Stainless steel and carbon steel cannot be used in direct contact, which is a crucial principle in materials science and engineering practice. Mainly due to the occurrence of "galvanic corrosion", also commonly referred to as "galvanic corrosion" or "heterogeneous metal corrosion". This is like a piece of carbon steel sacrificing itself to protect stainless steel, causing the carbon steel to quickly rust.
Stainless steel cannot match with carbon steel core cause: galvanic corrosion
1. Potential difference is the driving force
Different metals have different electrochemical activities in electrolytes (such as water, humid air, acids, bases, salts, etc.), which can be understood as their varying degrees of electron loss. This difference in activity is measured by electrode potential.
Reactive metals, such as carbon steel, have lower electrode potentials and are more prone to losing electrons, making them less corrosion-resistant.
Inert metals (such as stainless steel) have higher electrode potentials and are less likely to lose electrons. The reason why stainless steel is "stainless" is that the chromium on its surface forms a dense chromium oxide passivation film, which prevents further corrosion.
When these two metals come into direct contact in the electrolyte, a complete primary battery circuit is formed.
2. Corrosion process
Anode (corroded end): Carbon steel, as an active metal, becomes the anode of the battery. It will actively dissolve (corrode) and release electrons. The reaction is: Fe → Fe ² ⁺+2e ⁻
Cathode (protected end): Stainless steel, as an inert metal, becomes the cathode of the battery. It does not corrode, but only receives electrons flowing from the anode and uses these electrons to react with electrolytes (such as oxygen in water). The reaction is: O ₂+2H ₂ O+4e ⁻ → 4OH ⁻
Result: In this battery system, current flows from carbon steel (anode) to stainless steel (cathode), causing a sharp increase in the corrosion rate of carbon steel, while stainless steel is protected by "cathodic protection" and is almost not corroded.
A vivid metaphor:
It's like having an "honest person" (carbon steel) and a "smart person" (stainless steel) partner up to do business. When faced with difficulties (corrosive environment), honest people will constantly sacrifice their own interests (being corroded) to ensure that smart people are unharmed.
Stainless steel cannot match with carbon steel key influencing factors
The severity of galvanic corrosion depends on the following factors:
Environment (electrolyte): This is the most critical factor. In dry air, galvanic corrosion does not occur because there is no electrolyte forming a circuit. But in humid environments, seawater, industrial areas, and salt spray environments, corrosion can be very rapid and severe.
Potential difference: The greater the potential difference between two metals, the stronger the driving force for corrosion. The potential difference between carbon steel and stainless steel is large enough to cause significant corrosion.
The ratio of anode to cathode area: This is one of the most dangerous situations. If the area of the cathode (stainless steel) is large and the area of the anode (carbon steel) is small, the corrosion current will be highly concentrated on the small carbon steel, causing it to be completely corroded and perforated in a very short period of time. For example, if a stainless steel tank is fixed with a carbon steel bolt, the carbon steel bolt will quickly rust and break.
How to prevent and solve stainless steel connecting with carbon steel?
In practical applications, we often need to connect stainless steel and carbon steel together, and isolation measures must be taken:
1. Electrical insulation: This is the most effective and commonly used method. Add non-conductive insulation material between two metals to cut off the current circuit.
- Use insulation gaskets/washers: Use plastic (such as PVC, nylon), rubber, or synthetic gaskets at flange connections.
- Use insulated bushings and washers: In bolted connections, use plastic bushings between bolts and carbon steel holes, and use insulated washers under nuts.
- Coating isolation layer: Spray epoxy resin, paint or use other coatings on the contact surface. It is usually recommended to coat both, or at least coat the cathode (stainless steel) surface, because if only the anode (carbon steel) is coated, once the coating is damaged, the corrosion at the damaged area will become more severe.
2. Control environment: Keep the connection parts as dry and clean as possible to avoid electrolyte accumulation.
3. Using transition materials: adding a metal with an electrode potential between two metals (such as aluminum), but this method is less commonly used and requires careful design.
4. Cathodic protection: The entire structure is artificially transformed into a cathode by applying an external current or sacrificing an anode (such as a zinc block), but this is typically used for large structures such as ships and pipelines.
Conclusion
Stainless steel and carbon steel cannot come into direct contact because they can form primary batteries in humid electrolyte environments, leading to accelerated galvanic corrosion of carbon steel as the anode. To avoid this situation, electrical insulation isolation measures must be taken during design and installation, such as using insulation gaskets, bushings, and coatings, to ensure the safety and long-term service life of the equipment.
Post time: Oct-29-2025