Oxidation-resistant metal coatings

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2010

Researchers develop a new process to apply a cerium-oxide coating to a metal surface. 

KEY CONCEPTS
• Cerium oxide made into a slurry or paste with a halide activator forms a coating that is very resistant to oxidation under extreme temperatures and pressures.
• In testing at temperatures between 650 C and 900 C for 4,000 hours, cerium-oxide treated alloys displayed a significant reduction in oxidation compared to untreated alloys.
• Cerium-oxide coatings are being evaluated in power generation and solid oxide fuel cell applications.

The lubricant industry continues to be challenged to develop products that provide long-term operating life under conditions that are becoming more and more severe from both temperature and pressure considerations. One example is found in the power generation industry.

Boilers and turbines used in power plant facilities must operate at temperatures up to 760 C and pressures up to 5,400 psi. These conditions have been designated as the “ultra supercritical” range and can even cause such metals as nickel-based superalloys and stainless steels to exhibit premature failure due to oxidation.

A similar operating environment occurs in a solid oxide fuel cell that is being evaluated as an alternative source of electric power. This type of fuel cell uses a solid oxide or ceramic electrolyte and operates at high temperatures between 500 C and 1,000 C, which can lead to oxidation problems.

Dr. Paul Jablonski, senior metallurgist in the Process Development Division of the U.S. Department of Energy’s National Energy Technology Laboratory in Albany, Ore., says, “One option that has been looked at to resist oxidation is to coat the metal surfaces with a rare earth oxide such as cerium oxide. The use of this oxide changes the oxidation mechanism so that it can be controlled by the diffusion of anions into the metal instead of by diffusion of cations out of the metal.”

The traditional process for using cerium oxide is to include a halide salt and a powder filler in a chemical vapor deposition process known as pack cementation. Dr. David Alman, director of the Materials Performance Division at the National Energy Technology Laboratory, says, “Pack cementation involves the use of an embedded active component in a pack. The main component in the pack is an inert material such as alumina that acts as a filler.”

In pack cementation, cerium oxide reacts with the halide salt under high temperatures in an inert atmosphere to form a coating that diffuses into the metal substrate. There is a need to devise a more efficient way to develop a cerium-based coating that can diffuse into the metal. Such a process has recently been developed.

CERIUM-OXIDE SLURRY
Jablonski and Alman have developed a process for simply applying a cerium-oxide coating to a metal surface. Jablonski says, “We found that cerium oxide can be made into a slurry or a paste with a halide activator to form a coating. The slurry can be applied by brushing, dipping or spraying onto the metal surface. Drying takes place at ambient temperature or by using a low-temperature oven.” Figure 2 shows a slurry of cerium oxide being applied to the surface of a metal substrate.


Figure 2. Application of cerium-oxide onto ferrous alloys that are chromium oxide film formers greatly improves oxidation resistance. (Courtesy of the U.S. Department of Energy’s National Energy Technology Laboratory)

Once the coating is in place on the metal substrate, heating is conducted at temperatures between 500 C and 1,300 C in an oxygen-free environment to enable the halide activator to transfer cerium from cerium oxide into the metal itself. This process can take up to 12 hours. After being cooled to room temperature, the remaining paste is removed by a process such as water washing.

Alman says, “We consider this process to be similar to pack cementation but without the pack.”

The researchers have applied cerium oxide to ferrous alloys that are chromium oxide formers. Many of these are nickel-based super alloys that also contain manganese. Jablonski adds, “An emphasis was placed on manganese because of its use in alloys recommended for solid oxide fuel cells.”

Testing of the cerium-oxide coating on an alloy such as Crofer 22 APU (a stainless steel alloy with 22% chromium) was conducted under aggressive air flowing conditions and a controlled infusion of water (3% by weight) in a furnace. Jablonski says, “We evaluated treated and untreated alloys at temperatures between 650 C and 900 C. Every so often samples of the test coupons were removed, weighed and then put back in the furnace. All cerium-oxide treated alloys displayed a significant reduction of oxidation in testing up to 4,000 hours.”

In contrast, untreated metal alloys absorbed more than twice the amount of oxygen during the same period. The cerium-oxide coating has been found to increase the antioxidant properties of most metal alloys by a factor of two to three.

The researchers indicate that no specific thickness is required for the cerium-oxide coating to be effective. Alman says, “We believe that the cerium-oxide slurry is really a treatment and not a coating. The use of the term coating is a misnomer.”

One other benefit is that scratching the metal surface leads to no impact on the performance of the cerium-oxide coating.

Jablonski says, “The mechanism for how the halide activator functions is not clear.” Any of a number of halide salts such as ammonium fluoride, cobalt chloride, titanium tetrabromide and bismuth triiodide can be used.”

The researchers have been awarded an R&D 100 Award for this work. Further details on the use of cerium-oxide coatings can be found in a recent U.S. Patent (1) or by contacting Jablonski at paul.jablonski@netl.doe.gov. 

REFERENCE
1. Jablonski, P. and Alman, D. (2009), “Method of Applying a Cerium Diffusion Coating to a Metallic Alloy,” U.S. Patent 7,553,517 B1.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.