One of the biggest challenges of using high temperature alloys is that their service life is limited by corrosion. Apart from maintenance downtime, the corrosion causes flaking, and these flakes can contaminate products during production.
A newly developed coating process can address extend the service life of high temperature alloys for chemical plants and furnaces, says Heraeus, the coating manufacturer. With the aid of aerosol deposition, the alloys are protected with a layer of ⍺-Al2O3. The patented coating prevents microstructural changes, voids and precipitations, says the company, so alloy corrosion is reduced. As a result, service life is up to eight times longer.
Possible applications include calcination and drying furnaces, which are used in metal substrate production or the manufacture of battery materials. Avoiding impurities means reducing scrap and increasing productivity.
Used in many process applications, high temperature alloys are characterized by high creep strength and mechanical resistance at higher temperatures. they are used in many applications where temperatures range between 752 and 2552°F (400 and 1400°C) due to these characteristics. If the alloys contain at least 0.5 percent aluminum, however, they have a limited ability to protect themselves by forming an Al2O3 layer on the alloy's surface.
At temperatures below 2552°F (1000°C), however, metastable phases such as θ- and ɣ-Al2O3 form. As the temperature is ramped up and down, phase transitions occur with corresponding volume changes. These phase transitions lead to voids that allow corrosion to occur. In addition, the aluminum required for Al2O3 layer formation is rapidly consumed, the alloy is depleted, and the properties suffer.
To prove the efficacy of the patented coating, Heraeus conducted a study. Alloy 602 (NiCr25FeAlY), an MCrAlY alloy with an aluminum content of 1.8 to 2.4 percent, was used. Samples were coated with a 5 µm thick ⍺-Al2O3 layer by aerosol deposition at room temperature. A 50-hour heat treatment at 1832°F (1000°C) then was performed, simulating the temperature in the actual application. In contrast to an untreated sample, the coated sample shows no microstructural changes in scanning microscopy cross-section.
These results can be transferred to all nickel alloys with a minimum aluminum content of 0.5 percent.