Metallic coatings for the protection of a basis metal such as iron fall into two broad classes – those which give sacrificial protection, and those which do not. Metals that are more active than iron, such as zinc and cadmium, give sacrificial protection to iron. Sacrificial protection is of value where the coating metal is likely to be damaged and the underlying metal exposed.

On the other hand, non-sacrificial metals, such as nickel, are more noble than iron and protect by forming a complete envelope. Nickel protects iron in a variety of environments and has been used as a coating for dowel pins in highway construction.

With reinforcing rods, there is always the possibility that rough handling will cause a break in the coating. A break in a non-sacrificial type of coating, depending on the pH of the corrosive medium, may cause the exposed basis metal to corrode more rapidly than it would if it were not coupled with the nobler metal. Galvanic-cell experiments were thus performed in connection with the National Cooperative Highway Research Program to establish the magnitude and direction of these possible galvanic effects.

It was concluded that, under the alkaline conditions prevalent in sound concrete, a break in a nickel coating would not be a serious problem. In case of neutral or slightly acidic environments, heavier coatings of nickel (3-5 mils thick) are appropriate. The heavier coating can resist rough handling, and it is unlikely to be disrupted to the point of exposing the steel. Cut ends will, of course, expose the steel, but because of their location, there is little threat of the reinforcing rod failing even if corrosion does occur.