The critical heat exchange components in boilers (water
wall tubes, superheater tubes, reheater tubes, and economizer tubes) are
susceptible to premature failure under high temperature, high pressure, and
complex corrosive media. This remains a significant challenge for the
reliability and economic operation of thermal power plants, especially with the
trend towards higher parameters and capacities. Statistics indicate that
failures of these components account for nearly half of all unplanned shutdowns
in power plants, approximately 60% of which are directly attributable to
high-temperature corrosion, resulting in substantial economic losses. A primary
cause of such failures is hot corrosion induced by the deposition of molten
salts such as NaCl, KCl, and Na2SO4, which originate from
impurities in low-grade coals or biomass fuels. These salts form
low-melting-point eutectic mixtures that destroy protective oxide scales and
initiate catastrophic corrosion. To mitigate this issue, thermal spray
coatings, particularly those applied via high-velocity oxygen-fuel (HVOF) spraying
technology, offer an effective solution due to their low flame temperature,
high deposition efficiency, and excellent bond strength. While traditional
NiCrBSi coatings provide good protective properties, their performance limits
are being challenged in increasingly harsh environments. This study introduces
ZrB2 as a reinforcing phase to enhance performance. ZrB2 possesses an ultra-high melting point and high hardness. Most importantly, it
oxidizes to form protective ZrO2 scales, effectively blocking sulfur
and chlorine permeation. However, its standalone use is limited due to poor
sintering properties and high brittleness. In this work, NiCrBSi–ZrB2 composite powders with high sphericity and uniform composition were
successfully prepared through planetary ball milling, spray granulation, and
vacuum sintering. Subsequently, dense and well-bonded composite coatings were
fabricated using the HVOF spraying process. The hot corrosion behavior of the
coating was systematically investigated in a NaCl–KCl–Na2SO4 molten salt mixture at 600, 700, and 800 °C for 100 h. The results demonstrated
that the composite coating exhibited a dense layered structure with a bond
strength exceeding 70 MPa and uniform distribution of phases. The hot corrosion
resistance was highly temperature-dependent. At 600 °C, the coating displayed
optimal performance with a mere weight gain of (13.13 ± 0.08) mg/cm2 and a kinetic constant of 1.19 mg2/(cm4·h). Analysis via
XRD, SEM, EPMA, and TEM revealed that the corrosion products primarily consisted
of protective oxides (such as ZrO2, SiO2, and Cr2O3)
alongside spinels and chromates (including NiCr2O4, K2CrO4,
and Na2CrO4). The preferentially formed ZrO2 and SiO2 created a mixed oxide layer that acted as a robust barrier,
significantly hindering the diffusion of corrosive elements and resisting
alkaline dissolution. Thermodynamic analysis indicated that the ZrCl4 and SiCl4 generated within the coating possessed low oxidation Gibbs
free energy, leading to a sluggish oxidation process that effectively
interrupts the destructive chlorine cycle and reduces the corrosion rate.
However, at 800 °C, the chlorination-oxidation cycle accelerated dramatically. The
thermal stress generated within the oxide scales exceeded their cohesive
strength, leading to extensive cracking and spallation. Consequently, the
corrosion rate increased significantly, and the protective capability
deteriorated. This study identified a critical temperature threshold of
approximately 700 °C for the safe long-term application of this coating.
