{Paper read by Professor Henry M. Howe at the International Congress on Testing Materials of Construction (under the auspices of the Paris Exposition, 1900).

The facts presented were based on the determination of the loss of weight by oxidation of several plates of wrought iron, soft steel, 3 per cent. nickel steel, and 26 per cent. nickel steel, after an exposure to sea water, river water, and also to the weather, for two periods of about one year each. Each of the plates was about 24 ins. long, 16 ins. wide, and 1/8 in. thick, the total weight of all the plates was 2,597 lbs., and the total area exposed was 928 square feet. Professor Howe stated that the scale of these requirements was therefore not only much larger than that of any previous experiments of which he knew, but also larger than that of all previous experiments taken collectively. His paper includes tables and comparisons of all previous accessible reliable investigations on this subject.

Professor Howe sums up the results of his experiments in the following table: -

Relative Corrosion of Soft Steel, Wrought Iron, and Nickel Steel

(Wrought iron taken as a standard.)

He therefore found that soft steel corroded less than wrought iron in fresh water, but more than wrought iron in sea water; the difference, though always moderate, was in each case sufficiently constant to raise a considerable presumption that it was a real, and not merely an apparent, one. In Krupp's very important experiments the opposite results were obtained, for soft steel corroded much less than wrought iron in sea water; and here, too, the difference was so constant as to raise a considerable presumption that it was real, and not merely apparent.

Professor Howe draws two inferences: -

1. That the difference in the rate of corrosion between wrought iron and soft steel is rarely enough to be of great moment, except perhaps in marine boilers.

2. That the ratio of the corrosion of a given soft steel to that of a given wrought iron may vary greatly with the conditions of exposure.

He suggests two chief causes for the apparent discrepancies between the results not only of different observers, but even of the same observer: -

1. The quasi-accidental variations, individual peculiarities, etc.

2. That the susceptibility to corrosion of soft steel, taken as a whole, does differ somewhat from that of wrought iron taken as a whole; but that this difference is of such a nature that wrought iron, as a class, corrodes, on an average, faster than soft steel under certain conditions, but slower than soft steel under others.

He referred to the strong and wide-spread belief, at least in the United States, that soft steel corrodes much more rapidly than wrought iron; and stated that this belief has greatly retarded the introduction of soft steel for tubes and other purposes in which oxidation is a matter of vital importance; and stated that, having before him the results of such extensive experiments indicating the reverse of this belief, he was led to study the cause of the discrepancy.

He looks upon the cinder of wrought iron, and the cementite of soft steel, as offering a protection to the pure iron or ferrite. The particles of ferrite on the surface are, of course, in each case, oxidised at once, but it may be possible that the mechanical protection of the flakes of cinder in the wrought iron increases with time, much more than the flakes of cementite of soft steel, so that it is quite possible that, though wrought iron and soft steel corrode at the same rate initially, yet later the wrought iron should oxidise much less than soft steel. He stated that, fortunately, data for testing this hypothesis were at hand, for, in his own experiments and in another very extensive series, the oxidation of soft steel and of wrought iron, for each of two successive long periods, was given. Comparing these, he does not find that the oxidation of the soft steel accelerates relatively to that of wrought iron as the period of exposure continues. The hypothesis is therefore weakened, and he has hence concluded to continue his experiments by re-exposing all the plates, and he hopes to reweigh and report on them again after a further period of several years.

Referring finally to the nickel steels, he stated that the above table showed that, on a general average, the 3 per cent. nickel steel corroded 77 per cent. as fast as wrought iron, and the 26 per cent. nickel steel about one-third as fast. The superiority of the 3 per cent. nickel steel, though decided, is hardly enough to weigh heavily in determining its introduction. The 26 per cent. nickel steel, while having an enormous advantage over wrought iron and soft steel as regards corrosion, can still not be classed as a non-corroding metal under common conditions, but simply as a slowly corroding one.