326 SCIENCE PROGRESS 



The determinations of corrodibility were carried out with 

 samples subjected to heat treatment 



" designed to resolve the pearlite into the principal varieties 

 in which it usually exists in carbon steels — viz. : (a) Diffused, 

 (b) Laminated and (c) Emulsified and also into Hardenite, 

 which is the essential constituent of hardened carbon steels. 

 The treatments employed were (i) As Rolled, (2) Normalised, 

 (3) Annealed, (4, 5) Hardened and Tempered at (a) 400 C. and 

 (b) 500 C, (6) Hardened." 



The results indicate 



" that the carbon exerts two types of influence upon the 

 corrodibility, dependent upon the condition of the carbide in 

 the steel. In the rolled, normalised and annealed specimens, in 

 which the carbide (as shown by microscopic examination) exists 

 entirely as the diffused, normal variety or as the laminated 

 variety of pearlite, the corrodibility rises to a maximum at 

 saturation point (0*89 per cent, carbon) and then decreases upon 

 the appearance of cementite in the steel. The rise in corrodi- 

 bility in such steels with increase of carbon from o'io per cent, 

 to o'8a. per cent, is not regular but is much more rapid in the 

 range o'3 per cent, to 089 per cent, carbon than in the low 

 carbon range o'io per cent, to 03 per cent, carbon. 



" In the hardened and tempered specimens, however, in 

 which the carbide has been converted respectively either to 

 hardenite or the emulsified variety of pearlite, it has been found 

 that the corrodibility rises continuously from o'io per cent, to 

 0^96 per cent, carbon, no maximum being observed at the 

 saturation point. The proportional increase of corrodibility in 

 these steels with rise of carbon per cent, is very rapid up to 

 about C25 per cent, carbon in the case of the hardened steels 

 and about 0*40 per cent, carbon in the tempered specimens. 

 After these points the rate of increase of corrodibility with rise 

 of carbon per cent, is small but regular up to 0*96 per cent, 

 carbon, being in this respect the reverse of that found in the 

 type first described. 



" The state of division of the carbide in the pearlite is found 

 to exert very considerable influence on the rate of corrosion of 

 iron-carbon alloys. In general, the annealed steels in which 

 the carbide exists entirely in the laminated condition show 

 a minimum corrodibility, whilst the tempered steels containing 

 the carbide as the emulsified variety show a maximum corrodi- 

 bility- except in the very low carbon steels. The normalised 

 steels, moreover, in which the carbide is in an intermediate 

 state of division, being mainly of the diffused variety, take up an 

 intermediate position. This indicates that the finer the state of 

 division of the carbide in the pearlite, the greater is the liability 

 to corrosion." 



