To better understand whether copper and stainless steel press-fit fittings can be directly connected, we need to consider some key material properties and research data. Below are the key data comparisons for copper and stainless steel, as well as research on electrochemical corrosion:
1. Basic Data Comparison Between Copper and Stainless Steel:
| Property | Copper | Stainless Steel |
|---|---|---|
| Electrical Conductivity | High (around 5.8 × 10^7 S/m) | Low (around 1.4 × 10^6 S/m) |
| Thermal Expansion Coefficient | 16.5 × 10^(-6)/℃ | 16.0 × 10^(-6)/℃ |
| Specific Gravity | 8.96 g/cm³ | 7.85 g/cm³ |
| Tensile Strength | 210–250 MPa | 485–620 MPa |
| Corrosion Resistance | Good, but prone to corrosion in chloride environments | Excellent, especially in oxygenated environments |
| Weldability | Good, but prone to oxidation | Good, with proper welding techniques |
2. Electrochemical Corrosion:
When two different metals (such as copper and stainless steel) are in direct contact, galvanic corrosion occurs. The severity of this corrosion depends on the following factors:
Electrode Potential Difference: Copper has a lower electrode potential (around +0.34 V), while stainless steel has a higher electrode potential (around +0.2 V). This difference means copper will oxidize when in contact with stainless steel, leading to corrosion.
Corrosion Rate: In humid, chloride-rich environments, the rate of galvanic corrosion increases. Copper, acting as the anode, will corrode more quickly, while stainless steel, as the cathode, will be protected. Therefore, the corrosion rate of copper will accelerate, potentially leading to leakage or damage at the connection points.
Research shows that galvanic corrosion occurs more rapidly in wet and chloride-rich environments. For instance, ASTM G82 studies indicate that contact between different metals speeds up corrosion, especially in wet and saline conditions.
3. Thermal Expansion Coefficient Difference:
Thermal Expansion Coefficient of Copper: 16.5 × 10^(-6)/℃
Thermal Expansion Coefficient of Stainless Steel: 16.0 × 10^(-6)/℃
The thermal expansion coefficients of both materials are very close, but due to copper's lower tensile strength, prolonged thermal cycling could cause copper to deform permanently or cause the connection point to loosen.
4. Seal Compatibility:
The types of seal rings required for copper and stainless steel pipes may differ:
Copper Pipes typically use EPDM or NBR seals.
Stainless Steel Pipes often use EPDM or FPM seals, especially in high-temperature environments.
The properties of these materials are not entirely compatible, and they could impact the sealing performance at the connection points.
5. Relevant Research Data:
"Corrosion of Copper in Seawater" (J.C. Scully, 1991): This study examined copper corrosion in seawater and found that copper is prone to electrochemical corrosion, particularly when in contact with other metals like stainless steel.
"Galvanic Corrosion of Copper and Stainless Steel in a Chloride Environment" (D.G. Doolan, 2001): This research explored the galvanic corrosion between copper and stainless steel in chloride environments. The findings indicated that copper's corrosion rate is significantly higher in wet and chloride-rich conditions, especially when in contact with stainless steel.
6. Conclusion and Recommendations:
It is not recommended to directly connect copper and stainless steel fittings due to the increased risk of electrochemical corrosion, particularly in wet and chloride-rich environments.
If it is necessary to connect these two materials, it is recommended to use copper-stainless steel transition fittings or electrically insulating gaskets to reduce the occurrence of galvanic corrosion.
Choose an appropriate connection method based on the materials' corrosion resistance, thermal expansion properties, and sealing requirements.