86 REPORTS ON THE STATE OP SCIENCE. 



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 mimimum corrodibility, 

 whilst the tempered steels containing the carbide as the emulsified 

 variety show a maximum corrodibility — 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 when immersed in sea-water — a conclusion which 

 is in complete accordance with the electrolytic theory advanced by 

 Cushman ~ and Walker. 3 This is also supported by the fact that in the 

 case of tempered steels a rise in the tempering temperature from 

 400° 0. to 500° 0. produces a marked decrease in the corrodibility, this 

 no doubt being due to the slight decrease in the fineness of division 

 of the pearlite which is produced by the rise in tempering temperature. 

 The conversion of the pearlite into hardenite is accompanied by a very 

 considerable rise in corrodibility, the hardened steels corroding more 

 rapidly than any of the unhardened or tempered steels. 



Careful examination of the deposits on the corroded bars on 

 removing them from the sea-water shows the rust deposit to be made 

 up of two different types. These may be briefly described as 



(a) Light brown deposit, flocculent and easily removed, forming 

 an even coating over the whole of the exterior of the rust deposit. 



(b) A deposit of bluish-black colour, somewhat greenish in some 

 cases, underlying the previous mentioned deposit. This is mainly 

 found at the lower end of the bars, to which it is usually somewhat 

 firmly adherent. In the case of the hardened and tempered specimens 

 a thin easily removed layer of a similar colour is found covering the 

 whole of the bar underneath deposit (a), in addition to the firmly 

 adherent deposit at the lower end. Whether this deposit is merely a 

 form of deposit (b), or whether it is really a chemically different type 

 of deposit of the same colour, is not known. A detailed examination 

 of these respective deposits might probably throw considerable light on 

 the phenomena involved in the corrosion of steel. 



Determination of Solution Pressures in Sea-water. 



Determinations of the solution pressures of the steels after pro- 

 longed immersion in sea-water have also been made with a view to 

 ascertaining the influence of carbon on the electro-chemical positions 

 of the various steels under conditions of galvanic corrosion. The results 

 up to the present, however, are not sufficiently conclusive to warrant 

 any definite statements in this direction being made in this report. 



2 Journ. Iron and Steel Inst., 1909, vol. 1, p. 33. 

 » Ibid., 1909, vol. 1, p. 69. 



