Advances in the understanding of corrosion and performance of hot-dip galvanized rebar in concrete structures

2.1 Initial reactions in fresh concrete: passivation and the hydrogen evolution debate

When hot-dip galvanized rebar is first embedded in fresh concrete, its zinc coating is exposed to a highly alkaline pore solution, typically with a pH between 12.5 and 13.5, saturated with calcium hydroxide. As zinc is an amphoteric metal, it is reactive in such a strong alkaline environment, leading to a series of immediate chemical reactions that define the nature of the interface for the entire service life of the structure (Sistonen 2009). The initial anodic reaction involves the dissolution of zinc into zinc ions (Zn→Zn²⁺+2e⁻), while the corresponding cathodic reaction is predominantly the reduction of water to produce hydrogen gas (2H₂​O+2e⁻→H_2​↑+2OH⁻), as the high pH suppresses other cathodic reactions like oxygen reduction (Yeomans 2004).

This initial period of active corrosion is transient. The dissolved zincate ions react with the abundant calcium ions (Ca²⁺) present in the cement pore solution to precipitate a dense, adherent layer of a complex salt known as calcium hydroxyzincate (Ca[Zn(OH)₃​]₂​⋅2H₂​O), often abbreviated as CHZ (Bellezze et al. 2021). The formation of this calcium hydroxyzincate (CHZ) layer is widely considered to be the key passivation mechanism for galvanized steel in concrete (Garnica-Rodríguez et al. 2022). Once the surface is covered by this stable, insoluble film, the vigorous initial corrosion and hydrogen evolution cease, and the zinc coating enters a passive state with a very low corrosion rate (Maeda et al. 2020). The formation of this layer typically consumes about 10 µm of the outer pure zinc (eta) layer of the galvanized coating (Mandal et al. 2009). The thickness of the zinc coating plays a critical role in this process. A thicker coating provides a larger reservoir of zinc available for CHZ precipitation, ensuring that the initial 10 µm consumption represents a smaller fraction of the total coating. This generally results in a denser and more continuous CHZ layer and prolongs the duration of passivation (Al-Negheimish et al. 2021; Mandal et al. 2009). However, excessively thick coatings, particularly on reactive steels, may promote the growth of brittle intermetallic phases, which could adversely affect bond performance (Baig and Ahmad 2022). Thus, the balance between adequate thickness for passivation and avoidance of brittleness remains an important consideration in practice. Figure 1 provides a schematic representation of these initial reactions at the zinc–concrete interface. This diagram illustrates the electrochemical processes occurring when galvanized rebar is first placed in wet concrete. It shows the zinc coating (anode) dissolving to form zincate ions (Zn²⁺), while the cathodic reaction on the zinc surface reduces water to evolve hydrogen gas (H₂). The diagram depicts the subsequent reaction of zincate and calcium ions (Ca²⁺) from the pore solution to precipitate a protective, crystalline layer of calcium hydroxyzincate (Ca[Zn(OH)₃​]₂​⋅2H₂​O) on the rebar surface, which passivates the zinc and arrests the initial reaction.

Figure 1:

Schematic diagram of initial passivation of hot-dip galvanized rebar in fresh concrete (author-created).

The morphology of this critical passive layer has been studied using advanced microscopy. Figure 2 presents a representative scanning electron microscope (SEM) micrograph, illustrating the characteristic faceted morphology of the calcium hydroxyzincate layer formed on a galvanized surface.

Figure 2:

SEM micrograph of the CHZ passive film on galvanized steel (Zheng et al. 2020). Reproduced with permission from Elsevier.

While the formation of the CHZ layer is an accepted phenomenon, the consequences of the initial hydrogen evolution are a central point of scientific controversy, leading to two divergent viewpoints on its impact. It is important to note, however, that galvanized coatings are not applied to high-strength or prestressing steels, as hydrogen uptake during the early corrosion reactions can induce embrittlement of the steel matrix. This risk has led to prohibitions in standards and specifications for prestressing applications, confining the use of hot-dip galvanized reinforcement primarily to conventional reinforcing steels (mild or medium-strength grades) (Pernicova et al. 2017; Pokorný et al. 2024a, b). The first viewpoint, often promoted by industry associations and supported by a significant body of performance testing, treats the hydrogen evolution as a manageable and temporary side-effect. Proponents of this view argue that the reaction is short-lived, ceasing as soon as the concrete hardens and the CHZ layer passivates the surface. Laboratory studies are cited to show that the liberated hydrogen does not permeate the zinc coating to cause embrittlement of the underlying steel substrate. Furthermore, it is often suggested that any small voids or increased porosity at the interface created by the hydrogen bubbles are subsequently filled by the zinc corrosion products, negating any long-term detrimental effect on the bond. This perspective emphasizes that the reaction can be effectively controlled or suppressed through post-galvanizing passivation treatments. Historically, chromate-based methods (as required by ASTM A767) were widely adopted; however, the carcinogenicity of hexavalent chromium has led to their progressive replacement with environmentally safer alternatives. Recent developments include trivalent chromium (Cr(III)) passivation systems (e.g., ADL-3P, ADL-32), as well as organic and zinc-based coatings such as zinc phosphate, silicate, ceramic, or silane treatments, all of which provide effective protection against white rust and hydrogen evolution. Furthermore, passivating oils and molybdate/polyphosphate conversion processes have also been reported as viable non-chromate options for maintaining the integrity of galvanized reinforcement in concrete environments (Liu et al. 2012; Saravanan and Srikanth 2019).

In stark contrast, a more critical school of thought, supported by detailed mechanistic studies, posits that the initial hydrogen evolution is an inherently detrimental process that imparts a permanent weakness at the steel-concrete interface (Pernicova et al. 2017). The core of this argument is that the formation of hydrogen gas bubbles creates a zone of increased and irreversible porosity within the cement paste immediately adjacent to the rebar surface, known as the interfacial transition zone (ITZ) (Scrivener et al. 2004). Contrary to the first viewpoint, these researchers present evidence that this porosity is not sufficiently filled by corrosion products and thus remains as a permanent microstructural defect. This weakened, more porous ITZ is theorized to directly reduce the mechanical bond between the rebar and the concrete. Moreover, for high-strength steels or prestressing strands under tension, the hydrogen can cause embrittlement and micro-cracking of the zinc coating itself, compromising its barrier function from the very beginning. This debate is crucial, as it establishes a direct causal link between the chemistry of the first few hours of curing and the ultimate mechanical performance and long-term durability of the reinforced concrete element. The initial chemical reaction may therefore define the service life trajectory by creating a permanent microstructural feature that impacts both bond and the future ingress of aggressive ions.

2.2 The stability of the passive layer

The long-term performance of galvanized rebar hinges on the stability and protective quality of the CHZ passive layer (Al-Negheimish et al. 2021). The conventional understanding is that this layer, once formed, provides robust and lasting protection. It acts as a stable physical barrier, isolating the zinc from the concrete pore solution, and is particularly effective in the typical pH range of sound concrete (approximately pH 12.5 to 13.2) (Pokorný et al. 2022).

However, this foundational assumption has been challenged by recent, highly detailed research that suggests the passivity of galvanized steel in concrete is not an absolute state but rather a conditional and potentially precarious equilibrium. Work by Roventi et al. (2014) provides compelling evidence that the protective nature of the CHZ layer is highly dependent on both pH and moisture availability. Their electrochemical and microscopic analyses showed that while a protective film forms at a pH of 12.6, at slightly higher pH levels (13.0 and above), which can occur in some modern concrete formulations, the zinc coating remains in a state of active corrosion, continuing to produce hydrogen for extended periods. Their findings indicate that the CHZ layer formed at these higher pH levels is less compact and non-protective, and that the presence of calcium ions may actually destabilize the formation of more desirable passive films like zinc oxide (ZnO) or zinc hydroxide (Zn(OH)₂​) (Tan and Hansson 2008). This research raises the critical possibility that the apparent passivity observed in many older tests may not be true electrochemical stability, but rather a state of very slow corrosion dictated by the drying of the concrete and the consequent lack of a continuous electrolyte path. If the concrete becomes re-saturated with water, this “dormant” active corrosion could be re-initiated (Roventi et al. 2013). This nuanced perspective implies that in environments with persistent moisture and high alkalinity, the long-term protective performance of the CHZ layer may be significantly overestimated, suggesting its stability is conditional rather than absolute (Pokorný et al. 2024a, b).

2.3 Depassivation mechanisms

The protective passive layer on galvanized rebar, whether fully stable or conditionally so, can be broken down by two primary mechanisms: chloride ingress and carbonation.

2.3.1 Chloride-induced corrosion

The ingress of chloride ions, typically from de-icing salts or marine environments, is the most common cause of premature corrosion of reinforcement in concrete (Shi et al. 2023). Interestingly, galvanized rebar can maintain a protective passive film in pore solutions with pH values between approximately 12.0 and 13.5, but once localized pH declines below about 10.5, depassivation becomes increasingly likely (Pu et al. 2017). Environmental factors strongly modulate this process. Roventi et al. (2014) demonstrated that carbonation can degrade the original CHZ layer, and subsequent chloride-rich wet–dry cycles substantially reduce the chloride threshold for corrosion initiation, while wet–dry cycles in tap water alone have little accelerating effect. Furthermore, González and Andrade (1982) emphasized that ambient humidity and fluctuating moisture availability significantly affect the persistence of passivation. These findings highlight that both chemical (pH) and environmental (wet–dry cycling, humidity) conditions are decisive in governing the durability of galvanized reinforcement in chloride-contaminated concrete. The concentration of chlorides at the rebar surface required to initiate corrosion is known as the chloride threshold (CT). For black steel, this threshold is generally accepted to be around 0.4 % total chloride ion content by weight of cement. For galvanized steel, the literature consistently reports a much higher threshold, typically ranging from 2 to 5 times that of black steel, with some long-term field studies suggesting a tolerance up to 10 times higher. Reported values for the CT of galvanized rebar vary, but a value of 1.2 % by cement weight is often cited as a conservative average, with some studies finding initiation not occurring until chloride levels exceed 2.0 % (Conciatori et al. 2015). This elevated threshold significantly extends the corrosion initiation phase of the structure’s service life.

Once the chloride threshold is breached, the aggressive chloride ions locally disrupt the CHZ passive film. The exact mechanism is complex, but it is understood to involve the formation of soluble zinc chloride complexes, which leads to the breakdown of the passive layer and the precipitation of new, less protective corrosion products (Yu et al. 2020). The primary solid corrosion products identified in chloride-contaminated concrete are zinc oxide (ZnO) and a complex salt called zinc hydroxychloride, or simonkolleite (Zn₅​(OH)₈​Cl₂​⋅H₂​O) (Ghazaee et al. 2024). The formation of simonkolleite is of particular concern, as some studies suggest it is more voluminous than ZnO and could potentially generate expansive pressures, though this point is debated. Figure 3 illustrates the mechanism of chloride-induced depassivation. This diagram shows a cross-section of a galvanized rebar embedded in chloride-contaminated concrete. It illustrates chloride ions (Cl⁻) migrating through the concrete pores and attacking the protective CHZ layer. The schematic depicts the local breakdown of the CHZ film, the formation of new corrosion products like simonkolleite (Zn₅​(OH)₈​Cl₂​⋅H_2​O), and the subsequent active corrosion of the underlying zinc and zinc–iron alloy layers. The relative performance of different reinforcement types with respect to chloride tolerance is a critical factor for material selection in aggressive environments. Table 1 compiles typical chloride threshold values from the literature for a comparative overview.

Figure 3:

Schematic diagram of chloride-induced corrosion of galvanized rebar (author-created).

Table 1:

Comparative chloride thresholds for various reinforcement types in concrete.

Reinforcement type	Typical chloride threshold (% Cl− by weight of cement)	Relative threshold (vs. black steel)	Key references
Black (carbon) steel	0.2 %–0.4 %	1x	Hurley and Scully (2006)
Epoxy-coated steel (ECR)	Similar to black steel if damaged; very high if intact	∼1x (at defects)	López-Calvo et al. (2013)
HDG steel	0.8 %–2.0 % (or higher)	2x – 5x (or higher)	Kenny and Katz (2019)
Stainless steel (e.g., 316)	> 2.0 % (often much higher)	>5x – 24x	Darwin et al. (2007)

4.1 The controversy of bond strength: a critical analysis

The composite action of reinforced concrete is entirely dependent on the transfer of stress between the rebar and the surrounding concrete, a mechanism known as bond (Khaksefidi et al. 2021). Any corrosion protection system applied to the rebar must not compromise this critical property. For galvanized rebar, the literature presents a deeply divided and conflicting picture of its effect on bond strength, representing one of the most significant areas of debate in the field (Kobayashi et al. 2021).

One body of research, comprising numerous pull-out and beam-end tests, concludes that the bond strength of hot-dip galvanized rebar is equivalent to, or in some cases superior to, that of uncoated black steel (Mohammed and Roopa 2022). The primary mechanism proposed to explain this enhanced performance is the formation of the crystalline CHZ layer at the interface during concrete curing. It is theorized that these crystals grow into the cement paste, creating a strong chemical adhesion and enhancing the mechanical interlock between the rebar’s surface and the concrete matrix, thereby increasing the force required to cause slip (Khalaf et al. 2016). Studies supporting this view often find that while the initial hydrogen evolution reaction occurs, its effect on the final, fully cured bond strength is negligible.

Conversely, a compelling and growing body of evidence, often from studies employing more detailed microstructural analysis, reports a significant reduction in bond strength for galvanized rebar compared to black steel (Jaśniok et al. 2021). The principal mechanism cited for this reduction is the detrimental effect of hydrogen evolution during the initial 24–48 h of curing. As discussed in Section 2.1, the formation of hydrogen gas bubbles at the interface is argued to create a permanent zone of increased porosity in the adjacent cement paste (Pernicova et al. 2017). This porous and weaker ITZ provides a less effective medium for stress transfer, leading to a lower overall bond capacity. Some researchers also suggest that the formation of zinc corrosion products themselves can be non-adherent or voluminous, leading to local disintegration at the interface that further compromises the bond (Groshek 2017). Another contributing factor can be the physical nature of the coating itself; a very thick or non-uniform galvanized coating, particularly on reactive steels, can smooth the profile of the rebar’s ribs, reducing the mechanical bearing component of the bond (Baig et al. 2022). This fundamental disagreement in the literature highlights the complexity of the interfacial zone, where competing mechanisms – chemical adhesion from CHZ versus microstructural damage from hydrogen – are at play. The net effect likely depends on a sensitive balance of factors including cement chemistry, concrete mix design, curing conditions, and the quality of the galvanizing itself. Table 2 summarizes the key arguments and findings from this ongoing debate.

4.2 Comparative performance against alternative systems

The decision to use galvanized rebar is often made in the context of other available corrosion protection systems, primarily epoxy-coated rebar (ECR) and stainless steel rebar (Gagné et al. 2020).

ECR has been a widely used alternative to black steel, relying on an organic coating to provide a purely physical barrier against moisture and chlorides (Yadav et al. 2022). When the epoxy coating is perfectly applied and remains undamaged, it can provide excellent corrosion protection. However, its primary weakness lies in its susceptibility to damage during transportation, handling, and installation on the construction site. Even minor nicks or scratches can create holidays in the coating, exposing the underlying steel. At these defect sites, corrosion can initiate and propagate aggressively underneath the coating, a failure mode known as under-film or filiform corrosion, which can cause localized debonding of the epoxy and intense pitting of the steel (Zhang et al. 2019). In contrast, the metallurgically bonded zinc coating of HDG rebar is significantly harder and more abrasion-resistant, making it more robust against construction site damage (Pinger et al. 2024). Crucially, if the HDG coating is damaged, the surrounding zinc provides sacrificial cathodic protection to the exposed steel, preventing the initiation of corrosion until the local zinc is consumed (Choe et al. 2019). Life-cycle cost analyses frequently conclude that while ECR may have a lower initial cost in some cases, HDG often proves more cost-effective over the life of the structure due to its greater durability, reduced need for field repairs, and superior performance when minor damage is considered (Ni et al. 2022).

Stainless steel reinforcement represents a premium corrosion protection solution, offering exceptionally high resistance to chloride-induced corrosion, with a chloride threshold many times higher than that of both black steel and galvanized steel. For structures in the most aggressive environments requiring a service life of 100 years or more, stainless steel is often considered the most reliable option (Rabi et al. 2022). However, this superior performance comes at a substantial cost premium, which can be prohibitive for many projects. Hot-dip galvanized rebar is therefore positioned as a highly effective intermediate solution. It provides a significant and well-documented extension of service life over uncoated steel at a much more moderate initial cost than stainless steel, offering a balanced approach to durability and economics for a wide range of applications (Coppola et al. 2022).

While laboratory tests provide mechanistic insights, the true measure of a corrosion protection system is its performance over decades in real-world structures. Field studies of aged structures containing galvanized rebar provide invaluable data on its long-term durability. One of the most widely cited and compelling case studies is the Athens Bridge in Pennsylvania, USA, constructed in 1973 using HDG rebar in its deck (Shear 1973). Inspections conducted 8, 18, and 28 years after construction consistently found the galvanized rebar to be in excellent condition with no signs of active corrosion, despite the presence of high chloride concentrations in the concrete (up to 7.9 lbs/yd³, or approximately 4.7 kg/m³) from the use of de-icing salts – levels that would have caused severe corrosion in black steel. Even after nearly three decades of service, the zinc coating thickness was still well in excess of the minimum requirements for new bars, suggesting many more decades of maintenance-free life. Similar positive long-term performance has been documented in other structures, including bridges in Bermuda and Florida, and various projects undertaken by Departments of Transportation. Table 3 summarizes the findings from several key field studies.

A critical aspect of this long-term performance is the behavior of the zinc corrosion products. Unlike the voluminous, expansive iron oxides that cause cracking and spalling, zinc corrosion products are less dense, more soluble in the alkaline pore water, and tend to migrate away from the rebar surface into the surrounding concrete matrix. This migration prevents the buildup of localized stress at the interface, meaning that even when the zinc coating does begin to corrode, it does not typically cause the destructive spalling associated with black steel corrosion (Karlsson 2014). There is also evidence that these migrating zinc compounds can fill the pores in the ITZ, leading to a densification of the concrete around the bar, which can further slow the ingress of aggressive species and may contribute to the observed good long-term bond strength. This fundamental difference in the failure mechanism is a key advantage of galvanized rebar (Belaı̈d et al. 2001). Figure 4 provides a schematic comparison of the failure modes.

Figure 4:

Schematic comparison of corrosion-induced failure mechanisms for black steel and hot-dip galvanized steel (author-created).

While the majority of long-term field data is positive, it is important to acknowledge reports of mixed or poor performance. These instances are often linked to specific, highly aggressive local conditions, poor quality or highly permeable concrete, or issues with the galvanizing process itself, such as the use of highly “reactive” steels (with specific silicon or phosphorus contents) that can result in overly thick and brittle zinc–iron alloy coatings that are prone to flaking. This highlights that the performance gap between controlled laboratory tests and real-world field observations is a significant source of the ongoing controversies. Accelerated lab tests can fail to replicate the slow, complex chemical evolution of the concrete environment and may accelerate incorrect failure mechanisms, leading to pessimistic predictions (Mao et al. 2025). Conversely, positive field results demonstrate the system’s potential under real service conditions. This underscores that the selection of a corrosion protection system is not merely a materials science decision but also a comprehensive risk management and economic calculation, balancing initial cost, construction practicalities, and desired service life against the specific environmental challenges of the project.

5.1 Innovations in galvanizing processes: continuous galvanized rebar (CGR)

One of the most significant recent technological advancements is the development and commercialization of CGR (Liu and Stroia 2020). This technology represents a fundamental shift from the traditional batch HDG process. In the CGR process, blast-cleaned and pre-heated rebar is passed continuously through a molten zinc bath at high speeds (e.g., 10 m per minute), resulting in a very short immersion time of only a few seconds. This is in sharp contrast to the batch process, where bundles of fabricated rebar are immersed for several minutes.

A critical feature of the CGR process is the addition of a small amount of aluminum (typically around 0.2 %) to the molten zinc bath. The combination of the short immersion time and the presence of aluminum effectively retards the metallurgical reaction between the iron and zinc. This prevents the formation of the thick, relatively brittle iron-zinc intermetallic alloy layers (zeta and delta phases) that characterize traditional HDG coatings. Instead, the CGR process produces a coating that is typically thinner (around 50–60 µm) and consists almost entirely of a ductile, pure zinc (eta) phase over a very thin iron-aluminum-zinc bonding layer (Marder 2000). Figure 5 schematically compares the microstructures of the two coating types.

Figure 5:

Schematic comparison of coating microstructures: Traditional HDG versus CGR. This diagram presents a side-by-side cross-sectional view of (a) a traditional HDG coating and (b) a CGR coating. Panel (a) shows the distinct, thick layers of the HDG coating: The gamma, delta, and zeta iron–zinc intermetallic alloys, topped with a layer of pure eta zinc. Panel (b) illustrates the CGR coating, highlighting its much thinner overall profile, the near-absence of thick intermetallic layers, and a composition that is almost entirely pure, ductile eta zinc over a very thin Fe–Al–Zn bonding layer (author-created).

This distinct microstructure endows CGR with significant performance advantages. The most notable is superior formability. The ductile, pure zinc coating allows the rebar to be bent and fabricated after galvanizing without the risk of cracking or flaking that can occur with the brittle intermetallic layers of traditional HDG bars (Andrade and Alonso 2001). This simplifies the supply chain and reduces the need for costly and time-consuming post-fabrication galvanizing or field repairs. From a corrosion perspective, the thicker layer of pure zinc is theoretically ideal for the passivation reaction in concrete and provides a larger, more uniform reservoir for sacrificial protection.

The CGR process also offers substantial economic and environmental benefits. It is a more energy-efficient, in-line process that uses significantly less zinc per ton of steel compared to batch galvanizing (typically reducing zinc consumption by about half), making it a more cost-effective and sustainable manufacturing method (Viklund-White 2000). A new standard, ASTM A1094/A1094 M, has been established to govern the properties of CGR, formalizing its place as a modern alternative to traditional HDG. The development of CGR can be seen as an implicit acknowledgment by the industry of the known limitations of traditional HDG, particularly its brittleness and fabrication constraints. CGR is a targeted technological solution designed to overcome these specific weaknesses, representing a significant evolutionary step. Table 4 provides a direct comparison of the key features of the two processes.

5.2 Advances in zinc alloy coatings

Beyond the Al–Zn system used in CGR, research continues into other zinc alloy coatings to further enhance performance. For instance, studies on Zn–Al-Mischmetal coatings have demonstrated the potential for creating thinner, more ductile coatings with improved corrosion resistance in both simulated concrete pore solution and chloride environments (Wang et al. 2019). These investigations point toward an active field of materials development focused on tailoring the composition of the zinc bath to optimize the final coating’s mechanical properties and electrochemical behavior, offering pathways to even more durable and specialized reinforcement solutions.

The ability to accurately assess the condition of galvanized rebar in concrete and predict its remaining service life is crucial for infrastructure management. The field has seen significant advances in both laboratory testing and in-situ monitoring techniques. A variety of electrochemical methods are used to evaluate the corrosion performance of galvanized rebar in laboratory settings. The most common is the macrocell corrosion test outlined in ASTM G109, which measures the corrosion current between a top, chloride-exposed rebar and a bottom, passive rebar (Alonso et al. 2000). While widely used for comparative studies, the standard G109 test is often criticized for being slow and labor-intensive, leading many researchers to develop modified or accelerated versions (Darwin et al. 2007). Results from these tests generally show that galvanized rebar outperforms black steel, but performance can be marginal under certain conditions (Presuel-Moreno and Rourke 2009).

For assessing structures in the field, several non-destructive testing (NDT) techniques are employed. These include corrosion potential mapping (ASTM C876) to identify areas with a high probability of corrosion, linear polarization resistance (LPR) to measure the instantaneous corrosion rate, and ground-penetrating radar (GPR) to determine concrete cover and locate reinforcement (Zaki et al. 2018). However, the interpretation of data from these methods, which were primarily developed for black steel, can be more complex for galvanized rebar due to the different electrochemical nature of zinc and its corrosion products.

5.3 Computational modeling

In parallel with experimental work, computational modeling has emerged as a powerful tool for service life prediction. Finite element method (FEM) models are increasingly applied to simulate chloride ingress, carbonation, and the initiation of corrosion in reinforced concrete (Dominicini and Calmon 2017). These can be implemented in widely available software platforms. COMSOL Multiphysics has been frequently employed to solve coupled diffusion–migration–reaction equations, including zinc-specific electrochemical parameters, and allows flexible incorporation of environmental variables. Similarly, ANSYS and Abaqus are commonly used to couple transport models with mechanical response, enabling simulation of cracking caused by corrosion product expansion. For engineering practice, dedicated durability software such as Life-365 and the European DuraCrete model provide probabilistic frameworks for chloride ingress and carbonation, and although originally calibrated for black steel, these models have been adapted by researchers for galvanized reinforcement by inputting higher chloride thresholds and modified corrosion kinetics. The selection of software depends on the modeling objective: while Life-365 provides accessible and probabilistic service-life prediction for design, multi-physics packages such as COMSOL and ANSYS remain indispensable for fundamental research requiring coupled transport–mechanical–electrochemical analysis. This integration of numerical tools and experimental calibration represents a crucial pathway toward reliable full-service-life modeling of galvanized reinforcement.

These models are currently most reliable for predicting the initiation stage, i.e., the time to depassivation, defined either by exceeding a critical chloride threshold or by carbonation-induced pH reduction. Some recent models also attempt to represent the propagation stage, incorporating zinc consumption rates and the mechanical effects of corrosion products on concrete cracking (Jaśniok et al. 2020). However, such propagation modeling remains limited due to uncertainties in parameterizing zinc corrosion kinetics and product morphology. It should be noted that explicit modeling of the passivation stage – including the kinetics of CHZ film formation and stabilization – is still lacking, with current approaches treating passivation qualitatively rather than quantitatively (Angst 2018). Likewise, the so-called protection stage, in which sacrificial zinc continues to shield exposed steel areas, is often simplified or omitted from current frameworks. Future research should therefore develop multi-stage models that encompass passivation, initiation, protection, and propagation in a unified framework, calibrated against long-term field evidence (Roventi et al. 2014). Such integrated modeling would allow engineers to more reliably predict the full service life of galvanized reinforcement under diverse exposure conditions.

6.1 Synthesis of key findings and unresolved controversies

The collective evidence overwhelmingly confirms that hot-dip galvanizing provides a substantial improvement in the durability of reinforced concrete compared to traditional uncoated steel. The established benefits are clear and significant. First, the zinc coating possesses a much higher chloride threshold for corrosion initiation, consistently tolerating chloride concentrations 2 to 5 times greater than black steel, which significantly prolongs the corrosion-free initiation phase of a structure’s service life (Yeomans 2019). Second, galvanized steel exhibits excellent resistance to carbonation-induced corrosion due to the inherent stability of zinc over a wider pH range (Jiang et al. 2018). Third, the failure mechanism of galvanized rebar is fundamentally more benign; its corrosion products are less voluminous and tend to migrate into the concrete matrix rather than generating the destructive expansive pressures that lead to spalling and cracking, a key failure mode for black steel (Xu et al. 2016).

Despite these established advantages, this review has highlighted several critical and persistent scientific controversies where the literature remains divided. These unresolved issues represent the frontier of knowledge in this field and can be synthesized as follows:

1. Nature and Stability of the Passive Layer. The true protective capacity of the CHZ layer remains debated. Some studies suggest it provides long-term passivity, while others indicate that its stability is conditional on moisture and alkalinity, with potential for reactivated corrosion under wet conditions (Pokorný et al. 2022).

2. Net Effect on Bond Strength. Conflicting findings persist on whether CHZ-induced adhesion enhances bond, or whether hydrogen evolution and associated porosity reduce bond strength. This divergence highlights the need for improved interfacial characterization (Pokorný et al. 2023; Pokorný et al. 2024a, b).

3. Predictive Power of Laboratory Testing. Accelerated laboratory tests often fail to reproduce field behavior, leading to discrepancies in service-life prediction. Laboratory protocols may accelerate unrepresentative mechanisms, making long-term field validation essential (Mao et al. 2025).

6.2 Future research directions and knowledge gaps

Addressing these controversies and advancing the technology requires targeted research in several key areas. Based on the knowledge gaps identified in this review, the following future research directions are proposed:

1. Long-term performance of CGR: CGR requires long-term field validation in marine, de-icing, and carbonation-prone environments. Studies should also clarify the Al–Zn passivation mechanism in modern cement chemistries.

2. Resolving bond strength debate: Advanced micro-analytical techniques, such as X-ray microtomography and nano-indentation, are necessary to distinguish between hydrogen-induced porosity and CHZ-induced adhesion at the steel–concrete interface.

3. Behavior in modern concrete binders: Galvanized rebar performance in sustainable concretes (high SCM content, geopolymers, alkali-activated systems) remains underexplored, despite their growing use in infrastructure.

4. Development of advanced service life models: Models must incorporate zinc-specific electrochemical parameters, variable chloride thresholds, and the non-expansive nature of zinc corrosion products. This requires better accelerated testing protocols validated against long-term field data.

6.3 Concluding remarks

HDG reinforcement has demonstrated, across decades of laboratory and field research, a quantifiable enhancement of durability compared to black steel. Long-term bridge surveys in the United States and Bermuda have shown galvanized rebar remaining corrosion-free even after 25–40 years in chloride-rich or marine environments where black steel would have failed. These findings confirm that galvanized reinforcement can sustain chloride concentrations 2–5 times higher than conventional steel before depassivation occurs. Similarly, galvanized rebar maintains passivity down to pH ≈ 9.5, offering superior protection against carbonation compared to black steel, which depassivates below pH 11.5.

A second distinguishing advantage lies in the benign nature of zinc corrosion products. Unlike expansive iron oxides that generate internal stresses and spalling, zinc corrosion products are less voluminous and can migrate into the cement matrix, sometimes even densifying the interfacial transition zone. This fundamentally different degradation mechanism explains why galvanized reinforcement often avoids the catastrophic cracking associated with black steel, even after significant zinc loss. At the same time, scientific controversies remain unresolved. Studies diverge on whether hydrogen evolution during early curing permanently weakens the steel–concrete interface or whether the formation of CHZ crystals provides compensatory adhesion. Likewise, while accelerated laboratory tests frequently predict shorter service lives, field studies often demonstrate far superior durability, raising questions about the representativeness of current testing protocols. These debates highlight the need for refined test methods and advanced microstructural characterization.

Finally, the field is rapidly evolving with new technologies. CGR with its ductile eta-phase zinc layer and superior formability offers both sustainability and performance advantages. Early results are promising, but multi-decade field validation is still required before it can fully replace traditional HDG in critical infrastructure. Service-life modeling tools, including FEM-based chloride ingress simulations and probabilistic durability frameworks, are being adapted to account for galvanized-specific parameters, yet they require better calibration against long-term evidence. In conclusion, galvanized reinforcement should be regarded as a proven and cost-effective mid-tier solution: more durable and reliable than epoxy-coated rebar, yet more economical than stainless steel. With continued research into interfacial chemistry, performance in sustainable concrete mixtures, and validated service-life prediction, HDG and CGR technologies can play a central role in achieving infrastructure lifespans exceeding 100 years. The balance of field evidence strongly supports their inclusion in modern durability design, provided specifications are tailored to environmental exposure and supported by ongoing scientific refinement.