July 15, 1922] 



NA TURE 



tion due to the radiations from the Trapezium stars, 

 and, when in this field, the gas is set shining — faintly 

 near its outskirts, and without excitation of the nebular 

 lines ; more strongly, and with the nebular lines, near 

 the middle. 



According to unpublished investigations by Hubble, 

 it appears probable that the absorbing clouds in Orion, 

 not far from the nebula, weaken the light of stars 



behind them by at least ten magnitudes. The exciting 

 radiations probably penetrate to a relatively small 

 depth into the mass and, even if they went deeper, 

 little of the excited light could get out again. The 

 Orion nebula, on this hypothesis, may be regarded 

 almost as a superficial fluorescence of the gaseous por- 

 tion of this vast dark cloud, in the limited region where 

 it is stimulated by the influence of the exciting stars. 



The Corrosion of Ferrous Metals. 



r ~PHE fact that iron readily perishes when exposed 

 J- to the forces of Nature must have been observed 

 by man practically as long as the metal itself has been 

 recognised. But it is only comparatively recently that 

 the problem of the preservation of iron from corrosion 

 has developed into one of such stupendous economic 

 importance as at the present day. Sir Robert Hadfield 

 estimates the wastage of the world's steel on account 

 of rust alone to reach some 29 million tons for the year 

 1920. This, at an average figure of 20Z. per ton, repre- 

 sents a loss to the community of at least 580,000,000/. 

 One interesting feature of this calculation is that 

 the annual increase in the world's total stock of iron 

 and steel is only some 30 per cent, of the annual 

 production, the remaining 70 per cent, being absorbed 

 in replacing wastage consequent upon fair wear and 

 tear and upon corrosion. 



In a paper read before the North-East Coast Institu- 

 tion of Engineers and Shipbuilders in April last, Mr. A. 

 Pickworth most opportunely directs the attention of 

 marine engineers to this aspect of the subject. It is 

 pointed out that the repairs necessary to counteract 

 the ravages of rusting during ten years' service in the 

 case of a single battleship have, for structural work 

 alone, been known to cost some 150,000/. Any one, 

 therefore, who can assist in combating this destructive 

 plague to iron and steel merits the gratitude of the 

 community. 



It is now well recognised that, in addition to air, the 

 presence of liquid water is essential to corrosion. Rise 

 of temperature accentuates the evil, the rate of oxida- 

 tion of iron immersed in water at 8o° C. being more 

 than seven times as great as that at 0° C. It is easy, 

 therefore, to offer an explanation, as Mr. Pickworth 

 points out, for the fact that corrosion assumes more 

 serious proportions in the vicinity of boiler-room tanks 

 on steamers than elsewhere. The tanks, whether used 

 for ballast or feedwaters, are alternately filled and more 

 or less completely emptied, but are rarely, if ever, 

 thoroughly dry, for obvious reasons. The residual 

 moisture, coupled with unequal distribution of waste 

 heat from the boilers, and erratic cooling from the floor 

 and shell plating in contact with the sea under the 

 vessel, results in rapid corrosion. The steel work of 

 the bunkers of a vessel frequently requires patching or 

 renewing on account of corrosion. This is particularly 

 the case with pocket bunkers at the sides of the boilers 

 or 'tween-deck bunkers above the boilers. Not only 

 does the heat from the boilers and the moisture in the 

 coal tend to produce a corrosive atmosphere, but the 

 abrasive action of the coal as it enters from the shoots 

 and while it is settling as the lower layers are worked 

 out all tend to accentuate the destruction of the metal. 

 In practice it is found that the lower parts of the bunkers 



NO. 2750, VOL. I 10] 



are the most seriously affected, and this is attributed to 

 the accumulation of a mixture of small coal and mud 

 which is rarely removed except at special surveys. 

 Although in exceptional cases the outside surfai 

 the shell plating of a ship may be seriously pitted and 

 corroded, as, for example, when the vessel has been 

 lying in very foul waters, the general rule appears to 

 be for the wastage to be greater on the inside surfaces. 

 This is due, in the main, to the extra cleaning and 

 general attention received by the outside surfaces. 

 Special care should be paid to those portions of the 

 inside shell immediately under the sidelights, for the 

 constant trickling of rain or water of condensation from 

 the glass induces most serious corrosion, resulting at 

 times in actual perforation of the plating. 



As might be expected, the most serious external 

 corrosion of the shell plating occurs in the neighbour- 

 hood of the wind and water line, the metal being 

 alternately drenched and exposed to air. The surface 

 water also is in motion relatively to the plates, and 

 this tends to stimulate corrosion. One interesting 

 point deserving of consideration is the fact that the 

 external portions of a vessel above the water-line 

 receive deposits of sodium chloride in consequence of 

 the evaporation of sea spray. Owing to the hygro- 

 scopic character of the deposit, as well as to its chemical 

 activity, corrosion is readily induced thereby. 



It is difficult to obtain trustworthy comparative data 

 on the relative efficiencies of different methods of avoid- 

 ing corrosion. Early man frequently surrounded his 

 iron with copper or bronze, so that his implements 

 might possess the strength of the former combined 

 with the incorrodibility of the latter metal. Modern 

 developments of this process are galvanising, tinning, 

 electroplating, and the like. The Bower-Barff process 

 consists in coating the iron or steel with magnetic oxide, 

 which is an excellent protection so long as it remains 

 unbroken. The metal is cleaned, heated in a closed 

 chamber by means of producer gas, and finally oxidised 

 in a current of superheated steam for a couple of hours. 

 If, however, the resulting coat of oxide should crack or 

 chip off at any point, the layer of unprotected metal 

 thereby exposed is rapidly attacked. The oxide 

 functions as cathode and the metal as anode, so that 

 deep pitting ensues. Mr. Pickworth mentions that a 

 certain shipowner, relying on the protective action of 

 mill scale, gave instructions for a ship to be built, the 

 shell plates of which were to retain their mill scale as 

 completely as possible. For a time all went well on 

 service, but suddenly the vessel developed leaks and 

 was almost lost. Upon docking it was found that many 

 of the plates had been deeply pitted and even perforated 

 in a number of places where the mill scale had been 

 destroved either during construction or through 



