When ions of noble metals such as copper come into contact with more active metals such as steel or aluminum, the noble metal will spontaneously plate out on the active metal surface. The active metal is oxidized and the noble metal is reduced in accordance with the following chemical reaction:
Cu ++ + Fe (s) → Cu (s) + Fe++
This process is quite useful in the mining industry where it is known as cementation. It was first used in China a thousand years ago to extract copper from mine water1. It is still used in the copper mining industry today where copper is leached from low grade ores and the solution is then trickled over scrap iron to recover the copper. The same process is also used by high school science teachers to dazzle students by dipping a steel nail into a copper sulfate solution where copper will plate out on all wetted surfaces of the nail. The process happens quickly enough to hold the student’s attention.
There is a less useful side to the cementation process. When water passes over a copper surface, it will pick up enough copper so that when it subsequently passes over aluminum (or other active metal) surface, copper will plate on the active metal surface. This can occur even when copper concentration is in the parts per million range. A galvanic cell is formed which leads to pitting corrosion of the active metal. This process is sometimes referred to as deposition corrosion.
Copper/copper sulfate reference electrodes will leach very minute amounts of copper and sulfate ions through the membrane. It is the diffusion of these ions which allows the measurement circuit current to pass through the membrane. The amount of material being leached is extremely small and it will rapidly diffuse into the surrounding environment. However, when the reference electrode membrane is located within a couple millimeters of a steel surface, some of the copper ions will deposit on the steel. This creates a local galvanic cell which alters the corrosion behavior of the steel.
1 The history of copper cementation on iron – The world’s first hydrometallurgical process from medieval china. T. N. Lung; Hydrometallurgy, Vol. 17, No. 1; Nov. 1986, P 113 – 129.