Bimetallic corrosion or contact corrosion?
Bimetallic corrosion, also known as contact corrosion, is another type of corrosion that occurs, according to DIN EN ISO 8044, when two different metallic materials come into contact and are exposed to an electrolyte. Both terms refer to a corrosion mechanism resulting from electrochemical reactions between metals that have significantly different potentials in the electrochemical series.
Mechanism of bimetallic corrosion
The mechanism of bimetallic corrosion is that of galvanic corrosion, in which the anode and cathode are made of different metals. In most cases, the metals are separate components, but they are connected via an electrolyte or other metallic components and thus form a corrosion cell. Like galvanic corrosion, contact corrosion is also based on the electrochemical series, which describes the tendency of different metals and alloys to oxidize—that is, to lose electrons. When two different metals that are far apart in the electrochemical series come into contact and an electrolyte is present, a galvanic cell is formed. The metal with the lower (less noble) potential becomes the anode, while the more noble metal becomes the cathode. This electrochemical difference promotes the flow of electrons from the anode to the cathode. This galvanic cell becomes a corrosion cell when the more noble and less noble metals are connected in an electrically conductive manner. If a copper-plated screw or even just a ground wire is screwed into an aluminum housing, the screw acts as the cathode and the aluminum as the anode (Figure 1), causing the housing to corrode more quickly. Conversely, a galvanized screw in a steel housing acts as the anode and corrodes more quickly (Figure 2).
The corrosion rate at the anode is determined not only by the potential difference but also by the current density, which is calculated as the ratio of the actual current flowing to the anode. The larger the area of the cathode compared to the anode, the lower the current density and thus the corrosion rate at the anode. This underscores the importance of the geometric arrangement of the metals involved in practical applications. Aluminum/stainless steel contact corrosion or aluminum/copper contact corrosion is therefore not significantly influenced by the contact area, e.g., of a bolted joint, but rather by the surface and potential of the combined components (Contact corrosion table link: Screw Encyclopedia: Corrosion, Types, Causes, Protection Against Corrosion, Contact Corrosion (schrauben-lexikon.de)).
Exacerbation of corrosion types due to contact corrosion
Bimetallic corrosion shares several mechanisms with surface corrosion and crevice corrosion, particularly the role of electrolytes in accelerating the corrosion reaction. The direct contact between different metals can accelerate both surface corrosion and crevice corrosion. Stress corrosion cracking is also relevant, as mechanical stresses can be amplified by the different coefficients of thermal expansion of the metals.
How can you prevent contact corrosion?
Material separation
- Avoiding direct metal-to-metal contact
- Use of plastic or coated interlayers
Coatings
- Protective coatings or special corrosion protection systems
- Sealing of contact points
Reduce moisture
- Waterproofing of building components
- Prevention of water accumulation
Structural measures
- Select appropriate material combinations
- Take the electrochemical series into account
Preventive measures and protection strategies against contact corrosion
Passive corrosion protection is an effective method for preventing contact corrosion. The application of corrosion-inhibiting waxes or other coatings creates a physical barrier that prevents direct contact between the metals and the electrolytes. This approach prevents the electrochemical reaction, the flow of current, and thus corrosion. Corrosion protection wax is particularly suitable for areas subject to high stresses and vibrations, as its self-healing properties allow it to seal any cracks or damage that may occur in the protective film.
In contrast, cathodic corrosion protection merely reduces the current flow by lowering the potential difference between the metals. This is often achieved through the use of sacrificial layers made of zinc alloys, which have a lower potential than the base steel material to be protected. For larger surfaces, sacrificial anodes or impressed current anodes, which artificially maintain a lower potential, are also used. Although effective, cathodic corrosion protection merely reduces the rate of corrosion rather than preventing it entirely. Contact corrosion between aluminum and copper—for example, caused by a screw connection between a copper cable and an aluminum housing—can be more easily prevented using corrosion protection wax than with sacrificial anodes.
Typical real-world examples
Contact corrosion is particularly common in automotive manufacturing and industry:
- Screw connections between aluminum and steel
- Body components made of different metals
- Underbody areas exposed to water and salt
- Fasteners, brackets, and joints
Especially in the automotive sector, it often leads to hidden damage that goes unnoticed for a long time.
Summary
Bimetallic corrosion is a critical aspect of corrosion protection that requires a deep understanding of material interactions. Passive corrosion protection through coatings offers a robust method for preventing direct contact with electrolytes and atmospheric oxygen, thereby halting electrochemical reactions and completely preventing corrosion.