CLADDING TECHNOLOGY SHANXI CO., LTDRapid Laser Cladding of Nickel-Stainless Steel Overlay Layers Microstructure and Corrosion Resistance
Literature Overview and Application Context
The study by Dong Hui, Han Yan, Fu Anqing, Zhou Yong, and Li Xiao, published in Surface Technology in 2019, investigates the microstructure and corrosion resistance of nickel/stainless steel overlay layers produced by rapid laser cladding. This research was supported by the Xi'an Shiyou University Provincial Advantageous Discipline of Materials Science and Engineering (YS37020203) and the State Key Laboratory of Material Processing and Die & Mold Technology Open Research Fund (P2018-17). The authors from Xi'an Petroleum University and the China National Petroleum Corporation Petroleum Tube Engineering Technology Research Institute addressed a critical need in the oil and gas industry: developing corrosion-resistant overlay layers for downhole tools and tubular goods exposed to aggressive sour environments containing hydrogen sulfide (H2S), carbon dioxide (CO2), and chlorides.
The research is particularly relevant to the petroleum industry, where tubular goods and downhole tools are frequently subjected to multi-phase corrosion environments that can cause severe damage to conventional carbon steel and stainless steel materials. The overlay of nickel-based and stainless steel layers on carbon steel substrates is a well-established approach for corrosion protection, but conventional welding methods often produce overlay layers with excessive dilution, coarse microstructures, and inadequate corrosion resistance. Rapid laser cladding offers a promising alternative by enabling the production of overlay layers with minimal dilution, fine microstructures, and superior corrosion resistance.
Microstructural Analysis and Phase Composition
The rapid laser cladding process employed in this study utilized a fiber laser with output power of 2–6 kW, scanning speed of 200–800 mm/min, powder feed rate of 100–300 g/min, and helium shielding gas flow of 20–30 L/min. The substrate preheating temperature was maintained at 100–200 °C to minimize thermal stress cracking at the interface. The laser power density was in the range of 10^5–10^6 W/cm², which produced a shallow melt pool with a depth of 0.5–2 mm and a width of 2–5 mm.
| Microstructural Feature | Single-Layer Cladding | Multi-Layer Cladding (3 passes) |
|---|---|---|
| Grain morphology | Columnar | Equiaxed (top), columnar (bottom) |
| Average grain size | 50–100 μm | 30–60 μm (top) |
| Dilution ratio | 8–12% | 5–10% |
| Hardness (HV) | 250–350 | 300–400 |
| M23C6 carbide | Moderate | Fine, dispersed |
| Sigma phase | Absent | Trace (at layer interface) |
| Cracking tendency | Low | Very low |
The microstructural analysis revealed that the rapid laser cladding process produced overlay layers with a fine dendritic microstructure composed of austenite (gamma) and ferrite (delta) phases, with M23C6 carbides precipitated at the dendrite boundaries. The dilution ratio was significantly lower than that achieved by conventional arc welding methods, typically falling within the 5–12% range compared to 20–35% for submerged arc welding (SAW) and 15–25% for gas metal arc welding (GMAW). This low dilution was directly attributed to the shallow melt pool depth and high solidification rate characteristic of laser cladding.
The multi-layer cladding strategy produced overlay layers with more uniform microstructures and improved mechanical properties compared to single-layer cladding. The top layers exhibited equiaxed grain structures with fine grain sizes, while the bottom layers adjacent to the substrate retained columnar grain structures. This microstructural gradient was beneficial for achieving good bonding between the overlay layer and the substrate while maintaining the desired properties in the functional surface layers.
Corrosion Resistance Evaluation and Mechanisms
The corrosion resistance of the laser-cladded overlay layers was evaluated through potentiodynamic polarization testing in simulated sour environment (3.5% NaCl solution with 500 ppm H2S at 60 °C), electrochemical impedance spectroscopy (EIS), and immersion testing. The results demonstrated that the laser-cladded Ni/stainless steel overlay layers exhibited significantly superior corrosion resistance compared to the bare carbon steel substrate and conventional arc-welded overlay layers.
| Material | Corrosion Potential (mV vs. SCE) | Corrosion Current Density (μA/cm²) | Corrosion Rate (mm/y) |
|---|---|---|---|
| Carbon steel substrate | -750 to -800 | 50–80 | 1.5–2.5 |
| SAW overlay layer | -500 to -550 | 20–35 | 0.5–0.8 |
| Laser-cladded overlay layer | -200 to -300 | 2–8 | 0.05–0.15 |
| 316L stainless steel | -250 to -350 | 3–10 | 0.08–0.20 |
The corrosion resistance of the laser-cladded overlay layers was attributed to several factors: (1) the low dilution ratio preserved the high chromium and nickel content of the overlay layer, ensuring adequate passivation; (2) the fine microstructure with dispersed carbides promoted the formation of a stable and uniform passive film; (3) the absence of sigma phase and other detrimental intermetallic phases prevented localized corrosion attack; and (4) the compressive residual stresses at the surface inhibited crack initiation and propagation.
The immersion testing results confirmed that after 30 days of exposure in the simulated sour environment, the laser-cladded overlay layers exhibited negligible weight loss and minimal surface attack, while the bare carbon steel substrate suffered severe general corrosion and localized pitting. The SAW overlay layer, while showing improved corrosion resistance compared to the bare substrate, still exhibited localized corrosion attack at carbide-rich regions and at the overlay-substrate interface.
Engineering Practice Integration and Study Insights
This research has direct implications for the design and specification of corrosion-resistant overlay layers for petroleum industry applications. The key finding is that rapid laser cladding can produce overlay layers with corrosion resistance comparable to or exceeding that of solid stainless steel, while retaining the structural strength and cost-effectiveness of carbon steel substrates. This makes laser cladding an attractive option for extending the service life of downhole tools and tubular goods in aggressive sour environments.
From a quality assurance perspective, the study recommends performing the following quality control checks on laser-cladded overlay layers: (1) chemical analysis to verify the dilution ratio and overlay layer composition; (2) metallographic examination to assess microstructure, grain size, and the presence of detrimental phases; (3) hardness testing to ensure the overlay layer meets the specified hardness range; (4) corrosion testing to verify the corrosion resistance meets the required performance criteria; and (5) non-destructive testing (preferably ultrasonic testing) to detect any lack of fusion or porosity at the overlay-substrate interface.
The engineering takeaway is that rapid laser cladding represents a significant advancement in corrosion protection technology for the petroleum industry. The ability to produce overlay layers with minimal dilution, fine microstructures, and superior corrosion resistance makes this technology particularly suitable for high-value components where service life and reliability are critical. However, the higher equipment costs and lower deposition rates compared to conventional arc welding methods must be considered when evaluating the overall cost-effectiveness of laser cladding for specific applications.