16 REPORT—1843. 
not doubt the fact that an iron ship kept constantly in motion through the 
water will present much less signs of corrosion than she will do if laid up for 
an equal time, but the fact does not warrant the conclusion; on the contrary, 
this fact rightly interpreted is the surest possible proof, and that too from the 
testimony of those most advantageously circumstanced for judging, that rapid 
corrosion does take place. 
It has been heretofore shown, that when iron oxidates in sea water, the rust, 
when first formed, is soft and pulverulent; it has also been shown that every 
metal, iron included, is electro-positive to its own oxides ; in other words, that 
the peroxide of iron formed acts as an acid towards the iron upon which it lies, 
in the same way exalting the rate of corrosion as the plumbago formed on 
cast iron has been shown by the present set of experiments to do upon it. 
Now it is admitted that an iron ship at rest does corrode: if so, peroxide 
of iron is formed if the ship continue long at rest. This coat of oxide gets 
harder and forms a scale of oxide, which yet more promotes the rate of cor- 
rosion ; but if the ship be kept in motion, the oxide formed, soft and pulve- 
ruient at first, is swept off by the passage of her sides through the water 
nearly as fast as it is formed, and hence, while corrosion is still going on, the 
exposed surface of iron, when examined, presents a clean and apparently 
uncorroded appearance. 
Thus it is not true that an iron ship constantly in motion is incorrodible 
by sea water; on the contrary, corrosion does go on, and just at whatever 
rate the conditions of exposure warrant, in a surface of iron whose oxide is 
removed nearly as fast as it is formed, that is to say, which is exposed only 
to the corroding effects of the salt or other water, &c., and not to this to- 
gether with the effect of its own peroxide ; but it also follows, from the expla- 
nation above given of the phzenomena, that the real rate of corrosion of an iron 
ship is less, and probably a good deal less, while she is kept in motion than 
while she may be at rest ; and for the same reasons her tendency to “foul” 
is less while in motion than at rest. 
334. By others it has been fancied that magnetism in some occult way 
interfered with corrosion in iron ships. There is no doubt that every iron 
ship becomes a magnet by induction from the earth, but the intensity will 
depend upon the ship’s bearing, at any moment, as well as upon other obvious 
conditions. Admitting however that an iron ship were at all times a perma- 
nent magnet, no known fact warrants the supposition that its rate of corrosion 
would be in the slightest degree altered thereby (1st Rep. 66). 
The experiments cited by Levol, and alluded to in 1st Rep. 67, as appa- 
rently leading to a different conclusion, I have since found do not sustain the 
view of that author. The deficiency in rate of precipitation, &c. observed 
by him, arose from mechanical impediments introduced by the evolution of 
gas bubbles, and affected by the different position of his wires in the solution, 
and had nothing to do with their magnetism. 
335. I therefore look upon it as perfectly certain that iron vessels corrode 
just as any other mass of iron in similar conditions will. I would add, that 
no mere inspection of surface is sufficient to determine in this case whether 
oxidation has taken place or to what extent, nor can any sufficiently precise 
determination of amount of corrosion be obtained by drilling holes through 
the plates and measuring their thickness. ‘This method might give some 
answer after a quarter of a century’s corrosion; but for any moderate period 
no correct data as to the loss of metal can be had, but by a plate of large 
size and known weight, attached to the ship’s hull by rivets or screw-bolts, 
detached after exposure and again weighed ; and this experiment has not to 
my knowledge ever yet been made. 
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