In the arena of electrochemistry, the pairing of electrode materials is by no means arbitrary. When the
calm copper acts as the cathode (negative electrode) and the lively aluminum acts as the anode (positive
electrode), and they are jointly immersed in a suitable electrolyte solution, a sophisticated drama containing
protection and transformation is quietly staged. This seemingly unusual combination plays an
irreplaceable role in key areas of modern industry.
Core principle: Differences in potentials drive reactions
The central driving force behind the pairing of copper and aluminum lies in their significantly different
electrode potentials. The standard electrode potential of aluminum (Al) (Al³⁺/Al = -1.66 V) is much more
negative than that of copper (Cu²⁺/Cu = +0.34 V). This means that aluminum is much more reactive than
copper and is more likely to lose electrons in an oxidation reaction.
When the two are connected by a wire and immersed in a conductive solution (e.g. salt-containing solution,
specific electrolyte) to form a primary cell or electrolyzed at an applied voltage:
Aluminum Anode (Sacrificial Anode): Aluminum, because of its more negative potential, is the source of
electron flow in a circuit. It undergoes a spontaneous (or current-driven) oxidation reaction and loses
electrons to aluminum ions (Al³⁺) which dissolve into solution.
Core reaction: Al(s) → Al³⁺(aq) + 3e-
Phenomenon: Gradual dissolution and depletion of the surface of the aluminum anode, possibly accompanied
by the production of a small amount of hydrogen gas (depending on the pH of the solution).
Copper cathode: Electrons flow through the external circuit to the copper cathode, which has a positive potential.
At the copper surface, some oxidizing substance in solution (e.g., dissolved oxygen O₂, or H⁺ ions in solution)
gains electrons and a reduction reaction occurs.
Common cathodic reactions (neutral/weakly alkaline environments):
O₂ + 2H₂O + 4e- → 4OH- (oxygen reduction reaction)
Common cathode reaction (acidic environment):
2H⁺ + 2e- → H₂(g) (hydrogenation reaction)
Phenomenon: The copper cathode surface remains stable and does not dissolve. Hydrogen bubbles
are observed in an acidic environment.
Core Application I: Cathodic Protection with Sacrificial Anodes (SACP)
This is the most valuable application of copper cathodes in combination with aluminum anodes, where the
“self-sacrifice” of the aluminum is used to protect the copper (or other more valuable metal structures).
Objects of protection: Copper equipment or components submerged in seawater, soil or conductive solutions
(e.g. copper alloy propellers for ships, seawater-cooled copper pipes, underground copper pipeline joints, copper
components for docks).
Working Principle:
An anode block made of aluminum (or aluminum alloy) is connected directly to the copper structure to be
protected through a wire.
Both are exposed to a corrosive environment (e.g. seawater, i.e. electrolyte).
The aluminum acts as an anode to preferentially corrode and dissolve (sacrificing itself), continuously exporting electrons.
These electrons flow through the wire to the copper structure (cathode), causing a reduction reaction (usually
oxygen reduction) to take place on its surface, inhibiting the copper's own tendency to lose electrons to oxidation
(i.e. corrosion).
Key Advantages:
No external power supply required: driven entirely by the potential difference between metals, simple structure,
relatively easy to install and maintain.
Significant protection effect: effectively reduce the corrosion rate of copper structure, extend the service life several times.
Wide range of application: Especially suitable for occasions where it is difficult to access the power supply or where
explosion-proof is required (e.g. ships, offshore platforms, underground pipeline networks).
Aluminum Anode Characteristics: High purity aluminum or specific aluminum alloys (e.g., Al-Zn-In system) are usually
used to optimize current efficiency, dissolution uniformity, and drive voltage. Anode blocks require periodic
inspection and replacement.
Core Application II: Aluminum Electrolytic Capacitors (Indirect Collaboration)
In aluminum electrolytic capacitors, which form the “heart” of modern electronic devices, copper and aluminum work
together in a subtle and indirect way:
Core structure:
Anode (Al Foil): Electrochemical etching increases the surface area and forms an extremely thin, insulating dielectric layer
of aluminum oxide (Al₂O₃). This is the core energy storage unit of the capacitor.
Cathode (electrolyte + cathode foil): The cathode usually consists of an absorbent paper (diaphragm) impregnated
with a conductive electrolyte (ionic conductor) and another piece of aluminum foil (cathode foil, usually also etched).
The primary function of the cathode foil is to provide a low-resistance current lead path at a potential close to the
“ground” or negative terminal in the circuit. In capacitor packages, the cathode lead is usually connected to the external
circuit via a copper pin.
The role of copper (as an extension of the cathode lead/terminal):
The aluminum foil of the cathode inside the capacitor is connected to the copper pins or terminals by soldering, etc.
The aluminum foil is used for the cathode of the capacitor.
When the capacitor is plugged into a circuit for operation, the copper pins/terminals become the final physical interface
and conductor for the exchange of electrons between the external circuit and the internal cathode (electrolyte/cathode foil).
Copper provides excellent electrical conductivity, mechanical strength, and solder reliability, ensuring that current flows
efficiently and consistently into/out of the cathode system.
Role of the aluminum anode: The dielectric layer of aluminum oxide on its surface stores the electrical charge. When a
positive voltage is applied (positive to the aluminum foil and negative to the cathode system), the dielectric layer is
subjected to the voltage and energy is stored.
Synergistic nature: While the aluminum foil is the core functional material (anode), the copper pins/terminals, as the
final conductor of the cathode current and the external connection point, are an indispensable, efficient and reliable
physical extension of the cathode function. Without a low-resistance, reliable copper connection, the capacitor's cathode
performance would be greatly compromised.
Technical Points and Challenges
Electrolyte selection: In sacrificial anode protection, the ambient medium (seawater, soil) is itself the electrolyte. In
capacitors, specially formulated electrolytes such as ammonium borate and organic amine salts are used to ensure ionic
conductivity, stability and compatibility with the alumina layer.
Passivation of Aluminum: Aluminum naturally forms an oxide film in air. In sacrificial anode applications, it is necessary to
ensure that the environment maintains the active dissolution of the aluminum (avoiding excessive passivation). In capacitors,
this oxide film (capacitively thickened) is the core dielectric.
Corrosion product management: In sacrificial anode applications, the dissolution of aluminum produces corrosion products
such as aluminum hydroxide, which need to be considered in terms of their impact on the environment or on localized crevices
(which are usually controlled). Capacitors require a high degree of sealing to prevent the electrolyte from drying out.
Connection reliability: In cathodic protection systems and capacitors, the electrical connection between copper and aluminum
(or aluminum alloy) is critical. Contact corrosion (corrosion of dissimilar metals in contact) must be prevented, and special
connectors, conductive pastes, or surface treatments (e.g., tin plating) are often used to ensure long-term, low-resistance
conduction.
Conclusion: Wisdom of guarding and connection
The combination of copper cathode and aluminum anode perfectly illustrates the electrochemical philosophy of “sacrificing
the small to achieve the big” and the engineering wisdom of efficiently connecting and transferring energy. In the vast depths
of the ocean to guard the copper ship, is the silent dissolution of the aluminum block; in the square inch electronic components
to store the release of energy, is the aluminum foil surface of the subtle oxidation layer and copper pins constructed by the
reliable pathway. They are not the most glamorous protagonists, but they are the “silent guardians” and “energy bridges” that
are indispensable for the smooth operation of modern industry. Understanding and optimizing the application of this pair is
critical to improving the life of critical infrastructure and the performance of electronic devices. With advances in materials
science and electrochemical engineering, this duo will continue to increase in effectiveness and reliability, bringing their
unique value to a wider range of applications.