Countering the Effects of Corrosion

Large iron artifacts can require years of conservation. Multi-component objects such as Monitor’s steam engine and condenser must be disassembled to guarantee thorough treatment. Following documentation and disassembly, iron artifacts are conserved through a process called electrolytic reduction.

Electrolytic reduction is a process in which an artifact is placed in an alkaline solution of sodium hydroxide or sodium carbonate in deionized water. Metal electrodes are then suspended around the artifacts and a low-volt, low-amp current is passed through the object. The negative charge applied to the artifact forces negatively charge chloride (salt) ions from the artifact and into the storage solution. The positively charged electrodes help attract the negatively charge ions, increasing the rate at which they diffuse into solution. During this process, oxygen and hydrogen bubbles form at the artifact’s surface and help loosen and remove concretion from the artifact. The electrical current also consolidates and stabilizes weakened iron and reduces iron corrosion products to more stable forms. The chlorides are trapped in the electrolyte solution, which is changed when it becomes contaminated with chlorides. The process is complete when no more chlorides can be detected in the solution.

Electrolytic Reduction



In order to complete the electrolytic reduction circuit in the conservation tank, there has to be a place for the electricity to go when it leaves the artifacts. Platinum coated wire, or electrodes, complete the circuit. Conservators also use stainless steel mesh in some tanks for the same purpose.

When the electric current flows out of the artifacts into the solution in the tank, it causes water molecules to break down into their component parts. This creates bubbles of hydrogen and oxygen.

It is estimated that it will take fifteen to twenty years to reduce the corrosion and to remove all the salt from the turret. Other objects, such as the engine, may also take more than a decade to desalinate.

Most materials made or refined by humans become altered by minerals and salt in the chemically and biologically active medium of sea water. If left untreated after recovery, these artifacts would rapidly corrode and disintegrate.

When the reduction and chloride removal phases of iron conservation have been completed, the artifacts are removed from the tanks. Their surfaces are stabilized with phosphates and tannates and conservators then apply special coatings to protect the artifacts from humidity, moisture, and other damaging substances.