Section III., 1913. [183] Trans R.S.C 



The Corrosion of Metals by Water. 



With special reference to relative corrosivity of " mechanically filtered " 



and "raw" waters and to the effect of water on unlike 



metals in contact. (Summary). 



By A. T. Stuart, B.A. 



Presented by Frank T. Shutt, M.A., F.R.S.C. 



The object of this investigation was primarily to ascertain the 

 effect that ''mechanical filtration" or treatment witli alum might 

 have on the corrosivity of the Ottawa River water. In the course 

 of the work other points of interest suggested themselves and were 

 included. 



Without attempting to give here even a brief review of the sub- 

 ject of corrosion, it may be stated that the "electrolytic theory" is 

 now generally accepted as furnishing an explanation of the various 

 phenomena. 



We may imagine the water itself, more or less, and especially 

 such substances in solution as carbonic acid, being partly dissociated 

 into ions carrying positive and negative charges of electricity. The 

 hydrogen ions, carrying positive charges, attack the metals in contact, 

 giving up their positive charges of electricity, and, changing from the 

 ionic to the molecular or gaseous condition, are expelled from the 

 sphere of action. In the case of iron, the metal goes into solution 

 in the ferrous state carrying a positive charge of electricity. If 

 oxygen be present the iron is oxidized to the ferric state and precipi- 

 tates out as hydrate in the comparatively neutral state of some waters. 

 It is this precipitate, which is quite apparent to the eye, that is the 

 cause of the so-called "red water trouble" common to certain water 

 systems employing the alum treatment of purification. On the other 

 hand, in some waters iron remains in a state of either ordinary or 

 colloidal solution and, not being so apparent, does not render the 

 water so objectionable as a household supply. 



Further complications, however, than those so far outlined, ensue 

 when other materials than the pure metal are to be considered. In 

 actual practice there is no such thing as pure metal ; all contain greater 

 or less amounts of foreign admixtures. For example, iron in the 

 arts and manufactures always contains silicon, sulphur, phosphorous, 

 manganese and carbon. All the various elements exhibit a difference 



