"Corrosion Behavior of Niobium, Tantalum and Their Alloys in Hot Hydrochloric Acid and Phosphoric Acid Solutions"

Table 1 Chemical structures of niobium, niobium-tantalum alloys and tantalum

Table 2 Densities of niobium, niobium-tantalum alloys and tantalum

Corrosion Behavior of Niobium and Niobium-Tantalum Alloys in Boiling Hydrochloric Acid:

Figure1 Corrosion rate of niobium in boiling hydrochloric acid solution as a function of exposure time

Figure2 Corrosion rate of niobium-20wt% tantalum alloy in boiling hydrochloric acid solution as a function of exposure time

Figure3 Corrosion rate of niobium and niobium-20wt% tantalum alloy in boiling hydrochloric acid solution as a function of asid concentration

Figure4 Corrosion rates of niobium, tantalum and their alloys in hydrochloric acid solution at 150℃ and corrosion rates of arsenic

Figure5 Corrosion rate of niobium, tantalum and their alloys in hydrochloric acid solution at 200℃ and corrosion rate of arsenic

Figures 1 and 2 show the concentration and exposure time. The corrosion rate decreased with the test time. The corrosion rates of the two materials over the longest test time are shown in Figure 3. For niobium and niobium-20 wt % tantalum alloys, the corrosion rate increases with the doping level. We note that the niobium-20 wt% tantalum alloy is more wear resistant than niobium. In 17.5 wt% hydrochloric acid and 20 wt% hydrochloric acid boiling at 75°C, the corrosion rates of niobium after 14 days of exposure were 97 and 33 μm/year, respectively.

Corrosion rates for all materials and all gases increased with temperature (Figure 3-5).

The corrosion rates of niobium, tantalum, and their alloys in boiling H3PO4 solutions are shown in Figures 6-11 as a function of corrosion rate and exposure time. As observed in boiling hydrochloric acid solutions, the corrosion rate decreases over time and stabilizes after prolonged exposure.

Figure6 Corrosion rate of niobium in boiling H3PO4 solution as a function of exposure time

Figure7 Corrosion rate of niobium-20wt% tantalum alloy in boiling H3PO4 solution as a function of exposure time

Figure8 Corrosion rate of niobium-40wt% tantalum alloy in boiling H3PO4 solution as a function of exposure time

Figure9 Corrosion rate of niobium-60wt% tantalum alloy in boiling H3PO4 solution as a function of exposure time

Figure10 Corrosion rate of niobium-80wt% tantalum alloy in boiling H3PO4 solution as a function of exposure time

Figure11 Corrosion rate of tantalum in boiling H3PO4 solution as a function of exposure time

After 14 days of exposure, tantalum was formed in boiling 40% H3PO4. The corrosion rate obtained with the longest exposure time increases with exposure time and decreases with tantalum content.

The corrosion resistance of niobium, tantalum and niobium-tantalum alloys in hydrochloric and phosphoric acid solutions depends on gas concentration, temperature and tantalum content. An increase in temperature and pressure resulted in an increase in the corrosion rate, and an increase in tantalum resulted in a decrease in the corrosion rate.

Table 3. Corrosion rates of niobium and niobium-tantalum alloys in H3PO4 solution at different temperatures

The corrosion behavior of niobium, tantalum and niobium-tantalum alloys in hydrochloric acid and phosphoric acid solutions from the boiling point to 200℃ shows that:

• Increased arsenic content results in lower corrosion rates for all materials and properties;

• Loss rates for all materials and items increase with temperature;

• The addition of tantalum increases the resistivity of niobium;

• Niobium-tantalum alloys are very corrosion resistant from a 60% tantalum content, and the corrosion rate decreases with exposure time due to the formation of supervisible oxides.