PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 157 



removed and hammered, worked or heated, and replaced, it will show that 

 it is electropositive to the unworked piece, and it will corrode if placed in 

 water of a certain temperature in which the other piece may or may not be 

 affected according to the treatment and chemical composition of the original 

 sample. Composition castings are generally of a very uniformly stressed 

 surface, due to the heat treatment naturally given in coaling. Those placed 

 as sea chests in ships require no protective coating and seldom if ever give 

 any trouble, the highest temperatures to which they are subjected are not 

 sufficient to raise their potentials to equal that of the sea water in which 

 immersed. If the copper compositions are segregated and these segregated 

 spots are of higher potential than sea water, they dissolve away and pit to 

 the depth to which the segregations extend. 



These compositions are negative or lower in electrical potential than 

 ordinary or sea waters, while steels are electropositive or of higher potential 

 than the same waters at ordinary temperatures. If the bare steel is immersed 

 in the water it will corrode or dissolve at all points in its wetted surface, 

 while the composition casting in the same waters will not corrode at any 

 of its points. If these two metals are connected by a perfect metallic contact 

 and immersed in the same water, the rate of corrosion over the steel will be 

 greatly increased. The rate at which it will be increased all over its surface 

 will depend upon the perfection of the metallic contact; if there is no such 

 contact the presence of the composition will not increase the rate of corrosion 

 on the steel, no matter how close together they may be. If the steel is 

 covered with a moisture-proof paint and the same contact as above made, 

 it will not corrode when connected by a perfect metallic contact with the 

 compositions, but if the film of the moisture-proof paint is broken so that 

 any part of the steel surface is exposed to the water it will corrode rapidly 

 over the exposed part. 



Where these composition castings are secured to the steel hull of 

 a vessel it is customary to secure plates or rings of rolled zinc as a protection 

 to the steel, the idea being that the zinc being of higher potential than the 

 steel, will corrode and preserve the steel. That this assumption is thought 

 to be correct is evidenced by the great use of zincs at a very large cost in 

 these places. Yet the writer has failed to see that the assumption is posi- 

 tively correct for the following reasons: (i) Zinc quickly corrodes and 

 becomes covered with zinc oxide, and zinc oxide from boiler, condenser and 

 hull zincs has been found to be electronegative to steel in every case tried. 

 (2) In no case has the writer ever had a piece of steel connected to a zinc 

 plate corrode less in the same water in thirty days than a piece of steel from 

 the same plate placed alongside the first plate, but not connected to zinc. 



