Faraday to Samuel Brown   28 December 1835¹

Royal Institution, 28 December 1835.

Dear Sir,

Your letter and pamphlet² have very much interested me, and as regards the point on which I am authorized to form a judgment, I do not perceive that you will meet with any serious difficulty.

Cast-iron is certainly liable to great injury from constant immersion in salt water, and I think you would find few, if any, exceptions, provided the water and the iron are in contact. Where the iron is out of the water, even though subject to the action of the spray, and that to a considerable extent, the case is very different, especially if you call in the aid of a coat of tar or of paint from time to time.

Bronze does not altogether resist the action of sea water, and it suffers more near the surface of the sea, or where sometimes under and sometimes out of water, than at great depths; plates of bronze placed vertically in sea water are more acted on in the water at the part near the surface than at the lower edge. But any action of this kind (which is by comparison small) could, I have no doubt, be entirely prevented by the use of protectors, in the manner directed by Sir H. Davy for ships' bottoms³.

Will you allow me to mention to you that bronze is not a combination of copper with tin or zinc? The alloy of copper and tin is bronze; the alloy of copper and zinc is brass; a subject very much worse for your purpose than bronze, and I would strongly recommend that no zinc should ever be allowed to enter into the composition of the bronze which you will, I hope, have occasion to use. A little might do no harm; but I know that in the proportions which constitute brass, the zinc occasionally dissolves out by the action of sea water, and the copper is left in what may be called a comparatively rotten state. I quite agree with you that in many cases the upper part might be iron, the lower of bronze, and I do not think the galvanic action would trouble you.

I am, &c. | (signed) W. [sic] Faraday