In which scenarios are black-oxidized screws suitable for use?

I. Core Performance Limitations and Applicable Boundaries

1. Extremely Weak Corrosion Resistance

• Salt spray test lasts only 24-72 hours. The oxide film is easily damaged by water, salt spray, sweat, etc., exposing the base material to rapid rusting.
• Comparison: In the same environment, the service life of electro-galvanized screws is 3-5 times longer than black-oxidized ones.

2. Low Wear Resistance

• The hardness of the oxide film is only 100-150HV, lower than galvanized coatings (80-120HV) and chrome plating (500HV+). It is prone to peeling off under friction or impact.
• Application Limitations: Unsuitable for frequently disassembled, high-vibration, or high-friction scenarios (e.g., mechanical moving parts).

3. No Hydrogen Embrittlement Risk but No Strength Enhancement

• The process does not involve acid etching or electroplating, making it suitable for high-strength steel screws (e.g., grade 10.9 and above) to avoid hydrogen embrittlement. However, it does not enhance the base material's strength.

II. 5 Typical Application Scenarios for Black-Oxidized Screws

1. Non-Critical Components in Dry Indoor Environments

• Typical Scenarios:
  • Furniture assembly (e.g., screws for tables, chairs, and wardrobes): Can remain rust-free for 1-2 years in dry indoor environments.
  • Internal non-exposed screws in appliances (e.g., TV and computer cases): Prevents metal burrs without requiring long-term rust protection.
  • Temporary installations (e.g., exhibition equipment, short-term structures): Typically used for less than 1 year.
• Advantages: Low cost (approximately $0.01-$0.03 per screw), simple process, and no impact on appearance (the black oxide film hides screw marks).

2. Decorative Scenarios Requiring Uniform Aesthetics

• Typical Scenarios:
  • High-end furniture/lighting: The black oxide film matches dark wood or metal frames, enhancing overall aesthetics (e.g., Nordic-style furniture).
  • Art installations/models: Model screws need to match the main body's color; black oxidation achieves a matte black effect more easily than galvanizing.
• Note: Must be paired with antirust oil or varnish (e.g., vaseline); otherwise, decorative parts will still rust slowly when exposed to air.

3. Precision Instruments Requiring Slight Friction Reduction

• Typical Scenarios:
  • Tiny screws in clocks, cameras, and other precision machinery: The oxide film slightly reduces the thread friction coefficient (about 10% lower than bare steel), facilitating fine adjustment during installation.
  • Internal screws in optical devices: Black oxidation eliminates reflections, avoiding interference with light paths (e.g., microscopes, telescopes).
• Limitations: Only suitable for static, low-load scenarios; the oxide film easily wears off in high-speed or vibrating environments.

4. Workpieces Requiring Magnetic Particle Inspection

• Specialized Scenarios:
  • Components requiring magnetic particle flaw detection in aerospace or pressure vessels: The black oxide film does not interfere with magnetic particle adsorption, allowing direct use as a pretreatment for detection.
  • Temporary fasteners in military equipment: Facilitates detection operations more easily than coatings in scenarios requiring regular inspection for substrate cracks.

5. Low-Cost Consumables for Laboratories/Education

• Typical Scenarios:
  • School physics/chemistry experiment setups (e.g., temporary circuit brackets, lab bench fixtures): Disposable after use, no need for long-term durability.
  • R&D sample test parts: Screws in new product prototypes are only for functional verification and may be replaced with other processes during mass production, reducing trial costs.

III. 5 Scenarios Where Black Oxidation is Absolutely Unsuitable

IV. 3 Tips to Extend the Service Life of Black-Oxidized Screws

1. Apply Post-Coating Protection
   • Smear grease or molybdenum disulfide lubricant: Extends rust protection to 1-2 years in dry environments (e.g., indoor mechanical bearing screws).
   • Spray transparent electrophoretic paint or epoxy resin: Forms a composite protective layer, increasing salt spray resistance to over 200 hours (suitable for decorative scenarios).
2. Control Service Cycle
   • Mark replacement dates: Label installation dates to ensure replacement with higher-protection screws within 1 year (6 months in humid environments).
3. Select Appropriate Base Materials
   • Prioritize low-carbon steel (e.g., Q235): High-carbon steel (e.g., 45# steel) is more prone to substrate rusting, causing oxide film peeling and shorter lifespan.

V. Cost-Life Balance Recommendations

• Environments with humidity >60% or potential liquid contact;
• Screws bearing structural loads or requiring frequent disassembly;
• Expected service life exceeding 1 year.