45 2 SCIENCE PROGRESS 



From this point of view, it is interesting that Dunstan and 

 Hill appear to have been successful in detecting the presence 

 of traces of the peroxide as a product of the rusting of iron. 



Sodium chloride and similar salts destroy the passivity of 

 iron, so that rusting takes place in their presence ; sulphates 

 and nitrates act similarly but less rapidly. It was pointed 

 out in the previous article that such neutral salts should 

 not promote rusting when alone. Dunstan and Hill have 

 shown by experiment that this is the case and that the strong 

 vivifying effect of sodium chloride is due to the combined 

 action of both air and the salt. It is to be supposed, 

 however, that the primary effect is due to acid — to carbonic 

 acid derived from the air — as electrolysis would not set in, 

 even though oxygen were present, if the metal were covered 

 with a non-conducting layer of oxide. 1 



To summarise the position — there are three classes of com- 

 pound, all electrolytes, with which iron may be brought into 

 contact under the ordinary conditions of practice: (i) acids 

 (carbonic and sulphuric in particular), (2) alkalies (usually lime) 

 and (3) salts (common salt, magnesium chloride, etc.) ; but 

 as oxygen is ever present, iron is never brought into contact 

 with these alone ; moreover, acid is always associated with 

 the oxygen owing to the presence of carbon dioxide in the air. 



Iron is one of those metals on which— given the necessary 

 electro-negative conductor — acids generally can act unassisted; 

 the interaction is always of the type 



Fe + 2HX = FeX 2 + H 2 



In other words, as the iron is dissolved, the hydrogen of 

 the acid is displaced by it ; in presence of oxygen this hydrogen 

 is more or less completely oxidised, according as the conditions 

 are more or less those which favour effective depolarisation. 



Apparently, no such direct attack is possible when an 

 alkali is used alone, the combination not being sufficiently 



1 The corroding effect of salt in conjunction with carbonic acid often is not 

 taken sufficiently into account in dealing with soft waters containing relatively large 

 amounts of sodium chloride and little if any calcium carbonate. A case in point 

 is that of the Goldfields Water Supply to Kalgoorlie, West Australia (see The 

 West Australian, Perth, March 16, 1910). The suggestion made to free the 

 water from oxygen before passing it into the steel main savours of the ridiculous 

 rather than of the sublime. 



