details on a specific fastener type or standard

Alternative Coatings (Preventive Approach) Zinc Flake Coatings (e.g., Dacromet, Geomet): No hydrogen generated during application. Mechanical Galvanizing: Zinc particles cold-welded to the fastener (no acid pickling or electroplating).

 

Author: Robby

Detailed breakdown of hydrogen removal methods for fasteners, categorized by fastener type, industry standards, and best practices:

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1. High-Strength Steel Fasteners (Grades 8.8, 10.9, 12.9, ASTM A490, etc.)

Risk: High susceptibility to hydrogen embrittlement due to hardness (>32 HRC).
Solution:

• Baking (Preferred Method)

  • Temperature: 190–230°C (375–450°F)

  • Duration: 8–24 hours (longer for thicker coatings or higher hardness).

  • Timing: Must be done within 1–4 hours after plating to prevent hydrogen diffusion deeper into the steel.

  • Standards:

    • ASTM B849 (Pre-treatment for electroplating)

    • ASTM B850 (Post-plating baking)

    • ISO 9587 (Hydrogen embrittlement relief for high-strength fasteners)

• Alternative Coatings (Preventive Approach)

  • Zinc Flake Coatings (e.g., Dacromet, Geomet): No hydrogen generated during application.

  • Mechanical Galvanizing: Zinc particles cold-welded to the fastener (no acid pickling or electroplating).

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2. Electroplated Fasteners (Zinc, Cadmium, etc.)

Risk: Acid pickling & electroplating introduce hydrogen.
Solution:

• Baking Parameters:

  • Cadmium-Plated: 190–220°C for 8–24 hrs (per AMS-QQ-P-416).

  • Zinc-Plated: 200–220°C for 4–12 hrs (per ASTM F1941).

• Delayed Baking Risk: If baking is delayed >4 hrs, hydrogen may migrate deeper, making removal harder.

• Low-Hydrogen Plating Processes:

  • Alkaline Zinc Plating: Generates less hydrogen than acid zinc.

  • Trivalent Chromium Passivation: Reduces hydrogen vs. hexavalent chromate.

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3. Case-Hardened or Carburized Fasteners

Risk: Hydrogen absorption during heat treatment.
Solution:

• Low-Temperature Baking (150–200°C for 4–8 hrs) to avoid tempering effects.

• Vacuum Heat Treating (Preventive): Reduces hydrogen exposure during hardening.

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4. Critical Aerospace & Automotive Fasteners

Standards:

• AMS 2759/9: Baking for cadmium-plated aircraft fasteners.

• SAE J2339: Hydrogen embrittlement testing for automotive fasteners.

• NASA & Military Specs (e.g., MIL-STD-1312): Require baking + testing (e.g., sustained load test).

Testing Methods:

• Notched Tensile Test (ASTM F1624): Measures embrittlement susceptibility.

• Bent Beam Test (ASTM F326): Evaluates hydrogen cracking resistance.

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5. When Baking Isn’t Possible (Field Repairs or Large Fasteners)

• Room Temperature Aging: 48–72 hrs (less reliable but used in low-criticality cases).

• Hydrogen-Diffusion Coatings: Sealants that allow gradual hydrogen escape.

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Key Mistakes to Avoid

❌ Delaying baking >4 hrs after plating (hydrogen penetrates deeper).
❌ Overheating (>250°C) – Can temper high-strength bolts, reducing hardness.
❌ Skipping hydrogen testing for critical applications (e.g., bridges, aircraft).