Puddled Iron Corrosion

by: Cushman & Gardner, 1910

Mr. J. P. Snow, Chief Engineer of the Boston and Maine Railroad, has called attention to a very significant case of corrosion in connection with the destruction of some railroad signal bridges erected in 1894, and removed and scrapped in 1902. These structures were built at the time that steel was fast displacing puddled iron as bridge material. (reference)

The result was that the bridges were built from stock material which was partly steel and partly wrought iron. The particular point of interest in this case lies in the fact that while some of the members of the bridge structures rusted to the point of destruction in eight years, others were in practically as good condition as on the day they were erected. This is clearly shown in the following illustration that is from a photograph of these bridge members. The specimen shown in the middle has suffered only very slight superficial rusting, while those shown on either side have gone to the point of destruction.

Figure showing unequal corrosion of different members of the same structure. The lacelike condition of badly corroded steel is also shown.

Tests carried on by Snow and examinations made by one of us appeared to indicate that the badly rusted parts were steel and the unrusted portion wrought iron. On first thought this observation would appear to indicate that wrought iron was far superior to steel as material for such structures. In the light of the electrolytic theory and of the other evidence that has been given, it appears highly probable that the steel in this case was electro-positive to the iron, which resulted in the protection of one metal at the expense of the other. From this point of view the steel is no more to be condemned for having failed than the iron for having been the destructive agent. With the results of recent investigations to guide them, it is not probable that engineers will permit the assembling of different types of metal in one and the same structure, without first considering the probable effect on its life.