Distinguishing Between Two Distinct Coating Failure Mechanisms
Hot-dip galvanized coatings occasionally exhibit coating integrity issues that manifest as visible separation or loss of zinc layers from the steel substrate. While casual observers might categorize any such coating separation under generic terms like "delamination" or "coating failure," galvanizing professionals recognize two fundamentally different phenomena—peeling and flaking—that arise from distinct metallurgical mechanisms, occur under different conditions, and require different prevention and remediation approaches.
Understanding the differences between peeling and flaking proves essential for multiple stakeholder groups. Engineers specifying galvanized coatings need to recognize which failure mode concerns apply to their specific applications and steel selections. Quality control inspectors must accurately diagnose coating issues to determine whether rejection, repair, or acceptance proves appropriate. Galvanizers require this knowledge to implement process controls preventing these conditions. Even project owners benefit from understanding these distinctions when evaluating coating performance claims or addressing field issues.
The confusion between peeling and flaking often stems from superficial similarities—both involve zinc coating separation from steel, both create aesthetic concerns, and both may compromise long-term corrosion protection. However, the underlying metallurgical processes, characteristic appearances, remaining coating configurations, and root causes differ substantially between these two phenomena. Mastering these distinctions enables informed decision-making about steel selection, galvanizing process parameters, handling practices, and quality acceptance criteria.
The Layered Structure of Hot-Dip Galvanized Coatings
Before examining peeling and flaking mechanisms, understanding the fundamental structure of hot-dip galvanized coatings provides necessary context. When steel enters molten zinc at galvanizing temperature—approximately 840°F (449°C)—a metallurgical reaction occurs between iron and zinc, producing a multi-layered coating structure growing outward from the steel substrate.
The Gamma (Γ) Layer
Immediately adjacent to the steel substrate, the gamma layer forms as the initial reaction product between iron and zinc. This layer comprises approximately 75% zinc and 25% iron by weight, representing the highest iron content of any coating layer. The gamma layer establishes the primary bond between the galvanized coating and steel substrate, typically exhibiting exceptional adhesion through direct metallurgical bonding at the atomic level.
The gamma layer's composition gives it intermediate properties between pure zinc and steel—harder and more brittle than pure zinc but more ductile than the outer alloy layers. Typical gamma layer thickness ranges from 0.2 to 1.0 mil (5 to 25 micrometers) depending on steel chemistry, bath temperature, and immersion time.
The Delta (δ) and Zeta (ζ) Layers
Above the gamma layer, progressively zinc-richer intermetallic layers form through continued iron-zinc diffusion. The delta layer contains approximately 90% zinc and 10% iron, while the zeta layer composition approaches 94% zinc and 6% iron. These intermediate alloy layers exhibit increasing hardness and brittleness as zinc content increases.
Together, the delta and zeta layers typically account for the majority of total coating thickness in normal galvanized coatings. Their combined thickness varies substantially based on steel chemistry, with reactive steel producing dramatically thicker alloy layers compared to non-reactive steel under identical galvanizing conditions.
The Eta (η) Layer
The outermost layer consists of nearly pure zinc (greater than 99% zinc) that solidifies atop the intermetallic layers as the galvanized article exits the zinc bath and begins cooling. This eta layer provides the characteristic bright metallic appearance of freshly galvanized steel and offers excellent ductility compared to the brittle intermetallic layers beneath.
The eta layer serves multiple protective functions including providing a ductile surface layer that accommodates minor mechanical stresses without cracking, offering a reservoir of zinc for continued galvanic protection even if outer portions weather away, and supplying zinc for continued alloy layer growth during slow cooling conditions.
Understanding this layered structure proves crucial for comprehending how peeling and flaking differ in terms of which layers separate and what coating configuration remains after failure.
Peeling: Eta Layer Separation Through the Kirkendall Effect
Peeling represents a coating failure mode characterized by separation of the outer eta (pure zinc) layer from the underlying intermetallic alloy layers. This phenomenon differs fundamentally from flaking in both its appearance and the coating configuration remaining after separation occurs.
Visual and Physical Characteristics of Peeling
When peeling occurs, the separated coating consists of relatively thin, flexible sheets or flakes of nearly pure zinc corresponding to the eta layer. These separated pieces can often be bent or flexed without breaking due to zinc's ductility. The underlying steel surface, after eta layer removal, exhibits a matte gray appearance reflecting the exposed intermetallic alloy layers rather than bare steel.
Coating thickness measurements taken on areas that have experienced peeling typically show residual coating in the range of 2 to 6 mils (50 to 150 micrometers), confirming that substantial alloy layer thickness remains bonded to the steel. This residual coating continues providing corrosion protection, though reduced compared to intact coating with full eta layer thickness.
Peeling often manifests as irregular patches or islands where eta layer has separated, creating a mottled appearance combining areas of bright zinc (intact coating), matte gray (exposed alloy layers), and sometimes visible separated eta layer sheets clinging to the surface. The separation may progress gradually over time or occur suddenly during handling or thermal cycling.
The Kirkendall Effect: Root Cause of Peeling
The metallurgical mechanism underlying most peeling cases involves a phenomenon known as the Kirkendall Effect, named after physicist Ernest Kirkendall who first documented this diffusion-related process. Understanding this effect requires examining what occurs during extended high-temperature exposure of galvanized coatings.
During normal galvanizing, steel remains immersed in molten zinc for relatively brief periods—typically seconds to minutes—then withdraws and begins cooling. As cooling progresses and temperature drops below approximately 600°F (315°C), zinc-iron diffusion rates decrease dramatically, effectively halting further alloy layer growth. The eta layer solidifies as a stable outer covering, and the coating structure remains essentially unchanged thereafter under normal temperature conditions.
However, when galvanized steel experiences extended exposure to elevated temperatures—either through extremely slow cooling after galvanizing or through subsequent high-temperature service exposure exceeding 400°F (200°C) for prolonged periods—zinc-iron diffusion reactions can resume. The eta layer, consisting of nearly pure zinc, serves as the zinc source for continued alloy layer growth at the interface between eta and zeta layers.
The Kirkendall Effect describes the phenomenon where diffusion rates of two species (in this case, zinc diffusing into iron-rich layers and iron diffusing outward) differ substantially. Zinc atoms diffuse inward faster than iron atoms diffuse outward, creating a net vacancy flux that accumulates at the interface between layers. These accumulated vacancies coalesce into voids that progressively weaken the bond between eta and zeta layers.
As void formation continues, the interface between pure zinc and alloy layers becomes increasingly discontinuous until mechanical stress—from thermal expansion/contraction, handling, or even coating weight—causes separation along the weakened interface. The eta layer peels away, leaving the intact but zinc-depleted alloy layers adhered to the steel.
Conditions Promoting Peeling
Several process and service conditions create elevated risk for peeling through Kirkendall Effect mechanisms:
Slow cooling of large, massive galvanized components represents a common cause. Heavy structural members, thick-walled vessels, or assemblies with significant thermal mass cool slowly after withdrawal from the zinc bath, maintaining temperatures above 600°F for extended periods. During this slow cooling phase, continued diffusion reactions consume eta layer zinc while creating interfacial voids that eventually cause separation.
Inadequate quenching or air blast cooling after galvanizing exacerbates slow cooling concerns. Galvanizers typically employ water quenching, air jets, or forced air cooling to accelerate cooling of components susceptible to peeling. When these cooling interventions prove inadequate or are omitted, slow cooling conditions persist.
High-temperature service exposure creates peeling risk for galvanized components operating at elevated temperatures. Applications including oven components, exhaust systems, or industrial process equipment where galvanized parts experience sustained temperatures exceeding 400°F may develop peeling over time as Kirkendall Effect mechanisms progressively weaken the eta-zeta interface.
Post-galvanizing thermal processes represent another risk scenario. If galvanized components undergo thermal treatments such as stress relief, paint baking, or assembly processes involving elevated temperatures, the thermal exposure may trigger Kirkendall Effect void formation and subsequent peeling.
Prevention Strategies for Peeling
Preventing peeling requires addressing the thermal conditions that enable Kirkendall Effect mechanisms:
Rapid cooling after galvanizing represents the primary prevention strategy. Galvanizers should employ water quenching, forced air cooling, or other accelerated cooling methods for components at risk of slow cooling, particularly heavy sections and massive assemblies. The goal involves reducing temperature below 600°F as quickly as practical to minimize time available for void-forming diffusion.
Design considerations affecting thermal mass and cooling rates warrant evaluation. Components designed with relatively uniform section thicknesses cool more predictably than those combining thin and thick sections. Where heavy sections prove unavoidable, design features promoting heat dissipation—such as increased surface area or internal cooling passages—can accelerate cooling.
Service temperature limits must be observed. Galvanized coatings should not be specified for continuous service above 392°F (200°C) per standard recommendations, with intermittent exposure limits somewhat higher. Applications requiring sustained high-temperature operation should employ alternative coatings better suited to thermal service.
Post-galvanizing thermal process scheduling requires attention. When galvanized components must undergo baking or thermal processing, limiting exposure temperature and duration to minimize zinc-iron interdiffusion reduces peeling risk.
Flaking: Cohesive Failure of Brittle Thick Coatings
Flaking represents an entirely different coating failure mechanism compared to peeling, involving cohesive separation of multiple coating layers rather than interfacial separation at a single layer boundary. The distinction proves crucial for understanding root causes and implementing appropriate prevention strategies.
Visual and Physical Characteristics of Flaking
Flaked coating material exhibits dramatically different physical properties compared to peeled material. Flaked pieces consist of thick, hard, brittle shards comprising the zeta, delta, and gamma layers that have separated en masse from the steel substrate. These shards typically measure several mils thick and demonstrate extreme brittleness—attempting to bend flaked coating pieces results in shattering rather than flexing.
The characteristic appearance of flaked areas shows near-complete coating removal, often exposing bare steel or leaving only an extremely thin gamma layer remnant. Coating thickness measurements in flaked areas typically register near zero—commonly 0.2 to 0.5 mils (5 to 12 micrometers) or even zero in some locations—confirming that essentially the entire coating has departed.
Flaked areas exhibit sharp, well-defined edges where coating material has cleanly separated from the substrate. The fractured edges show clear cross-sections revealing the multilayered structure of the departed coating. This sharply defined failure boundary distinguishes flaking from gradual coating wear or corrosive coating loss that produces irregular, fuzzy boundaries.
Flaking commonly occurs at edges, corners, and areas subjected to impact or mechanical stress during handling and installation. These locations experience stress concentrations that exceed the cohesive strength of brittle, thick coatings, initiating fracture propagation through the coating thickness and along the gamma layer-steel interface.
Root Cause: Excessive Coating Thickness and Brittle Behavior
Unlike peeling which involves interfacial weakness created by diffusion processes, flaking stems from the mechanical properties of excessively thick galvanized coatings. As coating thickness increases beyond typical ranges—particularly when exceeding 8 to 10 mils (200 to 250 micrometers)—the coating's brittleness increases disproportionately while its ability to withstand mechanical stress decreases.
This relationship between coating thickness and brittleness derives from multiple factors. Thick coatings contain proportionally greater volumes of hard, brittle intermetallic alloy layers compared to the ductile outer eta layer. The alloy layers possess extremely limited plastic deformation capability, behaving essentially as brittle ceramic-like materials.
Additionally, thick coatings develop high residual stresses during cooling from galvanizing temperature. The differential thermal contraction between zinc (which has a high coefficient of thermal expansion) and steel (with lower thermal expansion coefficient) generates tensile stresses within the coating as temperature decreases. Thicker coatings accumulate greater total strain energy, creating internal stresses that approach or exceed the cohesive strength of brittle alloy layers.
When these highly stressed, brittle thick coatings experience mechanical loading—from impact during handling, stress concentration at geometric discontinuities, thermal cycling in service, or mechanical strain from substrate deformation—crack initiation and propagation occur readily. Once initiated, cracks propagate rapidly through the brittle alloy layers and along the gamma layer-steel interface, causing large coating sections to separate.
The mechanical property mismatch between coating and substrate exacerbates flaking susceptibility. Zinc and its alloys exhibit much lower elastic modulus compared to steel, meaning the coating deforms more readily under equivalent stress. During handling impacts or mechanical loading, the soft steel substrate deforms elastically or plastically while the overlying brittle coating cannot accommodate equivalent deformation without fracturing.
Reactive Steel Chemistry: The Primary Driver of Excessive Thickness
While flaking fundamentally represents a mechanical failure mode, understanding why excessively thick coatings develop requires examining steel chemistry effects on coating growth kinetics. The relationship between steel composition—particularly silicon and phosphorus content—and galvanized coating thickness has been extensively documented through decades of research and industrial experience.
Steel silicon content exerts the most dramatic influence on coating thickness. Steel with silicon content below 0.03% produces relatively thin coatings through normal zinc-iron reaction kinetics. However, steel with silicon in the range of 0.04% to 0.14% exhibits highly reactive behavior, generating extremely thick coatings—often 8 to 12 mils (200 to 300 micrometers) or more—under standard galvanizing conditions.
The mechanism involves silicon's catalytic effect on zinc-iron diffusion. Silicon atoms in solid solution within steel lattice promote accelerated zinc penetration and iron-zinc compound formation. The accelerated reaction kinetics produce disproportionately thick alloy layers, particularly the zeta layer which can become extraordinarily massive in reactive steel.
Steel with silicon content between 0.15% and 0.22% demonstrates more moderate reactivity, typically producing coating thickness in normal to slightly elevated ranges. Silicon above 0.22% often returns to relatively non-reactive behavior, though coating thickness and appearance vary considerably in this range.
Phosphorus content presents similar but less pronounced effects. Steel with phosphorus exceeding 0.04% can exhibit reactive coating growth even when silicon falls within normally non-reactive ranges. The combined effects of silicon and phosphorus prove particularly problematic, with steel containing elevated levels of both elements presenting the highest flaking risk.
ASTM A385, "Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)," addresses reactive steel concerns by providing recommended steel chemistry ranges that minimize coating thickness issues. Following these recommendations—silicon below 0.04% or between 0.15% and 0.22%, phosphorus below 0.035%, manganese below 1.35%—substantially reduces flaking risk by preventing excessive coating development.
Prevention Strategies for Flaking
Preventing flaking requires a multi-faceted approach addressing both steel selection and galvanizing process parameters:
Steel chemistry specification represents the most effective prevention strategy. Specifying steel meeting ASTM A385 chemistry recommendations ensures that coating thickness remains within normal ranges less susceptible to flaking. This proactive approach addresses the root cause rather than attempting to compensate through process modifications.
When reactive steel must be galvanized, several process modifications can limit coating thickness. Minimizing immersion time in the zinc bath reduces the duration available for excessive alloy layer growth. However, care must be taken to ensure sufficient immersion time for complete steel heating and adequate coating formation on all surfaces.
Reducing galvanizing bath temperature by 10 to 20°F below normal operating temperature slows zinc-iron reaction kinetics, producing thinner coatings on reactive steel. However, excessively low bath temperatures create problems including incomplete zinc wetting, poor coating appearance, and difficulty maintaining adequate zinc fluidity for drainage.
Abrasive blast cleaning steel before galvanizing can significantly reduce coating thickness that develops on reactive steel. The mechanical roughening and work-hardening of the steel surface through blasting appears to interfere with zinc-iron diffusion, moderating the reactive coating growth. Blast cleaning proves particularly effective for reactive structural shapes and plate.
Accelerated cooling through water quenching immediately after zinc bath withdrawal limits the time available for continued coating growth during cooling, helping control final coating thickness. However, caution proves necessary as excessive thermal shock from overly aggressive quenching can itself cause coating damage or substrate distortion.
Post-galvanizing coating thickness reduction through light abrasive blasting or chemical etching offers a remedial approach for components that developed excessive thickness. However, these secondary processes add cost and may affect coating appearance.
Contractual Considerations for Reactive Steel
Galvanizers facing customers who consistently provide reactive steel should consider incorporating contractual language addressing the inherent risks. Purchase orders or processing agreements might include provisions limiting galvanizer liability when processing steel outside ASTM A385 recommended chemistry ranges. Such provisions recognize that flaking represents an inherent characteristic of reactive steel rather than a galvanizing quality defect.
Documentation requirements specifying that customers provide steel chemistry certifications enable galvanizers to identify reactive steel before processing and implement appropriate handling procedures. Pre-notification of reactive steel enables process planning including blast cleaning, immersion time reduction, or quotation adjustments reflecting additional processing requirements or increased rejection risk.
Inspection, Acceptance Criteria, and Remediation
Both peeling and flaking create coating integrity concerns requiring evaluation against specification acceptance criteria and potential remediation.
Inspection and Detection
Visual inspection readily identifies both peeling and flaking through characteristic appearances. Peeled areas show matte gray exposed alloy layers, often with loosely attached eta layer sheets. Flaked areas exhibit sharply defined bare or nearly bare regions with brittle coating pieces nearby.
Coating thickness measurement using magnetic thickness gauges confirms the diagnosis. Peeled areas show substantial remaining thickness (2-6 mils) confirming intact alloy layers. Flaked areas measure near-zero thickness indicating complete coating removal.
The simple field test of attempting to bend separated coating material provides immediate differentiation—peeled material (pure zinc) bends readily while flaked material (alloy layers) shatters when bent.
Acceptance and Rejection Criteria
ASTM galvanizing specifications including A123/A123M and A153/A153M establish that coatings must be adherent and continuous. Both peeling and flaking constitute coating discontinuities requiring evaluation.
For peeling, the acceptance decision considers the remaining coating thickness and affected area. Minor peeling leaving 2+ mils of adherent alloy layer coating may be accepted for many applications since substantial corrosion protection remains. However, extensive peeling or applications with critical appearance requirements may warrant rejection.
Flaking typically constitutes grounds for rejection due to near-complete coating removal creating bare or minimally protected areas. The lack of adequate remaining coating thickness compromises long-term corrosion protection. Additionally, flaking often indicates coating brittleness that will likely cause continued problems during subsequent handling and service.
Remediation Options
Components exhibiting limited peeling or flaking can be repaired through coating touch-up methods specified in ASTM A780/A780M. Zinc-rich paint, thermal spray zinc, or other approved repair materials can restore corrosion protection to affected areas when damage remains limited in extent.
For extensive coating failure, stripping and regalvanizing offers complete remediation. The hot-dip galvanizing process does not adversely affect steel mechanical properties, enabling repeated processing. During regalvanizing, process parameters can be adjusted to prevent recurrence—employing rapid cooling for peeling prevention or reduced immersion time for flaking prevention on reactive steel.
However, if flaking resulted from inherently reactive steel chemistry, regalvanizing may reproduce the same excessive thickness and brittleness unless process modifications or blast cleaning are employed. In some cases, acceptance of moderately flaked components with extensive repair may prove more economical than regalvanizing with uncertain outcomes.
Peeling and flaking represent distinct coating failure mechanisms affecting hot-dip galvanized steel through fundamentally different metallurgical and mechanical processes. Peeling involves interfacial separation of the outer eta zinc layer from underlying alloy layers through Kirkendall Effect void formation during slow cooling or high-temperature exposure. Flaking describes cohesive failure of excessively thick, brittle coatings subjected to mechanical stress, typically resulting from reactive steel chemistry that produces abnormally massive zinc-iron alloy layers.
Accurate diagnosis distinguishing between these phenomena enables appropriate prevention strategies and remediation approaches. Peeling prevention focuses on thermal management through rapid post-galvanizing cooling and observing service temperature limits. Flaking prevention emphasizes steel chemistry selection following ASTM A385 recommendations and, when reactive steel must be galvanized, implementing process modifications including immersion time reduction, bath temperature control, and pre-galvanizing blast cleaning.
Understanding these distinctions proves valuable for all stakeholders in galvanized steel projects. Engineers specifying galvanized coatings should recognize the importance of steel chemistry selection in preventing flaking while acknowledging thermal management requirements for preventing peeling. Quality inspectors must accurately diagnose which failure mode they encounter to apply appropriate acceptance criteria and remediation recommendations. Galvanizers benefit from understanding the metallurgical basis for each phenomenon, enabling informed process optimization and customer communication.
Both peeling and flaking remain relatively uncommon when appropriate steel grades are galvanized under controlled process conditions. However, when these issues do arise, the ability to quickly diagnose the specific failure mechanism, understand its root cause, and implement effective preventive or corrective measures distinguishes truly competent galvanizing practice from simple routine processing. Check out the original AGA resource on this topic here.
