Molybdenum crucibles depend on the use environment, temperature and contact substances. The following is a detailed analysis of its corrosion resistance:
1.Overall corrosion resistance
Advantages: Molybdenum has high chemical stability, especially in vacuum or inert gas environment, and exhibits good corrosion resistance to many chemicals.
Limitations: Under certain conditions (such as high temperature oxygen-containing environments or certain highly corrosive substances), the corrosion resistance of molybdenum will be significantly reduced.
2.Specific corrosion resistance
a.In vacuum or inert environment
Excellent performance: In vacuum or inert gas (such as argon, nitrogen), molybdenum crucible hardly reacts with the environment, has strong corrosion resistance, and is suitable for high-temperature smelting or heat treatment processes (such as semiconductor, rare metal smelting).
Temperature range: Can withstand 1700-2000°C or higher without significant corrosion.
b.Oxygen environment
High temperature oxidation: Molybdenum reacts with oxygen above about 500°C to form volatile molybdenum oxide (MoO₃), which leads to rapid corrosion and material loss. Therefore, in oxygen-containing environments, the corrosion resistance of molybdenum crucibles is poor, and the use temperature is usually limited to below 400-500°C.
Improvement method: Oxidative corrosion can be effectively avoided by surface coating (such as aluminum oxide, silicide) or using in vacuum/inert atmosphere.
c.Acid, alkali, salt solution
Acidic environment:
Molybdenum has good corrosion resistance to some non-oxidizing acids (such as hydrochloric acid and sulfuric acid), but the effect is better at room temperature or low temperature.
It has poor corrosion resistance to oxidizing acids (such as nitric acid and hot concentrated sulfuric acid) and is prone to corrosion.
Alkaline environment:
Molybdenum has average corrosion resistance in alkaline solutions (such as sodium hydroxide), especially under high temperature or strong alkaline conditions, where slow corrosion may occur.
Molten alkali (such as molten NaOH) is highly corrosive to molybdenum and contact should be avoided.
Salt solutions: Molybdenum has good corrosion resistance to neutral salt solutions (such as sodium chloride), but may be damaged in salt solutions at high temperatures or containing oxidants.
d.Molten metals and compounds
Tolerance:
Molybdenum crucible has good corrosion resistance to many molten metals (such as gold, silver, copper, aluminum, zinc) and is suitable for the smelting of precious metals or rare metals.
molybdenum at high temperatures (such as carbon, silicon), a slight chemical reaction may occur, resulting in corrosion of the crucible surface.
limit:
Molten glass or oxides (such as SiO ₂) may react with molybdenum at high temperatures, so the crucible material must be carefully selected or a protective layer must be added.
Certain fluorine-containing compounds or fluoride melts are highly corrosive to molybdenum and should be avoided.
3.Factors affecting corrosion resistance
Temperature: High temperatures accelerate corrosion reactions, especially in the presence of oxidizing or reactive species.
Purity: High purity molybdenum (above 99.95%) has better corrosion resistance than low purity molybdenum.
Surface state: The polished or coated surface of the molybdenum crucible is more corrosion-resistant.
Ambient atmosphere: Vacuum or inert atmosphere significantly improves corrosion resistance, while oxidizing atmosphere (such as air) weakens its performance.
4.Corrosion resistance in practical applications
Semiconductor industry: In vacuum or inert environment (such as sapphire crystal growth), molybdenum crucibles are almost corrosion-free and perform excellently.
Metallurgical industry: When smelting precious metals or rare metals, molybdenum crucibles have good corrosion resistance to molten metals, but they must be avoided from contact with highly active substances.
Glass/ceramic industry: May react slightly to certain molten glasses or oxides, needs to be evaluated based on the specific composition.
Laboratory research: In high temperature experiments, the atmosphere needs to be strictly controlled to protect the crucible.
5.Methods for improving corrosion resistance
Surface coating: Apply anti-oxidation coating (such as ZrO ₂, Al ₂ O ₃) or silicide coating to improve anti-oxidation and corrosion resistance.
Alloying: Molybdenum alloys (such as Mo-W, Mo-La) can improve corrosion resistance and high temperature strength.
Environmental control: preferably used in a vacuum or inert gas environment to reduce the risk of corrosion.
Regular maintenance: Clean the crucible surface to avoid residues that may cause local corrosion.
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