The fundamental reason for the corrosion resistance of stainless steel lies in its key alloying element—chromium (Cr), with a chromium content typically exceeding 10.5%.

Formation of the passive film: In an oxidative environment, chromium reacts with oxygen to spontaneously form an extremely thin and dense chromium-rich oxide film (mainly Cr₂O₃) on the surface of stainless steel. This film is only 2-5 nanometers thick and is invisible to the naked eye.

Dual protection functions:

• Physical barrier: This film completely isolates the metal matrix from external corrosive media (such as water, oxygen, acids, alkalis, chloride ions, etc.), blocking the electrochemical pathways of corrosion reactions.

• Chemical stability: Chromium oxide itself is highly chemically stable and remains inert in most environments, making it resistant to dissolution or degradation.

If this passive film is locally damaged due to scratching, the newly exposed metal surface will immediately react with oxygen and quickly self-repair, forming a new protective film. This endows stainless steel with a persistent "self-healing" ability.

In addition to chromium, the core element, other alloying elements commonly added to stainless steel also play a crucial synergistic role in enhancing its corrosion resistance in various harsh environments:

Molybdenum (Mo): It significantly enhances resistance to chloride ions (Cl⁻). Chloride ions are one of the main "culprits" that destroy the passive film, causing dangerous pitting and crevice corrosion. The addition of molybdenum can effectively inhibit this destruction. Therefore, heat exchangers for marine or salt-containing duties normally use Mo-bearing grades such as 316L (S31603), 317L (S31703), 2205 (S32205), 2507 (S32750) or 254SMO (S31254).

Nickel (Ni): Its main function is to stabilize the austenitic structure, improving the material's overall toughness and ductility. It also enhances the stability of the passive film and reduces the risk of stress corrosion cracking, which is crucial for heat exchangers subjected to thermal and mechanical stresses (e.g. 304L/S30403, 316L/S31603, 904L/N08904, 254SMO/S31254, Alloy 625/N06625).

Nitrogen (N): In duplex stainless steel (2205/S32205, 2507/S32750), nitrogen can effectively refine the grain size, increase the material's strength, and significantly enhance its resistance to pitting corrosion.

Although stainless steel has an excellent inherent corrosion resistance, the corrosion resistance of heat exchangers is also affected by actual operating conditions and the quality of design and manufacturing:

Medium characteristics: The composition of corrosive media (such as chloride ion concentration, pH value), temperature, and pressure are key factors. For example, when the chloride ion concentration exceeds 50ppm, ordinary 304 (S30400) stainless steel may face the risk of pitting corrosion. In such cases, more corrosion-resistant 316L (S31603) or duplex 2205 (S32205) needs to be selected.

Manufacturing process: Welding is a critical part of heat exchanger manufacturing, but the heat-affected zone of welding is prone to sensitization, increasing the sensitivity to intergranular corrosion. Therefore, it is usually necessary to use processes such as solution treatment to eliminate welding residual stress and restore the material's corrosion resistance (particularly for low-carbon grades such as 304L/S30403 and 316L/S31603).

Surface treatment: Electrolytic polishing or acid pickling and passivation of the stainless steel surface can remove minor surface defects, impurities, and contaminants, making the surface smoother. This promotes the formation of a more uniform and dense passive film, further enhancing its corrosion resistance.