CLADDING TECHNOLOGY SHANXI CO., LTDCorrosion Resistance of Nickel-Based Thermal Spray Materials
Literature Overview and Background
This study investigates the corrosion resistance of various nickel-based thermal spray materials in different aggressive environments, including acidic, alkaline, and chloride-containing solutions. Nickel-based alloys are widely used as corrosion-resistant coatings in chemical processing, oil and gas, and marine applications due to their excellent resistance to a wide range of corrosive media. The thermal spray process, however, introduces unique microstructural features—such as porosity, oxide inclusions, and unmelted particles—that can significantly affect the corrosion behavior compared to cast or wrought nickel-based materials.
Microstructural Factors Affecting Corrosion Performance
The corrosion resistance of nickel-based thermal spray coatings is governed by several microstructural factors identified in this study. The most critical factor is porosity, which provides pathways for corrosive media to penetrate the coating and reach the underlying substrate. Porosity levels in as-sprayed coatings typically range from 1% to 8%, depending on the spraying process and material. The study demonstrates that even small amounts of porosity (below 2%) can dramatically reduce the coating's protective effectiveness, as corrosion initiates at pore sites and propagates laterally along the coating-substrate interface.
Oxide inclusions, primarily consisting of NiO, Cr2O3, and Fe2O3, are another significant microstructural feature. These oxides are generally less noble than the surrounding nickel matrix and can act as cathodic sites in the electrochemical corrosion process, accelerating localized corrosion. The study finds that coatings produced by HVOF spraying exhibit fewer and smaller oxide inclusions compared to those produced by atmospheric plasma spraying, resulting in superior corrosion resistance.
Comparative Corrosion Performance
| Material | Corrosion Rate in 3% NaCl (mm/year) | Corrosion Rate in 10% H2SO4 (mm/year) | Corrosion Rate in 5% NaOH (mm/year) |
|---|---|---|---|
| Inconel 625 (as-sprayed) | 0.02–0.05 | 0.01–0.03 | 0.005–0.01 |
| Inconel 625 (remelted) | 0.005–0.01 | 0.003–0.008 | 0.002–0.005 |
| Hastelloy C276 (as-sprayed) | 0.01–0.03 | 0.005–0.015 | 0.003–0.008 |
| Hastelloy C276 (remelted) | 0.003–0.008 | 0.002–0.005 | 0.001–0.003 |
| Monel 400 (as-sprayed) | 0.03–0.08 | 0.02–0.05 | 0.005–0.01 |
| Monel 400 (remelted) | 0.008–0.02 | 0.005–0.015 | 0.002–0.005 |
The data clearly shows that remelting significantly improves the corrosion resistance of all nickel-based coatings by eliminating porosity and reducing oxide content. The improvement is most pronounced in chloride-containing environments, where pitting corrosion is the dominant failure mechanism.
Electrochemical Analysis and Passivation Behavior
The study employs potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) to characterize the corrosion mechanisms. The remelted coatings exhibit a higher corrosion potential and a lower corrosion current density compared to as-sprayed coatings, indicating superior passivation behavior. The passive film formed on remelted coatings is more uniform and adherent, providing better protection against localized attack.
In chloride environments, the study finds that the pitting potential of remelted coatings is shifted positively by 50–150 mV compared to as-sprayed coatings, indicating enhanced resistance to pitting initiation. This is attributed to the more homogeneous microstructure and the absence of oxide-rich inter-splat boundaries that would otherwise serve as preferential pitting sites.
Engineering Practice Implications
For pressure vessel and heat exchanger applications, the selection of nickel-based thermal spray materials must consider not only the inherent corrosion resistance of the alloy but also the microstructural quality of the coating. The study recommends that for critical applications, coatings should be remelted after spraying to ensure porosity-free surfaces. Additionally, the coating thickness should be sufficient to accommodate any localized thinning due to corrosion, with a minimum thickness of 0.5 mm recommended for most service conditions.
The study also emphasizes the importance of coating-substrate compatibility. When nickel-based coatings are applied to carbon steel substrates, the galvanic coupling between the two materials can accelerate substrate corrosion at coating defects. Therefore, any coating discontinuities must be carefully inspected and repaired, and the coating thickness should be maintained uniformly across the entire surface.
Key Reflections and Study Insights
The most valuable insight from this literature is the recognition that the corrosion performance of nickel-based thermal spray coatings is not solely determined by the alloy composition but is equally dependent on the coating microstructure. This means that process control during the thermal spraying operation is as critical as material selection. Engineers must ensure that the spraying parameters are optimized to minimize porosity and oxide content, and that post-spraying treatments such as remelting or hot isostatic pressing are applied when necessary.
Another important observation is that the corrosion resistance of thermal spray coatings can be significantly enhanced through microstructural engineering, such as grain refinement and oxide reduction. This opens up possibilities for developing advanced coating systems that combine the corrosion resistance of nickel-based alloys with the process flexibility of thermal spraying.
Summary
The corrosion resistance of nickel-based thermal spray materials is a multifaceted topic that involves the interplay between alloy chemistry, microstructural features, and environmental conditions. The study provides valuable guidance for selecting appropriate materials and processes for specific service environments. Engineers working in pressure vessel fabrication and overlay welding should adopt a holistic approach to coating design, considering not only the corrosion resistance of the base alloy but also the microstructural quality of the coating and the compatibility with the substrate material.