Oxidation Protection

 

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Advanced alloys of nickel, titanium, and stainless steel are often susceptible to rapid, non-passivating oxidation at temperatures above 500°C. In these cases, the native oxide is non-uniform—even porous—and therefore cannot provide a sufficient diffusion barrier against further degradation. Similar problems exist for metallurgical bondcoats, SiC, and SiC composites.

 

Application of ATFI’s Cerablak® UTF surface treatment on these advanced alloys can offer a significantly more effective barrier against oxidation. As a result, the treated material lasts much longer under high-temperature service or it can enable lower-grade and less-expensive alloys—a significant cost savings. Cerablak® UTF treatments are applied using a remarkably flexible deposition scheme (dip, spray, spin, flow) followed by a brief thermal cure to form a hermetic-quality glassy film of aluminophosphate. While Cerablak® UTF treatments provide excellent oxidation protection at thicknesses as low as 100 nanometers, the film can range up to two microns to accommodate substrate morphology and topology.

 

Read more about oxidation protection for some of the extensively-tested applications:

 

Steel Alloys

 

Oxide Scale Dynamics

Upon exposure to ambient moisture and temperatures greater than about 500°C, carbon steels rapidly corrode due to surface formation of various iron oxides. As the exposure progresses, fragments of the corroded layer are prone to flaking from the surface and exposing new areas underneath for progressive degradation, eventually compromising structural integrity. Appliance-grade stainless steels generally experience minimal structural degradation, but are nonetheless susceptible to aesthetic degradation such as tarnishing and pitting. During early stages of thermal exposure, the oxide scale is typically of a mixed metal character, as opposed to the chromia-rich film needed to provide a quality long-term barrier. Furthermore, the presence of ambient moisture serves to accelerate oxidation near surface pits and terminating grain boundaries. 

 

Today’s surface engineering solutions have only provided incremental improvements to oxidation performance.  Cerablak ® UTF offers a quantum improvement in performance and enables the use of lower-grade, less-expensive alloys.  For example, Cerablak® UTF-coated AUS 304 matches or even exceeds the performance of uncoated AUS 310 which is almost twice as expensive.

 

Providing an Effective Barrier

Cerablak® UTF treatment of carbon and stainless steels offers outstanding oxidation protection with the following primary differentiating factors:

  • • Effectively seals microscale surface including pits, grain boundary junctions, and other irregular topography/finishing due to excellent film forming ability and prevents pitting corrosion.
  • • Promotes the formation of chromia- and alumina-rich scales underneath the treatment to mitigate long term oxidation. For instance, a relatively porous, chromia-rich scale is formed on uncoated stainless steels while a much thinner, denser chromia scale is formed on Cerablak® UTF-treated stainless steels after equivalent exposures.
  • • Greatly mitigates tarnishing and other aesthetic defects.

Cerablak® UTF fundamentally changes the oxidation behavior of steel alloys, resulting in a scale chemistry and microstructure that is more uniform, less porous, and an order of magnitude thinner. Durable, long-lasting steels are of high technological importance in numerous applications and are particularly important for the construction, transportation, and white goods industries.

 

Nickel Alloys

 

High Temperature Service

Due to the excellent strength of nickel alloys at high temperatures, they are used in highly demanding applications such as gas turbine engines (aircraft and power generation) where temperatures can exceed 1200°C. Generally, nickel-based alloys show good performance in oxidizing environments below 1000°C. However, demand for increased temperatures to improve efficiency has triggered a search for either new nickel alloys or appropriate surface treatments to extend their performance limits. Cerablak® UTF  surface treatments offer excellent protection to enable lower cost nickel alloys and reduce unexpected shutdowns. As these materials are pushed into new temperature regimes, further oxidation and corrosion solutions have become increasingly necessary.


Providing an Effective Barrier

Cerablak® UTF treatment of advanced nickel alloys offers outstanding high-temperature oxidation protection with the following primary differentiating factors:

  • • Forms a high quality, hermetic glass coating stable to temperatures of at least 1000°C.
  • • Effectively seals typical Inconel and superalloy surface topographies and provides significant high temperature oxidation resistance.
  • • Increased mitigation of corrosive attack by sulfur, vanadium, and molten salts; nickel-based superalloys are of particular importance for the industrial gas turbine industry and for next generation aerospace vehicles.

 

Titanium

 

Grain Boundary Diffusion

Alloys of titanium have become ubiquitous in high-value applications due to their high strength-to-density ratio and resistance to corrosion. However, their high temperature use is being limited due to internal oxidation (dissolved oxygen in the lattice) and subsequent conversion to alpha-titanium, the brittle phase, which causes the material to disintegrate rapidly.
 

Providing an Effective Barrier

Cerabalk® UTF has been proven to be highly effective for a range of titanium alloys for protection against oxidation by limiting diffusion of oxygen from surface-terminating grain boundaries. An order of magnitude improvement in oxidation life has been demonstrated with Beta-21S titanium alloy with over 300% improvement in Ti-6242 alloys.

 

SiC and SiC Composites

 

Ultra-High Temperatures

Advanced ceramics, such as Silicon Carbide (SiC)-based composites, are emerging as promising next-generation turbine materials operable above 1200°C. While oxidation resistance of SiC composites is adequate below 1200°C, even higher temperature performance is being sought for many applications, making the oxidation problem more severe. In addition, SiC is particularly susceptible to corrosion by high-temperature water vapor, which is prevalent in engine environments. As these materials are prohibitively expensive, a 20% extension of life via surface treatments can result in significant cost savings.

 

Serving as a Barrier

Cerablak® UTF and Cerablak® HTS and Cerablak® HTS treatments of SiC monoliths and composites have shown great promise for limiting oxidation in ATFI’s laboratory and customer testing facilities. One primary advantage of Cerablak® HTS is its ability to infiltrate the inherent porosity of SiC composites so that the protection is available locally throughout the bulk, such that protection does not rely solely on a topcoat layer.

 

High-temperature SiC materials are also used in heating elements, furnace parts, abrasives, bearings, and many other applications.

 

Metallurgical Coatings

 

Improving Reliability

Either as an alternative or an enhancement to costly high-temperature alloys, thick (~1-200mils) metallurgical overlay coatings or bondcoats are often used to provide oxidation and corrosion protection. These coatings (MNiCrAlY and the like) are deposited in high throughput using EBPVD, plasma spray, or other thermal spray processes. Many of these coatings are porous or have non-uniform surface chemistry such that their reliability is less than desired. With the increasing demand for higher operating temperatures in turbines, this problem becomes more severe with rapid degradation of substrate alloys. Upon oxidation, a thermally-grown oxide forms, which can grow uncontrollably and lead to cracking, spallation, or delamination. Cerablak® UTF and Cerablak® HTS can reduce the oxidation rate and improve performance of metallurgical coatings.

 

Providing an Effective Barrier

Cerablak® UTF and Cerablak® HTS treatments of overlays and bondcoats offer outstanding thermal growth oxidation protection with the following primary differentiating factors:

  • • Ensures uniform native oxidation of the overlay, helping prevent further degradation.
  • • Through an innovative proprietary coating process, the typically porous and permeable microstructure of overlay coatings and bondcoats can be sealed with a high quality film that does not negatively impact the originally designed thermal or mechanical properties.
  • • Provides sintering and enhanced corrosion resistance.

Overlays and bondcoats are of particular importance to power generation industries, such as the industrial gas turbine community, to help withstand the harsh environments faced by advanced substrate alloys.

 

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