ACID SOLUTIONS
Niobium is resistant to most organic and mineral acids at all concentrations below 100° C, except hydrofluoric acid. This list of acids includes the halogen acids (hydrochloric, hydroiodic and hydrobromic), nitric acid, sulfuric acid, and phosphoric acid. It is especially resistant under oxidizing conditions such as: concentrated sulfuric acid and ferric chloride or cupric chloride solutions. Niobium is completely resistant in nitric acid, having a corrosion rate of 0.025 mm/y (1 mpy) in 70% nitric acid at 250° C. It is completely resistant in 20% sulfuric acid at 100° C. In concentrated sulfuric acid, at the same temperature, it has a corrosion rate of 0.25 mm/y (10 mpy). In chrome plating solutions, niobium experiences only a slight weight change, and in the presence of small amounts of fluoride catalyst, it exceeds the corrosion resistance of tantalum. Niobium is inert in mixtures of nitric acid and hydrochloric acid. It has a corrosion rate of less than 0.025 mm/y (1 mpy) in aqua regia at 55 °C. In boiling 40% and 50% phosphoric acid with small amounts of fluoride ion impurity (5 ppm), niobium has a corrosion rate of 0.25 mm/y (10 mpy). In mixtures of nitric acid and sulfuric acid, niobium dissolves readily.

ALKALINE SOLUTIONS
In ambient aqueous alkaline solutions, niobium has corrosion rates of less than 0.025 mm/y (1 mpy). At higher temperatures, even though the corrosion rate does not seem excessive, niobium is embrittled even at low concentrations (5%) of sodium hydroxide and potassium hydroxide. Like tantalum, niobium is embrittled in salts that hydrolyze to form alkaline solutions. These salts include sodium and potassium carbonates and phosphates.

SALT SOLUTIONS
Niobium has excellent corrosion resistance in salt solutions, except those that hydrolyze to form alkalis. It is resistant to chloride solutions even with oxidizing agents present. It does not corrode in 10% ferric chloride at room temperature, and it is resistant to attack in sea water. Niobium exhibits resistance similar to tantalum in salt solutions.

GASES
Niobium is easily oxidized. It will oxidize in air above 200° C. The reaction, however, does not become rapid until above red heat (about 500° C). At 980° C, the oxidation rate is 0.025 mm/y (17,000 mpy.) The attack is catastrophic at 390° C in pure oxygen which freely diffuses through the metal causing em-brittlement. Niobium reacts with nitrogen above 350° C; with water vapor above 300° C; with chlorine above 200° C; and with carbon dioxide, carbon monoxide and hydrogen above 250° C. At 100° C, niobium is inert in most common gases, e.g., bromine, chlorine, nitrogen, hydrogen, oxygen, carbon dioxide, carbon monoxide, and sulfur dioxide (wet or dry).

LIQUID METALS
Niobium is resistant to attack in many liquid metals to relatively high temperatures. These include bismuth below 510° C; gallium below 400° C; lead below 850° C; lithium below 1000° C; mercury below 600° C; sodium, potassium, and sodium-potassium alloys below 1000° C; thorium-magnesium eutectic below 850° C; uranium below 1400° C; and zinc below 450° C. The presence of excessive amounts of gas impurities may reduce niobium's resistance to these liquid metals. Since liquid metals are excellent heat-transfer media, they can be used in very compact thermal systems, such as fast breeder reactors, reactors for space vehicles, and fusion reactors. Niobium resists attack both by sodium vapor at high temperatures and pressures. The Nb-1% Zr alloy is in use as the end caps on high pressure sodium vapor lamps.

GALVANIC EFFECTS
If niobium is polarized cathodically either by galvanic coupling or chemical attack, it can be destroyed by hydrogen embrittlement. If niobium is polarized anodically, however, it forms a very stable, passive film which protects the metal from corrosion. The stability of the passive film, combined with good electrical conductivity (13% that of copper) and good mechanical properties, has led to the use of niobium as a substrate for platinized anodes used in cathodic protection systems. Niobium's anodic breakdown potential in chloride solutions is about 115 V compared to 10 V for titanium. Platinized niobium anodes are used in high resistivity waters and other environments requiring high driving potential to obtain good current distribution. In this application, niobium has an advantage over tantalum, because it is less expensive. This cost advantage is further enhanced by using a composite electrode with a copper core, which increases the conductivity of the anodes.

SUMMARY
Niobium's corrosion properties are similar to those of tantalum; however, it is less expensive and should be considered in all applications requiring tantalum. Niobium has replaced tantalum in some hydrochloric acid applications.

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INDEX  •  CORROSION RESISTANCE PROPERTIES  •  FABRICATION   
NIOBIUM PRODUCTS   •  SPECIFICATIONS of NIOBIUM   
NIOBIUM ELECTRONICS APPLICATIONS