The Thick Coating Misconception
A persistent misunderstanding occasionally surfaces in customer-galvanizer relationships: the belief that galvanizers intentionally apply excessive coating thickness to increase revenue through weight-based pricing. This misconception stems from incomplete understanding of galvanizing economics and the metallurgical factors governing coating formation. In reality, thick coatings reduce galvanizer profitability while creating quality concerns for customers. Understanding the economic drivers, metallurgical causes of excessive thickness, and available mitigation strategies clarifies the true relationship between coating thickness and galvanizing business economics.
Hot-Dip Galvanizing Pricing Structure
North American galvanizing facilities predominantly employ weight-based pricing for batch galvanizing services, charging customers per pound of finished galvanized weight:
Industry Pricing Range
Survey data from galvanizing industry members indicates typical pricing ranges from $0.20 to $0.50 per pound of finished galvanized weight. This range varies based on several factors:
Geographic Location: Regional market conditions, competition density, and local economic factors influence pricing. Urban industrial centers typically show more competitive pricing than rural areas with limited galvanizing capacity.
Article Complexity: Simple, easily handled items (flat plates, standard structural shapes) command lower per-pound pricing than complex assemblies requiring special handling, extensive preparation, or difficult processing.
Production Volume: Large-volume repeat orders often receive discounted pricing compared to small one-off jobs requiring setup and parameter adjustment.
Special Requirements: Articles requiring non-standard processing (blast cleaning, specific appearance standards, tight tolerance work) may incur premium pricing.
Market Segment: Structural steel, rebar, hardware, and other market segments exhibit different pricing dynamics based on competition and value proposition.
The weight-based pricing model means heavier finished articles generate proportionally higher revenue—creating superficial appearance that galvanizers benefit from thick coatings. However, examining cost structures reveals the opposite reality.
Zinc: The Dominant Material Cost
Zinc represents the single largest material cost input in hot-dip galvanizing operations, typically constituting 35-50% of total operating costs. Understanding zinc pricing dynamics and consumption relationships illuminates why thick coatings harm rather than help galvanizer profitability.
Zinc Market Pricing
Zinc trades as a commodity on global metals exchanges with prices fluctuating based on supply-demand dynamics, mining production, economic conditions, and currency exchange rates. Recent 52-week pricing (in USD per pound) demonstrates typical volatility:
- 52-Week Low: $1.04/lb
- 52-Week High: $1.64/lb
- Typical Range: $1.10-1.40/lb
- Current Market: Approximately $1.19/lb (varies continuously)
This 58% spread between annual low and high illustrates the price volatility galvanizers must manage. Unlike many manufacturing operations with stable raw material costs, galvanizing facilities face continuous commodity price risk affecting profitability.
Zinc Consumption and Coating Thickness
The relationship between coating thickness and zinc consumption is direct and linear. Each mil (0.001 inch) of coating thickness adds approximately 0.044 pounds of zinc per square foot of coated surface. For a typical structural steel article with 100 square feet of surface area:
Zinc Consumption Calculation:
- 1 mil coating thickness = 4.4 pounds zinc
- 2 mils coating thickness = 8.8 pounds zinc
- 3 mils coating thickness = 13.2 pounds zinc
- 4 mils coating thickness = 17.6 pounds zinc
- 5 mils coating thickness = 22.0 pounds zinc
Excessive coating thickness directly translates to proportional zinc consumption increase.
The Profitability Equation
Comparing typical galvanizing service pricing against zinc material costs reveals why thick coatings reduce profit margins:
Economic Analysis at Current Zinc Pricing
Using current zinc market price of $1.19/lb and typical galvanizing service price of $0.35/lb finished weight (mid-range industry pricing):
Per-Pound Economics:
- Galvanizer receives from customer: $0.35/lb finished weight
- Zinc material cost: $1.19/lb zinc consumed
- Net loss per pound of zinc: $0.84/lb
This calculation demonstrates that for every additional pound of zinc coating beyond the minimum required for specification compliance, the galvanizer loses $0.84 in material cost that exceeds the revenue received for that weight.
Economics Across Pricing Range
Conservative Pricing Scenario ($0.20/lb service price):
- Revenue: $0.20/lb
- Zinc cost: $1.19/lb
- Net loss: $0.99/lb zinc
Premium Pricing Scenario ($0.50/lb service price):
- Revenue: $0.50/lb
- Zinc cost: $1.19/lb
- Net loss: $0.69/lb zinc
Even at the highest typical industry pricing, each pound of excess zinc costs the galvanizer $0.69 more than the revenue received. The economics universally favor minimizing zinc consumption while meeting specification minimums.
Profit Optimization Strategy
Rational economic behavior dictates galvanizers maximize profitability by:
- Meeting Minimum Requirements: Ensuring coating thickness satisfies ASTM A123 or other specification minimums
- Minimizing Excess Thickness: Avoiding coating thickness beyond minimum requirements
- Controlling Zinc Consumption: Optimizing bath chemistry, process parameters, and handling to limit zinc usage
- Volume Efficiency: Processing maximum article quantity per unit zinc consumed
This profit maximization strategy directly opposes intentionally producing thick coatings. Galvanizers have strong financial incentive to provide the thinnest coating meeting customer specifications.
Why Thick Coatings Occur Despite Economic Disincentives
If thick coatings harm galvanizer profitability, why do they occur? The answer lies in metallurgical factors beyond galvanizer control and the steel characteristics customers supply.
Reactive Steel Chemistry: The Primary Cause
Steel chemical composition—particularly silicon and phosphorus content—exerts dominant control over zinc-iron reaction kinetics and resulting coating thickness. Certain composition ranges produce excessively thick coatings regardless of galvanizing process parameters.
Silicon Content Effects
Sebisty Range Steel (0.13-0.28% Si):
Steel with silicon content in this range exhibits dramatically accelerated zinc-iron alloying reactions. The silicon acts as a powerful catalyst, increasing reaction rates by 5-10 times compared to low-silicon steel. The mechanism involves silicon's effect on iron-zinc interdiffusion, promoting rapid zinc-iron alloy layer growth.
Characteristic Results:
- Coating thickness: 5-12+ mils (typical minimum specifications: 2-4 mils)
- Appearance: Matte gray finish from extensive alloy layer formation
- Texture: Rough surface compared to normal galvanizing
- Metallurgical structure: Predominantly zinc-iron alloy layers with minimal pure zinc eta layer
Galvanizers cannot control this reaction through process adjustments. Reducing immersion time, lowering bath temperature, or other parameter modifications provide only marginal thickness reduction while risking incomplete coating coverage.
Sandelin Range Steel (0.04-0.13% Si):
This intermediate silicon range produces unpredictable, variable coating thickness. Articles may show extremely thin coatings (below specification minimums) or excessively thick coatings, often with dramatic variation across a single piece.
The metallurgical unpredictability makes consistent processing nearly impossible. Galvanizers must immerse articles long enough to ensure adequate coating on thin-coating-prone areas, inevitably producing excessive thickness on reactive zones.
Weathering Steel Complications
ASTM A588 weathering steel (Cor-Ten and similar alloys) contains deliberate alloying additions—copper, chromium, nickel—that enhance atmospheric corrosion resistance through formation of protective rust patina. However, these same alloying elements, combined with typical silicon content of 0.15-0.50%, produce highly reactive galvanizing behavior.
Weathering Steel Galvanizing Characteristics:
- Very thick coatings: 7-15+ mils common
- Extremely rough, uneven surface texture
- Matte gray to dark gray appearance
- Potential coating adhesion concerns from excessive alloy layer thickness
Many specifications explicitly discourage or prohibit galvanizing weathering steel due to coating quality concerns stemming from excessive thickness.
Phosphorus Interaction
Phosphorus content above 0.04% can interact with silicon to modify coating formation behavior. The combined effect of moderate silicon (0.03-0.08%) with elevated phosphorus (0.08-0.12%) sometimes produces unexpectedly thick coatings through catalytic reaction enhancement.
Geometric and Thermal Factors
Beyond steel chemistry, article characteristics affect coating thickness:
Steel Section Thickness Effects
Thick Sections (≥1 inch thickness):
Heavy steel sections retain substantial thermal mass after immersion in 840°F molten zinc. The prolonged elevated temperature—extending 30-60+ minutes after bath withdrawal—continues zinc-iron diffusion reactions well beyond normal timeframes.
Consequences:
- Continued coating growth during slow cooling
- Coating thickness 50-100% greater than thin sections of identical chemistry
- Potential Kirkendall Effect void formation in extreme cases
Thick structural members, heavy plates, and large-diameter thick-wall tubes exhibit this behavior regardless of galvanizer's process optimization efforts.
Mixed Section Assemblies
Assemblies combining thick and thin components face coating thickness challenges:
Thermal Dynamics:
- Thin sections: Rapid heat loss, quick reaction cessation, controlled coating thickness
- Thick sections: Slow cooling, extended reactions, excessive coating thickness
Processing Dilemma: Galvanizers must ensure adequate immersion time for thin sections to achieve minimum coating while inevitably producing excessive thickness on thick sections that remain hot longer.
Example: A structural assembly with 1/4-inch angle members welded to 1-inch plate will show 2-3 mils coating on angles and 6-8 mils on plate despite identical steel chemistry and processing.
Design and Fabrication Factors
Several design characteristics beyond material selection affect coating thickness:
Dissimilar Steel Chemistry Combinations
Fabrications incorporating multiple steel types from different mills or production runs often contain varying silicon and phosphorus content:
- Structural shapes from Mill A: 0.03% Si (low reactive)
- Plate from Mill B: 0.18% Si (Sebisty range, highly reactive)
- Reinforcing components from Mill C: 0.08% Si (moderate reactivity)
The mixed chemistry produces coating thickness variation across the assembly that no galvanizing process adjustment can eliminate. Some components inevitably receive excessive coating while others achieve normal thickness.
Poor Venting and Drainage Design
Inadequate vent and drain holes affect coating thickness through multiple mechanisms:
Restricted Zinc Drainage: Molten zinc trapped in pockets, recesses, or poorly vented hollow sections cannot drain during article withdrawal, solidifying in place as excess coating thickness.
Extended High-Temperature Exposure: Trapped zinc maintains component temperature, prolonging reaction time and increasing alloy layer growth.
Zinc Entrapment: Complex geometries without adequate drainage pathways accumulate zinc that adds substantial weight and thickness.
Proper venting and drainage design per ASTM A385 guidelines prevents these issues, but many fabrications arrive at galvanizers with inadequate provisions.
Coating Thickness Control Strategies
While metallurgical factors limit coating thickness control, several approaches can moderate thickness when properly applied:
Reduced Immersion Time
Mechanism: Shorter zinc bath exposure limits zinc-iron reaction duration, potentially reducing coating thickness by 10-25%.
Limitations:
- Must ensure sufficient time for complete surface wetting and coating formation
- Risk of thin spots or incomplete coverage if immersion too brief
- Minimal effectiveness for highly reactive steel chemistries
- Requires careful calibration for each steel chemistry and geometry combination
Application: Most practical for moderate-reactivity steels where baseline coating slightly exceeds requirements. Ineffective for Sebisty range steels.
Pre-Galvanizing Abrasive Blast Cleaning
Mechanism: Complete mill scale removal and surface roughening can paradoxically reduce coating thickness for some steel chemistries by:
- Eliminating scale that can trap flux or contaminants affecting local reaction rates
- Creating uniform surface condition promoting consistent reactions
- Removing surface layers with modified chemistry from rolling or storage
Effectiveness:
- Variable results: 10-40% thickness reduction for some steels
- May increase thickness for other chemistries through enhanced surface area
- Requires trial evaluation to confirm benefit for specific steel types
Costs: Adds $0.50-2.00/lb processing cost, potentially offsetting zinc cost savings
Article Agitation During Immersion
Mechanism: Moving articles within the zinc bath may limit coating growth by:
- Disrupting boundary layer formation at steel-zinc interface
- Creating localized cooling from bath circulation
- Preventing stagnant zinc zones promoting extensive alloying
Limitations:
- Practical only for small-to-moderate articles that can be safely manipulated
- Minimal effectiveness for large structural assemblies
- Inconsistent results depending on agitation method and frequency
Bath Chemistry Modification
Mechanism: Adding aluminum, nickel, or proprietary additives to zinc baths can moderate reactivity with certain steel chemistries.
Aluminum Additions: Small aluminum additions (0.005-0.015%) can reduce coating thickness on Sandelin range steels while improving appearance.
Limitations:
- Impractical for most facilities to modify bath chemistry for individual customer jobs
- Bath change affects all production for weeks or months
- Requires substantial technical expertise and trial-and-error optimization
- Aluminum additions may negatively affect other steel chemistries
Application: Limited to specialized facilities serving markets with predominant reactive steel problems.
Thermal Management
Lower Bath Temperature: Reducing galvanizing temperature from 850°F to 820-825°F slows reaction kinetics, potentially reducing coating thickness 5-15%.
Limitations:
- Increased zinc viscosity affects drainage and coating appearance
- Risk of incomplete wetting or cold shuts at lower temperatures
- Energy efficiency reduction from operating below optimal thermal conditions
- Minimal effectiveness for highly reactive chemistries
Rapid Cooling: Forced air cooling or water quenching immediately after withdrawal arrests continued reactions during cooling.
Application: Beneficial for thick sections but requires equipment investment and careful implementation to avoid coating damage.
Proactive Communication and Collaboration
The most effective approach to managing coating thickness concerns involves early collaboration among designers, fabricators, and galvanizers:
Pre-Fabrication Steel Chemistry Review
Mill Test Report Sharing: Fabricators should provide steel mill test reports to galvanizers before fabrication begins, enabling:
- Identification of potentially problematic steel chemistries
- Discussion of expected coating thickness ranges
- Evaluation of whether steel substitution is advisable
- Planning for supplemental processes (blast cleaning, special handling)
- Realistic expectation setting regarding achievable coating appearance and thickness
Chemistry Database Development: Galvanizers can maintain historical databases correlating steel chemistries from various mills with actual coating thickness results, enabling more accurate predictions.
Design Review for Galvanizing
Collaborative Assessment: Engineers should engage galvanizers during design phases to review:
- Venting and drainage adequacy for complex geometries
- Fit-up criticality and dimensional tolerance requirements
- Appearance standards and acceptable thickness ranges
- Steel specification appropriateness for galvanizing
- Alternative design configurations reducing thickness sensitivity
Design Modifications: Simple design changes often prevent coating thickness issues:
- Separating thick and thin components for individual galvanizing
- Specifying steel chemistry ranges avoiding reactive compositions
- Adding drainage provisions to prevent zinc entrapment
- Designing with assembly clearances accommodating expected coating thickness
Expectation Alignment
Realistic Specifications: Project specifications should reflect galvanizing metallurgical realities:
Appropriate: "Hot-dip galvanized coating shall meet ASTM A123 minimum average thickness requirements. Coating thickness exceeding minimums is acceptable."
Problematic: "Galvanized coating thickness shall be 2.5-3.0 mils." (Narrow range difficult to achieve consistently with varying steel chemistries)
Quality Priority Communication: Clarify whether coating thickness minimization takes priority over other factors:
- Is fit-up critical or are generous clearances provided?
- Are appearance standards stringent or is functional protection primary?
- Can thick sections be accepted with excessive coating if minimum specifications are met?
When Thick Coatings Are Beneficial
While usually problematic, certain applications genuinely benefit from thick coatings:
Extended Service Life Requirements
Coating thickness directly correlates with corrosion protection duration. Environments requiring 75+ year service life benefit from thick coatings:
Coastal Installations: Marine atmospheres with continuous salt exposure Industrial Environments: Chemical plant surroundings with aggressive atmospheres Infrastructure: Bridges and structures where replacement is prohibitively expensive
For these applications, reactive steel chemistries producing 8-12 mil coatings provide value through extended protection.
Aesthetic Preferences
Some architectural applications prefer the distinctive matte gray appearance of thick, alloyed coatings for:
- Industrial aesthetic design themes
- Visual texture and character
- Contrast with other building materials
Customer Response to Thick Coating Concerns
When thick coatings occur despite neither party desiring them:
Functional Assessment
Fit-Up Verification: Test-fit articles before complaining about coating thickness. Dimensional measurements may show adequate clearances despite thickness concerns.
Appearance Evaluation: Assess whether appearance truly violates project standards or merely differs from expectations.
Protection Analysis: Recognize that thick coatings provide superior corrosion protection—a functional benefit partially offsetting aesthetic concerns.
Cost Negotiation
If reactive steel chemistry from customer-supplied material produced unavoidable thick coatings:
Shared Responsibility: Customer's steel selection drove coating thickness, suggesting cost-sharing for excess zinc.
Future Avoidance: Establish protocols for steel chemistry verification preventing future occurrences.
Alternative Approaches: Discuss steel grade changes, pre-blast cleaning, or other options reducing future thickness issues.
The economics of hot-dip galvanizing decisively refute the misconception that galvanizers intentionally produce thick coatings to inflate revenue. With zinc material costs ranging from $1.04-1.64 per pound substantially exceeding typical galvanizing service pricing of $0.20-0.50 per pound finished weight, galvanizers lose $0.69-0.99 per pound of zinc beyond minimum specification requirements. This economic reality creates strong financial incentive for minimizing coating thickness while meeting specifications. Thick coatings occur despite these disincentives primarily due to reactive steel chemistries—particularly Sebisty range silicon content (0.13-0.28%) and weathering steel alloys—that produce accelerated zinc-iron reactions generating 5-12+ mil coatings uncontrollable through normal process parameters. Additional contributing factors include thick steel sections retaining heat that extends reaction time, mixed-chemistry assemblies producing variable thickness across components, and poor venting designs trapping excess zinc. While coating thickness control strategies including reduced immersion time, pre-galvanizing blast cleaning, and thermal management offer modest benefits, metallurgical factors fundamentally limit achievable control. The most effective approach involves proactive collaboration among designers, fabricators, and galvanizers, with early steel chemistry review enabling realistic expectation setting, alternative material selection when appropriate, and design modifications accommodating galvanizing metallurgical realities. Understanding these economic and metallurgical fundamentals clarifies that thick coatings represent a mutual challenge rather than a profit-maximization scheme, fostering productive partnerships addressing coating quality concerns. For further discussion, see the original AGA resource.
Economic Data Note: Zinc pricing data represents market conditions at time of publication and fluctuates continuously based on global commodity markets.
