ELECTROLYTIC CORROSION OF IRON AND STEEL IN CONCRETE.

A. A. Knudson, electrical engineer of New York, in reply to the question as to “whether concrete will afford to iron and steel the same protection from stray currents of electricity that it does from ordinary corrosion or rust,” states that, in the course of his investigation of the effects of such currents upon metals covered or incased in concrete or Portland cement, he has come upon several instances of damage caused by electrolytic action upon metals so incased. He has, therefore, published the restlts of some laboratory experiments so as to obtain “definite data as to the amount of current and time necessary to cause corrosion upon metals incased in concrete or hydraulic cements, and, al..o, to test their value as electrolytic.” In March, 1903, he prepared three blocks of concrete containing iron tubes. They were made of equal parts of Portland cement and sand in ordinary metal pails slightly larger at the top than at the bottom. In the centre of each was placed a 2-in. wrought iron pipe. The tests lasted thirty days. One block was immersed in fresh water; another in sea water and connected in series, with 0.1 ampere continuously floating through them. A third block was also immersed in sea water; but no current was sent through it. This was for the purpose of comparison, after the blocks were broken open. The tests lasted thirty days. After having been thoroughly dried they were broken open, without any difficulty in the case of the two blocks that had been subjected to electrolysis—they had already become cracked. The third, which had not been so subjected, was broken with the greatest difficulty. The first two “showed on their interior strong evidences of electrolytic action in the form of what was apparently a deposit of iron rust extending from the pipe towards the outside of the block. Along certain lines which acted like lines of cleavage, the cement was found to be softened, so that it could readily be cut or scraped with a knife. This softening was such that the point of a blade of a pocket knife could be inserted into it far enough, so that the knife was supported in an upright position.” The first and third pipes inclosed were found to be considerably corroded. A portion of the scale on the corroded parts of these pipes was removed, and the pipes were weighed. The pipe which had not been subjected to any current was as bright as a new pipe. The concrete showed no evidence of a deposit of iron rust. An idea may be formed of the condition of the concrete blocks from the photographs given herewith. Another set of experiments were made, the source of current being a storagebattery, with the pressure increased or decreased according to resistance, in order to maintain a constant current of resistance during the thirty days’ test. On the second day water appeared round the pipe, having soaked through the concrete; later on cracks appeared and a yellow frothy substance, which afterwards turned rust-color. The symptoms increased till the end of the test, when one block that had been under test in the salt water was found to have a rustcolored deposit extending along the sides of a crack from the external surface to the pipe. In the other sample, which had been in the fresh water, the rust-colored deposit was only about the pipe; but there were soft spots in the concrete which could be readilv dug out with a knife. The one from the salt water lost 1.6 oz.; the other (fresh water), 1.1 oz. The tests showed that only a small fraction of an ampere is necessary to cause electrolytic action.

A. A. Knudson, electrical engineer of New York, in reply to the question as to “whether concrete will afford to iron and steel the same protection from stray currents of electricity that it does from ordinary corrosion or rust,” states that, in the course of his investigation of the effects of such currents upon metals covered or incased in concrete or Portland cement, he has come upon several instances of damage caused by electrolytic action upon metals so incased. He has, therefore, published the restlts of some laboratory experiments so as to obtain “definite data as to the amount of current and time necessary to cause corrosion upon metals incased in concrete or hydraulic cements, and, al..o, to test their value as electrolytic.” In March, 1903, he prepared three blocks of concrete containing iron tubes. They were made of equal parts of Portland cement and sand in ordinary metal pails slightly larger at the top than at the bottom. In the centre of each was placed a 2-in. wrought iron pipe. The tests lasted thirty days. One block was immersed in fresh water; another in sea water and connected in series, with 0.1 ampere continuously floating through them. A third block was also immersed in sea water; but no current was sent through it. This was for the purpose of comparison, after the blocks were broken open. The tests lasted thirty days. After having been thoroughly dried they were broken open, without any difficulty in the case of the two blocks that had been subjected to electrolysis—they had already become cracked. The third, which had not been so subjected, was broken with the greatest difficulty. The first two “showed on their interior strong evidences of electrolytic action in the form of what was apparently a deposit of iron rust extending from the pipe towards the outside of the block. Along certain lines which acted like lines of cleavage, the cement was found to be softened, so that it could readily be cut or scraped with a knife. This softening was such that the point of a blade of a pocket knife could be inserted into it far enough, so that the knife was supported in an upright position.” The first and third pipes inclosed were found to be considerably corroded. A portion of the scale on the corroded parts of these pipes was removed, and the pipes were weighed. The pipe which had not been subjected to any current was as bright as a new pipe. The concrete showed no evidence of a deposit of iron rust. An idea may be formed of the condition of the concrete blocks from the photographs given herewith. Another set of experiments were made, the source of current being a storagebattery, with the pressure increased or decreased according to resistance, in order to maintain a constant current of resistance during the thirty days’ test. On the second day water appeared round the pipe, having soaked through the concrete; later on cracks appeared and a yellow frothy substance, which afterwards turned rust-color. The symptoms increased till the end of the test, when one block that had been under test in the salt water was found to have a rustcolored deposit extending along the sides of a crack from the external surface to the pipe. In the other sample, which had been in the fresh water, the rust-colored deposit was only about the pipe; but there were soft spots in the concrete which could be readilv dug out with a knife. The one from the salt water lost 1.6 oz.; the other (fresh water), 1.1 oz. The tests showed that only a small fraction of an ampere is necessary to cause electrolytic action.

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