6i8 



NATURE 



{April 2^, 1889 



of iron " — and on to this moist surface the protective composi- 

 tion has to be painted. If now a rapidly-drying varnish is put 

 on, the rapid evaporation of the volatile solvent causes again 

 another sudden fall of temperature — evaporation being always 

 accompanied by loss of heat — and this fall of temperature again 

 causes a deposition of moisture, this time on the surface of the 

 protective, so that the coating is sandwiched between two layers 

 of moisture, both of them probably acting deleteriously upon the 

 resin or gum in the varnish, whilst the moisture on the iron also 

 prevents adherence of the varnish to the metal. If, instead of 

 a quick-drying varnish, the old-fashioned red lead and linseed 

 oil protector had been used, the second deposition would not 

 have taken place, but the sweating of the iron would have pre- 

 vented cohesion, and, when dry, any rubbing of the coating 

 would bring it off in strips. 



The condition of the outer skin of a ship, when she is being 

 coated with her protective composition, is one of the prime 

 factors in the discrepancies found in the way in which composi- 

 tions act. It being a very usual thing for a composition to give 

 most satisfactory results on several occasions, and then, appar- 

 ently under exactly similar circumstances, to utterly break down, 

 and to refuse even to keep on. Too much stress cannot be laid 

 upon the condition of the plates at the time of coating, and it is 

 absolutely essential either to have a perfectly dry ship or else a 

 composition which is not affected by water. 



When an old ship is broken up, you will often see on the 

 backs of the plates the numbers which had been painted on 

 them with white lead and linseed oil before the ship was built, 

 and, under the paint, the iron in a perfect state of preservation, 

 the secret being that the paint was put on while the plates were 

 hot and dry. 



Boiled linseed oil, mixed with red or white lead, is amongst 

 the oldest of the protective compositions in use, but of late years 

 has been but little employed, since it was proved by M. Jouvin, 

 of the French Navy, and also in this country, that compounds 

 of lead, when exposed by the wasting of the vehicle to the action 

 of sea water, are converted into chloride of lead, and this is 

 rapidly acted on by the iron, depositing metallic lead and form- 

 ing chloride of iron, the deposited lead carrying on the corrosion 

 of the iron by rapid galvanic acti )n. The drying of boiled lin- 

 seed oil is due to the fact that it has in it a certain quantity of 

 an organic cojapound of lead, and the drying properties are 

 given to it by boiling it with litharge (oxide of lead), so that, 

 even when red or white lead is not mixed with it, still lead com- 

 pounds are present, and this action will go on to a lesser extent. 

 When the boiled oil drys, it does so by absorbing oxygen from 

 the air, and becomes converted into a sort of resin, thi acid 

 properties of which also h,ave a bad effect upon iron, so that 

 protectives containing boiled oil are open to objection. Within 

 the last two months a good example of the action of sea water on 

 the bottom of an iron ship, coated with red lead, has been 

 afforded by H.M.S. Nile, which, after being painted over with 

 coats of red lead, was allowed to remain for some months in 

 Milford Haven, with the result that her bottom is very seriously 

 corroded, and, on examination of specimens of rust taken from 

 her, the crystals of metallic lead are in many cases easily identified. 



If red k-ad is used, it can only form a ground-work for an 

 anti fouling composition which has to protect the red lead as 

 well as the iron of the ship from the action of sea water, 

 and when the anti-fouling composition and the vehicle perish, 

 then serious corrosion must ensue. 



The second class of protectives, consisting of tar and tar pro- 

 ducts, such as pitch, black varnish, and also asphalt and mineral 

 waxes, are amongst the best protectives, the waxes especially 

 not being affected by the sweating of the plates, and forming 

 admirable coatings for the plates. Certain precautions, however, 

 must be taken in the case of tar and tar products, both of which 

 are liable to contain small quantities of acid and of ammonia 

 salts ; but if care be taken to eliminate these, and if it could be 

 managed to apply this class of protectives hot to warm plates, 

 the question of protection would be practically solved, bitu- 

 minous and asphaltic substances forming an enamel on the sur- 

 face of the iron which is free from the objections to be raised 

 against all other protectives, that is, that being microscopically 

 porous they are pervious to sea water. 



The third class of protectives consists of varnishes formed by 

 dissolving gums or resins in volatile solvents, such as spirit, 

 turpentine, naphtha, fusel oil, &c., and such varnishes are open 

 to several objections — in the first place, they are acted upon by 

 moisture, which causes a deposition of the resins or gums as a 

 non- coherent powder and destroys tenacity of the varnish. The 



amount of action which moisture has on such a spirit-varnish 

 depends to a considerable extent upon the proportion of resin 

 or gum to spirit, when the solvent is present in large quantities, 

 and the resin in comparatively small ; then the moisture has 

 apparently little action ; but it must be remembered that the 

 drying of such protectives means the rapid evaporation of the 

 solvent and concentration of the resin or gum, whilst the rapid 

 volatilization which is going on cools the hull of the ship, and 

 causes deposition of moisture on the drying varnish with most 

 disastrous results. 



Another point which must be borne in mind is that no such 

 varnish is impervious to gases and liquids. We are apt to think 

 of a coating of varnish as being perfectly homogeneous ; but, on 

 examining it through a microscope, it is seen to be full of minute 

 capillary tubes, which become gradually enlarged by the action 

 of water; and finally result in the destruction of the varnish, 

 whilst moisture and dissolved gases find their way to the metal, 

 and carry on corrosion. The application of several coats of 

 varnish tends to diminish this evil, as in many cases the holes in 

 the first coat will not correspond with the holes in the second, 

 and so each succeeding coat will tend to make the protective more 

 and more impervious. In using such varnishes, they must only 

 be applied in favourable weather, and must be allowed to 

 thoroughly harden before being brought in contact with the water. 



In the fourth class we have varnishes of this kind to which 

 body has been given by the addition of foreign constituents, 

 generally mineral oxides ; and this class is far preferable to the 

 last, if the solvent used is not too rapid in its evaporation, and if 

 care has been taken to select substances which do not them- 

 selves act injuriously upon iron or upon the gums or resins which 

 are to bind them together, and are also free from any impurities 

 which could do so. 



At present the favourite substance used to give colour and body 

 to such varnishes is the red oxide of iron, the colour of which 

 effectually cloaks any rusting which may be going on under it. 

 In Using the red oxide for tljis purpose, care should be taken 

 that it contains no free sulphuric acid or soluble sulphates, as 

 these are common impurities, and are extremely injurious, tend- 

 ing to greatly increase the rate of corrosion. The finest coloured 

 oxides are, as a rule, the worst offenders in this respect, as they 

 are made by heating green vitriol (sulphate of iron), and in most 

 cases the whole of the sulphuric acid is not driven off as the heat 

 necessary impairs the colour ; this acid is often neutralized by 

 washing the oxide with dilute soda solution, but very little trouble, 

 as a rule, is taken to wash it free from the resulting sulphate of 

 soda, which is left in the oxide. 



A sample of exceptionally good colour intended for using 

 in protective compositions was sent me a few weeks ago for 

 analysis, and proved to contain no less than 15 '3 per cent, of 

 sulphate of soda. 



The best form of oxide of iron to u-;e for this purpose is ob- 

 tained by calcining a good specimen of haematite iron ore at a 

 high temperature. When prepared in this way, it contains no 

 sulphates, but from 8 to 40 per cent, of clay ; if the percentage 

 does not, however, exceed 12 to 18 per cent, it is perfectly 

 harmless. 



Composition manufacturers can easily test their red oxides for 

 themselves, to see if it contains soluble sulphates, by warming a 

 little of it with pure water, filtering through blotting paper, and 

 adding to the clear solution a few drops of hydrochloric acid, and 

 a little solution of chloride of barium (easily obtained at any 

 druggist's). If a white sediment forms in the solution, the 

 sample should be rejected. 



In a previous paper ^ on the corrosion and protection of iron 

 and steel ships, I pointed out that when such a varnish perished, 

 the oxide of iron being left in contact with the iron plates, in- 

 creased the corrosion giing on at the surface of the metal, all 

 oxides being electro- negative to the metals froai which they are 

 produced, and on that occasion I advocated the use of finely 

 divided metallic zinc, which can be obtained as an impalpable 

 powder, in place of the oxide of iron, pointing out that such a 

 composition would last as long as any varnish of this class, and 

 that, when the varnish perished, as it must do after long exposure 

 to sea water, then the metallic zinc would, 011 coming in contact 

 with the iron, set up galvanic action ; but that, instead of being 

 electro-negative, as in the case of oxide of iron, and causing cor- 

 rosion of the plates, it would be electro-positive, and in conse- 

 quence would protect them, being itself slowly oxidized, and so 

 would give a fresh period of protection. 



' Transactions of the Institution of Naval Architects, vol. xxviii. 



