Galvanizing is the process of applying zinc coating to a more noble metal (popularly steel or iron) to prevent corrosion (rusting).

From: Comprehensive Materials Finishing, 2017

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Surface Coating Processes

F. Ozturk, ... S. Kilic, in Comprehensive Materials Finishing, 2017


Galvanizing is a well-established, economical, and durable method for protecting metals from corrosion. It is basically a surface treatment process to coat metal surfaces by submerging the metals into a molten Zn metal bath. Zn coating has been widely used as strong protection for metals against corrosive environments for a long time. The galvanizing process has three major steps: surface preparation, galvanizing process, and postsurface treatment. Coating is established with the metallurgical interaction between molten metal and iron. Two types of galvanizing processing methods have been known for a long time. These are hot-dip galvanizing and electrogalvanizing methods. In this chapter, details regarding the hot-dip galvanizing process and its critical processing parameters are addressed because of its broad application and low cost by changing the galvanizing processing procedures, such as immersion time and galvanizing temperature affecting the coating properties. Moreover, small amounts of elements present in the galvanizing bath also contribute to final characteristics of the coatings. In addition to the fundamental galvanizing issues, galvanizing in advanced high-strength steels and failure mechanisms of coatings are also discussed.

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Surface Coating Processes

P. Sahoo, ... J. Paulo Davim, in Comprehensive Materials Finishing, 2017 Galvanizing

Galvanizing is the process of applying zinc coating to a more noble metal (popularly steel or iron) to prevent corrosion (rusting). Hot-dip galvanization is the most well-known method in which the steel part is submerged in a bath of molten zinc. In the bath, the iron in the steel metallurgically reacts with the molten zinc to yield a tightly bonded uniform alloy coating, which provides superior corrosion protection to steel. The simplicity of the process provides it with a distinct advantage over other corrosion protection methods. Moreover, galvanizing produces higher thickness coating (about 80 µm) of zinc compared to electroplating (about 25 µm).

Galvanizing is widely used for its ability to resist corrosion. The nature of protection offered by zinc coatings is threefold. First as a barrier, the dense coating covers the total surface and prevents contact with any corrosive environment. Second, if the coating is damaged, due to zinc’s sacrificial behavior, corrosion occurs only in zinc. Finally, the natural weathering process helps in developing a natural passive layer on the surface which can then resist corrosion.

Galvanizing is used to protect large steel structural members, roofing material, etc. Tin is also often deposited using the hot-dip technique.

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Phenomena on galvanized coatings

Vlastimil Kuklík, Jan Kudláček, in Hot-Dip Galvanizing of Steel Structures, 2016

6.1.4 Mixed coating structures

Non-homogeneous substrate

Galvanizing an article with a smooth and consistent surface that, however, exhibits local variations in structure and chemical composition of the steel may consequently give the impression that material suffering from a strong corrosion attack has been galvanized (Figure 6.9). Local differences of reactivity of the substrate with zinc cause the applied coating to have a so-called mixed structure [40] (see also Section 5.4.4). A section through the coating very clearly shows places with a coating structure corresponding to silicon content in the Sandelin range that alternates with places exhibiting a coating structure formed on steel with a low silicon content (Figure 6.10) [18]. Adding nickel to the zinc bath may suppress occurrence of such mixed structures. After repeated galvanizing (after first removing the coating in a stripping bath) the mixed structure defect is not likely to be observed again.

Figure 6.9. Example of a mixed structure.

Figure 6.10. Metallography of a mixed structure.

Corrosive disruption of substrate surface

A specific case of occurrence of a rough structure is galvanizing of materials attacked by corrosion where after previous hot rolling the last operation performed was cold rolling. Corrosive attack (Figure 6.11) of the surface layer of such a substrate causes local defects of the crystalline lattice in the reinforced surface ferrite layer and disruption of the mutual bonds of iron atoms. During the metallurgical reaction of hot-dip galvanizing this supports easier release of iron atoms from the substrate, which promotes reactivity of steel with zinc. Mixed structures are formed in the coating (Figure 6.13) characterized by a raised pattern in the coating (Figure 6.12).

Figure 6.11. Corrosive disruption of substrate.

Figure 6.12. Raised pattern of thin galvanized sheet with the surface disrupted by corrosion.

Figure 6.13. Mixed structures on a thin sheet with a cold-drawn surface attacked by corrosion.

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Polymers for a Sustainable Environment and Green Energy

G. Crapper, in Polymer Science: A Comprehensive Reference, 2012 Galvanizing

Galvanizing is a method of pretreatment where the article to be coated is immersed in molten zinc. On removing the part from the zinc bath, there is oxidation of the zinc leading to a passivated surface with a characteristic pattern.

Galvanized steel is often used ‘as is’, but is also frequently coated. The issue with galvanized steel is that the galvanized substrate is changing with time as the zinc surface oxidized rapidly to zinc oxide, and more slowly to zinc carbonate. Hence, the substrate is changing with time, and this can result in variable overcoating performance. To stop this variability, many galvanizers will also carry out a pretreatment, such as a zinc phosphating prior to application of a powder coating.

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An Overview on the Texture Evolution of Cold Rolled IF Steels and Zn Coating During Galvanizing and Galvannealing

Somjeet Biswas, ... Shiv B. Singh, in Reference Module in Materials Science and Materials Engineering, 2019

4 Texture Development in Zn Coating during Galvanization (GI)

During GI treatment, the annealed steel is immersed in the liquid Zn bath (with <1 wt% Al). The coating layer so formed comprises Zn-rich η-phase (Fig. 5(a and b)) with hexagonal closed packed (HCP) structure (c/a=1.856) [8]. Hot-dip GI coating often have large grains and are referred as spangles [8]. The liquid Zn for GI is always alloyed with Pb, Sb, Bi, Mg, Sn and Cd other than Al. In the interface, Fe2Al5 intermetallic layer develops inhibiting formation of Fe-Zn intermetallics. The presence of Pb and Sn in liquid Zn lower the surface tension between the dendritic arms. This leads to faster growth of spangles. Presence of Bi also helps in spangle formations. The overlay coating layer is made up of dendritic structure polycrystalline Zn (η-phase). According to Marder, [8] the dendritic growth in the initial stage occurs sideways and parallel to interface. Therefore, (0002) i.e., basal planes develops parallel to the interface with [101¯0] growth direction of the dendrites. Thereafter, orientation dependent thickening of the solid Zn layer takes place. As the thickness of the Zn-rich η-phase grains increases, solid enrichment in the liquid in front of solid-liquid interface leads to precipitation of Pb between Zn dendritic arms. Bhattacharjee et al. [39] have shown that the GI coating show presence of a strong {0002}<uvtw> fibre in the η-phase above the substrate. It is well understood from the literatures [24] that directional solidification during columnar growth in the positive temperature gradient takes place perpendicular to the most loosely packed planes. Hu in 1974, [49] indicated that for Cadmium (c/a = 1.885) and Magnesium (c/a=1.624), the fibre axis of the columnar grains are [101¯0] and [112¯0] respectively. Corroborating with Mardar’s work [8] it can be well recognized that, where the temperature gradient is positive and constitutive super-cooling does not occur, directional solidification must take place. This results in columnar grain growth parallel to [101¯0] on the (0002) planes nucleated at the substrate interphase [8]. If constitutive supercooling takes place then dendritic growth may occur with weak crystallographic texture.

Fig. 5. Schematic showing layer formation and orientation relations with the IF steel substrate during (a) Galvanizing with &lt;0.15 wt% Al in liquid Zn, (b) Galvanizing with &gt;0.15 wt% Al in liquid Zn and (c) Galvannealing with optimum-alloyed Zn coating (~0.14 wt% Al).

In case, the Al content is less than 0.15 wt% in liquid Zn bath during GI, ζ (FeZn13 monoclinic) and δ (FeZn10 with HCP crystal structure) phase develops as shown in Fig. 5(a) without detectable Fe2Al5 [50–54]. Ohtsubo et al. [55] showed formation of ζ-phase with tooth/pillar type morphology. Specific crystallographic orientation relationship between ζ-phase and Fe-substrate was observed. These are (001)ζ || (111)α, (100)ζ || (101)α and (010)ζ || (121)α. If the Al content exceeds 0.15 wt %, Fe2Al5 become thermodynamically stable (Fig. 5(b)). In the δ-phase, {101¯3}<uvtw> fibre usually develops and are observed during GI when the holding time is large. However, no texture development was observed during CGL. No information regarding the crystallographic relationship of with the steel substrate is present in the existing literatures. The orientation relationship between the Fe2Al5 inhibition layer and the steel substrate is reported by Guttmann et al. [56]: (311)Fe2Al5 || (110)α or (221)Fe2Al5 || (110)α. It was also observed that the growth of Fe2Al5 phase occurs parallel to [100] direction [57]. High Al added galvanizing are called Galfan and Galvalume. In Galfan, more than 5wt% Al and in Galvalume ~55 wt % Al is present in Zn bath. A detailed information could be obtained from Mardar’s work [8].

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Hot-dip galvanizing

Vlastimil Kuklík, Jan Kudláček, in Hot-Dip Galvanizing of Steel Structures, 2016

2.2.1 Dry process

Most commercial galvanizing plants are equipped with a technology for suspension of batches of components by cranes and jigs (Figure 2.2) within the dry process (Figure 2.3). A benefit of this technology is relatively high productivity at a sufficiently high degree of mechanization. The dimensions of tanks of these plants are adapted to the market demand. If the design and production principles of hot-dip galvanized parts are observed, very large parts can be galvanized in these plants with transportation capacities becoming the main limiting factor (Figure 2.4).

Figure 2.2. Sling galvanizing.

Figure 2.3. Schematic representation of the process sequence of operations in a dry galvanizing process line.

Figure 2.4. Hot-dip galvanized steel structure ready for shipment.

The hot-dip galvanizing line may be arranged linearly (Figure 2.5), U-shaped (Figure 2.6), or as a combination of these options. Prepared parts are first chemically pre-treated. Individual process tanks are arranged in accordance with the operation sequence from the degreasing bath through the pickling and rinsing baths to the flux bath. High-quality surface decontamination requires quite a lot of time; therefore the chemical pre-treatment workplace is always equipped with a higher number of pickling baths. In commercial galvanizing plants galvanizing is done in batches that pass through the entire line as they were suspended on jigs at the beginning of the line. After the galvanizing bath, a cooling water bath may be used or the items are left to air-cooling.

Figure 2.5. Linear arrangement of the process line of a hot-dip galvanizing plant. 1. suspension beam (for fixation of galvanized items), 2. transversal conveyor, 3. suspension crane, 4. overhead traveling crane, 5. enclosed chemical pre-treatment workplace, 6. degreasing bath, 7. rinsing, 8. pickling bath, 9. flux, 10. drying furnace, 11. enclosed galvanizing workplace, 12. galvanizing crane, 13. water cooling crane, 14. stand for putting the suspension beam aside, 15. crane track for the overhead crane, 16. single-beam crane track, 17. stacking truck, 18. high-speed roller door.

Figure 2.6. U-shaped arrangement of the process line of a hot-dip galvanizing plant. 1. overhead traveling crane, 2. high-speed roller door, 3. enclosed chemical pre-treatment workplace, 4. degreasing bath, 5. bath with rinsing water, 6. pickling bath, 7. bath with rinsing water, 8. flux, 9. drying furnace, 10. mechanical stand with a chain conveyor, 11. enclosure of the galvanizing workplace, 12. galvanizing workplace with a suspension crane, 13. suspension beam, 14. stacking truck.

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Hot-dip galvanized coating formation

Vlastimil Kuklík, Jan Kudláček, in Hot-Dip Galvanizing of Steel Structures, 2016

4.3 High-temperature galvanizing

High-temperature galvanizing is a process conducted at higher temperatures than 530°C (usually at 550°C), i.e., above the stability temperature of phase ζ, which therefore cannot be generated under these conditions (Figure 4.11). High-temperature galvanizing is generally associated with centrifuging (to remove all zinc adhering to the surface of galvanized parts after removal of the batch from the zinc bath) and rapid cooling in water. These measures eliminate equilibrium conditions for a peritectic phase transformation at which the mixture of δ phase crystals and the zinc melt could produce an undesired mixture of crystals of phase ζ and phase η during slow cooling. The high-temperature metallurgical process involves significant dissolution of iron that bonds with zinc, producing phase δ. The conditions are not favorable for generation of phase Γ, as within this technology the immersion time of the batch in the zinc bath is relatively short – generally not exceeding 100 seconds. During this time period, ferrite does not get sufficiently saturated with zinc so that the oversaturated solution α can be generated and the quick cooling in water mainly prevents segregation of phase Γ. Due to the quick cooling, phase Γ1 cannot be segregated from phase δ either. In the alloy coating phase δ predominates, which is dense closer to the substrate, and further from the substrate there are fine crystals of phase δ overlayed with pure zinc (Figure 4.12). The thickness of coating applied by high-temperature centrifuge galvanizing is relatively even and the coating follows the surface of the galvanized part. Therefore, high-temperature galvanizing is preferably used, e.g., for coating of threaded fasteners (see also Sections 2.2.3 and 5.4.6Section 2.2.3Section 5.4.6).

Figure 4.11. The gray strip in the diagram indicates the area of high-temperature galvanizing.

Figure 4.12. Coating on S355 steel, bath at the temperature of 550°C, immersion time 5 minutes, cooled in water.

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Zinc: Alloying, Thermomechanical Processing, Properties, and Applications

R.F. Lynch, in Encyclopedia of Materials: Science and Technology, 2001

7.1 Hot-dip Galvanizing after Fabrication (Batch Galvanizing)

Hot-dip galvanizing has been employed to protect iron and steel since 1741. In hot-dip galvanizing after fabrication or batch galvanizing, the iron or steel article is completely immersed in a bath of molten zinc at ∼450 °C. The process is applied primarily to finished articles and semifinished shapes such as large steel beams, castings, or forgings. Galvanizing forms a durable, protective zinc coating that completely seals edges, rivets, seams and welds so that there are no unprotected areas from which rusting can start. In the hot-dip galvanizing process, a series of intermetallic iron–zinc compound layers are formed. The intermetallic phases are formed by a diffusion reaction during immersion with increasing zinc content moving out from the ferrous surface of: 75 wt.% (γ), 90 wt.% (δ), 94 wt.% (ζ), and 100 wt.% (η), respectively. The free zinc outer η phase forms during withdrawal of the steel article from the molten zinc bath. Hot-dip galvanized coatings are thoroughly bonded metallurgically to the steel substrate. Historically, in actual practice Prime Western zinc has been used which contains ∼1 wt.% lead to improve zinc fluidity and drainage.Because a 1 wt.% lead content is above the solubility level of lead in zinc, a molten lead layer forms on the bottom of the galvanizing kettle. This practice allows easier removal of the zinc–iron bottom dross that floats on the lead but under the zinc. Lead also promotes the formation of larger and more pronounced spangles or surface grains. Some galvanizers have started operating with Special High Grade or High Grade zinc but these are in the minority. The use of a 0.1 wt.% bismuth concentration in the zinc bath has been found to provide most of the benefits of lead with reduced environmental concerns. Aluminum at a low level of ∼0.005 wt.% is used to brighten the appearance of the galvanized coating and is normally added using master alloys. In some cases, tin is now substituted for aluminum producing a bright appearance having a deeper sheen, without the harmful interactions with the flux employed in galvanizing.

The thickness of the hot-dip galvanized coating depends on the time of immersion in the molten zinc bath, the bath temperature, the composition of the steel being coated, and the thickness of the steel being galvanized. Traditional steels rapidly approach a saturation coating thickness, which is self-regulating and not increased by additional immersion time in the bath. However, reactive steels such as those produced in continuous casting, which contain silicon and phosphorous in particular concentrations, exhibit an excessively thick dull gray galvanized coating, which in some instances is so thick and brittle it flakes off. The reason for this is that the structure is dominated by long needles of the hard and brittle ζ phase that extend to the surface, consuming the pure zinc η layer after withdrawal from the bath, and resulting in a brittle coating that can be up to three times thicker than normal. The use of a Zn–0.05 wt.% Ni concentration in the bath solves the problem with additions in the form of a Zn–0.5 wt.% Ni master alloy or the introduction of nickel powder. In other cases, strict process control via lower bath temperatures, shorter immersion times, rapid cooling, and smoother steel surfaces allows satisfactory galvanizing.

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Sustainable development

Vlastimil Kuklík, Jan Kudláček, in Hot-Dip Galvanizing of Steel Structures, 2016

Hot-dip galvanizing could be considered as a process with so-called “soft sustainability.” This means that the total economical value of the resources or products obtained will not be reduced in future. It admits that primary non-renewable resources can only be consumed if a respective equivalent is generated (i.e., the consumption is not loss-making). After the expiration of the useful life of a product obtained, the product must be completely recycled to avoid generation of loss.


Strong sustainability is considered to be difficult to achieve both from the short-term and long-term point of view at present. It requires a non-decreasing volume of resources, which means that the strong sustainability principle only makes it possible to consume renewable resources as a source of energy or raw materials. It does not consider non-renewable resources at all.

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Hot Dip Coatings

W.J. Smith, F.E. Goodwin, in Reference Module in Materials Science and Materials Engineering, 2017

6.5.1 Coating development and reactivity of steel

Conventional operating temperatures for galvanizing are between 445 and 465 °C. High temperature galvanizing is carried out between 530 and 560 °C. Galvanizing processes are not operated at temperatures between these two ranges.16 Operators of the higher temperature process use ceramic galvanizing baths to overcome the problem of the increased reaction rate between steel and zinc at these high temperatures. For silicon-free steel, the reaction rate between iron and zinc is parabolic with time in the temperature range 420–490 °C (the lower parabolic range). At temperatures between 490 and 510 °C, the reaction rate is approximately linear with time (known as the linear region). The reaction rate reverts to a parabolic relationship at temperatures above 530 °C (the upper parabolic region).

The thickness of the alloy layers within the coating formed on a steel article will depend principally on (1) the chemistry of the surface of the steel, (2) the surface roughness of the steel, (3) the temperature of the galvanizing bath, (4) the time of immersion, and (5) the rate of cooling.17

The role of silicon and phosphorus in development of alloy layers in a galvanized coating has been subject to intense research over many years.18,19

Silicon-killed steels, containing Si over 0.3%, produce a thick gray galvanized coating on processing. These thick gray coatings consist predominantly of all zinc–iron alloy (see Figure 6). While offering extended periods of corrosion protection they are often less resistant to mechanical damage. Steels containing between 0.15 and 0.25% Si often produce coatings that retain a proportion of zinc on top of the zinc–iron alloy layers.

Figure 6. Structure of a galvanized coating on a reactive (silicon killed) steel.

(UK Galvanizers Association, 1999. Publication; The Engineers and Architects’ Guide: Hot Dip Galvanizing).

Where semi-killed steels are processed, the results are less predictable. Steels containing between 0.04 and 0.14% Si often produce coatings that are very thick and exhibit poor cohesion. This reactivity range (see Figure 7) is often known as the ‘Sandelin Peak’ named after the researcher who described the effect some years ago.20 Phosphorous is more influential in determining coating characteristics at these lower Silicon concentrations. The International Lead Zinc Research Organization (ILZRO) has set out a classification system for reactivity of steels which is broadly in line with the Sandelin data.21 Hot dip galvanizing of reactive steels is often associated with increased zinc usage.

Figure 7. Illustration of the ‘Sandelin’ Curve – Hot dip galvanized coating development with increasing steel surface silicon concentration.

(General Galvanizing Practice; p. 1.8, Galvanizers Association, UK; 1999).

The effect of Silicon can be suppressed by galvanizing at higher temperatures (530–560 °C). High temperature galvanizing is often (but not exclusively) used to process small work also adopting the ‘centrifuge’ or ‘spinning’ process.15

Alloy layers tend toward increased thickness on rougher steel surfaces because of the increased surface area able to take part in the reaction between the zinc and the iron.22 In practice a maximum uplift in coating of approximately 50% can be achieved for steel that has been grit blasted prior to galvanizing compared with the coating developed on a steel of similar thickness and surface chemistry that has not been subject to blasting.

The alloy layers grow more rapidly as temperature increases – but there are practical limits to the extent to which this factor can be used to control alloy layer growth.23

Coating development normally tends to follow a parabolic reaction rate when articles are first immersed in the zinc melt. Very reactive steels (particularly those with high (>0.25%) or critical levels (0.04–0.14%) of silicon and phosphorus) exhibit a reaction rate for coating growth that is more linear with time (see Figure 8).

Figure 8. Illustration of galvanized coating development with time for normal (unkilled) and reactive (silicon killed) steel.

(General Galvanizing Practice; p. 1.8, Galvanizers Association, UK; 1999).

The alloy layer may continue to grow slowly during the cooling of the work after galvanizing. This is rarely an important consideration except when very heavy sections are being processed. In extreme cases the alloy layer diffuses to the surface to form gray patches. This is often referred to as a ‘gray coating’ or ‘gray bar’.24

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