Titanium dioxide (TiO2) is one of the most widely investigated and tested metal oxides due to its numerous desirable properties, relatively low cost and exceptional performance in a broad range of applications. Additionally, vast resources have been dedicated to studying different coating processes that can produce surfaces integrated with this chemical compound.
Also known as titanium (IV) oxide or titania, titanium dioxide is a naturally occurring oxide of titanium that comes in several chemical compositions depending on the nature of their environmental exposure. (Learn more about titanium in Your Guide to Corrosion-Resistant Metals.) Titanium dioxide, in particular, is created when titanium metal oxidizes in aqueous environments.
Titanium dioxide is mostly used as an active ingredient in paints and varnishes – accounting for more than 80% of the total global consumption. Printing inks, fibers, elastomers, sunscreen, food coloring and cosmetics account for the remainder of the usage.
What is Titanium Dioxide?
Among the numerous metal oxides that are used for coating applications, titanium dioxide has garnered the most attention from the scientific community. This compound has a powdery white appearance and, as mentioned previously, is typically used as an additive to paints and coatings due to its exceptional corrosion resistance and surface passivation properties.
Although titanium dioxide is formed by the reaction between oxygen and titanium, it may also be obtained via extraction from naturally occurring minerals. Ilmenite ore (FeTiO3) is currently the most prominent source titanium dioxide in the world. Rutile is the next most abundant source of titanium dioxide, with over 98% of the compound in its ore. Other sources of titanium dioxide include anatase, perovskite (CaTiO3) and titanite (CaTiSiO5).
Truth #1: Titanium Dioxide’s Corrosion Resistance Surpasses Other Comparable Metal Oxides
In addition to its hardness and wear resistance, titanium dioxide is renowned for its superior corrosion resistance. Titanium dioxide films are very stable and are vulnerable to attack by only a few substances (e.g., hydrofluoric acid). This gives components coated with titanium dioxide the ability to resist corrosion in saline and some acidic environments.
This corrosion resistance is primarily due to the lack of interconnected pores that usually provide corrosive media with a preferential path to reach the metal substrate. Paints and coatings can vastly benefit from even small amounts of titanium dioxide. Several studies have shown that the addition of as little as 3% of titania to alumina can significantly increase the toughness and corrosion resistance of the coating.
When formed naturally due to the exposure of titanium metal to the environment, titanium dioxide is highly stable and tightly adheres to the metal’s surface to create a protective barrier that prevents the intrusion of further air and moisture. At room temperatures, the titanium dioxide film forms immediately after a clean titanium surface is exposed to the air. The initial film thickness upon exposure is about 12-16 Å thick (1 Å = 1 x 10-10 m). The thickness of the film continues to grow steadily until it reaches a thickness of 50 Å in 70 days, eventually reaching 250 Å after four years.
Titanium dioxide film growth can be accelerated under extraordinary oxidizing conditions such as high temperatures, anodic polarization in an electrolyte or when exposed to specific oxidizing agents (e.g., HNO3, CrO3).
Truth #2: Titanium Dioxide Films are Not Invulnerable to Corrosion
Although titanium dioxide is a highly effective corrosion barrier, it is vulnerable to attack from specific chemical compounds. Degradation of the titanium dioxide film exposes the substrate to the external environment and gives rise to additional corrosion; thus resulting in significant material loss. (More information can be found in 5 Things to Know and Understand about Titanium Corrosion.)
Fluorides and hydrogen peroxide, for instance, are known to slowly deteriorate titanium dioxide films. Hydrofluoric acid, although considered to be a relatively weak acid, consists of highly reactive molecules that are extremely aggressive towards titanium. Studies have shown that hydrofluoric acid concentrations above 30 ppm can have a destructive effect on these protective oxide films.
The effectiveness of titanium dioxide corrosion protection can also be severely affected by anhydrous environments, i.e., environments with little to no moisture. Since titanium dioxide typically forms in aqueous environments, the lack of water impedes the film’s ability to self-heal or offer galvanic protection.
Truth #3: Titanium Dioxide is Highly Resistant to Bodily Fluids
Titanium dioxide’s combination of desirable mechanical properties, corrosion resistance, chemical resistance and overall biocompatibility makes it a popular choice for coating medical devices and implants. Its biocompatibility is due mainly to its inertness and potent passivation properties.
Although titanium dioxide is an effective protective layer, there are some instances where it may be insufficient to maintain stability under the action of bodily fluids to provide full corrosion protection. Degradation of the titanium dioxide layer can result in the release of metal ions into nearby tissue. This can have an adverse effect on the body’s defensive mechanisms and cellular activity, which can ultimately trigger inflammatory responses and even result in implant rejection. For example, some titanium alloying elements have been found to be cytotoxic (toxic to cells), thus producing detrimental effects on tissues.
Fortunately, titanium oxide surfaces can be modified by numerous methods to improve its corrosion resistance for applications requiring biocompatibility. Heat treatments have emerged as a simple yet effective technique to create thin and firmly adherent layers of anatase. This layer consists of a well-defined morphology and crystallography, which enhances the film’s overall biocompatibility by impeding the release of potentially hazardous ions.
Titanium dioxide films and coatings offer corrosion protection that is superior to other comparable metal oxides. This is attributed mainly to its lack of interconnected pores, which have a tendency to facilitate the formation of corrosion. This oxide’s biocompatible properties make it one of the few oxides that can be safely used in medical applications where contact with human skin and tissue is required.
Although titanium dioxide has a multitude of desirable characteristics, it should be noted that it is not invulnerable to corrosion. Specific chemicals and anhydrous environments can break down the protective oxide layer, leaving the underlying substrate vulnerable to deterioration.