1. Purpose
To standardize the electroless nickel plating process, parameter control, and quality requirements, ensuring stable coating quality that meets customer and international standards.
2. Scope
Applicable to electroless nickel plating on carbon steel, stainless steel, and copper alloys. Not applicable to magnesium alloys, titanium alloys, or other special processes.
3. Normative References
- • GB/T 13913-2008 – Autocatalytic nickel-phosphorus alloy coatings
- • GB/T 6461-2002 – Rating of test specimens and articles of metallic and other inorganic coatings on metallic substrates after corrosion testing
- • ASTM B733-18 – Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings
- • ISO 4527:2003 – Metallic coatings – Autocatalytic (electroless) nickel‑phosphorus alloy coatings
- • GB/T 5270-2005 – Metallic coatings on metallic substrates – Electroplated and chemically deposited coatings – Adhesion test methods
4. Process Principle
Electroless nickel plating is an autocatalytic chemical deposition process in which sodium hypophosphite reduces Ni²⁺ ions to form a Ni-P alloy coating.
Reaction mechanism:
- • Anodic oxidation: H₂PO₂⁻ + H₂O → H₂PO₃⁻ + 2H⁺ + 2e⁻
- • Cathodic reduction: Ni²⁺ + 2e⁻ → Ni
- • Phosphorus co-deposition: H₂PO₂⁻ + 2H⁺ + e⁻ → P + 2H₂O
Coating classification by phosphorus content:
| Type | Phosphorus content |
| Low-P | 1–5% |
| Mid-P | 5–10% |
| High-P | 10–15% |
5. Process Flow
Inspection → Degreasing → Hot rinse → Flowing rinse → Pickling → Rinse → Activation → Rinse → DI rinse → Electroless plating → Recovery → 3-stage counterflow rinse → Passivation → DI rinse → Drying → Inspection → Packing
6. Key Process Parameters
| Parameter | Control range | Remarks |
| Ni²⁺ concentration | 8–12 g/L | Recommended 9–10 g/L |
| Sodium hypophosphite | 25–35 g/L | Molar ratio to Ni²⁺: 2.5–3.5:1 |
| Complexing agent (sodium citrate) | 30–40 g/L | Stabilizes nickel ions |
| Buffer (sodium acetate) | 15–25 g/L | Maintains pH stability |
| pH value | 4.5–5.5 (Mid/high-P) | Adjusted with ammonia or sodium carbonate |
| Temperature | 85–90℃ | Optimum 88±1℃ |
| Loading | 0.5–1.5 dm²/L | Higher loading reduces deposition rate |
| Deposition rate | 15–25 μm/h | Decreases with bath aging |
| Time | 15–60 min | Depending on required thickness |
| Agitation | Mechanical or low-pressure air | Workpiece movement or circulation recommended |
| Filtration | Continuous filtration, 5–10 μm rating | Removes particles, prevents pitting |
7. Product Specifications & Coating Capability
7.1 Workpiece Size Range
| Item | Parameter |
| Max. size | 1500×800×600 mm |
| Min. size | 5×5×5 mm |
| Max. weight | 200 kg |
| Min. weight | 1 g |
7.2 Coating Thickness Capability
| Type | Thickness range | Typical application |
| Precision parts | 5–10 μm | Threads, valve cores, hydraulic components |
| Standard parts | 10–50 μm | General corrosion & wear resistant parts |
| Thick coating | 50–100 μm | Wear repair, severe corrosive environments |
Uniformity: ±10% (general parts); ±15% (complex shapes)
7.3 Coating Properties
| Property | As-plated | Heat-treated (400℃×1h) | Remarks |
| Hardness (HV) | ≥500 HV | ≥800 HV | Heat treatment increases hardness |
| Adhesion | ≤ Grade 1 (GB/T 5270) | — | No peeling, no flaking |
| Salt spray test (NSS, 5% NaCl, 35℃) | ≥24 h (Mid-P) ≥72 h (High-P) |
— | Per customer requirement |
| Porosity | ≤1 point/cm² | — | Usually non-porous ≥25 μm |
| Resistivity | 50–100 μΩ·cm | 30–60 μΩ·cm | — |
Heat treatment: 400℃ ±10℃, 1 hour, air/furnace cool.
8. Production Capacity
| Item | Capacity |
| Single tank volume | 2–5 m³ |
| Batch weight | 500–1500 kg |
| Daily capacity | 3–8 tons |
| Monthly capacity | 80–200 tons |
Note: Actual capacity depends on geometry, thickness, bath aging.
9. Quality Inspection
9.1 Inspection Items
| Item | Method | Frequency | Acceptance criterion |
| Thickness | XRF / metallographic microscope | ≥3 points per fixture per batch | Meet drawing; max dev ≤10% |
| Hardness | HV0.1 microhardness | Once per shift | ≥500 HV (as-plated); ≥800 HV (HT) |
| Adhesion | File/bend/cross-cut+tape | 1 piece per batch | Grade ≤1, no peeling |
| Appearance | Visual (standard light) | 100% | Uniform bright; no defects |
| Salt spray | NSS 5% NaCl, 35℃ | Per batch/change | Mid-P ≥24 h; High-P ≥72 h |
| Phosphorus content | EDS / chemical | Weekly / new bath | 1–5% / 5–10% / 10–15% |
9.2 Appearance Classification
| Grade | Description |
| Grade A | Uniform bright/semi-bright, no defects |
| Grade B | Slight water/rack marks, no pits/peeling |
| Grade C | Slight dullness/color variation, functional OK |
| Reject | Pitting, peeling, skip plating, burning, severe color difference |
10. Troubleshooting
| Problem | Possible cause | Solution |
| Poor adhesion | Incomplete degreasing/pickling; insufficient activation | Improve pretreatment; extend activation |
| Pitting/roughness | Particles in bath; high pH precipitates | Enhance filtration (5 μm); lower pH |
| Dull/grey coating | Low Ni²⁺/hypophosphite; low pH/temp | Add chemicals; pH 4.8–5.2; heat to 88℃ |
| Skip plating | Local passivation; residual oil/oxide | Improve activation; re-degrease |
| Low deposition rate | Low temp/Ni²⁺/reducer; off pH; aged bath | Heat to 88–90℃; adjust pH; refresh bath |
| Bath decomposition | Overheating; high pH; fast addition; low loading | Stop heating; lower pH; dilute bath |
11. Safety
- • PPE: Acid/alkali gloves, goggles, apron, respirator (ammonia/acids).
- • Chemical storage: Segregate nickel sulfate, hypophosphite, acids, alkalis; away from heat/reducers.
- • Prohibitions: Do not mix conc. acid + hypophosphite (toxic phosphine). Dissolve solids in DI water first.
- • Ventilation: Local exhaust above tanks (hydrogen removal).
- • Emergency: Eyewash, shower, dry powder/CO₂ extinguisher, respirator, spill kit.
12. Environmental Protection
- • Wastewater: Ni wastewater separate; precipitate pH 9–10 + PAC/PAM; Ni <0.5 mg/L.
- • Hazardous waste: Spent bath, filters, activator, Ni sludge – licensed disposal with manifests.
- • Emissions: Continuous ventilation; monitor ammonia/hydrogen per GB 16297.
- • Energy/water: Insulate tanks; counterflow rinse; concentrated replenishment.
13. Bath Composition (Mid‑Phosphorus, General Parts)
| Chemical | Concentration | Function |
| NiSO₄·6H₂O | 25–35 g/L | Main salt, Ni²⁺ source |
| NaH₂PO₂·H₂O | 25–35 g/L | Reducing agent, P source |
| Na₃C₆H₅O₇·2H₂O | 30–40 g/L | Complexing agent |
| CH₃COONa·3H₂O | 15–25 g/L | pH buffer |
| Lactic/propionic acid | 5–10 mL/L | Accelerator |
| Stabilizer (thiourea) | 1–2 mg/L | Prevents decomposition |
Make‑up:
1. Dissolve NiSO₄, citrate, acetate in 1/3 DI water, 60℃.
2. Dissolve hypophosphite + lactic acid in another 1/3.
3. Combine, fill to volume, pH 4.8–5.2.
4. Add stabilizer, heat to 88℃, filter, trial plate.
14. Equipment & Inspection Capability
14.1 Main Equipment
| Equipment | Specification |
| Plating tank | PP/PVC lined, heated, temp control |
| Filtration | Acid‑resistant pump + housing, 5–10 μm, 1–3×/h |
| Temperature control | PID, ±1℃ |
| Drying oven | Hot air, 60–120℃ |
| Ultrasonic cleaner | 40 kHz, adjustable power |
| DI water | ≥10 MΩ·cm |
14.2 Inspection Equipment
| Equipment | Accuracy/Standard |
| XRF thickness | 0–200 μm, ±0.1 μm |
| Microhardness | 10–1000 gf, GB/T 4340.1 |
| Salt spray chamber | GB/T 10125, ≥200 L |
| Metallographic microscope | 100–1000×, micrometer |
| pH meter | ±0.02, temp compensation |
| Analytical balance | 0.01 g |
15. Documentation & Traceability
- • Production record: Part no., quantity, temp, pH, time, thickness, operator.
- • Bath log: Daily Ni²⁺, hypophosphite, pH; additions; filtration/cleaning.
- • Inspection record: Thickness, hardness, adhesion, appearance, salt spray.
- • Maintenance: Calibration, pump maintenance, descaling, fan checks (quarterly).
Retention: ≥3 years (≥10 years for auto/medical). Traceable to batch/bath cycle.
16. Supplementary Provisions
- • Reviewed annually by Technical Department.
- • Customer special requirements (salt spray, hardness, distribution, H‑embrittlement) documented and followed.
- • New products: Small trial first; mass production after passing all tests.
- • For production control and customer audit.
Revision History
| Rev | Date | Revision content | Approved by |
| V1.0 | 2024-xx-xx | Initial release | — |
| V2.0 | 2026-04-30 | English-only full revision | — |
Post time: May-18-2026