When Does the Oxidation Resistance of Pure Titanium Decrease?
Elevated temperatures beyond the critical threshold
Pure titanium exhibits good oxidation resistance at temperatures below 400°C, as the surface TiO₂ film remains dense and adherent, effectively blocking further oxygen infiltration. However, when the temperature exceeds 400°C, the oxidation behavior of pure titanium changes drastically:
At 400–600°C: The TiO₂ film begins to grow rapidly in thickness, and its structure transforms from a dense, protective rutile phase to a more porous anatase or brookite phase in localized areas. Meanwhile, a small amount of interstitial oxygen atoms diffuse into the titanium matrix, forming a brittle oxygen-enriched layer beneath the oxide film, which weakens the metal's structural integrity while reducing the film's barrier effect.
Above 600°C: The oxidation process enters a "parabolic-to-linear" transition stage. The TiO₂ film loses its protective properties entirely due to severe cracking and spalling caused by thermal stress (arising from mismatches in thermal expansion coefficients between the oxide film and the titanium substrate). Oxygen penetrates into the matrix at an accelerated rate, and the formation of low-adhesion oxide layers (such as Ti₂O₃ and TiO in the sub-surface zone) leads to catastrophic oxidation of pure titanium, with the oxidation rate increasing exponentially with temperature.
High-temperature environments with specific corrosive gas impurities
Even if the temperature is within the nominal "safe range" (below 400°C), the presence of certain corrosive gas impurities will drastically degrade pure titanium's oxidation resistance:
Chlorine-containing gases (e.g., Cl₂, HCl vapor): Chloride ions can penetrate the TiO₂ film through microcracks or grain boundaries, reacting with titanium to form volatile titanium chlorides (e.g., TiCl₄). This "active corrosion" mechanism destroys the continuity of the passive film and prevents its self-healing, leading to localized pitting or uniform corrosion even at moderate temperatures.
Sulfur-containing gases (e.g., SO₂, H₂S): At temperatures above 300°C, sulfur atoms can diffuse into the titanium matrix, forming brittle titanium sulfides (e.g., TiS, TiS₂) at grain boundaries. These sulfides not only reduce the metal's ductility but also disrupt the integrity of the TiO₂ film, making it more susceptible to oxygen attack and accelerating oxidation.
Nitrogen-rich atmospheres at high temperatures: Above 500°C, nitrogen reacts with titanium to form hard and brittle titanium nitride (TiN) on the surface and within the matrix. While TiN has some oxidation resistance, its formation causes internal stress in the oxide film, leading to cracking and creating channels for oxygen to further infiltrate the underlying metal.
Cyclic thermal loading conditions
Repeated heating and cooling cycles (e.g., in industrial furnaces or aerospace engine components that undergo frequent start-stop operations) severely compromise pure titanium's oxidation resistance:
Thermal expansion and contraction of the TiO₂ film and titanium substrate cause cyclic stress, leading to the formation of microcracks and delamination in the oxide layer.
Each thermal cycle exposes fresh titanium metal to the oxidizing atmosphere before the film can fully self-heal, resulting in cumulative oxidation damage and a gradual reduction in the film's protective capacity over time.
Presence of molten salts or low-melting-point metal contaminants
In environments containing molten salts (e.g., NaCl, Na₂SO₄ in high-temperature industrial processes) or low-melting-point metals (e.g., aluminum, magnesium, lead), pure titanium's oxidation resistance is significantly impaired:
Molten salts can act as electrolytes, inducing electrochemical corrosion that breaks down the TiO₂ film, while also facilitating the formation of low-melting-point titanium compounds that accelerate film failure.
Low-melting-point metals can diffuse into the titanium matrix at high temperatures, forming eutectic alloys and causing intergranular embrittlement, which weakens the structural stability of the metal and makes the oxide film more prone to cracking during oxidation.
