Introduction to Tantalum

Tantalum has excellent corrosion resistance in most inorganic acids, very similar to glass, and has an important use in the chemical industry, in addition to hydrofluoric acid, fluorine, fuming sulfuric acid and alkali, almost resistant to the corrosion of all chemical media (including boiling point hydrochloric acid, nitric acid and sulfuric acid below 175 ℃).

Tantalum has excellent corrosion resistance to dilute sulfuric acid below 75% and can be used at any temperature; it can be used for concentrated sulfuric acid without gas at 160~170℃; it can be used for concentrated sulfuric acid with gas at 250~260℃, and corrosion increases beyond this temperature. Generally in the high temperature above 170 ℃ to be used before the first test study. Tantalum corrosion resistance to phosphoric acid is also good, but acid such as containing traces of fluorine (> 4ppm), the corrosion rate increases.

Tantalum is usually not resistant to corrosion in alkali, it becomes brittle and corrodes more quickly at high temperatures and concentrations.

Tantalum can react with high temperature gases (except inert gases), O2, N2, H2, etc. can penetrate into the interior to make it brittle, such as contact with the initial state H, will also absorb hydrogen and become brittle. Therefore, tantalum equipment should not contact with more active metals (such as Fe, Al, Zn), because it is easy to form a tantalum - iron (Al, Zn) primary cell, the hydrogen generated by the primary cell reaction will destroy the tantalum cathode and make the equipment fail. If a small piece of platinum (area of about one ten thousandth of tantalum) with a very small hydrogen overvoltage is connected to the tantalum, then all the hydrogen will be released on the platinum, which can avoid the destruction of the tantalum by hydrogen.

Tantalum excellent corrosion resistance, but expensive, so its application form is mainly composite panels and linings, and in order to reduce costs, tantalum layer thickness hope as thin as possible, so composite panels or linings welding is very difficult, because tantalum and steel melting point difference, (tantalum melting point of 2996 ° C, the melting point of the steel for the 1400 ° C) and Fe and Ta at high temperatures will form a brittle Fe2Ta Intermetallic compounds, if the measures are not appropriate, it is easy to lead to weld cracking.

In the welding of thin-layer tantalum steel composite plate or lining, the thickness of the compound layer has an important effect on its weldability, Figure 1 is a schematic diagram of Ta1/16MnR composite plate welding, the smaller the thickness of the compound layer h, the higher the temperature T on the composite interface. When T>1500℃, a melting zone of 16MnR on the interface will appear. And Fe and Ta at 1460 ℃ will occur eutectic reaction, resulting in Fe2Ta brittle intermetallic compounds, under the action of welding stress, it is easy to crack and tantalum at the interface on the side of the tantalum weld pool direction will be extended, when serious, it will produce penetrating cracks, the base layer of the melted iron will be through the penetrating cracks to the diffusion of tantalum weld pool, and tantalum reacted with the generation of Fe2Ta Brittle compounds, so that the weld cracking.
In order to prevent this phenomenon is the primary factor is to increase the thickness of the appropriate compound layer or other measures to reduce the interface temperature, such as in the interface of the pre-composite anomalies outside a layer of other rapid thermal conductivity of the metal, in order to weld the heat generated to the surrounding migration. On the thickness of the compound layer and the relationship between the weldability of the composite plate, after a large number of tests, you can establish a model of the relationship between the interface temperature Ts and the thickness of the compound layer h.
When h≤2.0mm, the interface steel melts, iron diffuses into the weld, and cracks appear in the weld. When h>2.0mm, the welding of tantalum steel composite plate can be realized, and the larger the thickness of the compound layer, the better the weldability. In general, the thickness of the compound layer is 2.5~4.0mm. When used as a lining, 0.3~0.5mm can be used.