Oresson.] 606 [June 18, 1875. 



The result of the introduction of a new sheet was to set up at once a 

 strong galvanic action by which every sheet in the boiler was corroded 

 except the new one. 



Samples of iron cut from the edges of the old and from the new sheets 

 were placed in a bath to which a few drops of dilute acid were added and 

 a connection made with a galvanometer, resulting in the production of a 

 strong current ; the purer iron corroding, and protecting that which con- 

 tained the greatest amount of carbon. 



The inciting cause of the galvanic action was therefore judged to be 

 the introduction of a sheet of iron electro-negative to those already in 

 the boiler, its position in the electro-chemical scale depending upon the 

 amount of carbon it contained. 



The injurious effect consequent upon the junction of masses of wrought 

 iron of varying electro-chemical properties, is, therefore, increased when 

 steelis joined to wrought iron, as is frequently the case in locomotive 

 boilers in the tubes and tube sheets. 



Again by the junction of cast iron to steel or to wrought iron, the de- 

 structive effect is greatly intensified, and at times becomes quite as vio- 

 lent as when copper is made an element in the galvanic circuit in con- 

 nection with wrought iron. 



The necessity for the selection of iron with reference to its electric con- 

 dition, applies equally to the material employed for Bridges or Vessels or 

 Boilers or any structure which is to be built up from separate sheets and 

 bars of iron. 



It is or ought to be the habit of careful constructors to cut sample 

 pieces from every sheet or bar of metal worked, and to make a trial of 

 their quality by bending hot and cold, and to make frequent tests of ten- 

 sile strength. Examinations as to electro-chemical condition can be 

 made with equal facility. Determinations of the composition of the 

 metal or of the percentage of carbon in it by chemical analysis are un- 

 necessary ; an ordinary workman furnished with a coarse galvanometer 

 and a weak acid bath can ascertain the exact electro-condition of each 

 sheet or bar more rapidly than he can examine the quality by the ordinary 

 tests of bending on an anvil, hot and cold. With the metal of Bridges, 

 Vessels, and especially Steam Boilers, the deterioration by corrosion is 

 more to be feared than is mechanical wear. 



Galvanic corrosion acts with greater vigor in loca.tions that are usually 

 inaccessible, such as the interior of joints or defective sheets or parts that 

 are closely approximated, and the mischief is only suspected when it has 

 progressed to such a degree as to become evidently dangerous and the 

 parts are in condition to require immediate attention and repair. 



Attention to the precautions enumerated for secui'ing mechanical and 

 chemical fitness of the metal to be used for structures of iron, will un- 

 doubtedly promote economy and safety. 



