Inconel 625 TIP-TIG Surfacing Coating on 316L Stainless Steel

Literature Overview

Zhang Zhen and colleagues from Lanzhou University of Technology present a comprehensive study on Inconel 625 nickel-based alloy surfacing coatings deposited on 316L stainless steel substrates using the Transferred Inert Plasma-Tungsten Inert Gas (TIP-TIG) welding process. The research employed orthogonal experimental design to optimize three key welding parameters: welding current, welding speed, and wire feed speed, with dilution rate as the primary evaluation criterion. Published in Corrosion and Protection (2026, Vol. 47, No. 3, pp. 1-8), this work addresses the critical challenge of producing high-performance corrosion-resistant overlays with controlled dilution.

Core Technical Findings

Through orthogonal experimental optimization, the optimal welding parameters were identified as: welding current of 150 A, welding speed of 2 mm/s, and wire feed speed of 3.13 m/min. Under these conditions, the overlay coating achieved optimal comprehensive performance. Mechanical testing revealed that the 316L substrate exhibited tensile strength of 633.50 MPa and elongation of 55.90%, while the overlay coating achieved higher tensile strength of 751.90 MPa with elongation of 50.20%.

Hardness measurements across the weld cross-section showed a characteristic gradient: the overlay coating had the highest hardness at 219.60 HV, the base metal at 174.30 HV, and the heat-affected zone at the lowest value of 155.10 HV. This hardness distribution reflects the microstructural differences between the as-deposited overlay, the unaffected base metal, and the thermally affected region.

The corrosion resistance testing in 600°C ternary chloride molten salt revealed a dramatic improvement: the base metal exhibited an average corrosion rate of 17.75 mm/a, while the overlay coating showed only 3.35 mm/a, representing approximately one-fifth of the base metal corrosion rate. This fivefold improvement in corrosion resistance demonstrates the effectiveness of Inconel 625 as a protective overlay for severe chloride environments.

Process Optimization Analysis

The TIP-TIG process combines the advantages of plasma arc and TIG welding, offering a focused heat source with deep penetration and high deposition efficiency. The orthogonal experimental design is an efficient approach to parameter optimization, requiring fewer experimental trials than full factorial designs while still identifying the optimal parameter combination. The selection of dilution rate as the core evaluation criterion is appropriate because dilution directly affects the overlay composition and consequently its corrosion resistance and mechanical properties.

The optimal current of 150 A provides sufficient heat input for good fusion with the 316L substrate while avoiding excessive melting that would increase dilution. The welding speed of 2 mm/s balances deposition rate against heat input, ensuring adequate cooling to promote favorable microstructural evolution. The wire feed speed of 3.13 m/min maintains a consistent metal transfer rate that matches the melting rate at the given current and speed settings.

The hardness gradient observed across the weld cross-section is expected and reflects the different thermal histories experienced by each region. The overlay coating achieves the highest hardness due to the solidification microstructure and potential precipitation of intermetallic phases during cooling. The HAZ exhibits the lowest hardness due to grain growth and potential phase softening during the thermal cycle.

Corrosion Performance Discussion

The 600°C ternary chloride molten salt corrosion test represents a severe accelerated corrosion environment that simulates conditions encountered in chemical processing, desulfurization systems, and nuclear waste treatment. The dramatic reduction in corrosion rate from 17.75 mm/a to 3.35 mm/a demonstrates that the Inconel 625 overlay provides effective protection against chloride-induced corrosion at elevated temperatures.

The corrosion mechanism in this environment involves the breakdown of the passive film by chloride ions, leading to pitting and crevice corrosion. Inconel 625, with its high nickel and chromium content, forms a more stable and self-healing passive film that resists chloride attack. The overlay's effectiveness is further enhanced by its low dilution rate, which preserves the high nickel and chromium content necessary for corrosion resistance.

Engineering Practice Considerations

For engineers selecting overlay materials for chloride-containing environments, this study provides validated data for Inconel 625 on 316L substrates. The TIP-TIG process parameters identified can serve as starting points for process development on similar substrates. However, engineers should note that the corrosion test was conducted at 600°C, which is above the operating temperature of most industrial applications. Long-term corrosion performance at lower temperatures should be evaluated for specific service conditions.

The dilution rate must be carefully monitored during production, as excessive dilution with the 316L substrate will reduce the nickel and chromium content in the overlay, potentially compromising corrosion resistance. In-situ monitoring of welding parameters and periodic dilution testing are recommended for quality control.

Study Insights and Reflections

This research provides a complete process-performance correlation for Inconel 625 TIP-TIG surfacing on 316L stainless steel. The orthogonal experimental approach demonstrates the efficiency of systematic parameter optimization, and the results provide a reliable foundation for industrial implementation.

One limitation of the study is the relatively short duration of the corrosion test. Long-term exposure to chloride environments may reveal different corrosion mechanisms, such as stress corrosion cracking or intergranular corrosion, which could affect the overlay's long-term performance. Additionally, the mechanical properties were tested at room temperature, whereas the corrosion test was conducted at 600°C. High-temperature mechanical property evaluation would provide a more complete picture of the overlay's performance in service.

The fivefold improvement in corrosion resistance achieved through this relatively straightforward surfacing process represents a compelling value proposition for industrial applications where material replacement is impractical or uneconomical. Engineers should consider this technology for extending the service life of existing equipment in aggressive chemical environments.