When purchasing new household appliances or during scheduled maintenance, few people think about what is hidden behind the decorative panels of the housing. However, when it becomes necessary to repair or replace a compressor, the issue of piping chemistry becomes critical. Understanding what material are the tubes in the refrigerator fromallows the master to choose the right type of solder, flux and soldering method, which directly affects the durability of the system.
The modern refrigeration industry has come a long way of evolution from the heavy structures of the last century to high-tech light alloys. Engineers are constantly looking for a balance between thermal conductivity, corrosion resistance and production cost. That is why different metals can be used in one unit, each of which performs its own unique function in a closed refrigerant circuit.
In this article we will analyze in detail the physical and chemical properties of the main alloys used in the production of refrigeration circuits, and also consider the nuances of their interaction during repairs. You will learn why some metals cannot be connected directly and how to correctly identify the material of the tube visually and tactilely.
Copper: The classic standard for the discharge line
Traditionally copper tubes considered the gold standard in refrigeration technology, especially in those areas of the system where high pressure and temperature prevail. At the outlet of the compressor, in the so-called discharge line, the gas is compressed to a state where its temperature can exceed 100 degrees Celsius. Copper ideally copes with such loads due to its high ductility and heat resistance.
One of the main advantages of copper is its exceptional thermal conductivity. This property is critical for condensers, where the refrigerant must release heat to the environment as quickly as possible. In addition, copper can easily be soldered with hard solders, which makes repairing such systems a predictable and reliable process for a qualified craftsman.
However, this metal also has its drawbacks, the main ones being high cost and a tendency to oxidize if stored improperly. If a copper tube has been left open for a long time in a damp room, an oxide film may form inside it, which will subsequently clog the capillary or filter drier.
- 🔸 High ductility allows you to easily bend the tubes without the risk of creases.
- 🔸 Excellent compatibility with most refrigerants, including R134a freons and R600a.
- 🔸 Resistance to high pressure in the discharge system.
- 🔸 Easy soldering with silver or copper-phosphorus solders.
⚠️ Attention: When soldering copper tubes, be sure to use nitrogen purge. Copper oxides formed inside the pipe when heated without an inert gas can cause blockage of the system several months after repair.
Aluminum: Lightness and availability in modern models
In pursuit of reducing the cost and weight of the final product, manufacturers of household appliances are increasingly introducing aluminum alloys. Most often they can be found in the evaporators of modern refrigerators, especially in No Frost systems, which require intense heat transfer at low temperatures. Aluminum is much lighter than copper and cheaper to mine, which makes it attractive for mass production.
The main difficulty when working with aluminum is its ability to instantly form a strong oxide film in air. This film has a melting point significantly higher than the melting point of the metal itself, which makes soldering difficult. To connect such parts, special fluxes are required that destroy the oxide layer, or the use of ultrasonic soldering.
Despite the difficulties in processing, aluminum tubes have good corrosion resistance in certain environments and conduct heat well. However, they are more sensitive to mechanical damage and vibrations, therefore, during installation they require more careful handling and reliable fixation.
- 🔹 Low cost of the material compared to non-ferrous metals.
- 🔹 Low weight of the structure, which simplifies logistics and assembly.
- 🔹 Good heat dissipation in low-temperature circuit.
- 🔹 Requires specific skills and consumables for repair.
Why is aluminum difficult to solder?
Aluminum actively reacts with oxygen, creating an oxide film (Al2O3) on the surface. This film melts at a temperature of about 2000°C, while aluminum itself melts at 660°C. Therefore, when heated, the metal simply spreads, but does not connect. Special fluxes that destroy the oxide film are required.
Steel: Reliability and savings in budget solutions
Steel tubes (usually copper-plated or stainless steel) are often used in budget models of refrigerators, especially in doorway heating circuits or loop heaters. Steel is stronger than aluminum and copper, which makes it resistant to external mechanical influences, but it is much heavier and less susceptible to bending.
In refrigeration systems, low-carbon steel with a copper coating is most often used. This is a compromise solution: steel provides mechanical strength and low cost, and a thin layer of copper on the surface makes soldering easier with standard solders. However, if this protective layer is damaged, the steel will quickly begin to rust, which can lead to a refrigerant leak.
Repairing steel areas requires special attention to the choice of solder. Regular tin-lead solder will not work here due to poor adhesion. Craftsmen use special high-temperature alloys or adapters to connect steel with other metals of the system.
- 🔻 High mechanical strength and resistance to vibrations.
- 🔻 Low cost of production of pipe products.
- 🔻 Risk of corrosion if the protective coatings.
- 🔻 Difficulty in soldering without proper surface preparation.
Comparative table of materials
To systematize the information and help you quickly navigate the characteristics of various metals, we have prepared a summary table. It will help you understand which material is best suited for specific operating and repair conditions.
| Characteristics | Copper | Aluminum | Steel (copper-plated) |
|---|---|---|---|
| Thermal conductivity | High (395 W/mK) | Medium (220 W/mK) | Low (50 W/mK) |
| Soldering difficulty | Low | High | Average |
| Cost | High | Low | Very low |
| Corrosion resistance | High | High (oxide film) | Medium (risk of rust) |
Galvanic couple problem and connection methods
One of the most common mistakes when repairing refrigerators is an attempt to directly connect a copper tube to an aluminum one. At the place of such contact, galvanic coupleoccurs. In the presence of even minimal humidity (which is always present in the air), an electrochemical reaction begins between two dissimilar metals, leading to the rapid destruction of the less noble metal - in this case, aluminum.
To avoid this, it is necessary to use special adapters. Most often, steel-copper or aluminum-copper inserts are used, which have a coating on one side or a composition that is compatible with both metals. Also widely used is the crimp-to-crimp technology using a steel bushing that physically separates the contacts of dissimilar metals.
When soldering aluminum to copper without adapters (which is highly discouraged for beginners), special high-temperature zinc-based solders are used. They create a weld that chemically bonds to both metals, minimizing the risk of electrolysis, but this repair is considered a temporary solution in a professional environment.
- ⚡ Never solder copper directly to aluminum with regular solder.
- ⚡ Use bimetallic adapters for a reliable connection.
- ⚡ Treat joints with anti-corrosion compounds after soldering.
- ⚡ Make sure the surfaces are clean before starting work.
⚠️ Attention: Galvanic corrosion may not appear immediately, but 6-12 months after repair. That is why using the correct adapters is not a recommendation, but a mandatory requirement for quality service.
☑️ Check before soldering
Specifics of working with different refrigerants
The material of the tubes is selected not only based on cost, but also depending on the type used refrigerant. Old freons, such as R12, were inert to most metals. Modern environmentally friendly gases, such as R134a or isobutane R600a, may have their own characteristics of interaction with the internal walls of the pipeline.
For example, isobutane (R600a)which is now used in most household refrigerators, is explosive. This imposes additional requirements on the tightness of connections. Copper tubes in such systems must be perfectly soldered, since any microcrack can lead to a leak of flammable gas. Aluminum in systems with R600a also performs well, but requires more careful quality control of the seams.
In addition, modern synthetic oils used with new freons are highly hygroscopic (absorb moisture). If moisture or acid remains inside the tubes from the combustion of old oil, this can cause “copper plating” of the steel parts of the compressor and its failure. Therefore, the cleanliness of the internal space of the tubes is a critical parameter.
How to determine the tube material: practical tips
For a master or amateur, it is important to be able to quickly identify metal to be worked with. Visually, copper has a characteristic reddish-golden hue, which can darken to brown over time. Aluminum is always silver-gray, matte or slightly shiny. Steel is most often coated with a copper layer, but gray metal is visible on cuts or in places where the coating is damaged.
Tactilely, copper is heavier and softer. Aluminum is very light, it is easy to scratch it with a fingernail (if it is not hardened), but bending a thick tube without a special tool is difficult - it can burst. Steel is hard, heavy and, as mentioned, magnetic. A simple refrigerator magnet will help you instantly distinguish steel from non-ferrous metals.
You can also use the filing method. If you make a notch and look at the color of the shavings or a fresh cut: for copper it is pink, for aluminum it is bright silver, for steel it is gray. These simple methods allow you to avoid mistakes when choosing a repair technology.
- 👁️ Visual inspection of the color and gloss of the surface.
- 🧲 Checking with a magnet (works only for steel).
- ⚖️ Estimating the weight of a piece of tube (copper is heavier than aluminum).
- 🔪 Color analysis on a fresh cut or file.
Is it possible to replace a copper tube with an aluminum one during repairs?
Theoretically, it is possible, but only if you replace the entire circuit (for example, the entire evaporator). Partial replacement of copper with aluminum in a high-pressure system or at joints requires the use of special adapters. Direct soldering is prohibited due to the risk of electrochemical corrosion and depressurization.
Why do the tubes in the refrigerator turn black or green?
Blackening of copper is a natural oxidation process upon contact with air and high temperatures. Green plaque (patina) is formed by prolonged contact of copper with a humid environment and carbon dioxide. This is not critical for the operation of the system if the oxides do not get inside the tube.
What solder is best used for copper tubes?
For copper in refrigeration systems, hard solders are best suited: copper-phosphorus (for copper-copper connections) or silver (for copper with steel or brass). Soft tin-lead solders do not withstand the pressure and temperatures of modern systems.
Is aluminum in the refrigerator dangerous to health?
No, it is not dangerous. The aluminum in the tubes is in a closed circuit and does not come into contact with food. Even in the event of a refrigerant leak, the metal of the tubes itself does not pose a threat to a person in the kitchen.
Do you need to change the tubes when replacing the compressor?
Not always. If the old tubes are intact, have no signs of severe corrosion, dents and have not been pinched, they can be reused after thorough purging with nitrogen. However, the filter drier must be changed in any case.