April 25, 1889] 



NATURE 



617 



detect the presence of oxychloride, and from the fact that a few 

 (.irops of alkali added to the sea water stop the corrosion, I am 

 of opinion that the simple rusting of iron in sea water is due to 

 the same cause as in fresh — i.e., the decomposition of the water 

 by the iron in presence of carbonic acid. 



The saline constituents of sea water, however, do undoubtedly 

 play an important part in a m.ore active form of corrosion, by 

 helping to excite galvanic action between the iron in the plates 

 and any foreign metal or impurities present, an action which is 

 also materially aided by want of homogeneity in the metal, by 

 particles of rust, by mill scale, by wrought and cast iron or steel 

 in contact with each other ; or even by the different amount of 

 work, such as hammering or bending, undergone by different 

 parts of the same plate ; and in all of these cases the galvanic 

 action set up causes rapid oxidation of the iron at the expense 

 of the oxygen of the water, hydrogen being evolved. 



We may therefore consider that on the skin of a ship two 

 processes of rusting are going on, the simple corrosion on ex- 

 po.sed surfaces of the metal, due to the presence of moisture, 

 carbonic acid and free oxygen, which forms a fairly uniform 

 coating of rust on the metal, and the more local corrosion due to 

 galvanic action, which results in pitting and uneven eating away 

 of the plates. 



As I pointed out in a previous paper, rust cones are due to 

 the most local form of galvanic action, caused by the presence of \ 

 a spec of deposited copper, lead, or other foreign metal, or even 

 a small particle of rust, or mill scale, left on the surface of the 

 iron, and covered by the compositions used as protectives and | 

 antifoulers ; as soon as the sea water penetrates to these, galvanic 

 action is set up, water is decomposed, rust formed, and the j 

 escaping hydrogen pushes up the composition, forming a blister, j 

 the hydrogen leaks out, the water leaks in, the action becoming 

 more and more rapid, and the blister gradually filling with the 

 result of the action — rust. The blister bursts, but the cone of rust 

 has by this time set fairly hard, and continues to grow from the 

 base, the layers of rust being perfectly visible in a well formed 

 cone, and when the rust cone is detached, the pitting of the metal 

 at the base of the cone is, as a rule, found to be of considerable 

 depth. 



The speck of foreign matter which has caused this destructive 

 action generally clings to the surface of the iron, and, being at 

 the bottom of the pitting, escapes detection and removal, 

 and when the vessel, newly coated with fresh compositions, again 

 goes to sea, the corrosion will again probably be set up in the 

 same spot. 



The corrosion of the plates in the interior of a vessel is a sub- 

 ject quite equal in importance to the external action of sea water 

 and dissolved gases on the metal ; and from the fact that certain 

 portions of the interior plates, from their position, escape the | 

 frequent examination and attention bestowed upon the exterior, 

 it becomes a still greater source of danger. 



Corrosion, like all other forms of chemical action, is much 

 accelerated by increase of temperature ; and in the bottom of a 

 ship, near the furnace-room and boilers, this has a considerable 

 effect in increasing the rapidity of rusting. Also in the coal 

 bunkers, the mere contact of moist coal with the iron plates sets 

 up galvanic action, carbon being electro-negative to iron, and the 

 coal dust which sifts down into the double bottom lends its aid 

 to the destruction of the plates ; whilst, if the coal contains 

 any "pyrites," which is nearly always the case, these double 

 sulphides of iron and copper are gradually oxidised into soluble 

 sulphates of the metals, and these, washing down into the bilge 

 water, would at once cause most serious corrosion, should they 

 come in contact with any bare portion of the plates. Repairs to 

 any portion of the inside plates will loosen rust and mill scale, 

 which, finding its way into the bottom, tends to set up galvanic 

 action : whilst the scale of oxide of c jpper from copper and 

 brass fittings and pipes is another great cause of danger, as the 

 bilge water would gradually convert it into soluble salts, which 

 will deposit their copper upon the iron wherever a crack or 

 abrasion enables them to come in contact with it ; and finally, j 

 leakages from stores and cargo are in many cases of a character 

 highly injurious to iron. 



Ill addition to all these sources of danger, we must remember 

 that the interior of the vessel is the part most liable to abrasion 

 from shifting and moving of cargo, coals, &c. 



The protection of the outsides of the bottoms of our ships 

 from the destructive agencies of sea water and dissolved gases 

 may be said to have been attempted in two ways, by metallic 

 and by non-metallic coatings. 



So far, all attempts at metallic coatings have proved failures, 

 and, as far as it is possible to judge, there is but small likelihood of 

 their ever being made to succeed, because if zinc is used in order 

 to protect the iron of the ship there must be galvanic action, 

 and this action must take place evenly all over the surface of the 

 iron plates, which means that the sheathing must be in uniform 

 metallic contact with the iron, in which case the wasting of th*^ 

 sheathing would be so rapid that it would have to be renewed 

 frequently, which, even leaving out the question of cost, is in 

 many cases impossible. 



Zinc is practically the only metal which could be used for this 

 purpose, in order to place the plates of the ship in an electro- 

 negative condition, and it is, therefore, to zinc that inventors 

 have turned from time to to time, the chief novelties introduced 

 being the method of attachment. As far back as the year 1835, 

 I believe Mr. Peacock tried zinc plates on the bottom of H.M.S. 

 Medea, and in 1867 Mr. T. B. Daft again brought the subject 

 forward ; Sir Nathaniel Barnaby, Mr. Mclntyre, and others, 

 also suggesting various plans of attachment, whilst as late as last 

 year Mr. C. F. Henwood read a paper at the United Service 

 Institute strongly advocating zinc sheathing as attached by his 

 system. 



Where the galvanic contact has been but small there the 

 sheathing has had a certain life, but has afforded but little pro- 

 tection to the iron, and has gradually decayed away in a very 

 uneven fashion ; whilst in those cases where galvanic contact 

 has been successfully made the ship has on several occasions 

 returned from her voyage minus a considerable portion of the 

 sheathing. 



Another drawback to the use of zinc sheathing is one which 

 was found when it was used to coat wooden ships, and that is 

 that zinc when in sheets, like every other metal, is by no means 

 homogeneous, and that for this reason the action of sea water 

 upon it, leaving out of consideration galvanic action, is very 

 unevenly carried on, the sheathing showing a strong tendency to 

 be eaten away in patches, whilst the metal itself undergoes some 

 physical change, and rapidly becomes brittle. 



Attempts have been made to galvanize the iron before the 

 building of a ship ; but Mr. Mallet showed as early as 1843 that 

 this coating was useless when exposed to sea water, as in from 

 two to three months the whole of the zinc was converted into 

 chloride and oxide, and that, when, therefore, galvanising is 

 used, care must be taken to protect the thin coating of zinc. 

 This does not, of course, apply to fresh water, in which 

 galvanised iron would answer very well, the rapid action being 

 due to the ^alts in the sea water; but even in this case the 

 galvanising would have to be done after the plates had been 

 riveted together, as any breaking of the surface would set up 

 rapid wasting of the zinc, and it could, therefore, only be used 

 on small craft. 



Copper, tin and lead have been proposed for coating ships, 

 but these metals are electro-negative to iron, and would rapidly 

 destroy the hull, should any abrasion of the coating or damage 

 to the insulating material take place. 



The non-metallic coatings which are intended to do away 

 with corrosion have been almost endless. At the present 

 moment there are upwards of thirty in the market ; whilst the 

 patent list of the last fifty years contains an enormous number 

 which were practically still-born. 



They may be divided for convenience into — 



{a) Oil paints. 



{b) Pitch, asphalt, tar, or waxes. 



\c) Varnishes, consisting of resins and gums dissolved in 

 volatile solvents. 



{d) Varnishes, containing substances to give them body. 



\e) Coatings of cement. 

 And, before going into these in detail, it is necessary to consider 

 the condition of the surfaces to which they will have to be 

 applied, and the effect this will have upon them. 



Air has the power of holding water vapour in suspension, the 

 amount so held being regulated by the tempeiature ; the higher 

 the temperature the more can the air hold as vapour, whilst any 

 cooling of the air saturated at the particular temperature causes 

 a deposition of the surplus moisture. When a ship is scraped 

 down to the bare iron in the dry dock, we have a huge surface 

 of metal which varies in temperature much more rapidly than 

 the surrounding air, and cools much more rapidly than the stone 

 walls of the dock ; as it cools, so it chills the layer of air in im- 

 mediate contact with it, and causes a deposition of the surplus 

 moisture on its surface — a phenomenon known as the " sweating 



