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

Abstract

The present study investigates the initial corrosion behaviour of HVAF-sprayed NiCr, NiAl and NiCrAlY coatings in two different environments, O 2 + H 2 O and O 2 + H 2 O + KCl at 600 °C for up to 168 h in order to evaluate the possibility of utilizing such coatings in biomass-and waste-fired boilers. SEM/EDX analysis showed that all coatings displayed a protective behaviour in O 2 + H 2 O. Upon addition of KCl (O 2 + H 2 O + KCl), the corrosion behaviour of the NiCr coating drastically changed as it formed a thick oxide layer and displayed major chlorine diffusion down to the substrate. The NiCrAlY coating displayed a significantly better corrosion resistance with only minor oxide formation. The NiAl coating exhibited a protective behaviour similar to when exposed in the absence of KCl indicating that a thin protective oxide has formed on the coating surface. The performance of the NiAl and NiCrAlY coatings is promising for future studies with long-term exposures in more corrosive environments such as in a biomass-and waste-fired boiler.

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

“…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].…”

Microstructure and Tribological Performance of Hvaf Sprayed Ti-6al-4v Coatings

“…Despite the insignificant size of a coating relative to a coated component, a dense homogenous coating offers similar protection, as will a bulk sample of the coating material feedstock [14]. The key to ensuring the controlled microstructure of a coating lies in the adopted spray technique [15], the spray parameters/conditions [16], and the characteristics of the feedstock materials [17]. The aerospace industry has explored this capability of thermal spray with the high-velocity oxy-fuel (HVOF) thermal spray technique for repair.…”

“…This is a concern for oxidation-sensitive materials like titanium alloys, which have also limited their application. For this work, the feedstock is a Ti-6Al-4V powder deposited via HVAF, a thermal spray technique with the capacity to deposit dense coatings with reduced/no oxide formation while transferring limited heat to the coated substrate-HVAF coating also maintains good interfacial bonding with the coated substrate [2,17]. The earlier stated benefits guided the choice of HVAF above the known industrially employed techniques APS and HVOF [18,19].…”