So, does tungsten rust? Pure tungsten does not undergo rust or corrosion as it develops patina on its surface. High-quality nickel-binder tungsten used for jewelry is chemically inert and undergoes corrosion in extreme conditions ie; high temperatures (above 600°C) to give a rusty look. Whereas Cobalt-binder tungsten undergoes oxidation easily due to external factors and gets corroded. Technically, rusting is a chemical process that occurs only in iron and iron alloys. Tungsten being an inactive metal, does not react at room temperature.
From the graph, it can be seen that the Tungsten has a low coefficient of thermal expansion. It means, even at high temperatures, tungsten does not distort much. This is because it undergoes cyclic oxidation, which leads to ultimate volatilization.
Does Pure Tungsten Rust?
Yes, it does, but at a very high temperature. The reason behind the corrosion of Tungsten is in its electronic structure. Let us discuss it with a look at its electronic configuration.
The image shows Tungsten has 4 unpaired electrons in the 5d shell making it eager to undergo oxidation to achieve a stable condition
It is abundant in the atmosphere and a single oxygen molecule needs 2 electrons to achieve a stable condition. Two molecules of oxygen together ask for 4 electrons which is the reason why Tungsten having 4 unpaired electrons to donate, is one suitable choice. Furthermore, this is the reason why Tungsten reacts with oxygen and corrodes at high temperatures.
The chemical reaction of Tungsten undergoing corrosion is:
WC (tungsten) + 2.5O2 (oxygen) ——-> WO3 (tungsten trioxide) + CO2 (carbon dioxide) It is tungsten trioxide which is rusty brown in color, making the tungsten look rusted. Even pure tungsten similarly undergoes oxidation as mentioned in the chemical reaction.
Does Tungsten Rings Tarnish?
No. For jewelry grades, tungsten is mixed with nickel-binder to make it more suitable to chisel. A binder helps with binding carbide and Tungsten together which oxidizes above 600°C. No human will ever encounter the ring at such an extreme temperature.
The tungsten gets affected by acidic conditions having pH 1 and 0. Again, these values are too strong and no human will ever deal with such strong acids in his or her life, until and unless works in a chemistry lab. Nickel-based tungsten carbide is chemically inert in nature. As a result, it does not undergo oxidation due to external environmental factors.
Is Tungsten Corrosion Resistant?
Partially Yes. All tungsten alloy corrodes as they react with chemical elements present in the atmosphere to undergo oxidation, sulfidation, or others. It is because of properties like high malleability, ductility, stiffness, and others which makes amalgamation a common practice. Pure Tungsten and its alloy react at very high temperatures making it one of the most durable elements existing on the Earth. Similarly, platinum also generates a coating of patina that protects it from corrosion. Check out the interesting article of the tarnishing of platinum.
Why tungsten is corrosion-resistant?
The oxidative nature of Tungsten can be studied with the graphs mentioned down below. The oxidation process is solely dependent on high temperature and time. The kinetics of oxidation is studied under three temperature regimes of:
From room temperature till 500 °C: A bluish layer of WO3 forms Above 500 °C: A yellow layer of WO3 forms on the bluish layer Above 1000 °C: Final volatilization of WO3 begins.
Unlike tungsten, sterling silver is an alloy containing copper that makes it vulnerable to corrosion sometimes. Read out the article regarding the rusting of sterling silver.
Mass change per unit area with the temperature
From the graph, it is seen that the volatilization takes place reaching 1000°C and above. It would have occurred way before in W and WC.
Photographic image of the Tungsten getting smoother at higher temperatures
The open porosity at the higher temperature makes the oxidation process move much faster, ultimately leading to volatilization.
Mass change per unit area with time
From the graph, it can be seen that the Tungsten changes its mass structure as time increases at high temperatures. So, time plays a crucial factor in the corrosion of the Tungsten.
How corrosion of the Tungsten begins?
This graph shows how Tungsten, specifically, responds to an increase in temperature. The chemical reactions of Tungsten with steam take its temperatures till 800°C resulting in the formation of hydrated Tungsten-oxide, having high vapor pressure and readily convected in the flowing atmosphere. Yes, it is humidity which begins corrosion of Tungsten.
Conclusion
Tungsten is a good choice for commercial and industrial purposes because it does not corrode at normal temperatures. If you have realized, all the aforementioned graphs are showing a comparison between Tungsten and other refractory metals, especially Molybdenum. It is so because Molybdenum is one of the most preferred choices for commercial purposes. A comparison is made with it to show that Tungsten can be another most preferred refractory metal throughout the globe for varied purposes. Besides this, where oxidation (corrosion) in metals is not considered good, for tungsten, it is the opposite. The oxidation of Tungsten helps with maintaining its properties at high temperatures.
Some more questions
What is the difference between Cobalt-binder and Nickel-binder? Tungsten is mixed with cobalt-binder and nickel-binder to make it more suitable to chisel. A binder helps with binding carbide and Tungsten together. For jewelry, Tungsten is mixed with nickel-binder as the alloy oxidizes above 600°C. For industrial purposes, Tungsten is mixed with cobalt-binder as it oxidizes much quickly but contributes to hardness, toughness, and resistance to wear, making it a suitable choice for metal rollers and various engine components. Why Tungsten is a suitable choice for jewelry products? Tungsten is of low to no maintenance and is not allergic to skin, making it a suitable choice for daily wear. Is Tungsten waterproof? Yes, water cannot penetrate the surface of the Tungsten, making it waterproof. It is because of the densely arranged molecular structure of Tungsten.
You might be surprised to see how densely each molecule is attached to another. How to remove corrosion on the Tungsten? Heat the electrode till 800 °C and provide low pressure of Hydrogen gas. It leads to the reaction: WO3 (s) + 3H2 (g) → W(s) + 3H2O (g) Another method is the high-temperature treatment in-vacuo. Keep titanium getter near the Tungsten where oxygen from Tungsten moves to titanium. Is Tungsten the strongest metal? Yes, Tungsten is one of the strongest metals having a melting point of 3422°C / 6192 °F.
