60 PROCEEDINGS OF SECTION B. 



ferrous ion, the hydrogen ion separates out to a certain small 

 extent as hydrogen on the plate ; it is not separated in sufficient 

 quantity to form hydrogen gas bubbles, and remains as a layer on 

 on the surface of the iron. 



If the iron differs in its physical or chemical composition from 

 one place to another, the layer of hydrogen will not be evenly 

 distributed over the surface ; the hydrogen will then appear at a 

 point where there is no iron going into solution, and a current 

 tends to flow. 



What now is the effect of oxygen under these conditions ? 

 The oxygen oxidises the layer of polarising hydrogen on the iron, 

 which then dissolves further in the water. According to this view 

 of the action of oxygen, the corrosion of iron would probably be 

 hastened by contact with other more electro-negative metals, which 

 indeed is found by the author to be the case. 



After ferrous ion has been formed in solution it is oxidised by 

 oxygen to ferric ion, and in this way ferric hydrate (rust) is produced. 



EFFECT OF OTHER CONDITIONS ON CORROSION. 



(1) Addition of Carbon-dioxide. — It would be expected that as 

 this substance when dissolved in water increases the concentration 

 of the H ion it would assist corrosion. It may be calculated that 

 the amount of hydrogen ion in a specimen of water which contains 

 sufficient carbon dioxide to be in equilibrium with the carbon 

 dioxide in the air is ten times as much as exists in absolutely pure 

 water. 



Experiment shows that in many cases carbon dioxide 

 accelerates corrosion. Other things being equal, carbon dioxide 

 does accelerate corrosion ; if in any case presence of carbon dioxide 

 should not assist corrosion, then there must be some other disturbing 

 element present. 



(2) Addition of other Acids. — Other acids besides carbonic acid 

 also accelerate this corrosion of iron, the acceleration being in 

 general greater the stronger and more concentrated the acid. 



(3) Addition of Alkalies. — As might be expected, free alkali 

 prevents corrosion if present in sufficient quantity. Alkaline car- 

 bonates do not prevent corrosion ; indeed they usually accelerate 

 it. 



(4) Presence of Neutral Salts. — Neutral salts, such as common 

 salt, sometimes appear to accelerate the corrosion process, but not 

 always. Very httle is definitely known of the action of salts on 

 corrosion. They make the water a better conductor. Arsenic in 

 water prevents corrosion. Bichromates also prevent corrosion. 



(5) Temperature E-ffect. — Rise of temperature accelerates most 

 chemical reactions, but in cases where one of the reacting substances 

 is a gas, there will be less gas present in solution the higher the 

 temperature, and this decreases the rate of corrosion. 



