Corrosion of Nonferrous Metals in Contact With Concrete or Mortar

Here is some handy info if you are considering any “alternative” metal for use as a framework or armature.  Keep in mind that “rebar” is specifically engineered for use in concrete.  The metallurgy is designed to match the thermal expansion & contraction rates of concrete and therefore move consistently within the mass without creating any additional stress.

Yes, it rusts.  Badly. But there are techniques & treatments today that can limit or eliminate the effects of this problem.  As for the metals below…you are on your own as “experimenters”, so just take this technical data into consideration before you invest too deeply in a project. And, bear in mind that the information below was intended for the construction trades, where a product life-cycle of 20 to 50 years and some measure of visible defects were considered acceptable.

Aluminum (Al)
Embedded aluminum roof flashing, aluminum water stops, aluminum electrical conduit, introduced aluminum powder (sometimes used to foam concrete), or embedded structural aluminum shapes may all corrode in concrete or mortar. In all cases, a reaction that forms aluminum hydroxide and hydrogen gas occurs and may cause expansion and cracking of the concrete or mortar.
The common use of calcium chloride (or other alkali compounds) and dampness of the concrete increases the reaction rate. Usually, coating the aluminum with bituminous paint, impregnated paper or felt, plastic or an alkali-resistant coating will prevent or sharply reduce the corrosion.

Copper (Cu)
Copper embedded in concrete and/or mortar is usually roof flashing. Embedded copper is practically immune to reaction with corrosive alkalis, even if exposed to constant moisture. Copper will not react with dry, hardened concrete and/or mortar. Rainwater leaching, however, may bring chlorides in contact with the metal. Corrosion may occur and result in a green discoloration or runoff. Consequently, chloride admixtures should not be used in concrete if contact with copper is expected.

Lead (Pb)
Lead will always corrode when in contact with fresh concrete and/or mortar. The high pH from calcium hydroxide is the cause of the corrosion. Cured, seasoned concrete or mortar will not react with lead. Corrosion of embedded lead flashing in mortar joints will usually result in the production of lead oxide, a white discoloration. A special case of lead corrosion, called differential aeration, occurs when a lead strip is partially embedded in concrete so that part of the strip is exposed to air.
The embedded section has a different electrical potential than the section exposed to air. The result is that the strip will become polar in the presence of moisture. Gradual corrosion and disintegration of the embedded lead will then follow. In such a case, and in all other cases, the embedded portion should be coated with epoxy, varnish, asphalt, or pitch.

Zinc (Zn)
Zinc is highly reactive with alkalis and will deteriorate to some degree upon contact with fresh concrete and/or mortar. The reaction is limited due to a corrosive film that forms on the outer layer of the zinc. It protects the underlying metal from further reaction. Zinc will not react with dry, seasoned concrete and/or mortar. Embedded zinc will react with moisture and calcium hydroxide to produce calcium zincates.
Zinc corrosion may also occur when galvanized iron, in the form of flat or corrugated sheets and rebar, comes in contact with fresh concrete and/or mortar. Galvanized iron is coated with zinc, and will react with moisture and chlorides in the concrete and/or mortar to produce zinc chloride. The result is expansion and cracking of the concrete and/or mortar. The metal should be protected with epoxy, varnish, asphalt, or pitch.