As one of the oldest metal materials used by human beings, copper has been widely used in many fields such as electric power, architecture, marine engineering and so on because of its excellent electrical conductivity, thermal conductivity and corrosion resistance. However, even in the most ideal use environment, a series of physical and chemical changes inevitably occur in the long-term service process of copper.
1. Surface oxidation: naturally formed protective layer
The most intuitive change of copper exposed to air for a long time is the change of surface color:
- Initial stage (weeks to months): the surface gradually changes from bright orange-red to dark red.
- Mid-stage (months to years): further change to brown and purple.
- Long-term stage (more than several years): stable blue-green or dark green patina is finally formed.
The patina formed by natural oxidation is actually a dense protective film of basic copper carbonate, which can effectively prevent further corrosion of internal metals. In the field of architecture, this is the key to keep the structural integrity of copper roofs after a hundred years.
2. Grain boundary corrosion: gradual evolution of microstructure
In a specific environment, the deterioration of copper begins with microstructure:
- Stress corrosion cracking (SCC)
Copper members subjected to long-term tensile stress are prone to microcracks along grain boundaries in specific media such as ammonia and sulfide. This phenomenon is especially common in heat exchanger pipes and electrical connectors, and it usually takes 8-15 years to show obvious signs.
- Dezincification corrosion
For brass (copper-zinc alloy), when it is used in seawater or brackish water for a long time, zinc will be selectively dissolved, leaving a porous and fragile copper skeleton. This process often starts from the surface and extends inward at the rate of 0.01-0.1mm per year.
3. The aging change of mechanical properties
Natural evolution of strength and hardness
- Work-hardened copper: In the first 3-5 years, the residual stress is gradually released, and the hardness may decrease by 10-15%.
- Annealed copper: under long-term cyclic load, there will be slight work hardening, and the strength will increase by 5-8%.
- Critical turning point: usually after 20-25 years of service, the fatigue strength begins to decrease obviously.
Attenuation of plasticity index:
After long-term service, the elongation of copper usually decreases by 20-30%, which is especially obvious in the cold bending area. According to the statistics of the power industry, the bending performance of copper bus bars used for more than 30 years may decrease by 40%.
4. The slow degradation of electrical performance
Decline curve of conductivity:
- In standard atmospheric environment, the conductivity of pure copper decreases by about 0.1-0.3% every year.
- In a 30-year cycle, the overall conductivity loss is usually between 5-8%.
- At high temperature (> 80°C), the degradation speed can be accelerated by 2-3 times.
Potential risks of contact resistance:
The thickening of oxide layer on the surface of copper connector will lead to the increase of contact resistance at the rate of 1-2% per year. This may cause local overheating at the high-voltage electrical connection point, which may pose a safety hazard.
5. Environmental Accelerated Aging
Impact of regional climate differences:
- Industrial/marine environment: the corrosion rate is 3-5 times that of rural environment.
- High temperature and high humidity areas: the aging process is 40-60% faster than that in temperate areas.
- Acid rain frequent areas: the destruction speed of surface protective layer is increased by 2-3 times.
Influence of installation details:
- Contact of dissimilar metals: In direct contact with steel and aluminum, galvanic corrosion can shorten the service life of copper by 30-50%.
- Water in crevices: the corrosion rate of the overlapping parts with long-term water accumulation is more than 10 times that of the exposed surface.
