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M18 A4-70 Nut
QS Fastener: M18 A4-70 Nut
Name: M18 A4-70 Nut
Standard: DIN 934
Material: Stainless Steel
Dimension: M18
Type: Hex Nut
Standard: DIN 934
Material: Stainless Steel
Dimension: M18
Type: Hex Nut
Quantity
M18 A4-70 Nut – Technical Specification & Passivation Process
The M18 A4-70 Nut strictly complies with the DIN 934 standard and is made of A4-70 (304) stainless steel, offering excellent corrosion resistance and mechanical strength (tensile strength: 700 MPa). Designed for high-strength connections, its hexagonal structure ensures stable wrench torque and secure fastening. The surface undergoes passivation treatment for enhanced acid and alkali resistance, making it ideal for demanding environments such as chemical processing, marine applications, food machinery, and outdoor installations. Custom non-standard options are available, with original material certifications provided, ensuring reliability in industrial fastening solutions.
The M18 A4-70 Nut strictly complies with the DIN 934 standard and is made of A4-70 (304) stainless steel, offering excellent corrosion resistance and mechanical strength (tensile strength: 700 MPa). Designed for high-strength connections, its hexagonal structure ensures stable wrench torque and secure fastening. The surface undergoes passivation treatment for enhanced acid and alkali resistance, making it ideal for demanding environments such as chemical processing, marine applications, food machinery, and outdoor installations. Custom non-standard options are available, with original material certifications provided, ensuring reliability in industrial fastening solutions.
1. Principle and Function of Passivation
Passivation is a chemical process that forms a dense chromium oxide (Cr₂O₃) layer on stainless steel surfaces, significantly improving corrosion resistance. For the M18 A4-70 Nut, its AISI 304 material contains 18% chromium (Cr), providing inherent corrosion resistance. However, during manufacturing processes (e.g., cold forging, machining), residual iron particles or sulfides may remain, compromising the passive film and creating weak points for electrochemical corrosion. Nitric acid passivation addresses this through:
- Free Iron Removal: HNO₃ dissolves surface iron particles (Fe⁰), preventing galvanic corrosion (pitting).
- Chromium Oxide Enrichment: Nitric acid selectively oxidizes chromium, forming a 3-5 nm Cr₂O₃ layer, enhancing the M18 A4-70 Nut's stability in salt spray environments.
- Surface Homogenization: Eliminates machining defects (e.g., scratches), ensuring uniform corrosion resistance.
Passivation is a chemical process that forms a dense chromium oxide (Cr₂O₃) layer on stainless steel surfaces, significantly improving corrosion resistance. For the M18 A4-70 Nut, its AISI 304 material contains 18% chromium (Cr), providing inherent corrosion resistance. However, during manufacturing processes (e.g., cold forging, machining), residual iron particles or sulfides may remain, compromising the passive film and creating weak points for electrochemical corrosion. Nitric acid passivation addresses this through:
- Free Iron Removal: HNO₃ dissolves surface iron particles (Fe⁰), preventing galvanic corrosion (pitting).
- Chromium Oxide Enrichment: Nitric acid selectively oxidizes chromium, forming a 3-5 nm Cr₂O₃ layer, enhancing the M18 A4-70 Nut's stability in salt spray environments.
- Surface Homogenization: Eliminates machining defects (e.g., scratches), ensuring uniform corrosion resistance.
2. Passivation Process (Example for M18 A4-70 Nut)
Step 1: Degreasing & Cleaning
- Alkaline solution (pH 10–12) at 60–80°C for 5–10 minutes to remove forging oils.
Step 1: Degreasing & Cleaning
- Alkaline solution (pH 10–12) at 60–80°C for 5–10 minutes to remove forging oils.
Step 2: Acid Activation
- Immersion in 10–15% HNO₃ + 2–5% HF for 1–3 minutes.
- Purpose: Dissolves surface oxides (e.g., Fe₂O₃) and exposes fresh chromium.
- Immersion in 10–15% HNO₃ + 2–5% HF for 1–3 minutes.
- Purpose: Dissolves surface oxides (e.g., Fe₂O₃) and exposes fresh chromium.
Step 3: Nitric Acid Passivation
- Solution: 20–30% HNO₃, 20–40°C, 20–30 minutes (adjusted per M18 A4-70 Nut size).
- Key Parameters:
- HNO₃ concentration: <15% risks incomplete film; >35% may cause over-etching.
- Temperature & Time: >50°C accelerates reaction but may form porous films.
- Solution: 20–30% HNO₃, 20–40°C, 20–30 minutes (adjusted per M18 A4-70 Nut size).
- Key Parameters:
- HNO₃ concentration: <15% risks incomplete film; >35% may cause over-etching.
- Temperature & Time: >50°C accelerates reaction but may form porous films.
Step 4: Neutralization & Rinsing
- Rinse with deionized water; neutralize residual acid with a weak alkaline solution (NaHCO₃, pH 8–9) for 1–2 minutes.
- Rinse with deionized water; neutralize residual acid with a weak alkaline solution (NaHCO₃, pH 8–9) for 1–2 minutes.
Step 5: Drying & Quality Control
- Drying at ≤100°C to protect the passive film.
- Testing:
- Copper Sulfate Test (ASTM A380): No red copper precipitation within 5 minutes = pass.
- Salt Spray Test (ASTM B117): Passivated M18 A4-70 Nut must withstand ≥500 hours without red rust.
- Drying at ≤100°C to protect the passive film.
- Testing:
- Copper Sulfate Test (ASTM A380): No red copper precipitation within 5 minutes = pass.
- Salt Spray Test (ASTM B117): Passivated M18 A4-70 Nut must withstand ≥500 hours without red rust.
3. Performance Enhancements from Passivation
- Salt Spray Resistance: Improved from 72 to >500 hours.
- Pitting Resistance (ASTM G48): Breakdown potential (Eb) increases by 200–300 mV, delaying chloride-induced pitting.
- Surface Resistance: Cr₂O₃ film raises resistivity to 10⁴–10⁶ Ω·cm, reducing galvanic corrosion risks.
- Salt Spray Resistance: Improved from 72 to >500 hours.
- Pitting Resistance (ASTM G48): Breakdown potential (Eb) increases by 200–300 mV, delaying chloride-induced pitting.
- Surface Resistance: Cr₂O₃ film raises resistivity to 10⁴–10⁶ Ω·cm, reducing galvanic corrosion risks.
4. Process Considerations
1) Isolation from Carbon Steel Equipment: Prevent iron ion contamination during M18 A4-70 Nut passivation.
2) Wastewater Treatment: Neutralize nitric acid effluent with lime (Ca(OH)₂) to pH 6–9 before disposal.
1) Isolation from Carbon Steel Equipment: Prevent iron ion contamination during M18 A4-70 Nut passivation.
2) Wastewater Treatment: Neutralize nitric acid effluent with lime (Ca(OH)₂) to pH 6–9 before disposal.
3) Specialized Adaptations:
- Marine Environments: Add potassium dichromate (K₂Cr₂O₇) to seal film pores.
- Food-Grade Use: Ensure heavy metal residues (Pb, Cd) <0.01 ppm (FDA 21 CFR 175.300 compliant).
- Marine Environments: Add potassium dichromate (K₂Cr₂O₇) to seal film pores.
- Food-Grade Use: Ensure heavy metal residues (Pb, Cd) <0.01 ppm (FDA 21 CFR 175.300 compliant).
5. Comparison with Alternative Processes
| Process | Advantages | Disadvantages |
|------------------|-----------------------------|----------------------------|
| Nitric Passivation | Low cost, uniform film | Ineffective for carbon steel |
| Electro-Polishing | Superior surface finish | High energy consumption |
| Electroless Nickel | Excellent wear resistance | Phosphorus waste challenges |
|------------------|-----------------------------|----------------------------|
| Nitric Passivation | Low cost, uniform film | Ineffective for carbon steel |
| Electro-Polishing | Superior surface finish | High energy consumption |
| Electroless Nickel | Excellent wear resistance | Phosphorus waste challenges |
Conclusion: Nitric acid passivation is the most cost-effective and efficient surface treatment for M18 A4-70 Nut and other high-strength stainless steel fasteners.
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