01 Imperative: Dual Pressures of Cost & Environmental Protection

In recent years, copper prices have soared, placing heavy cost burdens on air conditioning manufacturers that rely heavily on copper materials. Copper has long dominated the mainstream position in air conditioning heat exchangers and connecting pipelines, thanks to its excellent thermal conductivity and mature processing technology.

However, fluctuations in the raw material market have forced the industry to seek more stable and cost-effective alternatives. Austenitic stainless steel, especially low-carbon grades such as 304L and 316L, has come into view with superior corrosion resistance. Meanwhile, global environmental policies continue to tighten. The EU's new F-Gas Regulation explicitly prohibits the use of HFC refrigerants in split air conditioners starting from 2035. China has also introduced policies to promote the substitution of natural working fluids. Low-GWP or even zero-GWP refrigerants such as R290 (propane) and R774 (carbon dioxide) have become the core direction for the future.

The advantages of stainless steel are evident: it has high mechanical strength to withstand the higher operating pressures of next-generation refrigerants like R290 and R774 (the working pressure of R774 is about 10 times that of traditional refrigerants), and its material cost is relatively stable. Among them, 304L offers high cost-effectiveness and is suitable for conventional working conditions, while 316L, with enhanced corrosion resistance due to molybdenum content, is ideal for harsh environments. Both meet the steel-for-copper needs of most air conditioning systems. However, doubts follow: Can stainless steel coexist harmoniously with future refrigerants with complex chemical properties?

The answer does not lie in stainless steel or refrigerants themselves, but in a long-neglected factor: the system environment.

02 Compatibility Nature: It Is Not Refrigerant, But the "System" That Causes Corrosion

A fundamental misconception must be clarified: R290 and R774, the mainstream refrigerants of the future, are not corrosive to stainless steel in their pure form. R290 is a natural hydrocarbon working fluid with stable chemical properties, and its corrosion rate on 304L and 316L stainless steel is ≤ 0.01 mm/year. R774 is an inorganic natural working fluid free of fluorine and chlorine elements, featuring high chemical inertness and excellent compatibility with stainless steel.

The real culprit of corrosion hides within the system: a "triangular alliance" of refrigerant + moisture + impurities (air, metal debris, etc.).

Moisture is the key to triggering the corrosion chain reaction. When moisture exists in the system, two types of dangerous reactions may occur under the catalytic effect of compressor high temperature, high pressure and metal surfaces:

The first is hydrolysis reaction, which mainly occurs in older chlorine-containing refrigerants (such as the obsolete R22). These refrigerants react with water to produce hydrochloric acid. Chloride ions dissociated from hydrochloric acid are the number one enemy of the stainless steel passivation film, easily causing pitting corrosion and stress corrosion cracking. Stainless steel (including 304L) achieves corrosion resistance through a dense surface Cr₂O₃ passivation film, and strong acidic environments directly damage the film structure. Among them, 316L has better resistance to chloride ion erosion than 304L due to its molybdenum content.

The second is thermal decomposition and acidification. Even for chlorine-free natural working fluids like R290, water in the system and compressor lubricating oil (mineral oil or POE oil compatible with R290) will decompose at high temperatures to produce organic acids. Although R774 is chemically stable, mixing with system impurities may still accelerate slight pipeline corrosion under long-term high-temperature and high-pressure conditions. A long-term acidic environment will slowly corrode all metals, including stainless steel.

Therefore, the compatibility challenge of "steel-for-copper" is essentially the ultimate challenge of "system dryness and cleanliness control". The corrosion resistance advantages of 304L and 316L can only be fully exerted in clean systems, especially when adapting to R774, which imposes more stringent cleanliness requirements on the system under high pressure.

03 Risk Classification: Who Are the Safe Partners, and Who Are the Potential Threats?

Refrigerants of different generations vary in risk levels when paired with 304L and 316L stainless steel, due to differences in chemical composition, environmental performance and thermal stability. Aligned with environmental policies, R290 and R774 occupy the core track of the future with their ultra-low GWP values.

High-Risk Zone: Obsolete Chlorine-Containing Refrigerants

Chlorine-containing R22 not only damages the ozone layer but also reacts with water to generate hydrochloric acid, posing a clear threat to 304L and 316L stainless steel. With the accelerated implementation of the Kigali Amendment, these high-GWP refrigerants have gradually been phased out of the market and should be strictly prohibited in steel-for-copper systems.

Medium-Risk Zone: Current Mainstream Chlorine-Free HFC Refrigerants

Represented by R410A and R32, these refrigerants contain no chlorine and have an Ozone Depletion Potential (ODP) of 0. However, the risk is completely transferred to the control of system moisture and acid value. Among them, R32 has become the mainstream choice for steel-for-copper air conditioners due to its moderate GWP (approximately 675) and excellent energy efficiency. When paired with stainless steel, the system's corrosion resistance is further enhanced under mild pollution conditions. The core considerations for this type of refrigerant are: strict control of system dryness is required to avoid acidification reactions caused by moisture mixing with POE oil. Moreover, they will be gradually replaced by natural working fluids.

Low-Risk Zone: Future Core Natural Working Fluids

R290 and R774 are environmentally friendly working fluids promoted by global policies, with excellent compatibility with 304L and 316L stainless steel:

• R290: As a natural hydrocarbon working fluid, it has an ODP of 0 and a GWP of 3, demonstrating outstanding environmental performance. Its thermal performance is close to that of traditional refrigerants, with high latent heat of vaporization and excellent thermal conductivity, achieving energy-saving effects of up to 30%. It is well-compatible with 304L/316L stainless steel and adapted mineral oils. The only challenge is its flammability and explosiveness, which can be effectively controlled by reducing charging amount and optimizing sealing structures. After 2029, it will become the mainstream working fluid for household air conditioners.
• R774 (CO₂): As a natural inorganic working fluid, it has an ODP of 0 and a GWP of 1, posing no environmental burden. It features extremely stable chemical properties, is non-flammable and not prone to reacting with other substances, making it suitable for all types of stainless steel. Its core feature is high working pressure, which imposes strict requirements on pipeline strength. Stainless steel has become the preferred material for R774 systems due to its higher mechanical strength and corrosion resistance.

04 Winning Strategy: Five Non-Negotiable Engineering Lifelines

The success of "steel-for-copper" is not a simple material substitution in the field of materials science, but a systematic engineering project. In particular, the adaptation of refrigerants to 304L and 316L stainless steel requires full-chain process control. For R290 and R774, the following five lifelines are essential to ensure reliability:

1. Ultimate Dryness & Cleanliness

This is the cornerstone of all requirements, and standards need to be further elevated when adapting to R774. High-standard helium leak detection and ultra-long deep vacuuming must be adopted in the manufacturing process to ensure that the system dew point is well below industry standards. Any residual moisture will act as a catalyst for acidification reactions, posing corrosion risks. Even 316L, with stronger corrosion resistance, cannot withstand long-term acidification erosion in high-moisture environments. In addition, under the high pressure of R774, fine impurities may increase pipeline wear, so system cleanliness control needs to be strengthened.

2. Scientific Material Matching

For conventional indoor conditions and dry environments, 304L stainless steel is preferred. Its low-carbon design can avoid intergranular corrosion after welding, meeting the service requirements of most air conditioning systems for more than 10 years. For critical high-pressure, high-salt spray environments (coastal areas) or R774 high-pressure systems, 316L stainless steel must be selected. Its molybdenum content significantly enhances resistance to chloride ion pitting corrosion, and its higher mechanical strength can withstand the ultra-high operating pressure of R774. Connection welding must use argon arc welding, and pickling and passivation treatment is required after welding to repair the surface passivation film. For 316L welding, it is recommended to use welding materials of the same grade. Additional pressure testing is required for R774 system welds to ensure sealing and strength.

3. Appropriate Medium Selection

Special lubricating oil must be matched according to refrigerant type: R290 is suitable for mineral oil or special POE oil, avoiding conventional POE oil with excessive hygroscopicity. R774 requires special lubricating oil with high pressure and high viscosity stability to ensure compatibility with refrigerants and stainless steel. All systems should be equipped with high-efficiency acid-absorbing filters to absorb moisture and acidification products in real time, reducing erosion of the stainless steel surface passivation film.

4. Full-Life-Cycle Monitoring

Reliance solely on factory inspection is insufficient. A monitoring mechanism or maintenance reminder for refrigerant acid value and moisture content during operation should be established within the product design life cycle. R290 systems require additional attention to leakage and can be equipped with refrigerant concentration sensors to achieve safety warnings. R774 systems need regular checks on pipeline pressure stability to avoid high-pressure leakage risks.

5. Standardized Installation & Environmental Isolation

Strictly follow installation specifications to avoid potential corrosion caused by direct contact between 304L/316L stainless steel pipes and copper pipes (use insulating gaskets if necessary). Protective measures should be taken for exposed pipelines based on material characteristics and refrigerant working conditions: in coastal areas, 304L pipelines require additional anti-corrosion coatings; when 316L pipelines are adapted to R774, external protective structures should be strengthened to resist environmental erosion under high pressure. Meanwhile, the installation area should be well-ventilated to avoid safety risks caused by R290 leakage and oxygen deficiency caused by R774 accumulation.

05 Future Outlook: Reshaping & Upgrading of the Industry Chain

"Steel-for-copper" is far more than replacing a single pipeline. It forms a two-way drive with natural working fluids such as R290 and R774, forcing the entire air conditioning industry chain to undergo systematic upgrading. The large-scale application of 304L and 316L stainless steel also promotes iterative optimization of material processing and manufacturing processes.

Upstream Materials & Processing

It promotes the expansion of production capacity of precision thin-walled 304L and 316L stainless steel pipes, optimizes the process of special high-pressure pipes for R774 systems, and balances strength and heat exchange efficiency. At the same time, it drives the purification and production capacity improvement of R290 and R774 refrigerants, and improves supporting industries such as special lubricants and high-pressure sealing components. China has reached a leading level in R290 technology R&D and application. Welding processes have also been upgraded, and precision argon arc welding technology for stainless steel is gradually popularized.

Midstream Manufacturing

It puts forward almost harsh new requirements for factory environmental humidity control, vacuum technology and testing standards. The importance of clean rooms and automated production has reached an unprecedented level to meet the ultimate system cleanliness requirements of the combination of natural working fluids and 304L/316L stainless steel. Meanwhile, enterprises need to optimize production processes: R290 systems require explosion-proof production environments; R774 systems need to upgrade high-pressure testing equipment and establish targeted quality inspection processes for materials and working conditions.

Downstream Services

Standardization requirements for installation and after-sales maintenance are higher. The extensive era of "random vacuuming" must come to an end. Maintenance tools and process standards need to be updated to use high-pressure and explosion-proof tools compatible with R290, R774 and stainless steel. Installation personnel must master high-pressure welding and refrigerant leakage prevention skills. R774 installation requires strict control of pressure sealing.

The ultimate winners of this transformation will be enterprises that can integrate extreme process control into mass manufacturing. What is eliminated is not copper, but backward and extensive production models. It drives the refrigerant industry to shift from HFCs to natural working fluids, making 304L and 316L stainless steel the core supporting materials for air conditioning pipeline systems.

Stainless steel pipes shine with a cool luster under sunlight, circulating next-generation environmentally friendly refrigerants such as R290 and R774. This is no longer a concept, but an ongoing industrial reality.

The successful formula for "steel-for-copper" is clear and rigorous: High-Quality Stainless Steel + Future Environmentally Friendly Refrigerants (R290/R774) + Top-Tier Dry Cleaning & Working Condition Adaptation Processes = Reliable Backbone of Next-Generation Air Conditioners.

When the last water molecule is removed from the system, the corrosion chain is completely cut off. This material revolution, driven by cost and environmental protection, will ultimately test the determination and capability of Chinese manufacturing to move towards high precision and high reliability, and propel the air conditioning industry into a green new era of "natural working fluids + high-quality low-carbon stainless steel".