156 PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 



Two pieces of steel from the same plate may vary perceptibly in the way 

 in which they corrode and in the loss of weight per unit of area in a unit of 

 time due to the above causes; that is, certain constituents may be segregated 

 over the surface of one piece and not over the other, or different stresses may 

 exist in the two surfaces, due to the different ways in which they have been 

 worked or treated. An angle bar corrodes more quickly than a plate of 

 steel of practically the same constituents; a bent plate or angle bar corrodes 

 in and near the bend more quickly than the straight part. There is always 

 more corrosion around the entrance, run and bilge of a vessel where all of 

 the hull plates and frames are bent, than along the straight body. A rivet 

 that has been hammered and upset corrodes faster than the same rivet 

 before it was hammered. The assumption that any piece of steel is a 

 homogeneous body either as to chemical contents or as to physical stress is 

 one that can very easily be disproved by sufficient chemical analyses and 

 by photomicrographic work. The writer was much surprised to note the 

 differences in chemical composition and photomicrographic results in a boiler 

 tube, samples taken (i) over the expanded surface and (2) over the unex- 

 panded surfaces. He was also surprised to see the different ways in which 

 boiler tubes corroded over the same areas, as shown both by Cushman and 

 Walker's ferroxyl mount and by actual immersion. Steels are of higher 

 potential than water at ordinary atmospheric temperatures. 



Copper and copper compositions vary in the way they corrode. 

 These variations are due to chemical compositions, variable temperatures, 

 and to the way in which they have been worked, producing stresses in the 

 surfaces of the finished product. At ordinary atmospheric temperatures 

 copper and its compositions are of lower potential than water, i. e., they are 

 said to be electronegative to water. As their temperatures are increased or 

 as they are stressed to higher degrees their potentials increase faster than 

 does the water in which they are immersed, and if the temperatures or 

 stresses are raised sufficiently they become of higher potential than water 

 and corrode or dissolve similarly to steel. As a proof of this, from observa- 

 tion on shipboard for thirteen years, copper and composition piping corrode 

 faster at the bends and near flanges that at the straight parts, and pipes in 

 which the temperatures are different in different places, such as feed piping 

 through feed heaters, condenser and circulation piping, and other piping 

 which is at different temperatures over different parts, corrode faster than do 

 the fire mains and other piping which are at the same temperature through- 

 out. Two pieces of copper or composition cut from the same plate, treated 

 in the same way, may show no difference of potential when immersed together 

 in water and connected through a potentiometer. Then if one piece is 



