Turbine Component Crevice Corrosion Resistant Weld Overlay Technology
1. Definition and Technical Principles
Crevice corrosion is a localized form of corrosion attack that occurs in confined, stagnant zones where aggressive species accumulate and oxygen depletion drives active-passive cell formation. In steam turbine assemblies, crevice corrosion manifests at gland seals, shaft sleeves, bolted flange joints, bearing housings, and threaded fastener interfaces—areas where tight clearances trap moisture, chloride-laden condensate, and acidic deposits. The weld overlay technology described in this capability entry addresses this degradation mechanism by depositing a corrosion-resistant alloy layer onto base turbine components (typically carbon steel, low-alloy steel, or austenitic stainless steel) using qualified TIG or MIG welding procedures.
The fundamental principle relies on introducing a barrier layer of high-nickel, high-chromium, or duplex stainless alloy (e.g., 309L, 316L, 625, 2205, or Inconel 718) that raises the pitting resistance equivalent number (PREN) of the surface beyond the threshold required to resist chloride-induced crevice initiation. The overlay metallurgy creates a thermodynamically stable passive film in the crevice environment, preventing the autocatalytic dissolution cycle that drives crevice corrosion.
2. Category and Business Positioning
This technology sits within the TIG/MIG Weld Overlay route of the company's three principal technology platforms. It represents a high-value-added, precision overlay service targeting power generation equipment manufacturers and operators who face unplanned downtime due to turbine component corrosion. The business positioning is as follows:
- Industry segment: Thermal power generation, cogeneration, industrial steam systems, and petrochemical turbine auxiliaries
- Component scope: Shaft sleeves, gland rings, diaphragm seals, casing flanges, bolted joints, and bearing pedestals
- Service model: In-house shop fabrication, on-site field repair, and OEM pre-overlay for new-build turbine assemblies
- Competitive differentiation: WPS qualification specifically for crevice corrosion resistance, coupled with NACE/ISO 9001 quality management and full NDT traceability
3. Technical Purpose and Value
The primary purpose of this overlay technology is to extend the service life of turbine components exposed to aggressive condensate, flue gas, and cooling water environments by providing a durable, metallurgically bonded corrosion barrier at crevice-prone geometries. The value proposition includes:
- Reliability enhancement: Reduction of unplanned turbine outage events caused by crevice corrosion failure of seals, glands, and fasteners
- Cost avoidance: Elimination of premature component replacement and associated production losses
- Specification compliance: Meeting OEM requirements for corrosion-resistant overlay on turbine parts per manufacturer welding procedures
- Sustainability: Extension of component life cycles, reducing material consumption and waste
4. Key Process and Implementation Points
4.1 Base Material Preparation
Proper surface preparation is critical to ensuring metallurgical bond integrity and overlay performance. The following steps are mandatory:
- Removal of existing coatings: Shot blasting to SA 2.5 per ISO 8501-1, with surface profile 40–75 μm
- Geometric verification: Confirmation of fit-up tolerances, especially at crevice interfaces where overlay must not alter assembly clearances
- Preheat application: Localized preheat to 100–250°C depending on base alloy carbon equivalent and thickness
- Contamination control: Removal of oil, grease, and chloride residues using approved solvents; no chlorinated cleaning agents permitted
4.2 Overlay Welding Parameters
| Parameter | TIG Overlay (GTAW) | MIG Overlay (GMAW) |
|---|---|---|
| Shielding gas | Argon 100% or Ar/He mix (80/20) | Ar/CO₂ (98/2) or Ar/O₂ (99/1) |
| Welding current | 80–150 A (DC) | 150–250 A (DC+) or pulsed |
| Travel speed | 30–60 mm/min | 150–300 mm/min |
| Wire diameter | 2.4–3.2 mm | 1.2–1.6 mm |
| Interpass temperature | ≤ 150°C (controlled via IR pyrometer) | ≤ 200°C |
| Typical overlay thickness | 1.5–3.0 mm (single or multi-pass) | 2.0–4.0 mm (multi-pass) |
| Typical dilution control | < 25% base metal dilution (verified by optical emission spectroscopy) | < 30% base metal dilution |
4.3 Overlay Material Selection Matrix
| Service Environment | Recommended Overlay Alloy | PREN (Pitting Resistance Equivalent Number) | Standards Reference |
|---|---|---|---|
| Hot condensate with chlorides (< 150°C) | ASTM A240 316L / ER316L | 24–25 | ASTM A554 / AWS A5.9 |
| Aggressive condensate with chlorides (> 150°C) | ASTM A240 317L / ER317L | 27–29 | ASTM A554 / AWS A5.9 |
| High-chloride, oxidizing environment | ASTM B366 UNS N06625 / ERNiCrMo-3 | 36–40 | ASTM B366 / AWS A5.14 |
| Dual-phase requirement (strength + corrosion) | ASTM A240 2205 / ER2209 | 34–36 | ASTM A564 / AWS A5.9 |
| Transition layer (carbon steel to austenitic) | ASTM A554 309L / ER309L | 19–22 | ASTM A554 / AWS A5.9 |
4.4 Multi-Pass Strategy for Crevice Geometries
Crevice-prone turbine geometries—such as labyrinth seals, gland packing grooves, and stepped shaft journals—require careful multi-pass planning. The recommended approach includes:
- Pass 1 (Bond/Transition): ER309L or ER310L deposited at 0.5–1.0 mm thickness to bridge carbon steel base to austenitic overlay
- Pass 2–N (Build-up): Final overlay alloy applied in 1.0–1.5 mm increments per pass, with each pass fully penetrating the previous layer to minimize dilution
- Final pass (Surface finish): A thin, smooth cap pass applied with reduced current to achieve Ra ≤ 3.2 μm surface finish, critical for crevice resistance
4.5 Post-Weld Treatment
- Solution heat treatment: 1050–1100°C for 30–60 minutes followed by water quench (for Ni-base alloys); or 1050°C/1h + air cool for austenitic stainless overlays
- Pickling and passivation: Per ASTM A380 or AMS 2700, using citric acid or nitric/hydrofluoric acid blend to remove heat tint and restore passive film
- Dimensional verification: Coordinate measuring machine (CMM) or laser scanning to confirm overlay thickness uniformity and dimensional compliance with OEM drawings
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- WPS/PQR qualification: ASME Section IX, Part QW-400 through QW-460 (GTAW and GMAW); AWS D10.9M/D10.9 (Welding Procedure Qualification for Overlay)
- Welding execution: AWS D8.1M/D8.1 (Recommended Practices for Welding Dissimilar Metals); ISO 14732 (Welding — Welding Procedure Qualification)
- Quality management: ISO 9001:2015; NACE MR0175/ISO 15156 (where applicable for sour service turbines)
- NDT requirements: ASME Section V, Articles 2, 4, 6, 7, 16; ASTM E1417 (PT), ASTM E2318 (EMAT), ASTM E3095 (UT for overlay thickness)
- Corrosion testing: ASTM G63 (Crevice Corrosion of Stainless Steels in Immersion Tests); ASTM G28 (Electrochemical Corrosion Testing); NACE TM0169 (Intergranular Corrosion Testing)
- Product standards: ASTM A554 (Welding Overlay Clad Plate); ASTM B366 (Nickel-Alloy Strip); GB/T 4237 (Chinese standard for stainless steel plates)
5.2 Acceptance Criteria
| Inspection Method | Acceptance Criteria | Reference Standard |
|---|---|---|
| Visual Inspection (VT) | No cracks, undercut > 0.25 mm, porosity > 1.0 mm, or surface discontinuities | ASME Section V, Article 9 |
| Penetrant Testing (PT) | No linear indications ≥ 1.6 mm; no clustering of round indications | ASME Section V, Article 7 / ASTM E165 |
| Magnetic Particle Testing (MT) | No linear indications ≥ 1.6 mm in ferromagnetic base/overlay | ASME Section V, Article 7 / ASTM E709 |
| Ultrasonic Testing (UT) | No lack-of-bond indications; overlay thickness verified within ±0.5 mm | ASME Section V, Article 5 / ASTM E3095 |
| Macrographic Examination | No centerline cracking, no excessive dilution (> 30%), uniform microstructure | ASTM E381 |
| Chemical Composition | Overlay composition within ±0.5% of nominal for Cr, Ni, Mo | ASTM E415 / Optical Emission Spectroscopy |
| Crevice Corrosion Test | No pitting or crevice attack after 30 days in ASTM G63 test solution | ASTM G63 |
6. Common Risks and Controls
| Risk | Mechanism | Control Measure |
|---|---|---|
| Excessive dilution | High thermal input or single-pass deposition dilutes overlay alloy below required PREN | Multi-pass strategy with reduced interpass temperature; OES verification after each pass |
| Intergranular sensitization | Carbon precipitation at grain boundaries in the heat-affected zone (HAZ) of 304/316 overlay | Use of low-carbon alloys (309L, 316L, 317L); solution heat treatment post-weld | Cracking in HAZ | Residual stress combined with carbon steel HAZ microstructure transformation | Controlled preheat and interpass temperature; post-weld stress relief per ASME Section IX QW-407 |
| Geometric interference | Overlay thickness exceeds assembly clearance tolerance, preventing turbine component fit-up | Pre-weld dimensional planning; maximum overlay thickness specified in WPS; post-weld machining allowance |
| Porosity from moisture | Hydrogen absorption from wet flux, contaminated surfaces, or humid shielding gas | Gas cylinder dew point monitoring (≤ -40°C); surface drying; preheat for hydrogen embrittlement prevention |
| Crevice corrosion at overlay edge | Discontinuity at overlay terminus creates a new crevice site | Gradual taper of overlay terminus; full encapsulation of base metal edge; PT inspection of overlay boundaries |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
This is the principal technology route for turbine component crevice corrosion protection. TIG (GTAW) is preferred for thin overlays on precision turbine shafts, gland rings, and labyrinth seals where dimensional control is paramount. MIG (GMAW) is employed for heavier build-up on casing flanges, bearing housings, and large-diameter shaft sleeves where productivity is prioritized. The WPS qualification program for this technology includes:
- Procedure qualification per AWS D10.9M for overlay welding on carbon steel, low-alloy steel, and austenitic stainless base materials
- Welder performance qualification per ASME Section IX Part QW-300/QW-400
- Non-destructive examination program covering VT, PT, MT, UT, and dimensional verification
- Corrosion resistance validation per ASTM G63 for each overlay alloy/base metal combination
7.2 Hydraulic Explosive Bonding (Complementary Route)
While hydraulic explosive bonding is not directly applicable to turbine component repair, it serves as a complementary technology for manufacturing clad turbine components. For example, large turbine casing plates can be fabricated as explosion-bonded clad steel (e.g., 2205 duplex on carbon steel per ASTM A491) and then machined to final dimensions. The crevice corrosion protection is then achieved through the inherent properties of the bonded cladding layer, supplemented by localized TIG weld overlay at bolted joint interfaces and crevice-prone geometries. This hybrid approach combines the large-area coverage of explosive bonding with the precision of weld overlay.
7.3 Explosion Welding (Complementary Route)
Explosion welding provides an alternative for producing clad turbine shaft blanks and sleeve stock. A typical configuration involves explosion-bonding of a 2205 or 625 alloy strip onto a carbon steel shaft bar, followed by machining to final turbine shaft dimensions. The explosion-bonded interface provides a metallurgically sound, dilution-free cladding layer that resists crevice corrosion throughout the shaft's service life. Localized crevice-prone areas (gland grooves, seal surfaces) receive additional TIG weld overlay as a secondary protection measure. This approach is particularly valuable for OEM pre-overlay programs where new turbine shafts are delivered with factory-applied corrosion protection.
8. Qualification Building and Customer Value
This technology entry represents a critical qualification asset for the company's power generation business development. The WPS and PQR documentation generated through this program enables the company to:
- Qualify for OEM supply chains: Demonstrate compliance with major turbine OEM requirements (GE, Siemens Energy, Mitsubishi Power, Dongfang Electric) for corrosion-resistant overlay on turbine components
- Enter after-market repair services: Offer competitive turbine component repair and refurbishment services to power plant operators facing crevice corrosion challenges
- Support new-build projects: Provide pre-overlay services for new turbine assemblies specified with corrosion-resistant protection, particularly for units operating in coastal or high-chloride environments
- Build regulatory confidence: Full ASME Section IX and AWS D10.9M qualification documentation provides the regulatory and insurance framework required for critical turbine component modification
The customer value proposition is quantifiable: a single turbine outage event due to crevice corrosion failure can cost $500,000–$2,000,000 in lost generation, emergency repair, and replacement parts. The overlay technology extends component life by 5–10 years, delivering a return on investment within the first major overhaul cycle. For power plant operators managing aging turbine fleets in aggressive environments, this technology provides a cost-effective, standards-compliant solution that preserves asset reliability and operational availability.
9. Conclusion
The Turbine Component Crevice Corrosion Resistant Weld Overlay Technology represents a specialized, high-value capability within the company's TIG/MIG weld overlay portfolio. By combining qualified welding procedures, carefully selected overlay alloys, rigorous NDT protocols, and validated corrosion resistance testing, this technology delivers a proven solution for one of the most persistent degradation mechanisms in steam turbine service. The qualification framework established through this program—spanning WPS/PQR documentation, welder certification, NDT procedures, and corrosion test validation—provides the technical and regulatory foundation for market entry into turbine OEM supply chains and after-market repair services. As the power generation industry faces increasing demands for reliability and asset life extension, this technology positions the company as a trusted partner for critical turbine component corrosion protection.