The Fastener Coating Market Landscape
Threaded fasteners—bolts, nuts, screws, washers, and related hardware—represent critical connection elements in structural assemblies, mechanical equipment, and countless other applications. These components face corrosion exposure ranging from benign interior environments to severe marine and industrial atmospheres, necessitating protective coatings matching exposure severity. While hot-dip galvanizing provides the industry standard for fastener corrosion protection, several alternative zinc coating technologies compete in specific market segments, each offering distinct characteristics, advantages, and limitations.
Understanding the technical differences among zinc coating processes—their metallurgical structures, coating thickness capabilities, application constraints, and performance attributes—enables informed coating selection matching fastener specifications to service requirements while optimizing cost, performance, and manufacturability.
Fundamental Principles: Coating Thickness and Longevity
Before examining specific coating technologies, a fundamental relationship governs all zinc coating performance:
Direct Proportionality
Service life is directly proportional to coating thickness across all zinc coating application methods. This relationship stems from zinc's sacrificial corrosion protection mechanism:
Zinc Corrosion Rate: Zinc corrodes at relatively predictable rates in atmospheric exposure:
- Rural atmospheres: 0.1-0.5 micrometers/year
- Urban/suburban: 0.5-1.5 micrometers/year
- Industrial: 1-3 micrometers/year
- Marine coastal: 2-4 micrometers/year
Service Life Calculation: Time to first maintenance (years) = Available coating thickness (micrometers) ÷ Corrosion rate (micrometers/year)
Example: 50-micrometer coating in urban exposure (1 micrometer/year corrosion rate): 50 ÷ 1 = 50 years to first maintenance
150-micrometer coating in same exposure: 150 ÷ 1 = 150 years to first maintenance
Practical Implication: Regardless of application method, thicker coatings provide proportionally longer protection. Marketing claims of coating superiority independent of thickness lack metallurgical foundation.
Metallurgical Versus Mechanical Bond
Coating adhesion mechanisms affect abrasion resistance and mechanical durability:
Metallurgical Bond: Zinc metallurgically bonded to steel through diffusion or alloying reactions (hot-dip galvanizing, thermal diffusion) demonstrates superior adhesion and abrasion resistance. Coating cannot be mechanically removed without removing substrate material.
Mechanical Bond: Zinc mechanically deposited on steel surface (electroplating, mechanical galvanizing) relies on physical interlocking and adhesion forces. While adequate for most applications, mechanical bonds show slightly reduced abrasion resistance compared to metallurgical bonds.
Process 1: Zinc Plating (Electroplating)
Zinc electroplating—termed "zinc plating" in fastener applications—represents the most common decorative and light-duty protective coating for threaded hardware:
Process Description
Electrodeposition Mechanism:
- Surface Preparation: Fasteners cleaned through alkaline cleaning, acid pickling, or mechanical methods removing oils, oxides, and contaminants
- Electrolytic Bath: Clean fasteners immersed in acidic zinc sulfate or zinc chloride electrolyte solution containing dissolved zinc ions (Zn²⁺)
- Current Application: Direct current applied with fasteners as cathode (negative electrode) and pure zinc anodes (positive electrode)
- Zinc Deposition: Electrical current reduces zinc ions at fastener surfaces, depositing pure metallic zinc:
Zn²⁺ + 2e⁻ → Zn (metallic) - Coating Growth: Continued current application builds coating thickness through sustained zinc deposition
- Post-Treatment: Optional chromate conversion coating or other post-treatments applied for enhanced corrosion resistance or appearance modification
Process Control: Coating thickness controlled through:
- Current density (amperes per square foot)
- Immersion duration
- Bath composition and temperature
- Agitation and anode configuration
Coating Characteristics
Coating Thickness:
Typical electroplated zinc thickness ranges: 5-12 micrometers (0.2-0.5 mils)
ASTM B633 classifications specify minimum thickness by fastener size:
- SC1 (Fasteners <1/4" diameter): 5 micrometers minimum
- SC2 (Fasteners ≥1/4" diameter): 8 micrometers minimum
- SC3 (Heavy service): 12 micrometers minimum
- SC4 (Very heavy service): 25 micrometers minimum
Appearance:
- Bright, shiny metallic finish
- Very smooth surface texture
- Excellent aesthetic appeal
- Uniform color and reflectivity
Physical Properties:
- Pure zinc composition (>99.5% zinc)
- Thin, dense coating structure
- Excellent ductility accommodating thread engagement
- Minimal dimensional change (<0.5 mils thickness addition)
Chromate Conversion Coatings
Post-plating chromate treatments enhance performance and modify appearance:
Clear Chromate (Clear Zinc, Bright Zinc):
- Transparent conversion coating preserving bright metallic appearance
- Modest corrosion resistance enhancement
- Typical for interior applications
Blue Chromate (Blue Zinc):
- Iridescent blue tint from thicker conversion coating
- Moderate corrosion resistance improvement
- Common for automotive and consumer hardware
Yellow Chromate (Yellow Zinc, Zinc Dichromate, Gold Zinc):
- Distinctive iridescent yellow to gold appearance
- Substantial corrosion resistance enhancement (2-4× improvement over clear)
- Traditional choice for moderate outdoor exposure
Black Chromate (Black Oxide, Black Zinc):
- Dark gray to black appearance
- Aesthetic rather than performance motivation
- Modest corrosion improvement
Regulatory Considerations: Traditional hexavalent chromium (Cr⁶⁺) conversion coatings face increasing restrictions. Alternative trivalent chromium (Cr³⁺) and chromium-free treatments provide similar benefits with reduced toxicity and regulatory concerns.
Advantages
Precision Dimensional Control: Thin coating thickness (5-12 micrometers) maintains thread dimensions, enabling:
- Close-tolerance thread engagement without interference
- Minimal impact on fit-up clearances
- Assembly compatibility with as-machined dimensions
Excellent Appearance: Bright, smooth finish provides:
- Attractive aesthetic for visible hardware
- Uniformity across production runs
- Multiple color options through post-treatments
Economic Production:
- High-volume automated processing
- Rapid coating application (minutes per batch)
- Lower material costs than alternatives
- Well-established infrastructure
Thread Lubrication: Electroplated coatings often incorporate lubricating topcoats or wax treatments providing:
- Consistent torque-tension relationships
- Reduced galling and seizing
- Improved assembly characteristics
Limitations
Limited Service Life: Thin coating thickness restricts corrosion protection duration:
- Interior applications: 5-15+ years
- Mild outdoor exposure: 2-5 years
- Moderate outdoor: 1-3 years
- Severe exposure: <1 year
Coating Discontinuities: Electroplating faces challenges on complex geometries:
- Recessed areas may receive inadequate coating thickness
- Sharp edges show reduced deposition
- Thread roots potentially under-coated
- Current distribution affects uniformity
Abrasion Vulnerability: Thin mechanical bond shows reduced abrasion resistance compared to metallurgical bonds. Handling damage can compromise protection.
Hydrogen Embrittlement Risk: High-strength steels (>150 ksi tensile strength) may experience hydrogen embrittlement during acid pickling and plating. Post-plating baking (bake-out) at 375-400°F for several hours drives hydrogen from steel, mitigating risk but adding processing steps.
Typical Applications
- Interior fasteners in non-corrosive environments
- Automotive hardware with mild exposure
- Consumer products requiring attractive appearance
- Electronic equipment and appliance assembly
- Small fasteners where dimensional control is critical
Relevant Standards
- ASTM B633: Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel
- ISO 2081: Metallic and other inorganic coatings — Electroplated coatings of zinc with supplementary treatments on iron or steel
Process 2: Mechanical Galvanizing (Mechanical Plating)
Mechanical galvanizing deposits zinc through impact peening rather than chemical or electrochemical processes:
Process Description
Impact Plating Mechanism:
- Loading: Cleaned fasteners loaded into rotating barrel with:
- Zinc powder (fine particles, 5-50 micrometer size)
- Glass beads or ceramic media (impact agents)
- Proprietary chemical activators
- Water or other carrier fluids
- Tumbling: Barrel rotates for extended periods (1-4 hours typical)
- Impact Peening: Glass beads strike fastener surfaces, generating:
- Localized plastic deformation of steel surface
- Zinc powder particles impacted into deformed surface
- Mechanical bonding through cold welding and interlocking
- Coating Building: Continued tumbling progressively builds coating thickness through layered zinc deposition
- Post-Treatment: Optional chromate conversion coating or sealer application
Temperature: Process occurs at ambient temperature, avoiding thermal effects on steel microstructure or mechanical properties
Coating Characteristics
Coating Thickness:
Mechanical galvanizing achieves substantially greater thickness than electroplating:
ASTM B695 classifications:
- Class 25: 25 micrometers (1.0 mil) minimum
- Class 50: 50 micrometers (2.0 mils) minimum
- Class 55: 55 micrometers (2.2 mils) minimum
- Class 100: 100 micrometers (3.9 mils) minimum
Common commercial fastener coating: 50 micrometers (2.0 mils)
Coating Structure:
- Pure zinc composition
- Layered, slightly porous structure
- Mechanical interlocking with substrate
- Less dense than electroplate or hot-dip coatings
Appearance:
- Matte gray finish
- Rough texture compared to electroplate
- Non-reflective appearance
- Uniform coloration
Advantages
No Hydrogen Embrittlement: Ambient temperature processing with no acid cleaning eliminates hydrogen introduction, making mechanical galvanizing suitable for:
- High-strength fasteners (>170 ksi ultimate tensile strength)
- Heat-treated components sensitive to hydrogen
- Spring materials
- Case-hardened parts
Substantial Coating Thickness: 50-100 micrometer coatings provide corrosion protection approaching hot-dip galvanizing
Thread Preservation: Lower processing temperatures compared to hot-dip galvanizing avoid thermal distortion risks
Edge Coverage: Unlike electroplating, mechanical galvanizing provides relatively uniform coverage including:
- Thread roots
- Sharp edges
- Recessed areas
Limitations
Size Restrictions: Tumbling barrel process limits part size:
- Maximum length: 8-10 inches typical
- Maximum diameter: 4-6 inches typical
- Not suitable for large structural fasteners
Reduced Edge Coating: Despite better coverage than electroplating, mechanical peening provides thinner coating:
- Under fastener heads
- At sharp edges
- In thread crests
Lower Coating Density: Mechanically deposited coating shows:
- Higher porosity than electroplate or hot-dip
- Slightly reduced corrosion resistance per unit thickness
- More irregular surface suitable for subsequent painting
Processing Time: 1-4 hour tumbling cycles limit throughput compared to minutes for electroplating
Cost: Higher than electroplating; comparable to or exceeding hot-dip galvanizing depending on volume
Typical Applications
- High-strength fasteners requiring embrittlement avoidance
- Automotive suspension and chassis components
- Case-hardened parts
- Spring washers and clips
- Fasteners requiring moderate corrosion protection without thermal exposure
Relevant Standards
- ASTM B695: Standard Specification for Coatings of Zinc Mechanically Deposited on Iron and Steel
- ISO 10683: Fasteners — Non-electrolytically applied zinc flake coatings
Process 3: High-Temperature Galvanizing (Delta Galvanizing)
High-temperature galvanizing represents a specialized hot-dip process employing elevated bath temperatures producing unique coating metallurgy:
Process Description
Elevated Temperature Immersion:
Standard hot-dip galvanizing: 815-850°F (435-455°C) High-temperature galvanizing: 1040-1165°F (560-630°C)
The elevated temperature dramatically alters zinc-iron reaction kinetics and alloy layer formation.
Coating Development:
At high-temperature galvanizing temperatures, zinc-iron phase diagram thermodynamics favor:
Gamma (Γ) Layer Formation: Thin interfacial layer (Fe₃Zn₁₀ composition)
Delta (δ) Layer Dominance: Compact, dense delta phase (FeZn₇ composition) forms primary coating bulk
Minimal Free Zinc: Limited or no pure zinc eta (η) layer on surface
The resulting coating consists primarily of hard, brittle delta phase intermetallic—fundamentally different from conventional galvanizing's layered structure.
Coating Characteristics
Coating Thickness:
Controlled through immersion time: 25-80 micrometers (1.0-3.1 mils) typical
Metallurgical Structure:
- Predominantly delta phase alloy
- Very hard (400+ Vickers hardness versus 70 for pure zinc)
- Brittle compared to conventional galvanizing
- Compact, dense structure
Appearance:
- Matte gray color
- Rough surface texture
- Non-reflective finish
- Consistent appearance across articles
Advantages
Dimensional Control: Coating thickness control through time/temperature enables:
- Predictable dimensional changes
- Accommodation of close-tolerance requirements
- Controlled fit-up for precision assemblies
Paint Adhesion: Rough, uniform delta surface provides:
- Excellent mechanical keying for paint systems
- No surface preparation required before painting
- Superior paint adhesion compared to smooth pure zinc
High Hardness: Delta phase hardness provides:
- Superior abrasion resistance
- Mechanical durability under service loading
- Resistance to handling damage
Corrosion Protection: Despite thinner coating than conventional hot-dip, delta phase density provides adequate protection for many applications
Limitations
Geographic Availability: High-temperature galvanizing facilities scarce in North America; more common in Europe
Coating Brittleness: Delta phase brittleness creates:
- Cracking risk under severe deformation
- Reduced ductility compared to pure zinc coatings
- Potential coating damage during thread engagement
Limited Thickness Range: Maximum practical thickness (~80 micrometers) less than conventional hot-dip galvanizing capability
Higher Processing Temperature: Elevated temperatures may affect:
- Steel mechanical properties in heat-treated materials
- Dimensional stability in precision parts
- Energy consumption and operating costs
Typical Applications
- Precision hardware requiring controlled coating thickness
- Fasteners destined for painting or powder coating
- Applications valuing abrasion resistance
- Moderate corrosion exposure where dimensional control outweighs maximum thickness
Market: Primarily European specification for specific industrial hardware categories
Relevant Standards
European standards address high-temperature galvanizing; no dedicated ASTM standard exists for this specific variant
Process 4: Thermal Diffusion Galvanizing (Sherardizing)
Thermal diffusion galvanizing—historically known as sherardizing—produces zinc-iron alloy coatings through solid-state diffusion at elevated temperatures:
Process Description
Diffusion Coating Formation:
- Loading: Cleaned fasteners loaded into rotating retort (sealed chamber) with:
- Zinc dust (fine zinc powder)
- Inert filler material (sand, glass beads)
- Proprietary additives
- Heating: Retort heated to 680-825°F (360-440°C)—below zinc melting point (787°F/419°C)
- Tumbling: Retort rotates continuously during heating maintaining part movement and zinc dust contact
- Zinc Diffusion: At process temperature:
- Zinc atoms diffuse into steel surface
- Iron atoms counter-diffuse into zinc
- Zinc-iron intermetallic alloy layers form (gamma, delta phases)
- No molten zinc; purely solid-state diffusion process
- Cooling: Controlled cooling in sealed retort
- Unloading: Parts removed, excess zinc dust reclaimed for reuse
Processing Time: 6-8 hours typical for complete cycle
Coating Characteristics
Coating Thickness:
ASTM A1059 minimum requirements:
- Grade 35: 35 micrometers (1.4 mils)
- Grade 45: 45 micrometers (1.8 mils)
- Grade 55: 55 micrometers (2.2 mils)
Typical commercial coatings: 25-75 micrometers (1-3 mils)
Metallurgical Structure:
- 100% zinc-iron intermetallic alloys
- Gamma and delta phase layers
- No free zinc eta layer
- Metallurgically bonded to substrate
Appearance:
- Matte gray finish similar to conventional hot-dip galvanizing intermetallics
- Uniform, rough texture
- Non-reflective
- Consistent appearance
Advantages
No Hydrogen Embrittlement: Process temperatures below typical bake-out temperatures and absence of acid cleaning eliminate hydrogen embrittlement risk for:
- High-strength fasteners (>170 ksi tensile strength)
- Heat-treated components
- Hardened steel parts
Excellent Dimensional Control: Diffusion process provides:
- Minimal dimensional change
- Uniform coating distribution
- Predictable thread fit-up
- Accommodation of close tolerances
Superior Thread Coverage: Solid-state diffusion coats all surfaces uniformly including:
- Thread roots and crests
- Under fastener heads
- Recessed areas and blind holes
- Sharp edges
Excellent Paint Adhesion: Uniform, rough alloy surface provides:
- Superior mechanical keying for paints and powder coatings
- No surface preparation required
- Durable paint system performance
High Abrasion Resistance: Metallurgically bonded intermetallic coating demonstrates:
- Superior abrasion resistance versus electroplate or mechanical coatings
- Durability under handling and service
- Coating integrity under thread engagement forces
Limitations
Extended Processing Time: 6-8 hour cycles limit throughput and increase cost
Higher Cost: Most expensive zinc coating option for fasteners; typically 2-4× conventional hot-dip galvanizing cost
Size Restrictions: Retort dimensions limit part size; typically suitable for:
- Small to medium fasteners
- Hardware components
- Limited to articles fitting in retort chambers
Limited North American Availability: Fewer facilities offering thermal diffusion compared to other processes
Moderate Coating Thickness: Maximum practical thickness (~75 micrometers) less than conventional hot-dip galvanizing achieves
Proprietary Variants
Several companies offer proprietary thermal diffusion processes:
ArmorGalv®: Thermal diffusion process marketed for fastener applications
Greenkote®: Environmentally-focused thermal diffusion variant
These proprietary processes follow similar principles with specific chemistry or process variations.
Typical Applications
- High-strength structural fasteners (ASTM A325, A490 tension control bolts)
- Precision hardware requiring dimensional control
- Fasteners for painting or powder coating
- Components where hydrogen embrittlement must be avoided
- Critical applications valuing coating uniformity and adhesion
Relevant Standards
- ASTM A1059: Standard Specification for Zinc Alloy Thermo-Diffusion Coatings (TDC) on Steel Fasteners, Hardware, and Other Products
Process 5: Conventional Hot-Dip Galvanizing (Batch Galvanizing)
Hot-dip galvanizing represents the baseline comparison for alternative fastener coating technologies:
Process Description
Standard batch galvanizing process as described in ASTM A153:
- Surface preparation (degreasing, pickling, fluxing)
- Immersion in molten zinc at 815-850°F (435-455°C)
- Coating formation through zinc-iron reactions
- Withdrawal and cooling
Coating Characteristics
Coating Thickness:
ASTM A153 requirements for fasteners:
- Fasteners <3/8" diameter: 43 micrometers (1.7 mils) minimum average
- Fasteners 3/8" to 1" diameter: 50 micrometers (2.0 mils) minimum average
- Fasteners >1" diameter: 86 micrometers (3.4 mils) minimum average
Typical achieved thickness: 85-150 micrometers (3.3-5.9 mils)
Metallurgical Structure:
- Zinc-iron alloy layers (gamma, delta, zeta phases)
- Pure zinc eta layer on surface
- Metallurgically bonded
Appearance:
- Bright metallic or matte gray depending on steel chemistry
- May show zinc spangle patterns
- Some surface roughness typical
Advantages
Maximum Coating Thickness: Substantially thicker coatings than alternatives provide:
- Longest service life per coating application
- Protection in severe exposure environments
- Decades of maintenance-free performance
Proven Performance: Extensive field history documenting:
- Long-term atmospheric corrosion protection
- Reliable performance across exposure environments
- Predictable service life
Metallurgical Bond: Alloyed coating layers provide:
- Superior abrasion resistance
- Excellent adhesion
- Durability under mechanical abuse
Economic Large-Volume Production:
- High throughput
- Established infrastructure
- Competitive pricing for volume production
Comprehensive Standards: Well-developed specifications (ASTM A153, ASTM F2329) provide:
- Clear coating requirements
- Acceptance criteria
- Quality assurance protocols
Limitations
Hydrogen Embrittlement Risk: High-strength steels (>150 ksi) may experience embrittlement during:
- Acid pickling (hydrogen introduction)
- Thermal exposure in zinc bath
Dimensional Changes: Thicker coating creates:
- Thread dimensional increase (2-5 mils diameter addition)
- Potential fit-up challenges for close-tolerance threads
- May require thread chasing or oversized nuts
Thermal Effects: 815-850°F process temperature may affect:
- Heat-treated materials below tempering temperature
- Dimensional stability in precision parts
- Steel mechanical properties if tempering temperature approached
Variable Appearance: Steel chemistry-dependent coating appearance creates:
- Bright or matte gray finish variations
- Rough texture on reactive steels
- Aesthetic inconsistency for appearance-critical applications
Typical Applications
- Structural fasteners (ASTM A307, F1554)
- Highway guardrail and sign hardware
- Utility pole hardware and line components
- Marine and coastal exposure applications
- Industrial plant fasteners
- Any application prioritizing maximum corrosion protection
Relevant Standards
- ASTM A153: Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
- ASTM F2329: Standard Specification for Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts and Special Threaded Fasteners
Comparative Performance Analysis
Coating Thickness Comparison
Coating Method Typical Thickness Thickness Range
Zinc Plating 8-12 μm (0.3-0.5 mils) 5-25 μm (0.2-1.0 mils)
Mechanical Galvanizing 50 μm (2.0 mils) 25-100 μm (1.0-3.9 mils)
High-Temp Galvanizing 40-60 μm (1.6-2.4 mils) 25-80 μm (1.0-3.1 mils)
Thermal Diffusion 45 μm (1.8 mils) 25-75 μm (1.0-3.0 mils)
Hot-Dip Galvanizing 100 μm (3.9 mils) 85-150 μm (3.3-5.9 mils)
Performance Implication: Hot-dip galvanizing provides 2-20× coating thickness advantage translating directly to proportional service life enhancement.
Service Life Projections
Estimated time to first maintenance in moderate outdoor exposure (1.0 micrometer/year zinc corrosion rate):
- Zinc Plating (10 μm): 10 years
- Mechanical Galvanizing (50 μm): 50 years
- High-Temp Galvanizing (50 μm): 50 years
- Thermal Diffusion (50 μm): 50 years
- Hot-Dip Galvanizing (100 μm): 100 years
Service life scales proportionally with coating thickness and inversely with environmental corrosivity.
Application Selection Decision Matrix
Select Zinc Plating When:
- Interior applications or mild exterior exposure
- Attractive appearance required
- Close dimensional tolerances critical
- Low-cost coating adequate for application
- High-volume production economics favorable
Select Mechanical Galvanizing When:
- High-strength fasteners (>170 ksi) requiring embrittlement avoidance
- Moderate corrosion protection needed
- Ambient temperature processing required for material property preservation
- Small fasteners suitable for tumbling
Select High-Temperature Galvanizing When:
- Controlled coating thickness for precise fit-up required
- Fasteners destined for painting or powder coating
- Abrasion resistance prioritized
- Geographic availability permits (primarily European markets)
Select Thermal Diffusion When:
- High-strength fasteners requiring hydrogen embrittlement avoidance
- Excellent dimensional control essential
- Superior thread coverage required including roots and crests
- Fasteners for painting or powder coating
- Application justifies premium cost for coating uniformity and adhesion
Select Hot-Dip Galvanizing When:
- Maximum corrosion protection required
- Severe exposure environments
- Long service life (50-100+ years) expected
- Structural fasteners in infrastructure applications
- Economic large-volume production
- Thread dimensional increases acceptable
Accelerated Testing Limitations
Claims of coating superiority often reference accelerated corrosion testing, particularly salt spray testing per ASTM B117. However, such test results require careful interpretation:
Salt Spray Test Methodology
ASTM B117 salt spray testing exposes coated specimens to continuous fog of 5% sodium chloride solution at 95°F (35°C) in sealed chambers. Test duration commonly ranges from 96 hours to 1000+ hours.
Fundamental Testing Limitations
Attacks Wrong Material:
Salt spray testing attacks metallic zinc rather than the protective zinc patina (zinc hydroxides, carbonates, hydroxycarbonates) that forms naturally in atmospheric exposure and provides actual corrosion protection. The test environment prevents stable patina formation, causing continuous zinc dissolution.
Non-Representative Conditions:
Real atmospheric exposure involves:
- Wet/dry cycling
- Temperature variations
- UV radiation
- Pollutant interactions
- Protective patina development
Salt spray provides continuous wet conditions at constant temperature without these critical variables.
Invalid Performance Predictions:
Salt spray test ranking of coating performance frequently contradicts actual field performance rankings. Coatings performing poorly in salt spray may excel in real atmospheres and vice versa.
Thickness Dominance:
Salt spray tests primarily measure coating thickness rather than coating quality. Thicker coatings survive longer regardless of metallurgical characteristics.
Appropriate Test Interpretation
Salt spray testing provides value for:
- Quality control detecting defects or process variations
- Comparative testing of similar coatings from different suppliers
- Accelerated screening of coating candidates
Salt spray results should NOT be used for:
- Predicting actual atmospheric service life
- Claiming coating method superiority across all environments
- Substituting for field exposure testing or documented performance history
Economic Considerations
Coating cost analysis requires lifecycle perspective:
Initial Cost Ranking (Relative, per 100 pounds of fasteners)
- Zinc Plating: $50-150 (baseline)
- Hot-Dip Galvanizing: $150-300
- Mechanical Galvanizing: $200-400
- High-Temperature Galvanizing: $250-450 (where available)
- Thermal Diffusion: $400-800
Cost-Per-Year-Of-Service
Dividing initial cost by projected service life in relevant exposure:
Moderate Outdoor Exposure Example:
- Zinc Plate (10-year life): $10-15/year
- Mechanical/Thermal/High-Temp (50-year life): $4-16/year
- Hot-Dip Galvanizing (100-year life): $1.50-3/year
Lifecycle Economic Winner: Hot-dip galvanizing delivers lowest annual cost despite moderate initial cost premium.
Specification Guidance
Clear specification language prevents coating selection ambiguity:
Explicit Process Designation:
Good: "Fasteners shall be hot-dip galvanized per ASTM A153"
Poor: "Fasteners shall be galvanized" (ambiguous—could mean any zinc coating process)
Performance-Based Alternative:
"Fasteners shall receive zinc coating providing minimum 50 micrometers average thickness with metallurgical bond to steel substrate. Hot-dip galvanizing per ASTM A153 or thermal diffusion galvanizing per ASTM A1059 acceptable."
Multiple zinc coating technologies serve fastener corrosion protection needs with distinct performance attributes, cost structures, and application niches. Zinc electroplating provides economical, attractive coatings for interior and mild exposure applications through thin (5-12 micrometers) pure zinc deposits offering excellent dimensional control but limited service life proportional to modest coating thickness. Mechanical galvanizing eliminates hydrogen embrittlement risks through ambient-temperature impact peening processes achieving moderate coating thickness (25-100 micrometers) suitable for high-strength fasteners requiring 20-50 year protection without thermal exposure. High-temperature galvanizing and thermal diffusion processes produce controlled-thickness zinc-iron alloy coatings (25-75 micrometers) offering dimensional precision, superior paint adhesion, and hydrogen embrittlement avoidance, though at premium costs and with geographic or size limitations. Conventional hot-dip galvanizing per ASTM A153 delivers thickest coatings (85-150 micrometers typical) providing century-scale corrosion protection through metallurgically bonded zinc-iron alloy layers and pure zinc, establishing the performance standard against which alternatives compete. The fundamental relationship between coating thickness and service life—wherein protection duration is directly proportional to available zinc thickness across all application methods—means hot-dip galvanizing's 2-20× thickness advantage over alternatives translates directly to proportional service life enhancement. Claims of alternative coating superiority often reference salt spray accelerated testing results that attack metallic zinc rather than protective atmospheric patina, producing rankings that contradict actual field performance and should not substitute for documented service life evidence. Application selection requires matching coating characteristics to specific requirements: electroplating for dimensional control and appearance, mechanical or thermal diffusion for high-strength fastener embrittlement avoidance, and hot-dip galvanizing for maximum protection duration in severe exposure or long-design-life applications where thicker coating justifies any dimensional accommodation. More information on Zinc Coatings for Fasteners is available at the original AGA resource article.
