Lasagna Cell - Overview

Overview

Dissimilar metals and alloys have different electrode potentials and when two or more come into contact in an electrolyte a galvanic couple is set up, one metal acting as anode and the other as cathode. The potential difference between the dissimilar metals is the driving force for the accelerated attack on the anode member of the galvanic couple. The anode metal dissolves into the electrolyte, and deposition is formed on the cathodic metal.

The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of an electrolyte and a electronic conducting path between the metals is essential for galvanic corrosion to occur.

In some cases, this reaction is intentionally encouraged. For example, low-cost household batteries typically contain carbon-zinc cells. As part of a closed circuit (the electron pathway), the zinc within the cell will corrode preferentially (the ion pathway). Another example is the cathodic protection of buried or submerged structures. In this example, sacrificial anodes work as part of a galvanic couple, promoting corrosion of the anode, rather than the protected subject metal.

In other cases, such as mixed metals in piping (for example, copper and cast iron), galvanic corrosion will contribute to accelerated corrosion of the system. Corrosion inhibitors such as sodium nitrite or sodium molybdate can be introduced to these systems to reduce the galvanic potential. However, the application of these corrosion inhibitors must be monitored closely. If the application of corrosion inhibitors increases the conductivity of the water within the system, the galvanic corrosion potential can be exponentially increased. pH is also a major consideration with regard to closed loop bimetallic circulating systems. Should the pH and corrosion inhibition doses be incorrect, galvanic corrosion will be accelerated. In most HVAC systems, the concept of anodes and cathodes are not an option as they would need to be applied within the plumbing of the system and, over time, would corrode and cause potential mechanical damage to circulating pumps, heat exchangers, etc.