The corrosion resistance principle of stainless steel tableware

The corrosion resistance principle of stainless steel tableware mainly stems from the dense oxide film formed on its surface and the optimization of the material's microstructure and electrochemical properties by alloying elements. The following is the specific analysis:

The formation of a surface passivation film

The role of chromium: The main component of stainless steel is iron, but at least 10.5% chromium is added. When stainless steel is exposed to air or an oxygen-containing environment, chromium reacts with oxygen to form an extremely thin (only a few nanometers) and dense chromium oxide (Cr₂O₃) film on the surface. This film is very stable and can effectively prevent oxygen, moisture and other corrosive media from further eroding the metal substrate.

Self-healing ability: If the passivation film is locally damaged due to scratches or corrosion, chromium will rapidly react with oxygen to re-form an oxide film at the damaged area, thereby restoring its protective effect. This self-healing ability is the key to the corrosion resistance of stainless steel.

2. Synergistic effect of alloying elements

The addition of nickel: The addition of nickel can enhance the toughness and corrosion resistance of stainless steel, especially in reducing environments such as acidic solutions. Nickel can also stabilize the austenite structure, enabling stainless steel to maintain good performance at low temperatures.

Molybdenum enhancement: In chlorine-containing environments (such as seawater or salt solutions), the addition of molybdenum can significantly improve the resistance of stainless steel to pitting and crevice corrosion. Molybdenum can promote the stability of the passivation film and inhibit the occurrence of local corrosion.

Other elements: The addition of elements such as nitrogen, titanium, and niobium can further refine the grains, enhance the material's strength, and improve its resistance to intergranular corrosion.

3. Electrochemical stability

The electrochemical properties of the passivation film: The chromium oxide film has a very high resistance, which can effectively prevent the transfer of electrons and thereby inhibit the progress of electrochemical corrosion reactions. In corrosive media, the electrode potential of stainless steel is relatively high, making it the cathode. Meanwhile, reducing substances in corrosive media (such as chloride ions) are difficult to undergo reduction reactions on the surface of stainless steel, further reducing the corrosion rate.

The challenge of chloride ions: Although stainless steel has good corrosion resistance, chloride ions (such as NaCl in table salt) can penetrate the passivation film, causing local corrosion (such as pitting corrosion). Therefore, in a chlorine-containing environment, the corrosion resistance of stainless steel will significantly decline.

4. The influence of microstructure

Prevention of intergranular corrosion: When stainless steel is heated at high temperatures or welded, carbon may form chromium carbide (Cr₂₃C₆) with chromium, reducing the chromium content near the grain boundaries and creating chromium-poor zones, which can cause intergranular corrosion. This problem can be effectively avoided by adding titanium or niobium (to form stable carbides) or reducing the carbon content (such as using ultra-low carbon stainless steel).

The influence of phase structure: Austenitic stainless steels (such as 304 and 316) have good corrosion resistance and toughness, while ferritic or martensitic stainless steels have relatively poor corrosion resistance. Therefore, tableware is usually made of austenitic stainless steel.

5. The influence of environmental factors

Oxidizing environment: In an oxidizing environment (such as air, fresh water), the passivation film can stably exist, and stainless steel has relatively good corrosion resistance.

Reducing environment: In a reducing environment (such as acidic solutions or oxygen-deficient conditions), the passivation film may be damaged, leading to an accelerated corrosion rate. At this point, the addition of molybdenum can significantly enhance the corrosion resistance of stainless steel.

Temperature and pH value: High temperature and low pH value (acidic environment) will accelerate corrosion reactions and reduce the corrosion resistance of stainless steel.