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Properties of HDG coatings

Zdenek BARTAK1, Katerina KREISLOVA1, Antonin SCHWARZER2

1SVÚOM Ltd., Prague, Czech Republic, bartak@svuom.cz 2 Signum Ltd., Hustopece, Czech Republic, antonin.schwarzer@signum.com

Abstract The HDG coating properties were studied in relation to steel chemical composition, surface treatment and galvanising technology. The thickness, adhesion and structure of zinc coating strongly depend on steel chemical composition. Content of aluminium in steel is very important in combination with silicon content. Keywords hot-dip galvanising, coating properties, steel chemical composition Introduction For more than 150 years the application of hot-dipped zinc coatings has been a standard method of corrosion protection of steel. Although hot-dip galvanizing has been utilized to protect steel for generations, the galvanizing process continues to evolve with many problems [1 - 5]. Appearance and thickness are basic parameters of zinc coating and may indicate the problems with their adhesion. Producers and/or customers have a lot problems to obtain required zinc coating thickness according to ISO 1461 in respect to steel thickness. In paper there are presented some results of steel surface treatment supposed to increase the zinc coating thickness for aluminium killed steel. The balance of suitable chenical composition of steel is very difficult to obtain - the new requirements are included in revised ISO 17413-2, but there are not covered Mn which content is specified according to ASTM A385 (Table 1). Table 1 - Specification of chemical composition of steel suitable for galvanising - class A

element ISO 17413-2 rev ISO 17413-2 ASTM A385 Si ≤ 0.03 % (< 0.02 % P) ≤ 0.03 % (< 0.02 % P) < 0.04 % Al - ≤ 0.035% - Mn - - < 1.30 %

Note : ASTM A385 limited content of carbon < 0.25% In cases the occurrence of below mentioned symptoms, the HDG has not characteristic phase structure resulting into lower adhesion:

- the coatings may have a matte, dull grey appearance and/or rougher surface due to the absence of the pure zinc layer,

- the coatings with very high thickness not corresponding to substrate thickness.

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The chemical composition and the surface condition (finish and roughness) of the basis metal, the mass of the parts and the galvanizing conditions may affect the appearance, the thickness, texture and physical/mechanical properties of the coating [6 - 9]. The effect of high content of aluminium In case of aluminium killed steels (≤ 0,03 % Si) the higher content of aluminium > 0,035% may results into zinc coatings with reduced adhesion. The steel sheet with thickness 1,7 mm was galvanised and coating delaminated (Figure 1). The thickness of coating was 185 μm instead of 55 μm specified according to ISO 1461. The problem with adhesion may be indicated by higher thickness and ʺmarbleʺ appearance of coating.

Figure 1 – Zinc coating delamination Table 2 - Chemical compostion of steel (wt. %)

C Si P Mn Al Fe 0.140 0.011 < 0.001 0.199 0.188 rest

The effect of steel surface pretreatment Opposite problem occurred for aluminium killed steel which does not keep the specified minimum average zinc coating thickness. The blasting of steel surface is recommended to obtain higher thickness of coating. The surface of the basis metal should be clean before dipping into the molten zinc. Degreasing and pickling in acid are the recommended methods of cleaning the surface. Standard steel surface pretreatment in galvanising process is degreasing in hot alkaline solution and pickling in HCl solution. The effect of different surface treatment was tested. In case of low silicon steel (Si ≤ 0.03 wt. % - Table 3) the zinc coating thickness is lower than specified ISO 1461 although the coating has typical phase structure and good adhesion.

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Table 3 - Chemical composition of steel (wt. %)

C Mn Si P S Cu Ni Cr Mo V Ti Al 0.497 0.452 0.016 0.003 0.003 0.033 0.019 0.033 0.002 0.003 0.002 0.032

The roughness of the steel surface has an influence on the thickness and structure of the coating. A rough steel surface, as obtained by e.g. grit blasting, gives a thicker coating than a surface that is obtained by pickling alone. The set of steel panels with dimension 100x150x5mm and different surface pretreatments had been galvanising in zinc bath alloyed by Pb and Ni. According to ISO 1461 the minimum average zinc coating thickness for such panels should be 70 µm. The zinc coatings were characterised by their appearance, thickness, adhesion, morphology, and phases´ structure. The results are given on Figures 2 to 5 and in Table 4. Blasting as a surface pretreatment helps to create zinc coating with higher thickness, practically twice of original ones, but the appearance of coating is different. The morphology of zinc coating surface was measured by 3D microscope Keyence VHX-5000 (Figure 1). The longer duration of pickling does not significantly affect the coating thickness – see comparison of sample C1 and D. The longer time of galvanising has positive effect on coating thickness and the minimal value was kept for immersion time 8 min.

pickled surface blasted surface

Figure 2 – Examples of zinc coating applied on the panels A with different pretreatment

sample pickled blasted

A

Figure 4 – Examples of zinc coating adhesion on the same panels with different pretreatment

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Table 4 – Properties of galvanising and formed coating

sample pickling duration

(min) blasting

galvanising duration

(min)

zinc coating appearance

coating thickness

(μm) adhesion

A1 120 - 2 gloss, zinc pattern 50 good A2 X matte, rought 119 good B1 165 - 5 gloss, zinc pattern 61 good B2 X matte, rought 150 acceptable C1 130 - 8 gloss, zinc pattern 72 acceptable C2 X matte, rought 157 good D1 210 - 5 gloss, zinc pattern 64 good

Figure 3 – Zinc coating thickness on the same panels with different pretreatment

Cross sections of coating layer (Figure 5) shows that it has mixed structure on all tested panels. Only partial occurence of η phase, mainly on pickled surfaces, but it has possitive effect onto adhesion tested by impact test. On pickled samples the structure of zinc coating became more uniform with increased time of galvanising. On blasted surfaces the coating structure is non uniform/non homogenous; the thickness of each phase varing, phases are mixed together. The effect of galvanising duration can be seen from Figure 6 – the crystals of ζ phase are more organised on sample C and the η phase thickness is practically twice on this sample than on sample A. The thickness of lower phases increased with time of glavanising, too, but not so significantly as η phase thickness. The effect of galvanising duration – time of immersion in zinc bath - is more important for pickled samples than for blasted ones. Conclusion To obtain the hot-dip galvanised coating with all required properties – aesthetical and technical ones – is not easy and the quality of HDG coating may be affected by many factors, some of them far from galvanisers´ possibilities to change or improve.

0

40

80

120

160

2 5 8

zinc

coa

ting

thic

knes

s (µ

m)

galvanising duration (min)

blasted surface pickled surface

5

sample pickled blasted

A

B

C

Figure 5 – Zinc coating cross section on the same panels with different surface pretreatment sample A1 – pickling for 120 min, galvanising for 2 min

sample C1 – pickling for 130 min galvanising for 8 min

Figure 6 – Zinc coating cross section on the same panels with different time of galvanising

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It is known that the exact structural nature of the galvanized coating may be modified in accordance with the exact chemical nature of the steel being galvanized, but problems with steel chemistry and zinc coating morphology are usually apparent after an item has been galvanized. But the appearance of galvanised coating it the first point which clients comment and claim. Steels with silicon in the range 0.04 % to 0.15 % or above 0.22 % can produce galvanized coating growth rates much higher than those for steels with silicon levels below 0.04 %. For aluminium killed steels the effect of aluminium content was not yet incorporated into technical standard ISO 17413-2. Other fact which negatively affects the acceptance of HDG coating is lower coating thickness, despite the good adhesion of coating. In the most atmospheric environment in Europe the corrosivity for zinc is so low that the reduced thickness does not significantly affect design durability of corrosion protection of structures. The most negative effect on durability has delamination of zinc coating around edges or screws repaired by zinc spray as is typically done. Some choices for users are to accept the lower coating thickness, apply a paint coating over the galvanized coating (duplex system). Other mentioned possibility is blasting the steel before hot-dip galvanizing to increase the coating thickness, but performed tests showed this approach cannot be applied in general. Acknowledgements This paper was prepared in frame of project TH02020550 Research of the effect of chemical composition and metallurgical surface conditions of substrate steel onto quality of galvanized coating. References 1. J.B. Nasr, A. Snoussi, C. Bradai, F. Halouani, “Effect of Physical parameters on the killed

Steel in hot-dip galvanizing”, in: Proceedings of EUROCORR 2007, 9-13.9.2007, Freiburg im Breisgau, Germany

2. S.Sepper, P.Peetsalu, V.Mikli, M.Saarna, “The effect of substrate microstructure on morphology of zinc coatings”, in: 8th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING, 19-21 April 2012, Tallinn, Estonia

3. S. Wegrzynkiewicz, “The Influence of Surface Preparation of Steel 41Cr4 on the Nature and Corrosion Resistance of the Hot-dip Zinc Coating”, in: METAL 2016: 25th International Conference on Metallurgy and Materials. Ostrava: TANGER, 2016, pp. 898-903

4. S. Wegrzynkiewicz, D. Jedrzejczyk, I. Szlapa, M. Hajduga, S. K. Boczkal, “Influence of a substrate surface on the Zn - coating formation”, in: Archives of metalurgy and materials, Vol. 59, No 4, 2014, pp. 1373-1378

5. H.Geiplova, L.Turek, “The effect of another processing on adhesion of HDG”, in: METAL2018: 27th International Conference on Metallurgy and Materials. Ostrava: TANGER, 2018 (in press)

6. S. Wegrzynkiewicz, D. Jedrzejczyk, I. Szlapa, M. Hajduga, S. Boczkal, “Diversification of intermetallic Zn phases growth on steel during hot-dip galvanizing”, in: Archives of metalurgy and materials, Vol. 61, No 2, 2014, pp. 515-520

7. P. Pokorny, J. Kolisko, L. Balik, P. Novak, “Effect of chemical composition of steel on the structure of hot – dip galvanized coating”, in: METABK, Vol. 55, No 1, 2016, pp. 115-118

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8. K.Kreislova, H.Geiplova, “Adhesion of zinc coatings affected by chemical composition of steel”, in: 22th Galvanising conference, 18.-20. 10. 2016, Mikulov, Czech Republic, 66-77

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