PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 155 



than itself, and a current flows and the original condition is maintained. If 

 the metal connection is broken, then the metal of higher potential raises that 

 of the liquid, which in turn raises that of the metal of the lower potential, and 

 eventually the potential of the whole becomes equal to that of the higher 

 metal, no current flows, the original condition is destroyed, and the metal- 

 water-metal series is polarized. A metal with different potentials at points 

 in its surface immersed in water or a water solution acts in the same way. 

 If all of the points are higher than that of the solution they all corrode until 

 the potential of the liquid is raised to that of the lowest point on the metal. 

 As the points of higher potential dissolve into the liquid and tend to raise its 

 potential, the liquid then discharges its potential to the points in the sur- 

 face lower than itself, a current flows from the high point to the liquid, from 

 the liquid to the metal at the points lower than itself; the circuit is then 

 complete and local corrosion as pitting is the result. 



This discussion on corrosion is made necessary by the subject-matter 

 that is to follow. The statements made above will be referred to in many 

 places and would not be understood unless explained beforehand. 



Abrasion is due to the wearing away of one substance by rubbing 

 against another. In vessels' hulls it is mostly noticeable on the bows where 

 rubbed and abraided by the anchors and chains. In their engines and 

 machinery it is noticeable in the bearings, guides and similar construction, 

 especially when not properly cared for. Its effect on the hull material is not 

 serious and cannot be prevented in wake of the anchors and chains. There 

 is no danger of damage from that source. 



Erosion is due to the wash or scouring action of liquids or gases over 

 metal at high velocities. With the exception of valves, and maybe in a few 

 other places about the machinery, its effect will not cause sufficient loss to 

 be noticeable. 



Metals used in the construction of the hulls of vessels are : Steel for 

 plates, angles, beams, rivets, struts, and external and internal braces and 

 hull fittings. Composition castings for sea chests and propellers. The 

 compositions generally used are manganese and phosphor bronzes; monel 

 metals (nickel bronzes) and an 88 copper, 10 tin, 2 zinc bronze. 



Steels are extremely variable in the way in which they corrode and 

 in their effect upon other steels to which they are connected by metallic 

 contact. These variations are due to the differences in chemical composi- 

 tions and to the difference in state of stress of the molecules of the metals 

 in their wetted surfaces. These different stresses are in turn due to the 

 different workings the metal has received, the temperatures at which it has 

 been worked, and to the heat treatment finally given the finished product. 



