Faraday to Thomas Byam Martin   12 June 1844¹

Royal Institution, June 12, 1844.

Sir,

I hasten to reply to your note, though not, I fear, with any very certain knowledge, for infirm health has prevented me from taking up the consideration of the action of sea-water on iron so practically as I should have liked to have done; but I will give you my opinion and views as far as my observation and judgment will permit. I conclude that the question is of cast-iron in sea-water. Between these two bodies there is a vigorous action; as far as I have been able to observe, it is greatest in the water near the surface, less in deep water, and least of all where the iron is buried in sand, or earth, or building materials, (into which the water may penetrate); for then the oxide and other results formed, are detained more or less, and form, sometimes, a cement to the surrounding matter and always a partial protection. Soft cast-iron, as far as my experience goes, (which is not much,) corrodes more rapidly than hard cast; and the soft, gray, and mottled iron, more rapidly than the brittle white iron. As to the amount of corrosion in any given time, I have nor [sic] had the opportunity of observing any good and satisfactory cases of illustration.

In estuaries and the mouths of rivers, it is very probable that great differences of corrosion will arise from the different circumstances of variable saltness, the soil of the river, if near a town, the matters brought down by the waters, &c., &c. The association of iron also with other substances, if metallic, will much affect it; thus a wharf of cast-iron might occasionally be greatly injured by making fast to it vessels that are coppered using iron cables.

As to the protection of iron, and first by a coating: the permanency of a coat of paint, or of tar, or bituminous matter, can only be ascertained by reference to experience; of this I have none; except that in a case where coated iron sheathing for ships was brought to me, I was much impressed with the very thorough adhesion of the coat to the iron; the process was patent, and I cannot remember whose it was. Zinced iron would no doubt resist the action of sea-water as long as the surface was covered by zinc, or even when partially denuded of that metal; but zinc dissolves rapidly in sea-water, and after it is gone, the iron would follow.

As to voltaic protection, it has often struck me that the cast-iron piles proposed for lighthouses or beacons might be protected by zinc in the same manner as Davy proposed to protect copper by iron²; but there is no doubt the corrosion of the zinc would be very rapid. If found not too expensive, the object would be to apply the zinc protectors in a place where they could be examined often, and replaced when rendered ineffective; in this manner I have little doubt that iron could be protected in sea-water. It is even probable that, by investigation and trial, different sorts of iron might be easily distinguished and prepared, one of which should protect the other; thus soft cast-iron would probably protect hard cast iron; and then it would be easy to place the protecting masses where they could be removed when required.

Hence though iron be a body very subject to the action of sea-water, it does not seem unlikely that it might be used with advantage in marine constructions intended to be permanent, especially if the joint effects of preserving coats and voltaic protectors were applied. Perhaps engineers are in possession of practical and experimental data sufficient to allow of the formation of a safe judgment on this point; for my own part I am not, and therefore am constrained to express the above opinions with much doubt and reserve.

I am, Sir, | Your very obedient faithful servant, | W. [sic] Faraday.

Sir T. Byam Martin, G.C.B., | &c. &c. &c.