Effect of LaB6 Particles on Microstructure and High-Temperature Oxidation Resistance of Plasma Overlay Welded Inconel 625 Coatings

Literature Overview

This paper, published in Surface Technology (2025, Vol. 54, No. 6), presents a comprehensive study on the effect of LaB6 particle addition on the microstructure, hardness, and high-temperature oxidation resistance of plasma overlay welded Inconel 625 (IN625) coatings. The research, conducted by scholars from Wuyi University and the Guangdong Provincial Key Laboratory of Material Joining and Advanced Manufacturing, was supported by multiple funding sources including the National Key R&D Program of China (2020YFE0205300). The study addresses a critical challenge in waste incinerator heat exchanger tube design: improving the high-temperature oxidation resistance of IN625 coatings to extend service life in increasingly demanding thermal environments.

Core Technical Points

Microstructural Evolution with LaB6 Addition

The addition of 1 wt.% LaB6 particles (average particle size 5 μm) to IN625 alloy fundamentally alters the microstructure of the plasma overlay welded coating:

The transition from columnar to mixed columnar-equiaxed grain structure is a significant finding. Columnar grains, which are typical of directional solidification in plasma overlay welding, can be detrimental to high-temperature performance because they provide preferential diffusion paths for cation transport during oxidation. The introduction of equiaxed grains disrupts this preferential path, forcing diffusing species to traverse more tortuous routes through the microstructure.

The La-Ti-O composite oxide particles, formed in situ from the reaction of LaB6 with the surrounding alloy matrix, serve a dual function: they act as nucleation sites for grain refinement during solidification, and they remain as fine, dispersed particles that impede grain growth during subsequent high-temperature exposure. This grain growth inhibition mechanism is critical for maintaining microstructural stability during long-term service at elevated temperatures.

High-Temperature Oxidation Mechanism

The oxidation behavior of IN625/LaB6 coatings at 1000°C is significantly improved compared to pure IN625, as evidenced by the oxidation constant reduction from (72 ± 4) × 10⁻² mg²/(cm⁴·h) to (26 ± 3) × 10⁻² mg²/(cm⁴·h), representing a 63.9% reduction in the oxidation rate constant.

The improved oxidation resistance is attributed to multiple synergistic mechanisms:

1. Grain refinement increases the number of grain boundaries available for Cr diffusion to the oxide/metal interface, promoting the formation of a more continuous and protective chromium oxide scale.
2. The high Cr consumption near the oxide interface creates a steep Nb activity gradient, driving Nb outward diffusion and forming a continuous δ-Ni3Nb layer at the oxide/matrix interface.
3. The formation of CrNbO4 on the oxide surface provides an additional diffusion barrier for both cations and anions.
4. La-Ti-O composite oxide particles precipitated at grain boundaries impede the outward transport of Ni, Fe, and Nb cations, preventing the formation of multiple oxide phases that would generate growth stresses and cause scale spallation.

Phase Formation and Stability

The formation of the δ-Ni3Nb intermetallic layer is a particularly important finding for high-temperature coating design. This phase is thermodynamically stable at elevated temperatures and provides an effective diffusion barrier due to its ordered crystal structure. The CrNbO4 phase formed on the oxide surface is similarly significant, as it represents a complex oxide that combines the protective qualities of chromium oxide with the structural stability of niobium oxide.

Engineering Practice Integration

In the context of waste incinerator and waste-to-energy plant operations, heat exchanger tubes are exposed to aggressive high-temperature flue gas environments containing SOx, HCl, and alkali metal chlorides. The tube surface temperature can exceed 1000°C in certain operating conditions, making high-temperature oxidation resistance a critical design consideration.

For pipeline and piping system engineers, this study has implications for the design of high-temperature process piping in petrochemical and power generation applications where Inconel 625 is commonly specified for its excellent corrosion and oxidation resistance. The LaB6 particle addition technique offers a pathway to extend the service life of IN625 components in applications where the base alloy's oxidation resistance is insufficient.

The plasma overlay welding process used in this study is particularly suitable for coating application on complex geometries such as heat exchanger tube bundles, where conventional cladding methods may be impractical. The process allows for controlled dilution and precise coating thickness, which are critical for maintaining the structural integrity of thin-walled tubes.

Key Questions and Reflections

Several important questions arise from this study. First, the long-term stability of the La-Ti-O composite oxide particles during extended high-temperature exposure is not fully addressed. If these particles coarsen or dissolve over time, their grain growth inhibition effect would diminish, potentially compromising the coating's long-term oxidation resistance. Second, the study does not evaluate the effect of LaB6 addition on the coating's resistance to other forms of degradation, such as hot corrosion by sulfur or chloride attack, which are common in waste incineration environments.

Additionally, the practical feasibility of incorporating LaB6 particles into plasma overlay welding powder formulations for industrial-scale production requires further investigation. Particle size distribution, powder flowability, and the consistency of LaB6 addition across production batches are all practical concerns that affect process reproducibility.

Study Insights and Implications

This study demonstrates a promising approach to enhancing the high-temperature oxidation resistance of Inconel 625 coatings through the strategic addition of LaB6 particles. The mechanism is well-established: LaB6 reacts in situ to form La-Ti-O composite oxides that refine the grain structure and impede grain growth during service, while simultaneously promoting the formation of protective oxide phases (δ-Ni3Nb and CrNbO4) that act as diffusion barriers.

For engineers designing high-temperature coatings for pipeline, heat exchanger, and process equipment applications, this research provides a clear pathway for improving coating performance through particle engineering. The 63.9% reduction in oxidation rate constant at 1000°C is a substantial improvement that could translate directly into extended service intervals and reduced maintenance costs. Future research should focus on validating these findings under realistic service conditions that include thermal cycling, mechanical loading, and exposure to corrosive species, to provide a complete qualification package for industrial implementation.