Titanium does not rust like iron, but it can experience localized corrosion under extreme conditions.
The following is a systematic analysis based on materials science and engineering practice.
I. Core Conclusion: The "Rust-Free" Mechanism of Titanium
Passivation Film Protection: Titanium rapidly forms a dense titanium oxide (TiO₂) film only 1–10 nanometers thick in air or water. This film is self-healing; even if mechanically scratched, it can quickly regenerate in an oxidizing environment, preventing further corrosion.
Chemical Inertness: Titanium exhibits extremely strong chemical inertness to most acid, alkali, and salt solutions (including seawater, chlorides, lemon juice, and vinegar), and does not release metal ions. Therefore, it is widely used in medical implants and food contact materials.
Terminology Clarification: "Rusting" specifically refers to the corrosion of hydrated iron oxides (Fe₂O₃·nH₂O). Titanium does not produce such products. In everyday language, "titanium does not rust" is actually a colloquial expression of "titanium does not corrode."
II. Comparison of Corrosion Resistance Data (Typical Environment)
| Environmental conditions | Corrosion rate(mm/a) | Material | Remarks |
|---|---|---|---|
| 3.5% NaCl Sea water | ≤0.001 | Grade 2,Pure Titanium | There is almost no uniform corrosion, and the risk of pitting corrosion is extremely low. |
| 5% HCl(35°C) | 1.27 | Grade 2,Pure Titanium | Although the figures seem high, there are still engineering risks due to the extremely thin wall thickness of the equipment (<0.5mm). |
| 3.5% NaCl + 110°C | >0.1 | Grade 2,Pure Titanium | Significant crevice corrosion led to equipment failure within 8 months. |
| 3.5% NaCl | ≤0.003 | Gr7 (Ti-Mo alloy) | Molybdenum enhances the stability of the passivation film and increases its corrosion resistance tenfold. |
| Human body fluids (simulated) | <0.01 | Grade 2,Pure Titanium | No corrosion or ion release after 20 years of implantation |
III. Corrosion Failure Mechanisms in Extreme Environments
Despite titanium's excellent corrosion resistance, localized corrosion can still occur under the following conditions:
**Crevice Corrosion:** In high-temperature (>80°C) and high-concentration chloride (e.g., ammonium chloride) environments, stagnant liquids form oxygen concentration cells, leading to honeycomb corrosion of TA2 pure titanium in welds and under gaskets, with corrosion pits reaching depths of up to 6 mm.
**Hydrogen-Induced Stress Corrosion Cracking (HISCC):** In deep-sea high-pressure (>5000m) or hydrogen-containing environments, titanium absorbs hydrogen atoms and becomes embrittled, reducing its fracture toughness to 22% of its original value. Typical scenarios: pressure hulls of deep-sea submersibles, marine oil and gas pipelines.
**Strain Corrosion Cracking (SCC):** Requires simultaneous tensile stress + a specific medium (Cl⁻, H₂S) + a sensitive alloy. The crack propagation rate can reach millions of times that of uniform corrosion, with no macroscopic warning.
IV. Engineering Applications
Successful Applications:
- Medical: Artificial joints, cardiac stents, dental implants (titanium integrates with bone tissue to form "osseointegration")
- Chemical: TA10 titanium alloy ammonium chloride evaporator, with a service life exceeding 15 years
Marine: Pressure-resistant hulls for deep-sea probes, seawater desalination equipment piping
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