Hot-Dip Galvanizing: Protect Your Fasteners

Hot-dip galvanizing (also known as hot immersion galvanizing) is a high-temperature immersion process that forms a composite coating—consisting of a zinc-iron alloy layer and a pure zinc layer—on the surface of fasteners. With its triple anti-corrosion mechanism, it has become the preferred anti-corrosion solution for harsh environments.

Why Can Hot-Dip Galvanizing Effectively Protect Fasteners?

1. Triple Anti-Corrosion Mechanism

Physical Barrier:

The 50–100μm thick zinc layer acts like "metal armor," isolating the steel substrate from corrosive media (water, salt spray, acids, alkalis, etc.).
Sacrificial Anode Protection:

Zinc is more chemically active than iron. When the coating is damaged, zinc corrodes preferentially, protecting the underlying steel (following an electrochemical principle similar to "self-sacrifice").
Self-Healing Passivation Film:

Zinc reacts with air to form a dense film of basic zinc carbonate (ZnCO₃·Zn(OH)₂), which covers scratches and prevents corrosion from spreading.

2. Core Advantages Compared with Other Anti-Corrosion Processes

Performance Indicator	Hot-Dip Galvanizing	Electroplating / Mechanical Galvanizing
Adhesion	Metallurgical bonding via zinc-iron alloy layer, no peeling	Mechanical adhesion, easy peeling under impact
Anti-Corrosion Lifespan	10–30 years outdoors (salt spray resistance ≥ 1000 hours)	1/3–1/5 of hot-dip galvanizing lifespan
Adaptability to Complex Parts	Uniform zinc coating on threads and blind holes	Thin coating in recesses, prone to rust
Cost-Effectiveness	Cost: 2–5 CNY/kg, suitable for mass production	Higher cost for high-end processes: 10–20 CNY/kg

Which Fasteners Require Hot-Dip Galvanizing?

1. Selection by Application Scenario

Outdoor Infrastructure:

Steel structure bolts, tower foundation bolts (resistant to rain, UV radiation, and industrial pollution).
Transportation Sector:

Automobile chassis bolts (resistant to road salt spray), fasteners for ships/marine engineering (high-salt environments).
Industrial & Chemical Fields:

Pipe flange bolts, fasteners for sewage treatment equipment (resistant to acid and alkali corrosion).
Agriculture & Outdoor Equipment:

Agricultural machinery bolts (resistant to fertilizer corrosion), hardware for outdoor furniture (moisture-resistant).

2. Selection by Material Type

Carbon Steel Fasteners: Grade 4.8, 8.8, and 10.9 bolts/nuts (most common application).
Cast Iron Parts: Flanges, pipe clamps (fill surface pores to improve overall corrosion resistance).
High-Strength Steel Parts: Require additional "hydrogen relief treatment" (baking at 180–200°C) to prevent hydrogen embrittlement and fracture.

Key Points of Hot-Dip Galvanizing Process

1. Simplified Process Flow

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Degreasing & Derusting (Pickling) → Fluxing (Improve zinc bath wettability) → Hot-Dip Galvanizing (3–5 minutes in 450–480°C zinc bath) → Cooling → Passivation (Chromium-free processes are more eco-friendly)

2. 3 Key Quality Control Factors

Coating Thickness:

≥50μm for general environments; ≥85μm for harsh environments (e.g., coastal areas) (tested with a magnetic thickness gauge).
Appearance:

Silver-gray / dark gray, free of uncoated areas, bubbles, and zinc nodules (which affect assembly).
Adhesion:

No peeling in hammer or bending tests (ensuring firm bonding between the coating and substrate).

Common Issues and Solutions

Issue	Cause	Solution
Uncoated Spots (Black Spots)	Incomplete pickling, residual rust	Extend pickling time or replace pickling solution
Excessively Thick Zinc Layer	Prolonged stay of workpiece when exiting the zinc bath	Optimize conveyor speed; shake off excess zinc via vibration
Surface Darkening	Excessively high zinc bath temperature or insufficient aluminum	Control temperature at 450–480°C; add 0.1% aluminum
Hydrogen Embrittlement Risk	Hydrogen absorption by high-strength steel during pickling	Bake at 180°C for 2 hours after galvanizing to relieve hydrogen

V. Environmental Protection and Sustainability

Chromium-Free Passivation:

Replaces traditional hexavalent chromium processes with trivalent chromium or silane treatment, complying with RoHS/REACH standards.
Recycling:

Zinc dross can be recycled and remelted; scrapped fasteners can be fully recycled in furnaces, with zinc and iron separated for reuse.

VI. How to Select a Hot-Dip Galvanizing Supplier?

Check Qualifications:

Ensure the supplier has ISO 1461 (hot-dip galvanizing standard) and ISO 9001 certifications, and can provide coating thickness test reports.
Inspect Samples:

Request salt spray test reports (e.g., no red rust after 1000 hours) and check the coating uniformity of complex parts (e.g., internal threads).
Clarify Requirements:

Specify the environmental class (e.g., C3–C5 corrosion categories) and whether a sealant (e.g., varnish to improve weather resistance) is needed.

Conclusion

With its long-lasting protection, high cost-effectiveness, and strong adaptability, hot-dip galvanizing has become the "gold standard" for fastener anti-corrosion. Whether it is for cross-sea bridges exposed to sea breeze erosion or precision bolts hidden in chemical equipment, hot-dip galvanizing can provide metal parts with "anti-corrosion armor" through scientific process design, ensuring reliable connections stand the test of time.

Choosing hot-dip galvanizing is like investing in "anti-corrosion insurance" for your fasteners!

Supplementary Notes on Key Terms

Hydrogen embrittlement: A phenomenon where hydrogen atoms penetrate high-strength steel during pickling, leading to reduced ductility and sudden fracture under stress. Hydrogen relief baking drives out excess hydrogen to mitigate this risk.
Corrosion categories (C3–C5): Defined by ISO 12944, they classify environmental corrosion severity: C3 (urban/industrial areas), C4 (coastal/chemical industrial areas), C5 (severe marine/industrial environments).
RoHS/REACH: International environmental standards. RoHS restricts hazardous substances (e.g., lead, hexavalent chromium) in electronics; REACH regulates the use of harmful chemicals in industrial products.

Hot-Dip Galvanizing: Protect Your Fasteners