Dissimilar metals – Aluminium and Copper suffer from Galvanic Corrosion and different thermal expansion properties when join together for Bubar, Switchgear terminals connections.
Aluminium and Copper both have different Properties for conductivity, tensile strength, weight, Melting point, Electrical Resistance, Tensile Strength, Hardness and environmental exposure.
While Joining Aluminium and Copper major factors comes into effect is Corrosion and Thermal Expansion.
Effects of Corrosion due to Exposure to Atmospheric Oxygen on Aluminium and Copper :
Aluminium – Effects of Corrosion :
- It Oxidize readily when exposed to Atmospheric Oxygen.
- Aluminium Oxidized layer is Strongly attached, hard.
- Aluminium Oxidized layer is Electrically comparatively more insulating
Copper – Effects of Corrosion :
- It Oxidize when exposed to Atmospheric Oxygen but comparatively to less extent.
- Copper Oxidized layer is Relatively Soft.
- Copper Oxidized layer is Electrically comparatively Conductive
Galvanic corrosion:
When two dissimilar metals like Aluminium and Copper comes in Electrical contact in the presence of Electrolyte.
In the ASSEMBLY, if Moisture / Humidity is Present, at Copper to Aluminium joints, Galvanic Corrosion occur. Reason being :
- Copper is More Cathodic compared to Aluminium.
- When Aluminium comes in contact with Copper, it becomes Sacrificial Anode.
- Due to this Aluminium becomes susceptible to corrosion.
- Since Copper is comparatively Noble metal, hence it does not exhibit Galvanic Corrosion
Hence, While joining Aluminium and Copper, attention to be paid to oxidation phenomenon during Manufacturing Process and also during Preventive Maintenance.
Also, there is standard industry practice to apply oxide inhibiting compound for Joints between Aluminium – Copper, which prevents Galvanic Corrosion and enhances Electrical joint Properties. Oxide inhibiting compound is natural petroleum based grease which seals out Oxygen and Moisture at the Joints.
Thermal Expansion :
As we understood from Above, Different metal joint of Aluminium – Copper suffers from corrosion effect.
Its due to Galvanic Corrosion of both metals reaction with Oxygen. This Corrosion leads to increase in contact resistance which leads to poor joint properties. This Additional Contact resistance adds into increase of I2R loss at the joints, due to which joint temperature further increases.
Due to Increased temperature at the Joints, one more phenomenon occurs, that is Thermal Expansion.
Both Aluminium and Copper both have different Linear Thermal Expansion Coefficient (Thermal Expansion Coefficients at 20 C)
- Copper : 17 x 10E-6 (m/m)/°C
- Aluminium : 23 x 10E-6 (m/m)/°C
Aluminium Thermal Expansion Coefficient is much higher than Copper around 35%, due to this Expansion – Contraction of Aluminium will be much more than Copper.
Thermal Expansion phenomenon gets more serious issue with change of Ambient Temperature and Change of Current flow through the Busbars/Conductors.
In case of Aluminium, due to higher Thermal expansion coefficient, with Increase of Temperature Aluminium with Expand with coefficient of 23 x 10E-6 (micro meter / meter) / °C and with reduction in Temperature Aluminium will contract with same coefficient.
In Practical application this change of Current flow is normal which leads to loosening of Joints of Busbar and Nut – Bolts, be it Al to Al OR Al to Cu. This Loose Joints will further increase joint Temperature and further deterioration of joint property.
Hence, to Protect ASSEMBLY from effects of Galvanic Corrosion due to dissimilar Al Cu Joints and Thermal Expansion due to Thermal Cycling, Proper care must be taken at the time of Manufacturing and also during Preventive Maintenance schedule attention should be given to Busbar Joints.