Corrosion resistant coatings in industry have been around for decades and over the years numerous coatings and methods to apply them have been developed. But how many are corrosion proof? While term “corrosion resistance” is very much overused and meaningless as it can apply to water in the kitchen or to industrial equipment in hot hydrochloric acid (HCl), for the scope of this blog, I am focusing on industrial applications in strong acid environments.
Probably one of the most popular industrial coatings for strong acids is PTFE or PFA coatings (common trade name is Teflon). But if polymers could not be used due to elevated temperatures, high pressures vacuum, or chemical compatibility what would be an improved option?
One very interesting option is tantalum metal coatings. Tantalum is best known for being one of the most corrosion resistant and inert metals on earth, even surpassing that of gold. In fact, in most extremely corrosive, strong and hot acid environments, like HCl, H2SO4, acetic acid or nitric acid, tantalum metal has no detectible corrosion rate.
- Atomic Number: 73
- Density: 16.6 g/cc
- Melting Point: 5468°F, 3020°C
- Boiling Point: 9856°F, 6100°C
- Coefficient of Thermal Expansion: (20°C) 6.5 x 10(-6) / °C
- Electrical Resistivity (20°C): 13.5 microhms-cm
- Electrical Conductivity: 13% IACS
- Specific Heat: 0.036 cal/g/°C
This is the key…tantalum has No Detectible Corrosion Rate in most acid environments. Because of this unique characteristic in strong acids, very little tantalum is require not just to be corrosion resistant….but to be corrosion proof. Since the metal is not being consumed or corroded over time in strong acid environments, tantalum lends itself well to a thin coating (0.001”-0.002”thick) that could be produce via CVD (chemical vapor deposition) to create corrosion resistant products. Few other metals could be utilized as a thin surface in highly corrosive environments because they slowly corrode. So while many other metals might be considered “corrosion resistant”, they actually are slowly corroding. As a bulk material this might be fine but they have limited usefulness as a thin coating in corrosive environments since it is only a matter of time before the surface is corroded through and the base metal attacked.
So because you only need the surface properties of tantalum metal and tantalum exhibits a nil corrosion rate in most acidic environments as well as tantalum being an expensive material, tantalum is a perfect material for a corrosion resistant coating.
The following HCl iso-corrosion curve shows a general comparison of the corrosion resistance of tantalum metal versus other common corrosion resistant alloys like Hastelloy.