Factors Influencing Galvanic Corrosion

There are a number of driving forces that influence the occurrence of galvanic corrosion and the rate at which it occurs. Among these influencing factors are the difference in the electrical potentials of the coupled metals, the relative area, and the system geometry. Other driving forces that factor into promoting or preventing galvanic corrosion include the polarization (the shift in electrode potential during electrolysis) of the metals, the electrical resistance and electrical current of the system, the type, pH, and concentration of the electrolyte, and the degree of aeration or motion of the electrolyte.

Potential Difference

The main driving force for galvanic corrosion is the electrical potential difference between the two dissimilar metals; thus, typically the bigger the difference, the more rapid the rate of galvanic corrosion. Galvanic corrosion occurs mainly at the contact area of the two metals and dissipates with distance from the junction. A fundamental expression that shows the electrical potential of a galvanic system is given in Equation 5.

where
E_c – electrical potential of the cathode
E_a – electrical potential of the anode
I – electrical current
R_m – resistance of the electrodes (external circuit)
R_e – resistance of the electrolyte solution path in the galvanic circuit (internal circuit)

There are numerous resources from which the standard electrode potentials of specific metals and alloys can be obtained. The rate of galvanic corrosion in specific environments, however, should not be determined based on the standard electrode potentials of metals. These standard potentials are determined as the potential of a metal in equilibrium with a specific concentration of the electrolyte.96 Furthermore, a galvanic system is dynamic and the reactions are dependent on a number of other factors, including electrolyte concentration, temperature, and pH, as well as oxygen content and fluid motion.

It is not always necessary, however, to have two distinct metals in order to create a galvanic couple. There are instances where galvanic corrosion occurs within the same metal. This can happen when the metal has both an active and passive state, for example, one part is covered with an oxide film and hence passivated, while another part of the metal is exposed to the atmosphere. This condition would create a potential difference causing the unpassivated area of the metal to galvanically corrode.

Relative Area

The size of the metal components in the galvanic system also influences the rate and degree of corrosion. For example, a system with a relatively large cathode (less reactive metal) and a relatively small anode (more reactive metal) will corrode via galvanic corrosion to a greater extent than will a system with electrodes of equal size. Furthermore, a system with a relatively large anode compared to a small cathode will not typically exhibit galvanic corrosion on the anode to a significant extent. In general, corrosion of the anode is proportional to the relative area of the cathode. That is, the induced electrical current increases proportionally with an increase in cathodic area relative to the area of the anode. The opposite is generally true as well: current decreases proportionally with a decrease in relative cathodic area. 

Geometry

Component geometry is another factor affecting the flow of current, which consequently influences the rate of galvanic corrosion. Current does not easily travel around corners, for instance.

Electrolyte and Environment

The rate of galvanic corrosion is partially dependent on the concentration, oxygen content and motion of the electrolyte, as well as the temperature of the environment. For instance, higher temperatures typically cause an increase in the rate of galvanic corrosion, while higher concentrations of the electrolyte will result in a decrease in the corrosion rate.96 The pH of the electrolyte solution may also affect the occurrence of galvanic corrosion in a dissimilar metal system. For example, a metal that is the cathode in a neutral or basic electrolyte may become the anode if the electrolyte becomes acidic.96 A higher oxygen content in the electrolyte also typically results in an increase in the rate of galvanic corrosion. Electrolyte motion can also increase the rate of corrosion, since it may remove some of the oxidized metal from the anode surface, allowing for further oxidation of the metal.