A Treatise On The Principles And Practice Of Dock Engineering

CORROSION OF IRON AND STEEL. 141 under such conditions, even if systematic expérimenta had been carried out from the earliest possible moment, which has not been the case. Again the variation in atmospheric conditions is extremely great, the seasons being marked by enormous fluctuations in sunshine, rainfall, and temperature not only for different seasons in the same year, but for the same season in consecutive years. The question is still further complicated by tlie factoi of locality. Then, as regards aqueous influences, there is no definite standard of comparison whatever. Plie salinity, acidity, density, and temperature differ in almost every unit volume of sea-water, so that it is never precisely the same at any two ports. Rivers, sewers, and ocean currents all contribute to differentiate its composition. It would, perhaps, be a comparatively easy solution of the difficulty to lay down one’s individual experience as a dogma for general acceptance, but the wiser and more judicious course will be to set forth such information on the subject as is available, and leave the reader to draw his own conclusions. The following coefficients, given by Thwaite and quoted by Molesworth,* represent the amount of corrosion in Ibs. per square foot of surface during twelve months’ exposure :— TABLE IX. Corroding Agents. Material. Foul Sea- Water. Clear Sea- Water. Foul River- Water. Pure Air or Clear River- Water. City Air or Sea Air. Sea- Water of Average Foulliess. Cast iron, Wrought iron, Steel, . • Cast iron (skin removed by planing), ,, (surface galvanised),. •0656 •1956 ■1944 •2301 ■0895 •0635 •1285 •0970 •0888 •0359 •0381 1440 1133 •0728 •0371 ■0113 •0123 ■0125 •0109 •0048 ■0476 •1254 •1252 •0884 •0199 ,, in contact with brass, „ ,, ,, copper, o ,, gun-metal, Best wrought iron in contact with brass, .....•• ,, copper, ,, gun-metal, . •1908 •2003 •3493 -2779 •4012 ■4537 If the metal be painted once a year the coefficient to be divided by 2; if once in two years, by 1’8; and if once in three years, by 1'6. Trautwine t states, in apparent contradiction of the above, that while “ fresh-water corrodes wrought iron more rapidly than cast, the reverse appears to be the case with sea-water,” and that “ the corrosion of iron or steel by sea-water increases with the carbon.” He admits, however, that * Poeket-book of Engineering Formidæ, 25th edition, p. 33. f Civil Engineers^ Poeket-book, 17th edition, p. 218.