
ON THE ACTION OF AIR AND WATER UPON IRON. 9° 
Wrought Iron and Steel. 
The loss of metal by corrosion is, in every sort of water tried, much more 
rapid in wrought iron than in cast iron, and the same is the case with steel. 
It takes place however much less locally than in cast iron. The particular 
phznomena presented by the corroded wrought iron and steel are given in 
Tables II., IV., VI., VIII. and X. respectively. In almost every case the fibre 
or crystalline texture of the iron becomes developed by the removal of the 
alternating portions of metal. This dissection commences usually at the ex- 
posed ends of the fibrous crystals, is most rapid in the direction of the prin- 
cipal axes of the crystals, and sometimes extends to a great depth. 
309. We thus learn that all wrought iron and steel consists of two. or more 
different chemical compounds coherent and interlaced, one of which is elec- 
tro-negative to the other, the electro-positive body being that which suffers 
first from corrosion. The electro-negative portions of the iron or steel re- 
main bright and hold a perfect metallic lustre until the whole of the other 
portions are removed, or at least are so to a great depth, when they begin 
likewise themselves to oxidate. Most of the specimens, when first taken: up, 
were found in this state, but soon tarnished on exposure to air. The great 
depth to which this removal of alternate layers takes place, is most remark- 
able in the case of Damascus iron (a 12) as it is called, made for the ma- 
nufacture of fowling-piece barrels, and purposely formed of irons of two or 
more different qualities, faggoted together: here specimens of about half an 
inch thick, chosen with straight parallel fibres of alternate kinds, had the elec- 
tro-positive strata (in § 26) removed quite through, so as to leave a grating 
of minute parallel rays that could be looked through. 
310. In general, the finer the quality of wrought iron and the more perfectly 
uniform its texture, the slower and the more uniform is its corrosion in water, 
as we before found in the case of cast iron; minute difference in chemical 
constitution has very little effect on the rate of corrosion ; thus the difference 
is slight between the index of corrosion of—a’ 1, Gloucestershire iron of fine 
quality, tough, both hot and cold; a! 2, Staffordshire iron, red-short, contain- 
ing sulphur and perhaps a trace of arsenic; and a! 3, cold-short Staffordshire 
iron, containing phosphorus in large proportion; but it is very great between 
these and @' 4, a common bar of inferior Shropshire iron. This iron, on ex- 
amination of its fracture with a lens, showed the presence of innumerable 
microscopic spots of silicate of oxide of iron and of magnetic oxide. These - 
immediately, on exposure to air and water, become partially converted into 
peroxide of iron, and being electro-negative to the iron itself, powerfully 
promote its corrosion. Hence such “slaggy” iron, as it is technically called, 
is to be avoided where durability is important. 
311. To the general fact of uniformity of texture giving a small index of 
corrosion, unfinished bars from the puddling furnace, before the second rolling, 
are an exception. These bars contain a large amount of silex, and are ex- 
tremely hard. They are of no constructive value, of course, but the experi- 
ments with them (a 10, 11, &c.) show that, as in cast iron, so here hardness or 
softness are elements in the rate of corrosion ; and this is further confirmed by 
the results relating to steel, wherein the small amount of corrosion for hard- 
ened cast steel is remarkable. 
The highly siliceous irons, however, corrode very locally, and appear to be 
partially defended from the reaction of air and water by a thin coat of silex 
formed upon them. i 
$12. Of all the wrought irons experimented on, that which was found most 
durable under all possible conditions of exposure was faggoted serap iron 
bar (a 14), that which had been most wrought, and which was not only most 
