Long-Term Flaking Concerns with Hot-Dip Galvanized Coatings: Causes, Prevention, and Remediation

Understanding Coating Flaking in Hot-Dip Galvanized Steel

Coating flaking represents a specific mode of galvanized coating failure characterized by separation and detachment of the zinc-iron alloy layers from the steel substrate. While properly galvanized steel demonstrates exceptional long-term durability with service lives extending 50-75 years in typical atmospheric environments, understanding flaking mechanisms, predisposing factors, and appropriate remediation protocols enables engineers to specify galvanizing with confidence and address the rare instances where flaking occurs.

Definition and Mechanism of Coating Flaking

Flaking Characteristics

Coating flaking manifests as separation of the galvanized coating—including the gamma (Γ), delta (δ), and zeta (ζ) zinc-iron alloy layers—from the underlying steel substrate. The separated coating typically detaches in discrete pieces exhibiting:

• Long, distinct, sharp edges along fracture boundaries
• Coating fragments ranging from several square centimeters to larger areas
• Exposure of the gamma layer or bare steel at detachment locations
• Coating thickness measurements approaching zero at flaked areas (only the thin gamma layer remains adhered, if any)

Flaking differs fundamentally from other coating defects such as:

• Coating spalling: Small-scale surface disruption without full layer separation
• Adhesion failure: Loss of coating integrity at the coating-substrate interface across large continuous areas
• Weathering erosion: Gradual zinc consumption through normal atmospheric corrosion processes

Metallurgical Basis for Flaking Susceptibility

The hot-dip galvanizing process produces a multi-layered coating structure formed through sequential iron-zinc alloy formation:

1. Gamma layer (Γ) - 75% zinc, 25% iron; directly adjacent to steel
2. Delta layer (δ) - 90% zinc, 10% iron; intermediate alloy
3. Zeta layer (ζ) - 94% zinc, 6% iron; harder, more brittle alloy
4. Eta layer (η) - >99% zinc; pure zinc outer layer

Normal coating thickness (3.5-5.0 mils or 85-125 microns) produces a balanced alloy structure with acceptable ductility and toughness. However, excessively thick coatings—typically exceeding 8-10 mils (200-250 microns)—exhibit altered mechanical properties predisposing to flaking.

Why Thick Coatings Become Brittle

As coating thickness increases substantially beyond normal ranges:

• Increased zeta layer proportion: The hard, brittle zeta phase constitutes a larger fraction of total coating thickness
• Residual stress accumulation: Differential thermal contraction between zinc-iron alloys and steel substrate generates internal tensile stresses proportional to coating thickness
• Reduced strain tolerance: Thicker coatings accommodate less mechanical deformation before fracture initiation
• Intermetallic embrittlement: Extended reaction time produces coarse intermetallic structures with reduced fracture toughness

These factors combine to reduce coating cohesion and increase susceptibility to impact-induced flaking.

Primary Cause: Reactive Steel Chemistry

The Sandelin Effect and Silicon-Induced Coating Growth

Steel silicon content profoundly influences zinc-iron reaction kinetics during galvanizing. The "Sandelin range" or "Sandelin effect" describes accelerated coating formation occurring when steel silicon content falls between approximately 0.04-0.15% (some references extend the upper bound to 0.22%).

Within this range:

• Zinc-iron reaction rates increase dramatically compared to low-silicon (<0.03%) steels
• Coating thickness can reach 8-15 mils (200-380 microns) or more under standard galvanizing parameters
• Coating appearance becomes dull gray rather than bright metallic
• Coating structure exhibits coarser, more brittle intermetallic phases

Phosphorus Effects

Phosphorus content below 0.020% can also produce thin, brittle coatings with poor mechanical properties, though this typically results in coating adhesion failures rather than flaking of excessively thick coatings.

Steel Chemistry Recommendations for Galvanizing

ASTM A385, "Practice for Providing High-Quality Zinc Coatings (Hot-Dip)," provides steel chemistry guidance to minimize coating thickness and brittleness issues:

• Carbon: <0.25%
• Phosphorus: <0.04%
• Manganese: <1.35%
• Silicon: <0.04% OR 0.15-0.22% (avoiding the Sandelin range)

Steels meeting these recommendations typically produce normal coating thickness with acceptable mechanical properties and minimal flaking risk.

Mechanism of Flaking Occurrence

Impact as Initiation Requirement

A critical characteristic of coating flaking is that it does not occur spontaneously. Flaking requires mechanical impact, shock loading, or rough handling to initiate coating separation. Common scenarios include:

During Transportation:

• Vibration and shock loading during truck or rail transport
• Impact from load shifting or improper securing
• Contact between stacked galvanized components
• Dropping or rough handling during loading/unloading

During Installation:

• Impact from rigging hardware during lifting operations
• Contact with tools or equipment during erection
• Dropping components or accidental impacts
• Walking on or striking galvanized surfaces

Key Point: Properly installed galvanized steel does not experience progressive flaking after installation. The coating does not spontaneously delaminate or continue flaking absent external mechanical forces.

Flaking Risk Factors and Identification

High-Risk Scenarios

Flaking risk increases when multiple factors coincide:

1. Reactive steel chemistry producing excessively thick coatings (>8 mils)
2. Rough handling during post-galvanizing operations
3. Complex geometries with sharp corners, edges, or constrained sections
4. Rapid cooling causing high residual stress
5. Extended immersion time in galvanizing bath

Visual Identification of Flaking-Susceptible Coatings

Coatings at elevated flaking risk often exhibit characteristic appearance:

• Dull gray color rather than bright metallic finish
• Rough, matte surface texture
• Visibly thick coating, particularly at corners and edges
• Lack of typical zinc spangle pattern

Coating thickness measurement using magnetic thickness gauges confirms excessive thickness (readings consistently >8 mils suggest elevated flaking risk).

Responsibility Assignment and Quality Assurance

Galvanizer Responsibility Domain

The hot-dip galvanizing facility bears responsibility for:

• Steel chemistry verification: Requesting mill test reports and identifying reactive steels before galvanizing
• Process control: Managing immersion time, bath temperature, and cooling rates to minimize coating thickness on reactive steels
• Pre-shipment inspection: Identifying and repairing any flaking damage occurring at the galvanizing facility
• Quality assurance: Ensuring coatings meet ASTM A123 or A153 requirements before customer acceptance

If flaking occurs at the galvanizing plant—during handling, cooling, or transfer operations—the galvanizer must repair affected areas or re-galvanize components before shipment.

Customer Responsibility Domain

Once galvanized components leave the galvanizing facility and transfer to customer ownership, responsibility for flaking damage shifts to the customer. This includes flaking resulting from:

• Transportation and handling practices
• Installation procedures and techniques
• Storage conditions and component stacking
• Any other post-acceptance activities

Critical Distinction: The transition of responsibility typically occurs at:

• Shipment from galvanizing facility (FOB origin terms)
• Delivery to customer site (FOB destination terms)
• Formal acceptance after customer inspection

Contract documents should clearly establish the transfer point and associated quality assurance protocols.

Inspection and Acceptance Criteria

Pre-Acceptance Inspection

Customers receiving galvanized steel should conduct incoming inspection including:

1. Visual examination: Inspect for coating uniformity, obvious damage, or flaking
2. Coating thickness measurement: Verify compliance with specification requirements and identify excessively thick coatings
3. Impact testing: Gently tap corners and edges with a rubber mallet to identify poorly adhered coatings before acceptance
4. Documentation review: Verify mill test reports document acceptable steel chemistry

Acceptance Decision Framework

ASTM A123 establishes acceptance criteria based on two primary considerations:

1. Effect on corrosion protection: Does the defect compromise long-term coating performance?
2. Interference with intended use: Does the defect prevent the component from performing its designed function?

Small-Scale Flaking: If flaking affects only limited areas and repair is feasible per ASTM A780 size limitations, the component may be accepted with repair.

Extensive Flaking: If flaking is widespread, occurs over large continuous areas, or cannot be adequately repaired, rejection and re-galvanizing may be warranted.

Repair Protocols and Standards

ASTM A780 Repair Procedures

ASTM A780, "Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings," establishes acceptable repair materials and procedures.

Shop Repairs (Galvanizer Responsibility)

Repairs conducted at the galvanizing facility follow ASTM A780 size limitations:

• Individual uncoated areas: ≤1 inch (25 mm) in one dimension
• Total uncoated area per article: ≤0.5% of accessible surface area OR ≤36 square inches per ton, whichever is less

Areas exceeding these limits require re-galvanizing rather than repair.

Field Repairs (Customer Responsibility)

Repairs conducted after customer acceptance have no size limitation under ASTM A780, recognizing the impracticality of returning installed components for re-galvanizing.

Approved Repair Materials (ASTM A780)

1. Zinc-rich paint (organic or inorganic binders):
   
   
   
   • Minimum 65% zinc dust in dried film (by weight)
   • Preferred: >92% zinc content for optimal performance
2. Metallizing (thermal spray zinc):
   
   
   
   • Arc spray or flame spray application
   • Minimum 4 mils (100 microns) coating thickness
3. Zinc-filled solder stick:
   
   
   
   • Heated application for small areas
   • Suitable for minor damage repair
4. Galvanizing repair paint (proprietary formulations):
   
   
   
   • Specially formulated zinc-rich coatings meeting ASTM A780 requirements

Repair Surface Preparation

Effective repairs require proper surface preparation:

1. Remove loose or poorly adhered coating
2. Remove rust or corrosion products exposing sound substrate
3. Abrade surrounding coating for profile (if painting)
4. Clean and degrease repair area
5. Apply repair material per manufacturer specifications

Prevention Strategies

Steel Specification and Procurement

Primary prevention begins with proper steel specification:

• Request mill test reports documenting chemistry before galvanizing
• Specify silicon content outside the Sandelin range (<0.04% or 0.15-0.22%)
• Reference ASTM A385 chemistry recommendations in purchase specifications
• Consider blasting before galvanizing if reactive steel cannot be avoided

Galvanizing Process Modifications for Reactive Steel

When reactive steel must be galvanized, process adjustments minimize coating thickness:

1. Reduced immersion time: Minimize bath residence time while achieving minimum coating thickness
2. Lower bath temperature: Reduce from standard 840-850°F (449-454°C) to lower bound of acceptable range
3. Rapid cooling: Accelerate cooling to terminate zinc-iron reaction quickly
4. Pre-galvanizing blast cleaning: Surface profile reduces coating growth on low-silicon steels

Design and Handling Practices

Minimize flaking risk through appropriate design and handling:

• Design to accommodate standard steel grades with controlled chemistry
• Specify protective packaging for transportation
• Implement careful handling procedures during installation
• Train installation crews on galvanized steel characteristics

Long-Term Performance Implications

Does Flaking Affect Service Life?

The long-term implications of flaking depend entirely on whether repairs are performed:

Unrepaired Flaking: Exposed steel at flaked locations will corrode, potentially compromising structural integrity and leading to premature failure. The surrounding intact galvanized coating cannot provide cathodic protection across large bare areas.

Properly Repaired Flaking: When flaking is identified and properly repaired per ASTM A780 using appropriate materials, long-term corrosion protection is restored. Zinc-rich repair materials (≥92% zinc content) provide corrosion protection approaching that of hot-dip galvanizing, with service lives of 20-40+ years depending on environment and application.

Post-Installation Stability

Critically, flaking does not continue or progress after installation absent additional mechanical impacts. Once installed and properly repaired, galvanized components demonstrate the expected 50-75 year service life characteristic of hot-dip galvanizing in typical atmospheric environments.

Contractual and Liability Considerations

Purchase Order and Contract Language

Clear contractual language protects all parties:

For Steel Suppliers: "Steel supplied for hot-dip galvanizing shall meet chemistry recommendations of ASTM A385, Section 3.2, particularly regarding silicon content outside the range of 0.04-0.15%."

For Galvanizers: "Galvanizer shall request and review mill test reports for all steel to be galvanized. Steel not meeting ASTM A385 chemistry recommendations may be refused or galvanized with notification to customer regarding potential for excessive coating thickness."

For Contractors/Customers: "Customer assumes responsibility for damage to galvanized coatings, including flaking, occurring after acceptance at the galvanizing facility."

Liability Limitation Clauses

Galvanizers may include contractual language limiting liability when galvanizing reactive steel:

"When Customer provides steel with chemistry outside ASTM A385 recommendations, Galvanizer is not liable for excessive coating thickness, unusual appearance, or coating brittleness/flaking resulting from steel chemistry, provided Galvanizer has notified Customer of non-conforming steel chemistry before galvanizing."

Case Study Context and Industry Experience

Industry experience demonstrates that flaking, while a recognized phenomenon, represents a relatively uncommon occurrence when proper practices are followed:

• Frequency: Flaking affects <1% of galvanized tonnage when steel chemistry recommendations are followed
• Primary cause: 80-90% of flaking cases involve steel with silicon content in the Sandelin range
• Prevention effectiveness: Pre-galvanizing chemistry verification and reactive steel identification reduce flaking incidence by >90%

Most galvanizers maintain decades-long track records with minimal flaking occurrences when working with properly specified steel grades.

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Coating flaking in hot-dip galvanized steel results from the combination of excessively thick, brittle coatings (typically associated with reactive steel chemistry) and mechanical impact during handling or installation. Critical understanding points include:

1. Flaking requires impact—it does not occur spontaneously or progress after installation
2. Steel chemistry represents the primary predisposing factor
3. Responsibility assignment depends on when/where flaking occurs relative to acceptance
4. Proper repairs restore long-term corrosion protection
5. Prevention focuses on steel specification and chemistry verification

Engineers specifying hot-dip galvanizing should reference ASTM A385 chemistry recommendations in steel procurement specifications, verify mill test report compliance before galvanizing, and establish clear contractual language regarding responsibility and acceptance criteria. When these practices are followed, flaking represents a manageable, preventable concern rather than a barrier to specifying hot-dip galvanizing for long-term corrosion protection. Read more at the original AGA resource on flaking concerns.

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