A Treatise On The Principles And Practice Of Dock Engineering

144 DOCK ENGINEERING. of mud deposited on the upper edges of the cover strips and on the rivet- heads, the mud being highly charged with acids derived from the deeaying river deposit and the salt-water and water from the moors conveyed by the lower Weser and the Geste. The corrosive influence of the deposit is proved by the fact that the decay in question is specially noticeable on the convex side of the curved floodgates, the outer skin of which is permanently immersed in the very muddy water of the outer harbour, whereas on their concave side they are often washed by the water in the harbour which is not so turbid. “III. The dock gates of the new harbour at Bremerhaven were erected in 1852, and removed as worn out in 1900. The thinning down of the plate was especially noticeable where projecting edges formed ledges upon which mud could settle. Those parts of the gates which had been in contact with oak timber were also in worse condition. At Bremerhaven the water is fairly full of salt and heavily laden with mud. “ IV. The inner gates of the great lock at Harburg, erected in 1880 and removed in 1901 for alteration, were found in very good condition with the exception of a strip about 2 feet wide near the low water-line, where the outer skin was very rough and showed rust spots penetrating j inch into the metal. The river-water is completely free from salt and almost free from mud at Harburg, but the water in the harbour is, as yet, strongly polluted by the surface and house drainage of the town, and several chemical factories, besides, discharge their waste water into it full of impurities, the oxidation of all which takes place on the surface of the water ; cousequently, the plating of the gates is principally damaged near the water-line.” The following statement of results, obtained by the author in some experiments, covering a period of twelve months, serves to illustrate the difficulty of deducing reliable coefficients of corrosion from any but the most extensive investigation. The data obtained are not without intrinsic interest, but in order to be of any practical value, such observations would have to be extended over a considerable number of years. It is a note- worthy feature that the galvanised specimens apparently suffered more than the ungalvanised, and that, during the first three months, the latter, instead of losing, actually gained, weight. This is due partly to the conditions of immersion, and partly to the fact that weight is, after all, no very reliable criterion of the amount of corrosion actually taking place, since some forms of oxidation involve no loss in this respect. The first six specimens were suspended in a disused clough-shaft, to which the tidal water of the River Mersey had free access, the specimens being placed at mean tide level, so that they were in and out of water for about equal periods. The water was somewhat impregnated with sewage discharged from a neighbouring outfall sewer, and the ungalvanised speci- mens became coated with a hard deposit, apparently of a calcareous nature, which was removed as far as possible before each weighing by washing in