Ti-Cr Casting Alloy Corrosion Resistance to Fluoride

Objectives: In a previous study, we reported that the corrosion resistance of cast Ti-Cr alloy to fluoride increased with rise in chromium content as evidenced by its electrochemical corrosion and dissolution behaviors. However, the correlation between chromium content in Ti-Cr alloy and surface reaction to fluoride remained to be clarified. This study investigated the mechanism of corrosion resistance to fluoride in cast Ti-Cr alloy containing from 5 to 20 mass% chromium by X-ray photoelectron spectroscopy (XPS). Methods: Ti-Cr alloys (5, 10, 15 and 20 mass%Cr) were made using an argon arc-melting furnace. Cast specimens (14 mm in diameter x 2 mm) were prepared using each alloy. They were then polished with silicon carbide paper and ultrasonically washed in acetone and distilled water. The specimens were immersed in 0.9% sodium chloride containing 0.2% sodium fluoride solution adjusted to pH 5.0 with lactic acid. Surface characterization of the Ti-Cr specimens before and after immersion in the fluoride solution was performed using XPS. Results: XPS analysis revealed that all the specimens had a thin oxide film prior to immersion consisting of the chromium oxide and titanium oxide. The [Cr]/([Ti]+[Cr]) ratio in the oxide film for each specimen was close to nominal composition. After immersion, the [Cr]/([Ti]+[Cr]) ratio in the oxide film increased, showing a higher level of chromium oxide. In addition, the [Cr]/([Ti]+[Cr]) ratio in the oxide film after immersion increased with rise in chromium content. The titanium alloy containing 20 mass% chromium showed the highest corrosion resistance to fluoride among the cast Ti-Cr alloys tested. Conclusion: Chromium content in Ti-Cr alloys increases the ratio of chromium oxide in surface oxide films. We suggest that oxide films which have a rich chromium oxide content have greater corrosion resistance to fluoride.