Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal alloy. The most common type of galvanic corrosion in a boiler system is caused by the contact of dissimilar metals, such as iron and copper. This differential cells can also be formed when deposits are present. Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction, including: scratches in a metal surface, differential stresses in a metal, differences in temperature, conductive deposits.  

Pitting of boiler tube banks has been encountered due to metallic copper deposits. Dissolved copper may be plated out on freshly cleaned surfaces, establishing anodic corrosion areas and forming pits.

This process is illustrated by the following reactions.

Using hydrochloric acid as the cleaning solvent, magnetite is dissolved and yields an acid solution containing both ferrous (Fe²⁺) and ferric (Fe³⁺) chloride. The latter are very corrosive to steel and copper.

Fe₃O₄ + 8 HCl > FeCl₂ + 2 FeCl₃ + 4H₂O

Metallic or elemental copper in boiler deposits is dissolved in the hydrochloric acid solution by the following reaction:

FeCl₃ + Cu > CuCl + FeCl₂

Once cuprous chloride is in solution, it is immediately redeposited as metallic copper on the steel surface according to the following reaction:

2CuCl + Fe > FeCl₂ + 2 Cu

Thus, hydrochloric acid cleaning can cause galvanic corrosion. A complexing agent is added to prevent the copper from redepositing on the steel surface. The following chemical reaction results:

FeCl₃ + Cu + Complexing agent > FeCl₂ + CuCl

This can take place as a separate step or during acid cleaning. Both iron and copper are removed from the boiler, and the boiler surface can then be passivated.