62 PROCEEDINGS OF SECTION B. 



0.80, 0.90, 1.18. After five weeks all the steels had corroded to 

 about the same extent. After about six months the steels with 

 the higher percentage of carbon had corroded considerably less 

 than those with the lower percentages of carbon. 



It is not possible at present, however, to lay down any general 

 rule regulating the manner in which corrosion depends on the 

 percentage of carbon in steel. 



Carbon in some varieties of cast iron is present as graphite. 

 Free carbon would be expected to accelerate the solution of the 

 iron in cast iron, and doubtless does so. Corroded cast iron pipes 

 are always found to have most of the iron oxidised or dissolved 

 away while the graphite remains. There is an impression that 

 cast iron often lasts better than steel or malleable iron. This is 

 probably because — 



(1) Structures of cast iron are usually much thicker than 



those of other forms of iron. 



(2) Cast iron structures have usually a uniform skin of 



magnetic oxide on them. 



(3) Cast iron often contains phosphorus. 



Magnetic Oxide of Iron. — This, if completely covering iron, 

 protects it from corrosion. However, it is easily chipped off in 

 patches, and the uncovered iron will then corrode more rapidly 

 than before, owing to contact with the more electro-negative iron 

 oxide. 



Slag or Cinder.— 'iX^.g is present chiefly in wrought iron. 

 Experience does not point to this being an objectionable con- 

 stituent. 



Manganese, Nickel, Silicon. — The effect of manganese or nickel 

 in such large proportions as 10 per cent, protects iron from corro- 

 sion, but in small quantities it is doubtful how these substances 

 act. The effect of silicon is also doubtful, being not very pro- 

 nounced. 



Phosphorus.- — Phosphorus usually acts so as to hinder corrosion. 



Arsenic (1%). — Arsenic also hinders corrosion. 



Sulphur. — Sulphur is undoubtedly a bad constituent, as it 

 accelerates corrosion. 



Occluded Gases. — Gases, in particular hydrogen, exist in small 

 quantities in iron. A small quantity of hydrogen, which is scarcely 

 detectable by analytical means, has probably a very pronounced 

 effect on iron. The hydrogen may come in during manufacture, 

 or it may be formed in pickling the iron in the acid. Iron which 

 has been cleaned by acid is more oxidisable than without this treat- 

 ment.^ Hydrogen accelerates corrosion of iron because it is 

 electro-negative to iron. 



Among accidental circumstances that effect corrosion must be 

 mentioned the electric currents which flow to earth in modern 

 cities. It appears^ that current flowing from iron pipes to earth 



1 Cushman and Gardner, op. cit., p. 69. 



2 Buyers; Trans. Amer. Elec. i,oc., 1908^ VJ, 19. ... 



