. Alkaline substance dissolved in pure distilled water increases its 

 potential. 



5. The decrease of the potential of water when an acid is dissolved 

 in it is due to the fact that the negative ion of the acid is lower in potential 

 than that of the hydroxyl OH of water, the H ion being common. 



The increase in potential of water when an alkaline substance is added 

 to it is due to the fact that the metallic or positive ion of the alkali is higher 

 in the potential series than hydrogen, while the hydroxyl or negative ion is 

 common to both. 



6. When any metal is wet by any water solution, if the potential of 

 the solution is (a) higher than that of the metal at every point on its surface, 

 the metal will not corrode; (b) higher than some points in the metal surface 

 and lower than that in others, the metal will corrode only over the areas 

 where the potential of the solution is the lower; and it will corrode faster 

 over those points than it would if immersed in distilled water alone, due to 

 the fact that though the difference of potential between that of the metal 

 and that of the alkaline solution is less than it would have been in distilled 

 water, yet the conductivity of the water has been greatly increased by the 

 addition of the alkahne substance; (c) lower than that of the metal at any 

 point it will corrode all over at the same time, faster over the higher potential 

 points than over the lower ones. The rate of corrosion over all of the points 

 will depend both upon the difference of potential between the metal at the 

 point and the solution, and also upon the conductivity of the solution. As 

 the alkalinity of the solution decreases, the difference of potential between 

 the metal and the water increases and the conductivity of the solution 

 decreases until pure water is reached. Any addition of an acid then increases 

 the difference of potential between the metal and the solution and also 

 increases the conductivity of the solution. 



When considered with a given metal the potential of some alkaline 

 solutions increase faster with a rise in temperature than does that of the 

 metal, therefore a very weak solution that may not be dangerous at ordi- 

 nary atmospheric temperature may be strongly active in the form of pitting 

 to the same metal at higher temperatures. 



Also a solution that may be strongly active to metals at atmospheric 

 temperatures may be non-corrosive entirely at still higher ones. 



The converse may be true with some of the alkaline substances, that 

 is, a rise in temperature may not increase the potential of the solution as 

 fast as it does the metal and a safe solution at atmospheric temperatures 

 may be unsafe at higher ones. In the writer's experiments with alkaline 

 solutions the general rule was that a one one-hundred-thousandth and a 



