Titanium (Ti) is a naturally occurring metallic element with many desirable chemical and mechanical properties. Among these properties, titanium is best regarded for two characteristics in particular: excellent corrosion resistance and a relatively high strength-to-density ratio. In its unalloyed form, titanium is resistant to corrosion in seawater, wet chlorine and organic chlorides – media that would typically cause pitting and crevice corrosion in steels.
Ever since the introduction of titanium alloys in the early 1950s, titanium has gone on to become one of the most commonly used construction materials in the aerospace industry. Over time, with decreasing costs and increasing availability, the use of titanium alloys has continued to spread and is now widely used in numerous industrial applications. Some of the many uses of titanium and its alloys include paints and coatings, steel alloying, jewelry, industrial storage tanks, electronic components, construction materials and automotive components.
Although titanium has superior corrosion resistance in comparison to many other naturally occurring metals, it is essential to understand the source of this resistance and its benefits and limitations, in order to make the best use of this metal in any given environment. What follows are five facts to know about titanium corrosion.
Titanium has Excellent Corrosion Resistant Properties
Titanium ranks relatively high among naturally occurring metals in terms of corrosion resistance. This is due to the stable, continuous and tightly adhered protective oxide film that forms almost immediately on the metal’s surface when exposed to air and moisture.
At room temperature, after a clean titanium surface is exposed to air and moisture, the oxide film produced is about 12 - 16 Å thick (1 Å = 1 x 10−10 m). After 70 days the film grows steadily to 50 Å, slowly increasing to 80 to 90 Å after 545 days, and 250 Å in four years.
Unlike some metals whose oxide layer can be flaky, brittle, and eventually spall off (such as steel), titanium’s tenacious oxide layer acts as a barrier that prevents corrosive agents from coming into contact with the underlying metal substrate. This layer becomes stronger and more resilient over time and can regenerate almost immediately if broken.
The Composition of Titanium’s Oxide Film Depends on the Surrounding Environment
The titanium passive oxide film does not consist of a single chemical compound. Rather, the film’s nature, thickness and chemical composition depend on the exposure environment. In oxidizing conditions, such as aqueous environments, the film consists mainly of TIO2 (titanium dioxide).
TiO (titanium oxide) is also another variation of the surface oxide film. In high-temperature oxidizing situations, the chemically resistant and highly crystalline form of TiO, known as rutile, is formed. Conversely, oxidation at lower temperatures can produce the more amorphous structure of TiO known as anatase. Other environmental conditions may also result in the formation of Ti2O3 (titanium (III) oxide).
Titanium is Not Corrosion Proof
Although titanium performs well in most corrosive environments, it can be susceptible to degradation under certain conditions. The corrosion resistance of titanium can be severely affected in anhydrous environments, i.e., environments containing little to no water. Titanium derives its corrosion resistance from its oxide film that forms in the presence of moisture and oxygen. Generally, even trace amounts of moisture can be extremely beneficial in passivating the titanium surface. However, in anhydrous media, the oxide film may not form, or if it does, it will be difficult to sustain.
Methanol, for example, can result in stress corrosion cracking (SCC) in unalloyed titanium at moisture contents below 1.5%. Hydrogen embrittlement has also been observed in high-temperature anhydrous conditions. (See An Introduction to Hydrogen Embrittlement for background information.) Other environments known to promote SCC are those containing red fuming nitric acid, nitrogen tetroxide and gaseous bromine and fluorine.
Titanium does Not Obey the Rules of Galvanic Corrosion
In galvanic corrosion (also known as bimetallic corrosion), if two metals are coupled together (directly or indirectly), one will act as the anode and the other as the cathode when exposed to an electrolyte. The metal that is less noble, according to the galvanic series, will become the anode and corrode preferentially. (To learn more about this topic, read Why Do Two Dissimilar Metals Cause Corrosion?)
Titanium, however, behaves a bit differently. Titanium is not as efficient at reducing dissolved oxygen as some other metals (such as copper). Therefore, it is possible for a less noble metal to dissolve more when coupled with copper than with titanium, despite titanium being much more electropositive in the galvanic series.
Similarly, coupling titanium with a more noble metal in an electrolyte does not result in accelerated corrosion of the titanium. In fact, in some cases the electrode potential of the titanium can be increased, reducing the corrosion rate instead of accelerating it. This phenomenon holds true except in environments where titanium does not passivate (e.g., in anhydrous conditions).
Titanium Alloys Well with Other Metals for Increased Corrosion Resistance
Titanium’s corrosion resistant properties can be increased even further by alloying it with other metals. Palladium has been one of the most successful alloying metals. Generally, small amounts of palladium can significantly increase titanium’s resistance to reducing chemicals such as sulfuric, hydrochloric and phosphoric acid. Additionally, this alloy combination is known to raise the critical temperature at which crevice corrosion can occur in seawater.
Other popular titanium alloys include Ti-0.8%Ni-0.3%Mo, Ti-6%Al-7%Nb and Ti-15%Mo-5%Zr.
Titanium possesses a level of corrosion resistance that is matched only by a few other naturally occurring metals. It is, however, important to note that like all other metals, titanium is not corrosion proof and is susceptible to degradation under specific, highly aggressive conditions. The advantages and limitations of the corrosion resistance of titanium alloys should be thoroughly understood to ensure that the metal delivers optimum and desired performance in a given environment.