Analysis Of Causes For Grayish Color in Hot-Dip Galvanized Bolts

Users have inquired about the grayish appearance of hot-dip galvanized bolts they previously purchased, wondering if it indicates a quality issue. Hot-dip galvanized bolts typically exhibit a matte silver-gray surface, distinct from the bright finish of electro-galvanized bolts. However, excessive grayness should be analyzed by considering process, material, and environmental factors. Below is a professional breakdown of key influences:

1. Impact of Zinc Bath Composition and Process Control

1.1 Inadequate Zinc Purity

Standard hot-dip galvanization requires zinc ingots with ≥99.99% purity (Grade 0 zinc). Adding zinc dross (containing FeZn₇ impurities), recycled zinc, or industrial waste lowers purity, causing iron content in the bath to exceed the normal limit (＞0.02% vs. standard ＜0.003%). Excessive iron promotes the formation of coarse Fe-Zn alloy layers on the bolt surface, reducing light reflectivity and resulting in a dull gray or blackish appearance. This indicates a quality issue, often accompanied by loose zinc coating and poor adhesion.

1.2 Abnormal Bath Temperature

The optimal hot-dip galvanization temperature range is 450–480°C:

 

Temperatures above 490°C: Intensified oxidation produces excessive zinc oxide scum, which adheres to the bolt surface as a porous layer, leading to gray discoloration.

Temperatures below 440°C: Incomplete zinc wetting causes uneven dark spots and weak coating adhesion.

2. Influence of Bolt Material and Heat Treatment

Different bolt grades exhibit color variations due to chemical composition and heat treatment, resulting in naturally occurring color differences within acceptable ranges (using 8.8 grade bolts as a reference):

 

Bolt Grade	Material Properties	Zinc Coating Formation	Color Appearance	Nature of Variation
4.8 Grade	Low-carbon steel (C≤0.25%), unheat-treated, high surface activity	Rapid zinc wetting and coarse alloy layer growth	Dark gray (matte and dull)	Normal process-related effect
8.8 Grade	Medium-carbon steel (C=0.25–0.55%), quenched and tempered, dense surface	Uniform alloy layer with fine crystal structure	Silver-gray (standard matte)	Typical normal state
10.9 Grade	Medium-carbon alloy steel (with Mn, Cr), quenched and tempered, high surface hardness	Moderate reaction with zinc, higher proportion of pure zinc layer	Light gray (slight metallic luster)	Optimal process compatibility

 

Mechanism Explanation:

 

4.8-grade bolts, unheat-treated, have highly reactive surfaces that form thick Fe-Zn alloy layers (Fe content 6–11%), reducing light reflectivity.

10.9-grade bolts, with hard surfaces, inhibit excessive alloying, retaining more pure zinc (Zn content ＞98%), thus showing higher luster.

3. Effects of Post-Treatment and Environmental Factors

3.1 Lack of Passivation Treatment

Bolts without passivation (e.g., trivalent chromium blue passivation) develop a natural zinc oxide film in air, reducing gloss by ~30% within 24 hours. This "dusty gray" appearance is a normal oxidation resistance process, not a quality defect.

3.2 Storage Environment Impact

In humid environments (humidity ＞60%), zinc surfaces form basic zinc carbonate white rust (initially white mist, turning grayish in severe cases). Unlike grayness from contaminated zinc baths, this rust can be mechanically removed without coating detachment.

4. Quality Assessment Standards and Recommendations

4.1 Signs of Abnormal Grayness (Quality Concerns)

Large-area dark gray patches with no metallic sheen

Powder-like residue after wiping (indicating poor coating adhesion)

Color difference ＞NBS 3 grade among same-batch, same-grade bolts (measured by color difference meter)

4.2 Professional Testing Methods

Zinc Coating Thickness: Magnetic thickness gauge test ≥85μm (complying with GB/T 13912 Class II standard)

Composition Analysis: X-ray fluorescence spectroscopy to verify Fe content < 0.005% in the zinc layer

Adhesion Test: Hammer test to ensure no coating peeling or bulging

4.3 Procurement Recommendations

Request furnace-specific test reports from suppliers (including coating thickness, composition, and adhesion data)

Keep standard samples for comparison to avoid misjudgment due to cross-grade color differences (e.g., 4.8 vs. 10.9 grade)

Conclusion

The primary reasons for grayish color in hot-dip galvanized bolts are:

 

Abnormal Grayness: Caused by contaminated zinc baths, temperature deviations, or missing post-treatment-indicates quality issues.

Natural Color Variation: Darker tones in 4.8-grade and brighter tones in 10.9 grade bolts are inherent to material-process compatibility, conforming to international standards.

 

Through composition analysis, thickness measurement, and adhesion testing, one can scientifically distinguish between normal color variation and quality defects, ensuring purchased products meet technical specifications.