KCl-Induced High-Temperature Corrosion Behavior of HVAF-Sprayed Ni-Based Coatings in Ambient Air

Abstract

KCl-induced high-temperature corrosion behavior of four HVAF-sprayed Ni-based coatings (Ni21Cr, Ni5Al, Ni21Cr7Al1Y and Ni21Cr9Mo) under KCl deposit has been investigated in ambient air at 600°C up to 168 h. The coatings were deposited onto 16Mo3 steel-a widely used boiler tube material. Uncoated substrate, 304L and Sanicro 25 were used as reference materials in the test environment. SEM/EDS and XRD techniques were utilized to characterize the as-sprayed and exposed samples. The results showed that the small addition of KCl significantly accelerated degradation to the coatings. All coatings provided better corrosion resistance compared to the reference materials. The alumina-forming Ni5Al coating under KCl deposit was capable of forming a more protective oxide scale compared to the chromia-forming coatings as penetration of Cl through diffusion paths was hindered. Both active corrosion and chromate formation mechanisms were found to be responsible for the corrosion damages. The corrosion resistance of the coatings based on the microstructure analysis and kinetics had the following ranking (from the best to worst):

Recent Development of Corrosion Factors and Coating Applications in Biomass Firing Plants

“…Both Ni20Cr coating sprayed by HVOF and Ni21Cr coating sprayed by HVAF have shown good corrosion resistance in different environments at 600 • C for 168 h, and the thickness of the corresponding corrosion product is relatively small between 1 and 22 µm. In ambient air, Ni21Cr coating sprayed by HVAF shows similar corrosion properties as Ni5Al and NiCrAlY coatings [62], while it has performed poorly in mixed atmospheres [58]. NiCrAlY material has high oxidation resistance, and its thermal expansion coefficient is close to that of the base Fe alloy materials [40], which is widely used in coating materials.…”

“…Under the simulated biomass boiler atmosphere of 550 • C, HVOF-sprayed Ni5Al coating has exhibited poor corrosion resistance and failed for 672 h [60]. However, under an air or simulated atmosphere of 600 • C, the Ni5Al coating has relatively low corrosion weight changes and corrosion product layer thickness [58,61,62]. With the increase in aluminum content, the corrosion resistance of NiAl(Ni31Al) coating has been improved under the condition in which the temperature is higher than 600 • C [24,49].…”

Advanced Coatings by Thermal Spray Processes

“…Specifically, nearly fully dense Ni-based coatings with the ability to form different protective scales (alumina, chromia, and alumina-chromia) have been studied in considerable detail and also subjected to both laboratory and field testing. The corrosion performance of candidate HVAF-sprayed coatings-Ni 21 Cr, Ni 21 Cr 7 AlY, Ni 5 Al, Ni 21 Cr 9 Mo, Ni 21 Cr 9 Mo-SiO 2 -has been evaluated through detailed laboratory studies in ambient air, moisture, and HCl-laden environments [36][37][38][39][40][41]. All coatings were highly protective in all environments in the absence of KCl due to the formation of corresponding protective scales of alumina or chromia on the coating surface.…”

“…The performance of the coatings in the complex Cl-containing environment was ranked as (from highest to lowest corrosion resistance): Ni21Cr9Mo-SiO2 > Ni21Cr7AlY > Ni5Al > Ni21Cr9Mo > Ni21Cr, confirming the enhanced corrosion protection of chromia-forming coatings in the presence of alloying elements and dispersed SiO2. While a typical result is summarized in Figure 4, further details regarding this work are available in the various publications mentioned previously [36][37][38][39][40][41].…”

High-Temperature Corrosion of HVAF-Sprayed Ni-Based Coatings for Boiler Applications

“…The corrosion behaviour of HVAF-sprayed NiCr, NiAl and NiCrAlY coatings has recently been investigated when exposed to ambient air at 600 °C with KCl present for 168 h [24]. It was shown that all three coatings managed to protect the substrate from corrosion attack during the time interval of the exposure.…”