PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. i6i 



internal or water side of the above cannot be protected by paints nor is it 

 generally believed by any other means, though lacquers, galvanizing, lead 

 lining and other means have been tried on pipes, they have all proved unsatis- 

 factorily, in some cases due to the unequal coefficients of expansion of the 

 coating and metal, in some to the solubility of the lacquer, and in others to 

 the fact that the metallic coating cannot be properly applied to the finished 

 article. As the life of piping, boiler plates and other parts is no greater than 

 the weaker part, the piece fails when improperly protected. Boiler, con- 

 denser, feed water, distiller, and evaporator tubes cannot be protected on 

 either side by paints, lacquers, and coatings. Therefore all such parts 

 should be made of homogeneous metals properly heat treated after being 

 finished and then fitted properly so that the least possible internal strains 

 are brought into it due to the fitting. 



It [has been definitely proved that any water made alkaline enough 

 to show 3 per cent of normal alkalinity with calcined sodium carbonate is 

 non-corrosive to steel at all temperatures up to 422° F. Therefore if the 

 water in the boilers, tanks, evaporators and feed-piping is kept at or above 

 that strength with sodium carbonate at all times, no corrosion will take 

 place, while if the strength is allowed to fall to about 1.8 to 2.5 per cent, 

 bad pitting will take place. In trying to reproduce the pitting continually 

 taking place in boilers, tanks, piping and on bilge-plates with acid solution, 

 or plain sea or distilled water, the writer failed in every instance on three 

 grades of nearly pure irons, three grades of boiler steel, and on four grades 

 of cast iron. It was easy to produce such similar pittings on all of them 

 when immersed in weak alkaline and very weak carbonate solutions. For 

 the above reasons it is believed by the writer that much more harm is done 

 in enclosed metal vessels by water made slightly alkaline, either artificially 

 or naturally, than is ever done by the small percentage of acids that ever 

 enters the average boiler. Boilers and other water containing metal vessels 

 or conduits containing acid water will go to pieces quickly, practically all 

 over at the same time; those containing sea or distilled water will go to pieces 

 all over at the same time but very slowly; those containing alkaline water of a 

 strength not high enough to stop all corrosion will go to pieces in the weaker 

 places, while other parts or places will remain perfectly good. If the per- 

 centage of alkalinity is high enough no corrosion whatever will take place. 

 The reason for this is believed to be as follows : — 



1. Water has a certain definite potential at any definite temperature. 



2. Pure distilled water has a potential lower than that of steels, irons, 

 and some alloys. 



3. Acids added to pure distilled water at any temperature decrease 

 its potential. 



