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DOI: 10.1361/15477020523509

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Metallurgical Investigations into the Genesis of Bare Spots, Exfoliation, and Matte Coating Appearance in Hot Dip Galvanized Steel Sheets S. Srikanth, C.B. Sharma, A. Bhattacharyya, and Amitava Ray

(Submitted March 15, 2005; in revised form March 22, 2005)

Continuously processed hot dip galvanized steel sheets that exhibited bare spots, exfoliation/peel-off, and matte coating appearance were investigated to gain an insight into the genesis of such defects. Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDS) and electron probe microanalysis (EPMA) were employed in conjunction with analysis of the galvanizing bath. Analyses revealed a bath condition sensitive to surface cleanliness of the steel substrate and showed significant levels of carbonaceous residues and rolling debris on the annealed steel substrates. This improper steel surface condition has been attributed to excess oil carry-over on the cold rolled strip surface and poor burnoff in the annealing furnace. Keywords: bare spots, bath analysis, EPMA, exfoliation, hot dip galvanized coating, matte appearance, SEM/EDS Introduction

In integrated steel plants producing both hot and cold rolled flat products, a significant proportion of cold rolled sheets and strips are galvanized for use in construction and automotive sectors. The continuous hot dip galvanizing operation entails joining,

cleaning, annealing, and cooling to 455 to 460 °C of cold rolled steel strip before hot dipping in a

minimizes dross generation by forming an oxide (Al2O3) skin on the surface of molten zinc. Lead

lowers surface tension and also increases the fluidity

of molten zinc. On the other hand, excess iron content in the molten zinc bath adversely affects the fluidity and leads to a dull-appearing coating that may have poor peel-off resistance. Although the surface quality of continuously pro-

molten zinc bath at 455 to 460 °C. The postcoating

cessed hot dip galvanized steel sheets has improved

treatment processes involve wiping, chromate

significantly in recent years, the production of defect-

passivation, temper rolling, and, finally, cooling to

and blemish-free coatings for automotive appli-

room temperature. The quality requirements of gal-

cations still poses a challenging task. Continuous

vanized sheet products encompass a prescribed uni-

galvanizing of steel sheets basically involves chemical

form coating weight/thickness, spangle size, surface

reactions between iron, zinc, and aluminum at

finish or luster, and formability, along with adequate

temperatures above 450

peel-off resistance and adherence of the coating with

bath-metal reactions, intermetallic particles con-

the steel substrate. Typically, hot dip galvanization

stantly form in the bath, while a zinc-aluminum

baths contain approximately 0.18 to 0.20 wt.% Al,

oxide skin is formed on the bath surface. The iron-

0.10 to 0.20 wt.% Pb, and restrict iron content to

zinc intermetallics, which constitute the bottom

0.30 to 0.40 wt.% maximum. These requirements

dross, are in a continuous state of levitation due to

are necessary for achieving high-quality coatings on

the turbulence created by the movement of the strip

a regular basis. Aluminum enhances the fluidity and

through the molten zinc. During continuous hot

wettability of the zinc bath and aids in suppressing

dip galvanizing, the levitated particles and surface

the occurrence of brittle iron-zinc intermetallics at

skimmings may be easily dragged onto the zinc

the coating-steel interface by forming an inhibition

coating. Intermetallic particles can also develop on

or barrier layer of Fe2Al5. This barrier layer also

the surface of submerged system hardware that

promotes coating adhesion. Aluminum also improves

operates inside the molten zinc bath. It is therefore

the luster or brightness of hot dip coatings and

not uncommon for deteriorated or improper surface

°C.

As a consequence of

S. Srikanth, C.B. Sharma, A. Bhattacharyya, and Amitava Ray, Metallurgical Services Laboratory Division, Research and Development Centre for Iron and Steel, Steel Authority of India Limited, P.O. Doranda, Ranchi, 834 002, India. Contact e-mail: [email protected].

Journal of Failure Analysis and Prevention

Volume 5(3) June 2005

73

Metallurgical Investigations into the Genesis of Bare Spots

(continued)

quality of the submerged hardware to result in

steel-coating interface. The EPMA was also used

inferior coating quality, because contaminants and

to corroborate the SEM/EDS findings and to

pickups from hardware surfaces can be transferred

pictorially depict elemental concentrations across the

to the coatings. The quality of the steel substrate

zinc coating, the steel-zinc coating interface, and

can also affect coating quality; defects on the sub-

the steel base.

strate surface, such as marks and indentations caused by mechanical damage, tend to be highlighted and much more noticeable through the coating. Additionally, surface contaminants on the steel substrate can inevitably intervene with the coating process and result in bare spots. Most surface defects in galvanized steel sheets occur due to a rough or mechanically damaged substrate surface, insufficient cleaning of the substrate, poor bath management, and inadequate line equipment maintenance.[1]

Visual Examination

Visual examination of defective galvanized steel sheets revealed bare, uncoated regions in the form of either small (2 to 5 cm in length and width) or massive (30 to 50 cm in length and width) irregular dark streaks. These streaks were associated with sporadically dispersed thin zinc ribbons adhering to the steel surface. The unwetted regions of the steel sheets exhibited a brownish-black sooty

Intermittent occurrences of bare spots, coating

appearance. Typical photographs of galvanized sheet

exfoliation/peel-off, and matte/dull appearance of

samples showing small and massive streaks of bare

coatings in galvanized steel sheets processed through

spots are shown in Fig. 1(a) and (b), respectively,

the hot dip galvanizing line of an integrated steel

and a sheet sample exhibiting coating exfoliation/

plant, under the Steel Authority of India Limited,

peel-off is shown in Fig. 2.

necessitated metallurgical investigations into the genesis of such defects. For this purpose, galvanized sheet samples were collected from specific coils exhibiting prominent coating defects. The defective galvanized sheet samples were subjected to metallographic examination through scanning electron microscopy coupled with energy-dispersive X-ray spectrometr y (SEM/EDS) and electron probe microanalysis (EPMA) techniques. The galvan-

Samples of thicker-gage ( ∼ 1.5 mm) hot dip galvanized steel sheet having a dull and matte coating appearance (Fig. 3) were also collected for investigation. It is important to mention that during the period of investigation, input cold rolled steel sheet coils in the hot dip galvanizing line exhibited substantial carry-over of cold rolling oil, especially on the bottom surfaces, and that the incidences of

ization bath was also analyzed. This paper elucidates the investigation procedures and discusses the apparent causes of bare spots, coating exfoliation/ peel-off, and matte/dull appearance of coatings in the galvanized coils manufactured during the period of investigation. Experiments

Samples exhibiting bare spots (i.e., uncoated patches) and coating exfoliation/peel-off were collected from the hot dip galvanizing line. The samples were initially examined visually and subsequently sectioned transversely for microstructural observations in the through-thickness direction. Scanning electron microscopy was performed on etched metallographic specimens for topographic examination of the zinc coating layer, the steel-zinc coating interface, and the steel substrate. Energy-dispersive spectrometric (EDS) analysis was concurrently used to identify elemental presence in the uncoated patches as well as at the

74


Fig. 1

Bare spots in hot dip galvanized steel sheet samples. (a) Small dark streaks. (b) Massive dark streak


bare spots were primarily on the bottom surfaces of

of investigation. Chemical analysis of the bath

galvanized steel sheets. A typical photograph of an

samples was made by X-ray fluorescence spec-

input cold rolled steel sheet sample exhibiting oil

troscopy for aluminum, lead, iron, and zinc. The

carry-over residue is shown in Fig. 4.

results are shown in Table 1.

Under normal processing conditions, surface

It is evident from Table 1 that the aluminum

contaminants such as residues of cold rolling oil,

content in the galvanizing bath is marginally higher

grease, or emulsion are expected to burn off and

than the operational norm of 0.18 to 0.22 wt.%.

vaporize from the steel sheet surface during in-line

Incidentally, a higher aluminum content is favorable

annealing/heat treatment in the direct-fired and

in galvanizing, because it facilitates the formation

radiant tube furnaces. However, if traces of any burnt

of a stable inhibition layer of Fe2Al5 at the coating-

carbonaceous residues are left on the sheet surface,

steel interface and ensures complete suppression of

problems may arise in the wettability of the steel by

iron-zinc intermetallic compounds. The iron-zinc

the molten zinc. Thus, preliminary investigations

intermetallics are brittle phases that impair adherence

were directed at ascertaining the presence of any

in hot dip coatings. The suppression occurs because

hydrocarbon residue on the sheet surface.

a higher aluminum content prolongs the incubation period for nucleation of iron-zinc intermetallic

Analysis of Galvanization Bath

In order to assess the compositional characteristics of the galvanizing bath, samples of molten zinc were collected during three different shifts over the period

crystallites.[2] However, if the aluminum is in excess

of 0.20 wt.% in the molten zinc bath, the surface cleanliness of the steel substrate becomes much more critical for good wettability of zinc, even though coatings produced in such baths visually appear more shiny.[1] Table 1 also reveals that iron content in

the galvanizing bath is close to the maximum operational limit of 0.08 wt.%; a higher iron level in the bath is conducive to iron-zinc crystallite formation and is known to impair the adherence and luster of galvanized coatings. The galvanizing

Fig. 2

Hot dip galvanized steel sheet sample exhibiting coating exfoliation/peel-off

Fig. 4

Typical photograph of input cold rolled steel sheet surface showing rolling oil carry-over

Table 1 Chemical Analysis of Galvanization Bath Samples

Fig. 3

Hot dip galvanized steel sheet samples showing nonspangled, dull/matte appearance of coating


Composition, wt.% Aluminum Lead

Sample

Zinc

1

99.33

0.23

0.039

0.091

Iron

2

99.24

0.22

0.069

0.080

3

99.31

0.24

0.053

0.083


75

Metallurgical Investigations into the Genesis of Bare Spots bath composition (Table 1) also shows that the lead

cursor) in Fig. 5(a)showed peaks of carbon,

content is considerably lower than the stipulated

oxygen, and iron, indicating the presence of

operational range of 0.10 to 0.25 wt.%. This lower

carbonaceous residue as well as some oxide of iron,

lead content is important because lead lowers the

which is presumably the cold rolling debris. On the

surface tension, improves fluidity, and consequently

other hand, the EDS plot in Fig. 6(b)corres-

improves the wettability of molten zinc with steel.

ponding to the dark region in Fig. 6(a)showed

Additionally, wettability usually deteriorates in the

prominent X-ray peaks of carbon and iron and a

presence of iron in the galvanizing bath. Lead in

very small peak of zinc. This implied that the dark

the galvanizing bath is also known to facilitate

area essentially consisted of carbonaceous residue

effective drainage of excess zinc from the strip surface,

(carbon) and the steel (iron) substrate. However, the

so the low lead content is important for several rea-

spectrometric analysis of a similar white feature in

sons. However, the main benefit that accrues as a

a small bare spot specimen revealed a prominent

result of the presence of lead in the galvanizing bath

peak of zinc in addition to carbon, oxygen, and iron

is that it renders the coating process less sensitive to

peaks, signifying the coexistence of carbonaceous

the surface preparation of steel.[1,2] It is therefore

(carbon) residue, oxidized iron (iron, oxygen) fines,

common in continuous galvanizing operations to

and top-dross (ZnO) particles. This observation was

maintain lead levels in the stipulated range.

reproducible during semiquantitative EDS analyses

The potential inferences from improper bath composition coupled with the low lead content undoubtedly indicate a sensitive galvanizing bath condition with respect to the surface cleanliness of steel. Moreover, excessive rolling oil carry-over on the surfaces of cold rolled sheets indicated the general vulnerability of the steel sheet to zinc wetting and coating adherence problems. Scanning Electron Microscopy

For SEM and EPMA, transverse specimens were sectioned from galvanized sheet samples in the vicinity of defects, including bare spots, exfoliation, and regions of dull/matte coating. The transverse surfaces of the specimens were mounted in electrically conductive copper resin, polished by conventional metallographic techniques to a scratch-free finish, and etched in an amyl alcohol/nitric acid (HNO3) reagent for selectively etching the galvanized layer. Additionally, samples of galvanized steel

sheets with small and massive streaks, or bare spots, were sectioned and mounted face up in copperresin powder for topographic observation and EDS analysis in the SEM. The SEM observations revealed the presence of irregularly shaped white features dispersed randomly and appearing in relief on a dark and rough-looking background that was presumably the uncoated steel surface. The SEM photographs and EDS plots corresponding to the white and dark features in a massive uncoated patch are shown in Fig. 5 and 6,

76

(continued)

corresponding to both white and dark regions obtained at different locations on a massive as well as a small bare spot in defective galvanized sheets (Table 2). It is important to note that because the secondar y electron image generates an inverse contrast vis-à-vis the corresponding optical image, the white features in the SEM images are in fact the regions of carbonaceous deposits that appear optically dark. Thus, the EDS analysis (Table 2) demonstrates that both dark and white features have significant levels of carbon. The presence of carbon is consistent with the presence of burnt carbonaceous residue on the annealed sheet surface. The white features, in addition, show a considerable amount of oxygen, iron, and, in some cases, zinc, along with carbon, suggesting the presence of oxides, either as oxidized rolling debris or top-dross particles. The dark areas, on the other hand, show the presence of iron and a small amount of oxygen and carbon, indicating possible contamination of the uncoated steel surface with burnt carbonaceous residue. These observations, in all likelihood, indicate poor burnoff of the cold rolling oil, leaving remnants to be carried over on the surfaces of cold rolled sheets to be galvanized. The presence of such carbonaceous residues on the surfaces of annealed steel sheets would inevitably hamper the wetting of molten zinc with the steel surface and impede the proper coating of zinc on steel.

respectively. The EDS plot in Fig. 5(b)corres-

Scanning electron microscopic observations on

ponding to the white feature (marked by a cross

polished and amyl alcohol/HNO3 reagent-etched



transverse sections of galvanized sheet specimens

small bare spot streaks are shown in Fig. 7 and 8,

with bare spots reveal the nature of coating-steel

respectively. The collages of micrographs reveal the

interfaces. The micrographs of the coating-steel

rough nature of the steel surface, demonstrate the

interface in galvanized samples with massive and

overall poor adherence, and reveal the delamination

Fig. 5

Fig. 6

SEM/EDS analysis of bare uncoated patch in defective

SEM/EDS analysis of bare uncoated patch in defective

hot dip galvanized steel sheet. (a) Secondary electron

hot dip galvanized steel sheet. (a) Secondary electron

image of uncoated patch showing white and dark

image of uncoated patch showing white and dark

features at 1300× magnification. (b) EDS spectrum of the

features at 1300× magnification. (b) EDS spectrum of the

white feature showing peaks of carbon, iron, and oxygen

dark region showing peaks of carbon and iron

Table 2 EDS Analysis of White and Dark Features Observed under SEM in Uncoated Patches on Defective Galvanized Sheets Sample

Feature

Location

Bare spot

White

Spot 1 Spot 2 Spot 1

6.01

Spot 2 Spot 1

(small streak)

(massive streak)

Dark

Bare spot

White

Dark

C

Composition, wt.% Zn Al Mg

O

Fe

Si

Cl

Cr

8.60

48.34

37.70

0.43

1.12

13.94

42.77

43.29

1.48

1.38

0.96

3.04

90.95

24.50

1.10

73.94

0.47

18.71

15.94

7.00

57.25

1.10

Spot 2

22.03

17.13

7.33

53.51

Spot 3

14.55

31.29

53.87

0.29

Spot 1

29.01

5.87

65.12

Spot 2

39.09

8.09

52.15

0.66

Spot 3

25.47

19.49

54.16

0.88



77

Metallurgical Investigations into the Genesis of Bare Spots

(continued)

at the coating-steel interface. The micrographs also

the general appearance of being cold welded/fused

show entrapped rolling debris, possibly iron or iron

with the substrate during cold rolling. It should be

oxide particles, resulting from the cold rolling of

noted that iron fines/particulates such as those gen-

the strip. The micrographs also show that the un-

erated during cold rolling are oxidized at the surface

coated bare surface of steel contains fragmented

due to the heat generated during rolling. This, in all

particulate matter on the surface. These particles have

probability, accounts for the oxygen peaks observed

Fig. 7

Scanning electron micrographs of transverse surface of galvanized steel sheet with a massive bare spot, showing (a) entrapped rolling debris at coating-steel interface, (b) rolling debrispossibly oxidized iron finesat a bare spot, (c) rolling debris and delamination at coating-steel interface, (d) delamination at coating-steel interface, (e) iron-zinc crystallite formation at coating-steel interface, and (f ) iron-zinc intermetallics at coating-steel interface

Fig. 8

Scanning electron micrographs of transverse surface of galvanized steel sheet with a small bare spot, showing (a) bare spot, (b) rolling debris and delamination at coating-steel interface, (c) hemispherical appearance of zinc ribbon with delamination at coating-steel interface, (d) hemispherical appearance of zinc ribbon with entrapped rolling debris and delamination at coating-steel interface, and (e) coating-steel interface with entrapped rolling debris

78



in the EDS analysis of the uncoated patches. There

ance of zinc ribbons is indicative of high surface

is also evidence for iron-zinc crystallite formation

tension at the molten zinc-steel interface and the

(possibly zeta [FeZn13] crystallites), indicating bath/

lack of good wetting of molten zinc with the steel

process conditions leading to ineffective inhibition and suppression of iron-zinc intermetallic compounds. The micrographs in Fig. 8 exhibit the hemispherical appearance of thin zinc ribbons adhering to the bare substrate, along with entrapped matter at the interface, as well as zinc delamination from the steel substrate. The hemispherical appear-

substrate. Examination of galvanized steel specimens with coating exfoliation/peel-off also revealed fragmented particulates and a considerable amount of rolling debris on the steel surface (Fig. 9). The micrographs also show entrapped top-dross particles in the coating. The SEM micrographs of galvanized sheet specimens exhibiting matte/dull coating appearance showed profusion and preponderance of Fe-Al-Zn intermetallics, possibly entrapped particles of levitated bottom dross. These intermetallics were distributed within the coating (Fig. 10) and conferred a lackluster appearance. In high-aluminum zinc baths, such as one encountered during the period of investigation, the bottom-dross particles ( δ , FeZn 7 ) that accumulate during a long gal-

vanizing campaign become levitated and undergo

Fig. 9

Fig. 10

Scanning electron micrographs of transverse surface of

Scanning electron micrographs of transverse surface of

galvanized steel sheet with non-spangled, matte and dull

galvanized steel sheet with coating exfoliation/peel-off,

appearance of coating showing profuse dispersion of Fe-

showing (a) and (b) rolling debris, and (c) entrapped top-

Al-Zn floating-dross crystallites within the coating. (a)

dross particles in coating

1000×. (b) 2500×



79

Metallurgical Investigations into the Genesis of Bare Spots the reaction:

distribution of iron, zinc, carbon, and oxygen at the

2FeZn7 (Bottom dross) + 5Al = Fe2Al5Znx

(Floating dross) + (14 -

(continued)

x)Zn

These floating-dross particles were presumably entrapped in the coating.

coating-steel interface. A typical BSE compositional image of the coating-steel interface along with the X-ray elemental mapping images of iron, zinc, carbon, and oxygen in a galvanized sheet specimen that exhibited a massive bare spot are shown in Fig. 11(a) to (e), while the line profile image of carbon

The SEM observations confirmed the abundance

is shown in Fig. 11(f ). As is evident from the micro-

of cold rolling defects in the form of mechanical

graphs, microprobe analysis reveals a strong presence

damage, oxidized rolling debris, and iron fines/

of carbon and oxygen at the coating-steel interface.

particulates on the steel substrate. The EDS analyses

This is indicative of the presence of entrapped

also suggest the presence of burnt carbonaceous

carbonaceous residues at the steel-coating interface.

residues on the annealed sheet surface. These sub-

Again, the carbonaceous residue is thought to have

strate deficiencies, together with a sensitive gal-

emanated from the burning of carry-over cold rolling

vanizing bath composition, resulted in the poor

oil. The poor adherence of the zinc coating is also

wettability of zinc and consequently impaired its

evident from its delamination with the steel surface,

adherence with the steel.

as is clearly revealed in the BSE image.

Electron Probe Microanalysis

Conclusions

Electron probe microanalysis was carried out on

The galvanizing bath contained higher aluminum

transversely mounted and polished specimens of

and iron and lower lead levels than the stipulated

galvanized sheets exhibiting bare spots to confirm

operational norms and increased the sensitivity of

the presence of burnt carbonaceous matter/residue

the galvanizing process to the surface cleanliness

at the delaminated coating-steel interfaces. Speci-

of the steel substrate. The occurrences of cold

mens from samples with massive and small streaks

rolling oil carry-over and the formation of carbon-

were observed in backscattered electron (BSE) imag-

aceous deposits on the sheet surface following

ing mode and qualitatively analyzed by wavelength-

annealing led to the incidences of bare spots and

dispersive spectrometr y to develop the relative

the exfoliation/peel-off phenomena.

Fig. 11

Electron probe microanalysis of delaminated coating-steel interface in galvanized steel sheet with a massive bare spot, showing (a) backscattered electron (compositional) image, (b) iron X-ray map, (c) zinc X-ray map, (d) carbon X-ray map, (e) oxygen X-ray map, and (f ) carbon line profile

80



SEM/EDS and EPMA studies of galvanized

istry to appropriate levels, by ensuring that carry-

sheet specimens exhibiting bare spots and

over of lubricant is minimized, and by ensuring

exfoliation confirmed the presence of burnt

that effective burnoff takes place prior to the

carbonaceous residues and oxidized rolling debris

initiation of the galvanizing process.

(possibly consisting of iron fines) on the annealed strip surface. Both of these substances impaired the wettability of molten zinc with the steel substrate and led to the formation of bare spots and/or poor adherence at coated areas, resulting in coating exfoliation/peel-off.

Acknowledgments The authors are extremely grateful to the management of R&D Centre for Iron and Steel (RDCIS), Steel Authority of India Limited, Ranchi, for their encouragement and support of this investigative

Therefore, the occurrences of bare spots and

study. The authors also wish to thank B.B. Patra of

exfoliation/peel-off in hot dip galvanized steel

RDCIS and Z. Imam and H.K. Mishra of Central

sheets were primarily attributable to the poor

Mine Planning and Design Institute Limited,

surface condition of the steel substrate that

Ranchi, for their valuable help with the EPMA and

contained carry-over oil. This oil was subsequently

SEM/EDS work, respectively.

burnt during annealing in direct-fired and radiant tube furnaces and formed the carbonaceous deposits observed at the defects. This observation

References 1.

N.Y. Tang and F.E. Goodwin: A Study of Defects in

suggests inferior burnoff of the cold rolling

Galvanized Coatings, Proc. of Fifth Int. Conf. on Zinc and

lubricant. The presence of oxidized rolling debris/

Zinc Alloy Coated Steel Sheet, M. Lamberights, ed., Galvatech 2001, June 26-28, 2001 (Brussels, Belgium), Verlag Stahl-

iron fines and the intrinsically rough steel undersurface generated during cold rolling of the sheet were also partly responsible for the occurrence of defects in the hot dip galvanizing line. These defects may be avoided by controlling bath chem-


eisen, Düsseldorf, Germany, pp. 49-55. 2.

E.V. Proskurkin and N.S. Gorbunov: Galvanizing, Sherardiz-

ing and Other Zinc Diffusion Coatings, Technicopy Limited, Gloucestershire, England, in association with Zinc Development Association, London, 1975, pp. 96-107.


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