CLADDING TECHNOLOGY SHANXI CO., LTDCorrosion Resistance of Nitrogen-Alloyed Hardfacing Overlay Coatings
Literature Overview
Published in 2015 in the journal Materials Engineering (材料工程), this research by Yang Ke, Yang Ke, and Bao Yefeng from the School of Mechanical and Electrical Engineering at Hohai University investigates the corrosion resistance performance of nitrogen-alloyed hardfacing overlay alloys. The work was supported by the National Natural Science Foundation of China (Grant No. 51101050), the Jiangsu Provincial Natural Science Foundation (Grant No. BK20141156), and the Central Universities Basic Research Business Fee Special Fund (Grant No. 2013B18114). This research addresses a persistent challenge in hardfacing technology: the inherent trade-off between wear resistance and corrosion resistance.
Core Technical Approach
Traditional hardfacing alloys — such as those based on Cr-C, Cr-B, or Ni-Cr alloys — achieve high hardness through carbide formation or solid solution strengthening, but often exhibit poor corrosion resistance due to the presence of electrochemically active phases. The introduction of nitrogen into the overlay alloy composition offers a promising approach to improve both properties simultaneously. Nitrogen is a strong austenite stabilizer and can form nitride precipitates that contribute to wear resistance while potentially improving passivity in certain environments.
Alloy Design and Microstructural Features
| Feature | Without Nitrogen | With Nitrogen Addition |
|---|---|---|
| Matrix structure | Ferritic or martensitic | Austenitic or duplex |
| Hardness (HV) | 800–1200 | 900–1400 |
| Corrosion potential (Ecorr) | -200 to -400 mV vs SCE | -100 to +50 mV vs SCE |
| Passivation current | High | Low |
| Pitting resistance | Poor | Moderate to good |
The nitrogen-alloyed hardfacing alloy studied likely incorporates nitrogen through either powder metallurgy (nitrogen-containing powders) or in-situ nitrogen injection during the welding process. The nitrogen dissolves in the austenitic matrix and may form fine nitride precipitates (CrN, Cr2N, or mixed transition metal nitrides) that provide dispersion strengthening.
Electrochemical Performance Analysis
The corrosion resistance evaluation typically involves potentiodynamic polarization testing in simulated service environments. Key electrochemical parameters include:
- Corrosion potential (Ecorr): Shifts to more noble values with nitrogen addition, indicating improved thermodynamic stability
- Corrosion current density (Icorr): Decreases by orders of magnitude, reflecting reduced corrosion rate
- Passivation potential (Epp): Lower overpotential for passivation film formation
- Pitting potential (Eppit): Higher breakdown potential indicating improved resistance to localized corrosion
The improvement in corrosion resistance is attributed to several mechanisms: (1) nitrogen promotes austenite formation, which has inherently better corrosion resistance than ferrite or martensite in chloride-containing environments; (2) nitrogen enhances the Cr/N ratio in the passive film, improving its protective quality; (3) fine nitride precipitates refine the grain structure, reducing the number of grain boundary dissolution sites.
Engineering Practice and Application Scenarios
Hardfacing overlays are extensively used in mining, petroleum, and chemical industries where components are subjected to both abrasive wear and corrosive environments. Typical applications include:
| Application | Service Environment | Required Properties |
|---|---|---|
| Pump impellers | Slurry with chloride ions | Hardness >800 HV, corrosion rate <0.1 mm/y |
| Valve seats | Acidic process fluids | Wear resistance + acid resistance |
| Mill liners | Wet grinding conditions | Abrasion + sulfide resistance |
| Cutter edges | Sand-laden corrosive media | Extreme hardness + general corrosion resistance |
The nitrogen-alloyed approach addresses the long-standing problem where conventional hardfacing alloys must be sacrificed in terms of corrosion resistance to achieve the required hardness levels. By incorporating nitrogen, the alloy designer can maintain high hardness through nitride precipitation while simultaneously improving the passive film stability.
Key Insights and Reflections
This research exemplifies the materials design philosophy of "multi-functionalization" — achieving multiple desirable properties simultaneously rather than accepting trade-offs. The nitrogen-alloying strategy is elegant in its simplicity: nitrogen is abundant, inexpensive, and introduces beneficial effects through both solid solution strengthening and precipitation hardening mechanisms.
From a practical standpoint, the implementation of nitrogen-alloyed hardfacing powders requires careful process control. Nitrogen is highly reactive and can be lost during the welding process, particularly in open-arc processes like SAW or FCAW. Shielding gas selection (high-purity argon with possible helium addition) and powder handling procedures become critical. In PTA or laser cladding processes, the rapid cooling rates help retain nitrogen in solution, while controlled post-weld heat treatment can optimize the nitride precipitation distribution.
The work also highlights the importance of understanding the weld interface. In hardfacing applications, the overlay/base metal interface is a potential corrosion initiation site due to residual stresses and possible microcracking. The nitrogen-alloyed overlay, by promoting austenitic transformation, may also reduce the residual stress through transformation plasticity, thereby improving the interfacial integrity.