CLADDING TECHNOLOGY SHANXI CO., LTDElectrochemical Corrosion Failure of NiCrBSi Cladding Layer
Literature Overview
This study note examines the electrochemical corrosion failure mechanisms of Ni-Cr-B-Si (NiCrBSi) cladding layers, which are widely used in applications requiring combined wear and corrosion resistance. Despite their excellent wear resistance, NiCrBSi alloys are known to be susceptible to electrochemical corrosion in certain environments, particularly acidic or chloride-containing solutions. Understanding the failure process is critical for proper material selection and service life prediction.
Material Characteristics
NiCrBSi cladding alloys typically have the following composition:
| Element | Content (wt%) | Role |
|---|---|---|
| Ni | Balance | Matrix element, corrosion resistance |
| Cr | 18–25 | Solid solution strengthening, oxide formation |
| B | 4–7 | Carbide formation, hardening |
| Si | 3–5 | Carbide formation, hardening |
| C | 1.0–2.0 | Carbide precipitation |
| Fe | 1–5 | Minor, affects corrosion behavior |
The microstructure consists of a Ni-Cr-Si solid solution matrix with dispersed CrB, Cr2B, Cr23C6, and Cr7C3 carbides. The hardness typically ranges from 45–55 HRC.
Corrosion Failure Mechanism
Microgalvanic Corrosion
The primary corrosion mechanism in NiCrBSi alloys is microgalvanic corrosion between the Ni-Cr matrix and the carbide phases. The carbides (particularly Cr23C6 and CrB) are more noble than the matrix, creating a galvanic couple at the microstructural level. In a corrosive electrolyte:
- The matrix acts as the anode and preferentially dissolves.
- The carbides remain as islands on the surface, creating a porous and non-protective corrosion product layer.
- The exposed matrix beneath the porous layer continues to corrode, leading to progressive material loss.
Pitting Initiation and Propagation
In chloride-containing environments, the corrosion process follows a pitting mechanism:
- Passive film breakdown: Localized breakdown of the Cr2O3 passive film occurs at carbide-matrix interfaces where the passive film is weaker.
- Pit initiation: Chloride ions penetrate the passive film at these sites, initiating localized dissolution.
- Pit growth: The acidic environment within the pit (due to hydrolysis of dissolved metal ions) accelerates dissolution, while the surrounding passive film inhibits general corrosion.
- Coalescence: Adjacent pits merge, leading to rapid material loss and eventual perforation.
Intergranular Corrosion
If the NiCrBSi layer has been sensitized by prolonged exposure to temperatures in the 500–800°C range, chromium carbides can precipitate at grain boundaries, depleting the adjacent matrix of chromium. This creates a chromium-depleted zone susceptible to intergranular corrosion.
Electrochemical Testing Results
| Test Parameter | Typical Value |
|---|---|
| Corrosion potential (Ecorr) in 3.5% NaCl | -0.4 to -0.6 V vs. SCE |
| Passivation potential (Epass) | -0.2 to 0 V vs. SCE |
| Pitting potential (Epit) | 0.1 to 0.3 V vs. SCE |
| Corrosion rate (icorr) | 0.1 to 1.0 mA/cm² |
| Impedance (Rct at Ecorr) | 100–500 Ω·cm² |
These values indicate that NiCrBSi has moderate corrosion resistance but is significantly inferior to austenitic stainless steels (e.g., 316L) in terms of pitting resistance.
Failure Case Analysis
A typical failure scenario involves a NiCrBSi-cladded pump impeller operating in a chloride-containing process stream:
- Service conditions: 5% NaCl solution, pH 4, 60°C, 1 year of operation
- Failure mode: Pitting corrosion leading to perforation at the leading edge of the impeller vanes
- Root cause: Microgalvanic corrosion between carbides and matrix, accelerated by cavitation damage that removed the passive film
- Depth of corrosion: Maximum pit depth of 2.5 mm in a 4 mm overlay layer
Countermeasures and Mitigation
- Increase Cr content: Higher Cr content (>25 wt%) improves passive film stability but may reduce hardness.
- Reduce B and Si content: Lower boron and silicon reduce the volume fraction of carbides, decreasing microgalvanic couples.
- Add Mo or W: Molybdenum or tungsten additions improve pitting resistance by stabilizing the passive film.
- Use a composite overlay: Deposit a NiCrBSi layer for wear resistance, followed by a thin 316L or Inconel 625 cap layer for corrosion protection.
- Control sensitization: Avoid prolonged exposure to sensitization temperatures; if unavoidable, perform a solution treatment (1100–1150°C, water quench) followed by aging.
Key Reflections
The corrosion failure of NiCrBSi cladding layers is a textbook example of the trade-off between wear resistance and corrosion resistance. The very carbides that provide excellent wear resistance also create the microgalvanic couples that drive electrochemical corrosion. Engineers must carefully evaluate the service environment: if the primary threat is abrasive wear in a non-corrosive medium, NiCrBSi is an excellent choice; if the environment contains chlorides or acids, a different material strategy is required. The composite overlay approach—combining a NiCrBSi wear layer with a corrosion-resistant cap layer—offers the best of both worlds but requires careful process control to ensure adequate bonding between the dissimilar layers.