Optimization of exposure time for hot dip galvanizing and study the effects of preflux bath additives on the microstructures of galvanized steel

International Journal of Emerging Technology and Advanced Engineering

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Optimization of exposure time for hot dip galvanizing and study

the effects of preflux bath additives on the microstructures of

galvanized steel

B.D.Deshmukh

, A.P.Patil

1_{Yeshwantrao Chavan College of Engineering, Nagpur, Department of Mechanical Engineering}

2Visvesvaraya National institute of Technology, Nagpur, Department of Metallurgical and Materials engineering

Abstract— The objective of this research work is to optimize the time of exposure for hot dip galvanizing of mild steel, so as to achieve such a microstructure of zinc coating that protects mild steel for long against atmospheric corrosion. In order to decide the above mentioned parameter the polished samples, one at a time, were dipped in to 500 g molten zinc at 460 to 470 oC in the crucible for a different period of time for example 2, 5, 10, 15 minutes. The samples of mild steel (25 mm x 40 mm x 5 mm) were abraded on series of emery papers (1/0, 2/0, 3/0 and 4/0 grades) before galvanizing. After abrading, the samples were washed in tap water and air dried before hot-dip galvanizing. The microstructures of the coatings were observed under optical microscope at 200X magnification. It is observed that defect-free uniform layers of different phases were produced on 5 and 10 minutes of exposure to molten zinc in hot-dip galvanizing process. Hot dip galvanizing normally produces four phases in the form of successive layers in the coating; starting with substrate, referred as gamma, delta, zeta and eta phases. These phases are found in Fe-Zn phase diagram on varying Fe content. These observations show that with 5 and 10 minutes time of exposure to molten zinc, good adhesion and uniform layers of different layers of Zn-Fe phases are formed.

Hot Dip Galvanizing process includes various pre treatment steps prior to immersion of the ferrous substrate in the molten zinc. The final step of these processes is fluxing which takes place right before immersion in the molten zinc. In the present investigation we have selected various preflux baths such as ZnCl2 .NH4 Cl, CdCl2, Nicl2 and SnCl2 with the

identical salt concentration (50%wt) so that, their effects on the microstructures of galvanized steel can be compared. As Cd and Zn belong to the same group of periodic table their properties are expected to be similar, hence their behavior concerning galvanizing have to be alike. But the anomaly is observed in the microstructure of specimen dipped in to the preflux bath containing CdCl2 is showing wavy layer of Zeta

phase. When the concentration of NiCl2 reaches 50% in

preflux bath the needle like crystals are not observed in zeta phase which is composed only by small grains thus resulted into compact coating layer.

Sn addition seems to be inactive with regard to coating

structure.

Keywords— Hot-dip Galvanizing, mild steel, coating and atmospheric corrosion, prefluxing baths

I. INTRODUCTION

Hot dip galvanizing is widely employed technique to coat a uniform layer of zinc on mild steel to protect it against atmospheric corrosion [1, 4]. While goods sector uses hot-dip galvanized mild steel sheets as basic material as it combines good formability and good corrosion protection [5, 6]. The requirement of such a galvanized steel sheet is so huge that there are automated plants for producing such sheets. These plants use imported machinery and technology. However, these plants are not suitable for small and medium scale galvanizer, who uses batch-galvanizing process for Varity of products due to economy of scale and demand. Therefore, it is imperative that indigenous technology is developed for such a sector. In present knowledge-based economy, it becomes all the more necessary.

There is considerable research work being done to study (a) effect of preflux bath additives on the morphology and structure of hot dip galvanized coatings. [7], (b) effect of deformation on corrosion [8], (c) effect of different coating materials [9], (d) ways of improving aesthetics [10] and cracking mechanism of hot dip galvanized steel [11]. During hot-dip galvanizing, a four layer coating of Zn and Zn-Fe intermetallic compounds are produced. In order to produce a uniform and defect free coating it is necessary that the steel surface is cleaned thoroughly, so that film of atmospheric corrosion product (i.e. Oxide) is removed completely. In the present work reported here is an attempt to identify the best duration of exposure in zinc bath for hot dip galvanizing

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Aqueous solution of 50wt% concentration was considered. The reason being that 50wt. % ZnCl2.NH4Cl bath is considered as the normal flux used for this purpose. [7]. In order to compare the effects of other fluxes same concentration (wt %) were taken into consideration.

II. EXPERIMENTAL PROCEDURE

Mild steel sheet of 5mm thickness, procured from local market was chosen deliberately at this stage so that steel/zinc layer interface could be easily subjected to microscopic examination under metallurgical microscope. From the sheet, 2 5mm x40 mm samples were cut and prepared for hot-dip galvanizing. The preparation procedure included, abrading entire surface, including edges, to remove existing atmospheric corrosion product, burrs and oil/grease. A series of emery papers were used (200, 400, 600 and 800grit). Then these samples were pickled in 10% HCl solution for 10 minutes, then washed in water, then in acetone and air dried. Meanwhile, zinc was melted in graphite crucible using muffle furnace maintained at 460 to470 oc. Then samples were dipped in molten zinc for varied duration of 2, 5, 10and 15 minutes as done by Pistifidis et.al. [6] during their studies on effect preflux bath additives on morphology and structure of hot-dip galvanized coatings. During the exposure i.e. after dipping the samples, the zinc bath along with the samples was kept in the furnace to maintain constant temperature. Then the samples were removed and cooled in air. These samples were then embedded in Bakelite. The specimens for micro structural examination were carefully cut from these samples so that a section across the thickness is revealed. The specimens were then carefully prepared for microstructural examination by abrading on series of emery papers (200, 400, 600 and 800 grit). These samples were then polished on velvet cloth smeared with 70 micron alumina powder suspension in water. Then these samples were etched with Nital, washed with water, rinsed with methanol air dried and subjected to microscopic examination. An inverted stage metallurgical microscope (Olympus make) was used for the purpose.

The mild steel samples were pickled in an aqueous solution containing 10% HCl for about 20 minutes. Following, they were fluxed in different solutions as shown in the following table. After fluxing for 20 minutes in the solution the specimens were dried with the air blower for 1 hour and finally they were dipped in galvanizing bath at temp of 4600-4700 c for 10 minutes.

After putting the samples were mounted in the Bakelite and then cut for microscopic examination. Etching reagent used for these samples was dilute Nitric acid 1 drop +10ml Ethyl alcohol.

III. RESULTS AND DISCUSSIONS

Microstructure of mild steel /zinc layer interface at 200x magnification is presented in Figs. 1 to 4. Figure 1 shows microstructure of mild steel/zinc layer interface of sample hot-dipped for 2 minutes. Similarly, Figs 2, 3 and 4 show microstructures of the interface of samples exposed for 5, 10 and 15 minutes, respectively. Different layers of Zn-Fe intermetallic compounds clearly seen. Figure 5 presents phase diagram of Fe-Zn alloy [3] and Fig. 6 below presents an example of the structure of galvanized layer [1].

Fig. 5 Fe-Zn alloy phase diagram [3]

SR.NO. SALT

SALT CONCENTRATION

(WT %)

SOLVENT

1. ZnCl2.NH4Cl 50% Water

2. CdCl2 50% Water

3. NiCl2 50% Water

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Fig 6. Structure of galvanized layer. [1]

As seen from these figures, the Gamma layer may have up to 75% Zn and balance 25% Fe. Similarly Delta layer may have 75 to 90% Zn at interface, and Zeta phase may have 91 to 94 % Zn and balance Fe. Top layer Eta is almost pure Zn (100%). From Fig. 6 it is clear that zinc content decreases and hardness increases from top zinc layer to mild steel substrate. It is also clear that thickness of the layers decreases towards mild steel substrate. The microstructures of test specimens are in agreement with the microstructure shown in Fig. 6. Therefore, different layers of microstructure were easily identified.Fig.1. Shows thin layer of Gamma, then layer of Delta then slightly thicker layer of Zeta. The layers are distinctly clear, but interfaces are slightly wavy. Fig. 2 shows a thin layer of Gamma, then a layer of Delta, a thicker layer of Zeta and an uneven layer of Eta with coarse grains. Such an uneven layer may cause problem during corrosion as well as during cold forming. Figure 3 shows very smooth interface between different phases. The Zeta and Eta layers are thick enough. Figure 4 also shows clearly different layers and smooth interfaces between them. Top layer has coarse grains. It is also observed the thickness of coating increases on increasing exposure duration. It is also clear that 5 to 10 minutes of exposure produces good coating. Therefore, the duration of hot dipping exposure of 5 to 10 minutes seems to be optimum. However, if the thick layer is required, 15 minutes exposure can be used safely. Thickness of different layers of Delta and Zeta increases on increasing exposure duration as more zinc can then diffuse through the top layer and interact with substrate of different layers. Further studies were carried out using different fluxes.

Fig. 1: Microstructure of sample which was hot- dipped for 2 minutes (200x).

Fig .2: Microstructure of sample which was hot- dipped for 5 minutes (200x).

Fig. 3: Microstructure of sample which was hot- dipped for10 minutes (200x).

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Fig.7. shows Microstructure of Base metal after Pre-fluxing in NiCl2 50wt% in H2O and hot dip galvanized

(200 X magnification).This is a mild steel structure. Fig.8.shows the microstructure of base metal and Zinc coating after prefluxed in ZnCl2.NH4Cl 50wt%in H20

which is considered as normal flux used in this process. At 200x magnification this structure shows Gamma layer at interface and subsequently above that delta layer and Zeta layer

Fig.9. shows the microstructure of base metal and zinc coating after prefluxed in CdCl2 50Ewt%in H20 and hot dip

galvanized. At 200x magnification structure shows the wavy layer of Zeta phase. This anomaly is observed with prefluxing with CdCl2.

Fig.10. In order to understand the detail about this anomaly we have observed the same microstructure at higher magnification i.e. at 500x magnification which clearly shows the wavy layer of Zeta phase. The reasons behind the formation of this wavy layer of Zeta phase need further investigation for this anomaly.

Fig.11. shows the microstructure of base metal and zinc coating after prefluxed in NiCl2 50wt% in H20 then hot dip

galvanized. This structure clearly revealed that there is densification occurred. Compacted coating consisting all phases will result into formation of thin coating layer.

Fig.12. shows the microstructure at 200x magnification of base metal and zinc coating after preflluxing in SnCl2

50wt% in H20 and then hot dip galvanized showing almost

similar structure obtained in normal zinc coating. This flux appears to be inactive as far as microscopic examination of coating structure is concern.

Fig.7.Microstructure of Base metal after Pre-fluxing in NiCl2 50wt% in H2O and hot dip galvanized (200 X magnification)

Fig.8.Microstructure of Base metal and zinc coating after Pre-fluxing in ZnCl2.NH4Cl 50wt% in H2O and hot dip galvanized (200 X

magnification)

Fig.9..Microstructure of Base metal and zinc coating after Pre-fluxing in CdCl2 50wt% in H2O and hot dip galvanized (200 X

magnification)

Fig10..Microstructure of Base metal and zinc coating after Pre-fluxing in CdCl2 50wt% in H2O and hot dip galvanizing (500 X

magnification)

Fig.11.Microstructure of Base metal and zinc coating after Pre-fluxing in NiCl2 50wt% in H2O and hot dip galvanized (200 X

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Fig12. Microstructure of Base metal and Zinc coating after Pre-fluxing in SnCl2 50wt% in H2O and hot dip galvanized (200 X

magnification)

IV. CONCLUSION

From the present study it can be concluded that exposure of 5 to 10 minutes produce good coating with uniform layers of different phases of Zn-Fe phases. Shorter or longer durations do not produce good coating. However, further work was carried out on this line to investigate of effect of prefluxing, revealed the facts that as Cd and Zn belong to the same group of periodic table their properties are expected to be similar, hence their behavior concerning galvanizing have to be alike. But anomaly in the structure of specimen dipped in preflux bath containing Cd is showing wavy layer of zeta phase.

When the concentration of NiCl2 reaches 50% in preflux

bath the needle like crystals are not observed in zeta phase which is composed only by small grains thus resulted into compact coating layer.

Sn addition seems to be inactive with regard to coatings.

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