Titanium exhibits excellent corrosion resistance in oxidizing environments such as nitric acid, chromic acid, hypochlorous acid, and perchloric acid due to the formation of a dense oxide film. However, its corrosion rate increases in reducing acids like dilute sulfuric acid and hydrochloric acid, especially with rising temperature and concentration.
In reducing acids, the addition of heavy metal salts can significantly mitigate corrosion. Alloys like titanium-palladium and titanium-nickel-molybdenum show enhanced corrosion resistance compared to industrial pure titanium by incorporating specific heavy metal elements.
For instance, titanium serves as one of the optimal materials for nitric acid heating equipment, displaying remarkable longevity even when exposed to 60% nitric acid at around 193°C. Despite initial rapid corrosion rates in boiling 40% and 68% nitric acid, titanium's passivity eventually restores, lowering corrosion rates noticeably.
In sulfuric acid at room temperature, industrial pure titanium tolerates solutions below 5%. However, as temperatures rise, its resistance diminishes. Notably, the corrosion rate of titanium increases significantly in nitrogen-infused sulfuric acid compared to air-exposed environments, a trend consistent across other reducing inorganic acids.
While industrial pure titanium withstands up to 7% hydrochloric acid at room temperature, its corrosion resistance decreases notably with higher temperatures. In contrast, titanium-nickel-molybdenum alloy endures 9% hydrochloric acid, while titanium-palladium alloy withstands up to 27%, showcasing the effectiveness of high-valence metal ion additions in enhancing titanium's corrosion resistance.
Furthermore, industrial pure titanium can resist solutions below 30% phosphoric acid at room temperature, with decreasing tolerance as temperatures escalate. However, corrosion rates do not accelerate further when phosphoric acid reaches boiling point, emphasizing titanium's stability under such conditions.
