Properly protected steel alloys demonstrate outstanding strength, formability, and damage resistance at elevated temperatures and provide extended service life under adverse conditions. However, alloy properties are compromised while operating in extreme service conditions due to the possibility of exposure of metals to steam, oxygen, and a variety of more corrosive species.
Corrosion at elevated temperatures can be especially challenging due to increasing kinetics of metal corrosion and oxidation at elevated temperatures, causing the degradation and decomposition of polymeric-based coatings. Glass coatings are an option but only for a limited number of applications due to their relatively limited range of thermal expansion and their tendency to interact with the underlying metals to form new corrosion products.
Nexceris’ Protective Coatings
The ChromLok™ family of oxide based protective coatings offered by Nexceris increases a metallic component’s high temperature stability under adverse process conditions. Nexceris has carefully engineered the process, right from the raw material oxide powder all the way through the formulation of the oxide-based coating suspension and succeeding deposition and heat treatment of the applied coating. This results in an oxide coating with optimized microstructure and performance.
It is possible to customize the ChromLok™ compositions to obtain a variety of electrical properties, from highly insulating to highly conducting at the operating temperature. Also, it is possible to tailor the coatings to allow bonding to standard borosilicate and aluminosilicate glasses. Selective application of ChromLok™ coatings to as-received metal surfaces enables patterning of chemical and electrical features. These crystalline oxide coatings attain outstanding chemical compatibility and adhesion with standard stainless steels via a simple production process.
Nexceris provides a variety of proprietary formulations as part of the ChromLok™ family to satisfy the requirements of customers. Leveraging its advanced capabilities, the company manufactures unique ceramic oxide powders to customize each coating’s process and properties. High value solutions can be obtained by applying the coating or multiple functional coatings on localized areas of intricate geometries.
Nexceris’ dedicated coating production space is used to apply ChromLok™ coatings through aerosol-spray deposition (Figure 1a). A nozzle-less ultrasonic spray head technology is utilized by this high performance platform, which allow the coating of a variety of oxide based coating suspensions. A precise coating application offers fine control of coating thickness and very high material transfer or low overspray. Alternative deposition methods such as slip coating and dip-coating are utilized for non-planar components as they are more suitable for the complex non-line-of-sight geometries. Figure 1b illustrates the ability of the ChromLok™ coated products to selectively coat regions and coat surface features.
Figure 1. ChromLok™ coating process: (a) Nexceris facility for coating deposition (left); and (b) multi-compositional coating demonstrating spatial resolution and conformal coating of features (right).
Following deposition, the green coated part is carefully heat treated to achieve a dense, adherent, and protective oxide coating. However, high sintering temperatures are needed for air firings to attain the desired densification, which can result in detrimental oxidation of the metallic substrate. Therefore, Nexceris have developed modified controlled atmosphere firings to allow densification of the coating at lower temperatures by controlling the pO2 level, reducing the chance of oxidation of the metal substrate.
ChromLok™ Coating Performance
Nexceris has devised a carefully controlled sintering processes to deposit high density oxide coatings over metal substrates. The cross-section and top-down microstructure of a ChromLok™ manganese cobaltite (Mn,Co)3O4 mixed spinel oxide coating applied over a ferritic stainless steel (441) substrate is depicted in Figure 2. An aerosol-spray deposition (ASD) process was used to deposit the manganese cobaltite (MCO) coating on the stainless steel substrate. The next step was to develop a dense protective coating through heat treatment.
Figure 2. SEM images of MCO coating, applied using Nexceris’ coating process: (a) top-down image (left); and cross-section image with superimposed EDS compositional analysis (right).
Excellent long-term stability was demonstrated by Nexceris’ oxide coatings at elevated temperatures. The oxidation resistance exhibited by a 18 wt. % Cr ferritic stainless steel substrate applied with and without an MCO protective coating at 800°C, and 900°C is shown in Figure 3. The ChromLok™ surface limits the formation of oxidation products at the surface and prevents Cr loss by acting as an adherent oxygen diffusion resistance layer.
Figure 3. Oxidation resistance of ChromLok™ coated and uncoated 18 Cr ferritic stainless steel.
The MCO coating not only has very high temperature stability in both reducing and oxidizing environments, it is also electrically conductive. The long-term electrical resistance (ASR) exhibited by MCO coated stainless steel under humidified air at 800°C is illustrated in Figure 4. The coating was able to maintain extremely low resistance even after a service period of 35,000 hours.
Figure 4. Electrical area-specific resistance (ASR) of ChromLok™ coated ferritic stainless steel (800°C, humidified air).
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The versatility of Nexceris’ coating process for the deposition of a variety of oxide-based coatings, including both perovskite and spinel based oxides, over metallic components has been demonstrated in this article. Nexceris has achieved highly dense and protective coating microstructures by carefully designing the properties of the starting oxide powder and the composition of the suspension, along with tailored firing and controlled coating application. These coatings are commercially used for advanced energy applications demanding high conductivity to achieve technical success. Nexceris sees opportunities for its coatings in other challenging service environments, where preventing corrosion and maintaining conductivity are critical, for instance, heat exchange applications, burner materials, and combustion liners.
This information has been sourced, reviewed and adapted from materials provided by Nexceris.
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