April 2^, 1889] 



NATURE 



619 



I hoped at the time that I had made it perfectly clear that the 

 zinc would in no way act until both the anti-fouling and protective 

 ▼arnish had perished, and had become spongy and porous, and 

 that the idea was a prolongation of the period of protection, the 

 great point which has now to be aimed at ; but the remarks made 

 afterwards in several journals which were kind enough to notice 

 my paper showed me that they had mistaken my intention, and 

 supposed that the zinc was put in to at once create galvanic action, 

 and predicted that if by any chance it did act, the hydrogen 

 generated would blow the composition into blisters, and defeat 

 its own purpose. I need hardly point out that nothing was 

 farther from my intention, as zinc in fine powder will be acted 

 on more rapidly than the dense metal in plates, and I have 

 Already pointed out that this is destroyed too rapidly by galvanic 

 action to render it of practical use as a protective per se. 



As to the hydrogen blowing off the composition, no gas could 

 he generated until both the anti-fouling and the protective coatings 

 had been perished and rendered perfectly porous by the action 

 of the sea water, a condition which would have permitted the 

 free escape of the generated hydrogen, which, it must be remem- 

 bered, will permeate through openings which other gases cannot 

 pass through. 



One of the largest firms of composition manufacturers had 

 •enough curiosity to try the effect of zinc versus oxide of iron, and 

 painted a patch of it upon a ship coated with his compositions, 

 and after a long voyage she returned with her protectives in per- 

 fectly good order, and had it not been for the patch containing 

 zinc having had its position fixed by careful measurements, its 

 whereabouts could not have been discovered. This is exactly 

 what one would have expected ; as long as the varnish remains 

 intai t oxide of iron, zinc, or, indeed, any substance which will 

 not damage the varnish, Hoes perfectly well ; but had the vessel 

 been allowed to continue un'il the varnishes had perished, then I 

 venture to say that the patch containing the zinc would have 

 shown better protection than those parts containing the oxide of 

 iron. My ideas have undergone con-;iderable modification during 

 the past two years, but I still consider the views I put forward in 

 my last paper were perfectly sound, and I am every day more 

 and more convinced that the great object the composition-maker 

 has to aim at is the prolongation of the life and effectiveness of 

 compositions, and not the multiplication of short-lived devices, 

 however admirable in their action. 



In the fifth class of protectives we have cement coatings ; but 

 these, together with such schemes ^s the covering the hull of the 

 vessels with vitreous glaze?, glass, &c. , have of late years, as far 

 as I know, entirely been abandoned. The action of cement on 

 iron, however, must later on be discussed in its important bearing 

 on the protection of the interior porti-^ns of the hull, for which it 

 is largely employed, its weight and the difBculty of attachment 

 rendering it unf tted for outside work. 



In selecting a protective composition for the bottom of a vessel, 

 one of the second or fourth class should be chosen, attention 

 being given to the points I have indicated, which are that in the 

 bituminous and asphaltic compositions all the original acids must 

 be eliminated, and that in the varnishes of the fourth group 

 quickly evapornt'ng solvents should be avoided, and, if possible, 

 zinc substituted for oxide of iron. 



The vessel should have her plates as dry as possible during the 

 application of ihe protective, and, if feasible, days on which the 

 air is fairly dry should be chosen. The protective should not be 

 too thick, a«, if it is, it does not readily fill into inequalities 

 in the plates ; and, if in this way any air is inclosed, change of 

 temperature will cause it to expand or contract, thus causing a 

 blister to form, which will fill with sea water and set up rapid 

 corrosion. The composition must either be elastic or else have 

 the same rate of expansion and contraction as the iron ; for, if 

 not, the change of temperature will cause cracking and tearing 

 of the composition with disastrous results. The vessel, if she has 

 to be scraped down to the bare metal, must be scrubbed free 

 from all traces of rust, and where a well -adhering coating of com- 

 position exists, it should be painted over and not disturbed. In 

 the case of a new ship, she must be pickled with dilute acid, to 

 get rid of every trace of mill scale, and then washed down with 

 some slightly alkaline liquid to neutralize every trace of acidity, 

 ihe alkali in turn being removed by clean water. Under these 

 conditions, and given a composition with good adhering pro- 

 perties, but little apprehension need be felt as to the ravages of 

 corrosion on the metal of a ship's bottom, the chief risk being 

 from abrasion and other mechanical injury to the composition, 

 coupled with improper constituents in the anti-fouling compo-i- 

 tions. The protection of the interior portion of the vessels, where I 



the plates are exposed to the corroding action of bilge water, 

 rendered more active by a high temperature, leakage from cai^o, 

 acids and sulphates from wet coals, and the presence of such 

 electro-negative factors as coal dust, scale, and nist, is a matter 

 of quite as great importance as the exterior protection ; whilst the 

 great chance of mechanical abrasion during coaling and shifting 

 of cargo, as well as the difficulty of getting at the lower portions 

 of the hold to examine the condition of the plates, renders it a 

 question of the gravest consideration. The corrosion found in 

 the portions underneath the engine-seats, the bunkers, and the 

 water-ballast chambers, especially near the engine-room, is 

 often very serious, and needs most careful watching, which, 

 from the position of these parts of the vessel, it is very hard to 

 bestow upon it. 



It must also be remembered that the bilge water in a vessel is 

 in constant motion, and that the air in these parts of the vessel 

 may be expected to be exceptionally rich in carbonic acid gas, 

 which, as I have before shown, is the most important factor in 

 corrosion. Under these conditions any abraded portion would 

 probably be continually washed over, and then exposed to the 

 foal air, a condition of things most conducive to rapid rusting. 

 There are three main classes of protectives for the interior of a 

 ship — 



(i) Cements. 



(2) Bituminous coatings. 



(3) Paints. 



The first of these, the cement coatings, have many good 

 points to recommend them, but they also have many serious 

 drawbacks. 



The rigidity, firmness of adherence and endurance, are all of 

 them points of the greatest importance, and there is no doubt 

 but that the silicates present in the cement in time, not only bind 

 the cement into a mass of wonderful hardness, but also bind that 

 cement to the iron. A point to which I should like to draw 

 your attention, however, is that a thin coating of Portland 

 cement is highly porous, and that it can be permeated by liquids 

 and gases. Suppose, now, that some copper scale from the 

 interior fittings had fallen into the bottom of the vessel, and had 

 been converted into soluble salts of copper by the saline bilge 

 water, this solution would soak through the capillary orifices in 

 the cement, until it came in contact with the iron below, when 

 the copper would be deposited on the iron, and rapid galvanic 

 action set up, the cement being loosened, and to a certain extent 

 lifted, by the formation of rust, whilst corrosion would gradually 

 extend under the cement, giving on the outside of the coating but 

 little sign of damage taking place below it. 



Also the hardness and rigidity of the cement gives it a tendency 

 to crack away from the metal when any strain isthrowTi upon the 

 plates, or during any expansion or contraction of the metal ; 

 whilst any repairs on the outside of the ship, such as making a 

 boring to test the thickness of plate, replacement of rivets, &c. , 

 would undoubtedly cause a loosening of the cement coating 

 within, and, wherever a loosening takes place, the space between 

 the cement and the plate will quickly be found to become a 

 starting-point for corrosion, which quickly spreads and loosens 

 the cement, and will only be discovered by chance. 



It is for this reason that I consider bituminous or asphaltic 

 varnishes, freed from any trace of acid, and applied hot, or sound 

 tough paint, preferable to cement ; as, although they are not so 

 hard, yet if serious corrosion should be set up, it is easily dis- 

 covered and stopped before much damage results, whilst, being 

 impervious to moisture, deleterious solutions, either from the co^ 

 bunkers or cargo, would be prevented from acting upon the skin 

 of the ship. 



In approaching the subject of fouling, one is impressed with the 

 apparent hopelessness of obtaining any reliable information from 

 the successes or failures registered by the bottoms of the vessels, 

 in the Service, or in the Mercantile Marine. Hundreds of ships 

 may be examined, and their condition and the nature of the 

 compositions used upon them registered, and just as one begins 

 to fed that the key to the mystery is within one's grasp, a whole 

 series of results so abnormal suddenly comes to light that it seems 

 impossible to reconcile them with one's previous experience, A 

 ship may sail half a dozen times to the same water*, coated with 

 the same composition — on four occasions she will come home 

 clean and in good condition, whilst on the other two voyages she 

 may accumulate an amount of weed and animal life sufficient to 

 knock down her speed from nine knots to five. Moreover, if the 

 compositions with which she was coated be examined, and scrap- 

 ings taken from her on her return, no cause will present itself that 



