Titanium is a corrosion-resistant metal used in demanding industrial, marine, and medical applications. Although titanium is chemically reactive, meaning it readily bonds with other elements, its durability against corrosive attack is the result of a spontaneous surface phenomenon. This protective mechanism creates a stable barrier that isolates the underlying metal from its surrounding environment.
How Titanium Achieves Corrosion Resistance
Titanium’s resistance is due to the formation of a thin, dense, and stable surface film. When titanium is exposed to air or any environment containing oxygen or moisture, it instantly reacts to form a layer of titanium dioxide (\(\text{TiO}_2\)). This process, known as passivation, occurs immediately upon contact, shielding the metal from further chemical reaction.
This protective oxide layer is thin, often measuring only a few nanometers, and is non-porous and tightly bonded to the surface. The layer prevents corrosive substances like water, acids, and salts from reaching the underlying titanium atoms. The stability of this \(\text{TiO}_2\) film allows titanium to resist environments that degrade many other engineering metals.
The passive film also has the capacity for self-healing. If the surface is scratched or mechanically damaged, the newly exposed titanium immediately reacts with available oxygen or water to reform the protective oxide layer. This instantaneous repair mechanism ensures the metal’s defense is continuous, even in conditions involving wear. This renewal process is beneficial in dynamic or abrasive environments.
Key Environments Where Titanium Thrives
Titanium’s passive oxide layer remains stable across a wide range of aggressive conditions. A common application is the marine environment, where the metal demonstrates near-total immunity to seawater and brine solutions. The \(\text{TiO}_2\) film resists chloride ions, which are notorious for causing pitting and crevice corrosion in stainless steels.
Titanium also performs well in chemical processing environments, especially when handling oxidizing acids such as nitric acid. The strong oxidizing nature of the acid reinforces the passive layer, stabilizing the titanium dioxide film. This makes it a preferred material for heat exchangers, pumps, and valves used in industrial chemical production.
Biomedical Applications
Titanium is widely used for biomedical implants, including joint replacements and dental fixtures. Its corrosion resistance ensures the metal remains stable when exposed to the physiological environment of the human body and body fluids. This stability prevents the release of metallic ions into the surrounding tissue, which is why titanium is considered biocompatible and non-toxic.
When Titanium Corrosion Can Still Occur
Titanium is not universally immune to corrosion and can fail under specific conditions. One vulnerability is crevice corrosion, which occurs in tight, stagnant gaps, such as under washers or flanges, particularly in high-temperature chloride solutions. In these confined spaces, the oxygen supply is depleted, which prevents the passive \(\text{TiO}_2\) layer from self-healing if damaged.
This localized breakdown typically requires high temperatures. Unalloyed titanium becomes susceptible to crevice corrosion in neutral brines above \(70^\circ\text{C}\) to \(80^\circ\text{C}\). The stagnant, low-oxygen conditions allow a localized, acidic environment to develop, eventually dissolving the protective film.
Certain reducing agents can also directly attack the passive layer, most notably hydrofluoric acid (HF). Titanium undergoes rapid dissolution even when exposed to minute concentrations of HF because the fluoride ion actively removes the titanium dioxide barrier. While titanium is effective at ambient temperatures, its resistance can be compromised in very high-temperature applications. At extreme temperatures, the layer may become excessively thick and brittle, or the metal may dissolve in strongly reducing media.