Why Do Two Dissimilar Metals Cause Corrosion?
The difference in electrode potential between dissimilar metals is the main contributing factor to bimetallic corrosion. This electrochemical process, depending on the application, can be both detrimental and beneficial.
The contact between two dissimilar metals frequently occurs in various situations in everyday life. An aluminum head on a cast iron block, zinc galvanized on a steel beam, solder on a copper pipe and steel fasteners in Aluzinc sheeting are just some common examples of different metals paired in constant contact.
Often this does not present any issues, however there are instances where the contact between dissimilar metals can be detrimental and result in a type of corrosion known as bimetallic corrosion (also known as galvanic corrosion). This type of corrosion occurs when certain criteria are met and environmental conditions are ideal. Although bimetallic corrosion can have many adverse effects on metals, there are instances where this electrochemical process can be beneficial and positively used in a number of applications. (To learn more about these electrochemical processes, read Corrosion Electrochemistry: The 6 Electrochemical Reactions Involved in Corrosion.)
What is Bimetallic Corrosion?
As mentioned previously, bimetallic (galvanic) corrosion occurs when two metals, with different electrode potentials, are in contact with each other (directly or indirectly) in the presence of an electrolyte such as water. It is characterized by accelerated or preferential corrosion that occurs in one of the metals while the other remains galvanically protected.
When a metal is exposed to or immersed in a conducting liquid it adopts an electrode potential. When two dissimilar metals are in contact, they form a bimetallic couple (one metal being the anode and the other being the cathode) due to their affinities for electrons. The difference between the anode’s potential to be oxidized and the cathode’s potential to be reduced is called the potential difference. This difference in potential allows for the free flow of electrons between the anode and the cathode, and results in an electrical current.
Generally, the reactions and processes that occur in bimetallic corrosion are similar to those that would occur in a single, uncoupled metal. However, when a dissimilar metal with a significant difference in potential is present then the rate of corrosion can be dramatically increased.
As electrons flow out of the anode (the metal higher in the galvanic series) into the cathode (the metal lower in the series), oxidation occurs at the anode and it begins to corrode preferentially, therefore sacrificing itself while the cathode remains protected. The further apart the metals are in the galvanic series, the greater the potential difference between the two metals, therefore the greater the corroding effect on the anode. (Get a background on the galvanic series in the article An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.)
Conversely, metals that are close to each other in the series have similar reactivity and little difference in potential between them, hence the effects of galvanic corrosion is less likely.
Conditions Necessary for Bimetallic Corrosion
The contact between two dissimilar metals, though essential, does not necessarily result in bimetallic corrosion. The three basic requirements necessary to cause bimetallic corrosion are:
1. An electrolyte linking the two metals
An electrolyte is basically a conductive solution that facilitates the flow of electrons between the anode and the cathode. The electrolyte does not necessarily have to be highly reactive to the individual metals when they are uncoupled, and may come in the form of rainwater, surface deposits due to the condensation of water vapor in the atmosphere, damp soil or salt deposits. In general, a more conductive electrolyte leads to greater corrosion.
2. Electrical connection between the metals
Electrical contact between the metals is essential for bimetallic corrosion to occur. Though this usually involves direct physical contact, electrical contact between the two metals may be achieved indirectly through the use of an insulation-coated conductor, structural steelwork or any other conductive material. It is not necessary for the connection medium to be exposed to the electrolyte for corrosion to occur.
3. Sufficient potential difference between the two metals
As mentioned previously, the difference in potential allows for the flow of electrons from the anode to the cathode, which gives rise to an electrical current. The further away the two metals are from each other in the galvanic series, the greater the potential difference. Oxidation occurs as the anode loses electrons. The metal at the anode gets converted to an ion, which dissolves such that it becomes pitted and is gradually consumed.
Advantageous Situations of Bimetallic Corrosion
Under most circumstances, bimetallic corrosion is to be avoided and steps are usually taken to prevent its occurrence. Despite its reputation as a destructive process, this electrochemical phenomenon provides some advantages, which are listed below:
1. Voltage production in batteries
The principle of an electrical current produced by potential differences between dissimilar metals is used in typical dry cell batteries. The anode usually consists of a zinc outer case while the cathode may come in the form of a graphite rod. The electrolyte solution separates the two metals and shuffles electrons between the anode and the cathode to produce electricity.
2. Galvanic/Cathodic protection of metals
Combing two metals with sufficient difference in potentials can be used to prevent corrosion of the cathode by allowing the anode to corrode preferentially, therefore sacrificing itself. This very same principle is used when galvanizing structural steel members, whereby the zinc coating is used to galvanically protect the underlying steel.
Prevention of Dissimilar Metal Corrosion
Methods to prevent or mitigate the effects of bimetallic corrosion are focused mainly on disrupting the electrical path in the metal or electrolyte part of the system. Some of the most common methods of preventing bimetallic corrosion include:
1. Electrical insulation
By insulating the metals from each other, they are no longer in contact and the flow of electrons between them is inhibited. This can be achieved through the use of a non-conductive material between the two metals, such as plastic or rubber bushes, washers and gaskets.
2. Isolation from the electrolyte
The electrolyte is the medium responsible for the transfer of electrons via the potential difference between the metals. Therefore, separating the metals from the electrolyte causes an interruption in the electrical circuit and thus prevents galvanic corrosion. Come common measures used to achieve this include the use of water-repellent compounds such as greases, paints, varnish, plastic coatings and electroplating.
3. Using metals with similar electrode potentials
Metals that are closely matched in the galvanic series have lesser potential differences and therefore less galvanic current. This ultimately results in a significant decrease in the probability of bimetallic corrosion.
Bimetallic corrosion, though detrimental to metal components, can be prevented by proper material selection and other insulation and separation techniques. However, despite its destructive reputation, the principles that govern this type of corrosion can be used in a number of beneficial applications.