A Treatise On The Principles And Practice Of Dock Engineering

CORROSION OF IRON AND STEEL. 143 surface of steel, and this received the confirmation of Sir W. H. White, at a later meeting of the institution, when he declared that “as regards the relative corrosion of iron and steel when immersed in sea-water, the experience of the Admiralty during the last six years (1876-1882) showed that if the manufacturera’ scale (black oxide) was thoroughly removed, and equal care taken in protecting the surfaces by paint or composition, iron and steel had about the same average rate of corrosion, the steel wearing somewhat more uniformly than the iron.* The question of corrosion principally concerns the dock engineer 111 regard to the duration and maintenance of metal gates and fittings. Decay mainly takes place below the water-line, where inspection and repairs are alike difficult. In this connection the following data taken from a reportt by Messrs. Brandt and Hotopp to the Nintli International Navigation Congress possess much interest : — “I. In the case of the floodgates at Glückstadt, erected in 1874 and to be renewed this year (1902), the first isolated rust spots on the outer skin are to be found at 4 inches below ordinary low water level; the spots increase in number at 6 inches below low water, and are thickly distributed all over the metal at a depth of 10 inches. The greatest depth to which decay has penetrated in the strip comprised between this line and another, lying about 3 feet 3 inches below low water, is about j-inch ; below this level the metal skin is covered with a layer of short-stalked moss, mixed with shells, the thickness of which increases downwards, and below which the depth and extent of decay grows gradually less and less (to about ^-inch deep near the sill), so that the plates near the sill are almost sound. A few of the rivet heads, starting at a depth of 14 inches below low water, begin to show signs of decay and are furrowed ; the decay gradually increases with the depth, so that when the rows of rivets, situated between 18 and 22 inches below low water, are reached, not only have all their heads been completely eaten off, but their shanks have also been already attacked in isolated cases. The decay in this case also becomes less and less with increased depth. The water of the River Elbe, at Glückstadt, is only on exceptional occasions somewhat brackish, but in the outer harbour there is a great deal of deposit, and several drains full of water from the moors empty into it. “II. The gates, and more especially the floodgates, in the harbour at Geestemünde, erected in 1861, show a furrow, the rust in places penetrating as deep as ^ inch into the outer metal skin, just above the cover stiips lying close below low-water line, and it may be assumed that similar rusty places exist also above the cover strips in lower situations, the upper portions of the outside rivet heads lying close under low water mark have also rusted away. The cause to which this damage is ascribed is the layer * Min. Proc. Inst. C.E., vol. Ixix., p. 35. + Brandt and Hotopp on “Iron, Steel, and Wooden Gates,” Int. Nav. Gong., Düsseldorf, 1902.