Pure titanium metal does not turn green or tarnish under normal conditions. This stability is why the silver-gray metal is widely used in demanding fields like aerospace, medical implants, and fine jewelry. Titanium resists the chemical reactions that cause discoloration in other materials, ensuring it maintains its original appearance indefinitely. The metal’s exceptional performance is linked to a fundamental chemical process that occurs the moment it is exposed to air.
The Science Behind Titanium’s Stability
Titanium’s resistance to tarnish is due to a natural process called passivation. When the metal is exposed to oxygen, it instantly reacts to form a layer of titanium dioxide (TiO2) on its surface. This oxide layer is extremely thin, typically only a few nanometers thick, and is transparent.
The titanium dioxide layer is dense, non-porous, and highly adherent to the underlying metal, acting as a protective barrier. This shield prevents the titanium atoms underneath from reacting further with oxygen or corrosive elements. Because this clear layer is chemically stable, the metal retains its original color and bright finish. The protective film is also self-healing; if scratched, it immediately reforms upon contact with air, ensuring continuous protection.
Why Other Metals Turn Green
The green discoloration commonly associated with metals is a form of corrosion known as verdigris or patina, which most frequently involves copper. Copper and its alloys, such as brass and bronze, are chemically different from titanium and lack its protective oxide mechanism. When these metals contact moisture, oxygen, and acidic substances, they undergo a visible chemical transformation.
This reaction forms copper salts, such as copper carbonate, chloride, or acetate. The green residue that transfers to the skin from jewelry is often caused by copper reacting with skin oils, sweat, or cosmetic chemicals. The Statue of Liberty has its famous green hue due to the formation of a copper sulfate-based patina over many years of exposure. This type of progressive, visible corrosion does not occur with pure titanium.
Identifying the Real Culprit of Green Discoloration
If a product labeled “titanium” turns green, it is almost certainly not pure titanium, but rather a low-grade alloy or a coated material. The most common culprit is a titanium alloy containing a significant percentage of copper, nickel, or cobalt. These less-stable metals within the alloy react with the environment, causing the visible discoloration.
Another possibility is that the titanium base metal has been coated with a cheaper finish or a base layer that is wearing off. When the coating wears away, it may expose a non-titanium alloy underneath that is prone to tarnish. Consumers can seek items stamped with specific quality designations, such as “Grade 1” or “Grade 2” (commercially pure titanium), or “Grade 5” (Ti-6Al-4V, a common aerospace alloy), to ensure metal purity and stability.
Intentional Coloring: Anodization and Surface Treatments
While titanium does not naturally turn green, its surface can be intentionally colored to virtually any shade, including vibrant greens, through anodization. This electrochemical process involves immersing the titanium in an electrolyte bath and applying a controlled electrical voltage. The voltage precisely thickens the natural titanium dioxide layer.
The color is not a dye or pigment but an optical effect achieved through light interference, similar to the colors seen on a soap bubble or an oil slick. As white light hits the thickened oxide layer, certain wavelengths are filtered out, and only a specific color is reflected back. Creating a green hue requires the highest voltage and the thickest oxide layer, making the color stable and resistant to tarnish.