Galvanizing

Abstract

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.

3.3.3.3 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.

6.1.4 Mixed coating structures

10.29.6.1.4 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.

4 Texture Development in Zn Coating during Galvanization (GI)

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, {$10\overline{1}3$}<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]: ${\left(311\right)}_{{\mathrm{Fe}}_2{\mathrm{Al}}_5}$ || (110)_α or ${\left(221\right)}_{{\mathrm{Fe}}_2{\mathrm{Al}}_5}$ || (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].

2.2.1 Dry process

4.3 High-temperature galvanizing

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.

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

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

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