Metallographic Preparation Technique for Hot-Dip
Galvanized and Galvannealed Coatings on Steel
C. E. Jordan, K. M. Goggins, A. O. Benscoter, and A. R. Marder
Lehigh University, Materials Science and Engineering Department, Bethlehem, PA 18015
A new metallographic technique for hot-dip galvanized and galvannealed coatings has been
developed. The new polishing procedure and etchant have shown excellent results on
commercial hot-dip galvanized and galvanneal coatings, as well as on laboratory-simulated
hot-dip galvanneal produced under a variety of thermal processing parameters.
INTRODUCTION
Galvanized coatings have been used for
many years, providing sacrificial and anodic
corrosion protection of steel. Zinc can be deposited onto steel by a number of different
processes, including hot-dipping, electrodeposition, and vapor deposition. Galvanneal is a galvanized coating that has undergone an annealing cycle to transform the
almost all zinc coating to an alloyed ironzinc coating. Hot-dip galvanneal has had expanded use in car body parts in the automotive industry because of its improved spot
weldability and perforation corrosion resistance over that of hot-dip galvanized coatings [1]. Because of the increased use of
hot-dip galvanneal by automobile manufacturers, there has been new interest in the
research and development of the older and
less costly hot-dip zinc coating process.
Metallographic inspection of these coatings provides a useful tool in the characterization of the iron-zinc phase layer growth
that occurs during the galvannealing process. Metallography alone cannot determine
the identity of the phases present, but it
can provide useful information w h e n used
in conjunction with other characterization
techniques.
Almost 45 years ago, Rowland [2] made
a significant contribution to the technique
of metallographic preparation and etching
of hot-dip galvanized and galvannealed coatings. In that work he discussed a number
of etchants to be used on coatings depending
upon their immersion time, chemical composition, and thermal history. The etchants
developed by Rowland were color etchants,
which could be used to identify phase layers
within the coating based on the color difference between adjacent phase layers. Rowland specified the use of different concentrations of picric acid, ethyl alcohol, and
water to etch short- and long-time immersion hot-dip galvanized coatings, galvannealed coatings, as well as coatings containing aluminum. He also developed two
alternative solutions for the etching of galvanized coatings containing aluminum.
Rowland's work in color etchants has since
been developed further by Kilpatrick [3].
The coatings discussed in this article are
short-time immersion coatings that are approximately 10~m in thickness. Metallographic preparation of thin zinc coatings can
be difficult because the outer edges of the
relatively soft coating can become rounded
during grinding and polishing, thus making
examination difficult. Etching of these coatings can also be a problem because of the
small anode to cathode reaction area ratio,
which causes the zinc coating (anode) to
react rapidly in acidic solutions. Other in-
107
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108
C. E. Jordan et al.
vestigators such as Giallourakis et al. [4] have
made an attempt to avoid the difficulties of
metallographic preparation and etching by
developing a cryogenic fracture technique
for the characterization of zinc coatings. This
technique avoids the need for polishing and
etching of the zinc coating altogether.
A new etchant based on Rowland's work
has been developed that is better suited for
today's thinner, short-time immersion, hotdip coatings. The etchant was found to work
well for hot-dip galvanized coatings containing 0.00-0.15 effective wt.% aluminum [5]
that were deposited on a number of different steel substrates. The etchant also performed well for coatings that were annealed
under a variety of temperature/time conditions. The preparation technique also uses,
in part, the work of Drewein et al. [6].
Although Drewein's techniques were developed for electrodeposited coatings, modihcations have been made for improved structural analysis of hot-dip zinc coatings in the
present investigation.
PROCEDURE
SECTIONING
For standard 31.75mm (1.25in.) mounts, the
sheet samples are cut to 25 x 13ram-size sections. Sectioning can be performed using
a tabletop hand shear so that the sheet can
be accurately cut to size. The samples are
then placed to form a stack (Fig. 1) with the
25mm-long freshly cut edges parallel to one
EdgesofI n ~
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GRINDING
When the mount has cured, excess epoxy
is ground off the surface of the mount until
the metal surfaces of all the samples have
Indicator
vf
pacs*
another. The stack is assembled so that the
long edges are aligned and flush with one
another. The flush orientation of the samples
is important during rough grinding where at
least 2mm of material must be removed from
each sample in the mount, and alignment
ensures this. If one side of each sheet sample
is of particular interest, it is necessary to form
the stack so that the sides of interest are all
facing in the same direction. The reason for
this orientation will be addressed later.
Samples can be separated from one another by placing a small piece of double-stick
tape (spacer) at each short end of the sample,
away from the edge of interest, as shown in
Fig. 2. Any spacer that separates the sheet
samples but keeps their distance apart to
a minimum is suitable. At least six to eight
sheet samples should be used in each stack.
Two additional dummy samples are needed,
one on each side of the stack, to maintain
coating flatness of the end samples. Because
the epoxy resin used for mounting is soft,
stabilizers, such as two cut pieces of steel
welding rod material, should be placed on
either side of the stack to ensure mount flatness during grinding and polishing. As a
point of reference, an indicator (scrap steel
material) can also be included in the mount
(Fig. 1) prior to filling the mould. Epoxy resin
and hardener are then used as the mounting media.
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FIG. 1, Planar view of the mount showing the stack
arrangement of the coated sheet samples.
EdgeofInterest
FIG. 2. Individual sheet sample in the process of being incorporated into the stack arrangement.
Preparation for Hot-Dip Coatings
been completely exposed. The thickness of
the mount is then measured using a micrometer. The mount is rough ground to remove at least 2mm of material, so that the
deformation introduced into the coating
during sectioning has been removed. The
amount of material removed can be routinely
checked during grinding if one uses the micrometer to monitor the thickness of the
mount. When a belt grinder is used for
rough grinding, the sheet samples must be
kept parallel to the direction of the belt during grinding to prevent edge rounding of
the samples. Edge flatness is critical for analyzing the zinc coating located at the outermost edges of the steel sheet samples.
If the paper used in rough grinding was
120 grit, then grinding should continue on
240-, 320-, 400-, and 600-grit papers, in that
order. The last step in grinding should leave
scratches parallel to the long edges of the
samples to minimize rounding. The mount
should be moved laterally back and forth in
the middle region of the spinning wheel,
off center, with the long edges of the stack
either perpendicular or parallel to the direction of spin of the wheel [6]. The mount
should then be rotated 90 ° and held in a similar manner while one grinds on a new grade
of grit paper; thus, the new scratches are
perpendicular to those of the previous step.
This method of grinding guarantees that all
of the scratches from the previous step have
been removed. During grinding, the edge
of interest is either the leading edge (first
edge to encounter the motion of the paper)
perpendicular to the spin of the wheel, or
it is parallel to the spin of the wheel, with
the edge of interest closest to the center
of the wheel. The placement of the leading edges in this manner again minimizes
rounding the edges of interest. After grinding on each paper, the surface is flushed with
alcohol, and the mount is blown dry and
inspected under a light optical microscope
to ensure that (1) all of the scratches are uniform in direction in all of the samples in the
mount, and (2) that no scratches remain from
the previous grinding step. The previously
described procedure can be performed on
an automatic grinder (with an applied load
109
of 25psi) by grinding for 60-90 s on each
grade of grit paper.
Immediately after the sample is removed
from the 600-grit paper, the surface is
swabbed with an alcohol saturated cotton
ball, and the mount is flushed with alcohol.
Ethanol (190 or 200 proof) or denatured alcohol is suitable for cleaning purposes. The
mount is ultrasonically cleaned for 30-60 s
while standing the mount on edge in a
beaker of alcohol. Precautions should be
taken to be sure that all of the samples in
the mount are submerged in the alcohol during ultrasonic cleaning. The mount is blown
dry and inspected. Immediate cleaning of
the mount in alcohol is crucial in maintaining a clean, corrosion-free sample. Ultrafine
grinding continues on an 8- and then 3~m
SiC papers (it is the author's preference to
use 8- and 3~tm papers, but 12- and 5~m
papers are also appropriate for fine grinding), and then the mount is cleaned with alcohol (as described earlier) after each paper.
POLISHING
Polishing can begin with a stationary napless cloth similar to the Leco Pan W cloth,
impregnated with 3~tm diamond paste.
Engis diamond extender solution is suitable
as a lubricating media for this and all subsequent polishing steps. A diamond slurry
and extender (pH = 9.6 + 0.2) combination
has also proven to be a successful polishing
media [6]. If a paste is used instead of a
slurry, a d u m m y mount should be used to
work the paste into the new polishing cloth.
Using a d u m m y mount prior to the actual
mount will prevent large scratches from being introduced into the samples. The mount
should be rotated in a clockwise direction
applying a heavy, even pressure. Polishing
continues for I minute, and then the mount
is cleaned and examined under the light microscope. The scratches should appear in all
directions, with no parallel scratches remaining from the last grinding step. This procedure is repeated using a new Pan W cloth
impregnated with l~tm diamond paste. Upon
examination after this step, the scratches
present should appear finer.
110
C. E. Jordan et al.
Polishing can then be continued on a stationary Struers DP NAP cloth charged with
l ~ m paste, or Struers DP spray, HQ. This
polishing cloth need only be charged infrequently and can be covered and stored for
future polishing of coatings. A heavy even
pressure must be applied for 30 s, and then
the mount should be cleaned and examined.
The samples should be almost free of
scratches. If large, significant scratches remain, polishing should continue for an additional 20-30 s, followed by cleaning and
examination. The finish polishing step is performed on a separate Struers DP NAP cloth
charged with 0.25~m diamond paste, or
0.25~m DP-spray, HQ. Heavy pressure for
20 s is required followed by cleaning and
examination. The samples should now be
ready for etching.
ETCHING
The etchant to be used should be prepared
prior to the start of any polishing procedures
so that the sample can be etched at room
temperature immediately after polishing has
been completed. The etchant found to give
the best results was a mixture consisting of
1% picric acid in amyl alcohol and 1% nitric
acid in amyl alcohol. The solution is-prepared by mixing equal parts of 1% picric acid
in amyl alcohol and 1% nitric acid in amyl
alcohol in a beaker. Equal amounts of the
mixed solution are poured into two crucibles.
Into one crucible, 3-4 drops of hydrofluoric
acid (to approximately 50ml of solution) is
added, and a beaker of ethanol is placed near
the two crucibles. It is critical that the etching solution be prepared with amyl alcohol
and not ethanol. Amyl alcohol-based etchants etch more slowly than ethanol-based
mixtures, thus allowing for more control during etching [6].
To etch the samples, the mount is held
with tongs so that the metal surfaces of the
samples face upward. The mount is immersed into the crucible containing no
hydrofluoric acid, and it is slightly agitated
for approximately 20 s. The sample is removed from the crucible and immediately
placed (metal surface side up) into the beaker
containing ethanol. The mount is then removed, the surface flushed with ethanol,
and then immersed into the second crucible
(HF added) and slightly agitated for 10 s.
The surface is flushed again with ethanol,
blown dry, and examined. If the samples are
underetched, the previous procedure is repeated using the ratio of 2:1 for the etching time of the first solution to that of the
second solution.
RESULTS
The etched coatings are shown in Figs. 3, 4,
and 5(a), and are approximately 8-10,m in
thickness. Figure 3 is a hot-dip galvanized
coating, Fig. 4(a-e) shows simulated galvanneal coatings, and Fig. 5(a) is a commercial galvanneal product. All three types of
coatings-hot-dip galvanized, simulated
galvanneal, and commercial galvannealexhibited good relief of structure using the
described preparation technique and etchant. Nomarski differential interference contrast in the light microscope allowed the
topographical features of the coatings in
cross section to be viewed.
Figure 6 is an x-ray spectrum of intensity
versus 2 0 values of the hot-dip galvanized
coating shown in Fig. 3. The major peaks
at 36.4°, 39.10, and 77.1° correspond to d
spacings of 24.6, 23.0, and 12.4nm that are
FIG. 3. Cross section of a hot-dip galvanized coating
(0.10 effectivewt.% A1-Zn)deposited o n t o a drawing
quality special killed (DQSK) steel.
Preparation for Hot-Dip Coatings
111
(a)
(b)
(c)
(d)
FIG. 4. Cross section of a hot-dip galvanized coating
deposited onto a titanium stabilizedinterstitialfree steel
that was annealed for (a) 1 s at 450°C; (b) 5 s at 450°C;
(c) 10 s at 450°C; (d) 20 s at 450°C; and (e) 60 s at 450°C.
(e)
consistent with those for the zinc-rich Fe-Zn
eta phase. Therefore, the x-ray data indicate
that the as-galvanized coating contained essentially all eta phase. Examination of the
coating using wavelength dispersive spectroscopy (WDS) analysis confirmed the presence of blocky crystals of an iron-aluminumzinc intermetallic c o m p o u n d located at the
coating/steel interface.
Figure 4(a-e) shows examples of the simulated galvanneal coatings generated during
thermal processing of an as-galvanized coat-
ing (like that s h o w n in Fig. 3) deposited onto
an interstitial free (IF) steel. Because the galvanized coating has u n d e r g o n e a diffusional
transformation u p o n heating, these coatings
have developed a more complex structure
of iron-zinc phases. The x-ray s p e c t r u m
p r e s e n t e d in Fig. 7 was obtained from the
coating s h o w n in Fig. 4(d) and shows that
the largest intensity peaks occur in the 2(9
range of 40-45 °. High-intensity peaks for the
Fe-Zn gamma, delta, and zeta phases all
occur at d spacings that c o r r e s p o n d to this
112
C. E. Jordan et al.
(a)
(b)
FIG. 5. (a) Cross section of a commercial galvanneal coating. (b) Scanning electron image of the surface structure
of the commercial galvanneal coating shown in (a). (See text for explanation of arrows.)
region. Therefore, it is difficult to identify
and quantify the phases present in the galvannealed (alloyed) coating by conventional
x-ray analysis.
For the shorter hold time annealed coatings [Fig. 4(a-c)], an eta phase layer remains
at the outermost part of the coating. Below
this layer there is thought to be a layer of
blocky zeta phase crystals, and a layer of
columnar grains of delta phase. The longer
hold time annealed coatings show no eta
phase remaining in the coating; instead, they
show the presence of a gamma Fe-Zn phase
layer at the coating/steel interface. Also
present in these longer hold time coatings
[Fig. 4(d, e)] are cracks in the delta and
gamma phases, running perpendicular to the
coating/steel interface. Similar metallographic
results were found for coatings deposited
on DQSK (drawing quality special killed),
DQSK preannealed, ultra-low carbon, and
rephosphorized steel substrates. The commercial galvanneal in Fig. 5(a) is very similar in appearance to the longer hold time
simulation coatings such as shown in Fig.
4(d). In the commercial product, a gamma
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FIG. 6. X-ray diffraction spectrum of intensity (counts) versus 20 values of the asgalvanized coating in Fig. 3.
113
Preparation for Hot-Dip Coatings
xiO 3 i
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FIG. 7. X-ray diffraction spectrum of intensity (counts) versus 2f) values of the simulated galvannealed coating
in Fig. 4(d).
phase layer is present as well as cracks perpendicular to the coating/steel interface.
DISCUSSION
One question that has been raised about the
metallographic procedure is whether the
cracks observed in the microstructure were
created by the technique itself, because of
the brittle nature of the Fe-Zn phases that
form, or are the cracks truly a characteristic
of the coating. Because the gamma phase
is known to be one of the most brittle of the
Fe-Zn phases that develop [7], it has been
proposed that the cracks may have been generated during polishing. It has been suggested that because of an excess of applied
pressure during polishing and the brittle
nature of the gamma phase, the gamma
phase initially undergoes cracking. The
cracks can then propagate along columnar
delta phase boundaries in the coating to form
cracks that extend the width of the coating.
However, scanning electron microscopy of
the surface of the coating, where no sample
preparation had been performed, revealed
that cracks were present, as shown in Fig.
5(b) (arrows). This hgure is a scanning electron image of the surface of the coating after
heat treatment but prior to sample preparation. Thus, the cracks appear to be present
prior to metallographic sample preparation.
It has been found, however, that upon
further polishing of the long hold time simulation samples [where a signihcant gamma
layer is present, as in Fig. 4(e)], the coating
can become more cracked and difficult to
work with. Care must be taken not to overpolish the samples.
The most essential part of the metallographic technique presented here is to keep
the sample surface extremely flat and clean.
During the last steps of grinding and all
throughout polishing, the samples should
be kept free of water, which can cause cor-
C. E. Jordan et al.
114
rosion of the coating. Precautions should
also be taken to maintain edge flatness. The
coatings are approximately 8-10~m in thickness and have a lower hardness than that
of the substrate steel; therefore, to have the
entire cross section of the coating in focus,
the softer outer edge of the coating must be
as fiat as possible relative to the substrate
steel. The stack orientation of the samples
helps to reduce this problem. Nomarski
differential interference contrast also proved
helpful in revealing the topography or texture of the coatings in cross section.
SUMMARY
A new etchant for hot-dip galvanized coatings has been developed. It has proven
successful for hot-dip galvanized, laboratorysimulated galvanneal, and commercial galvanneal coatings. The maintenance of coating sample flatness and cleanliness was
found to be critical in the metallography of
hot-dip galvanized and galvannealed coatings.
The authors thank National Steel, Armco Steel,
LTV Steel, Dofasco, Rouge Steel, Cockerill Sambre,
and Noranda for their sponsorship of this work.
References
1. Y. Hisamatsu, Science and Technology of Zinc and Zinc
Alloyed Coated Steel Sheet, Proc. Galvatech '89, The
Iron and Steel Institute of Japan, Tokyo, Japan, p. 3
(1989).
2. D.H. Rowland, Metallography of hot-dipped galvanized coatings, Trans. ASM 40:983 (1948).
3. J. R. Kilpatrick, A new etching technique for galvanneal and hot-dipped galvanized coatings, Practical Metallography 28:649 (1991).
4. N. M. Giallourakis, D. K. Matlock, and G. Krauss,
A cryogenic fracture technique for characterizing
zinc-coated steels, Metallography 23:209 (1989).
5. S. Belisle, V. Lezon, and M. Gagne, The Solubility
of Iron in Continuous Hot-Dip Galvanizing Baths, 21st
Meeting of the Galvanizers Association, Monterrey,
Mexico (October 1989).
6. C. A. Drewein, A. O. Benscoter, and A. R. Marder,
Metallographic preparation technique for electrodeposited iron-zinc alloy coatings on steel, Materials Characterization 26:45 (1991).
7. G. E Bastin, E van Loo, and G. D. Reick, A new
compound in the iron zinc system, Z. Metalkunde
65:656 (1974).
Received November 1992; accepted May 1993.