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 de- posited onto steel by a number of different processes, including hot-dipping, electro- deposition, and vapor deposition. Galvan- neal is a galvanized coating that has under- gone an annealing cycle to transform the almost all zinc coating to an alloyed iron- zinc coating. Hot-dip galvanneal has had ex- panded use in car body parts in the auto- motive industry because of its improved spot weldability and perforation corrosion resis- tance over that of hot-dip galvanized coat- ings [1]. Because of the increased use of hot-dip galvanneal by automobile manufac- turers, 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 coat- ings provides a useful tool in the character- ization of the iron-zinc phase layer growth that occurs during the galvannealing pro- cess. Metallography alone cannot determine the identity of the phases present, but it can provide useful information when used in conjunction with other characterization techniques.

Almost 45 years ago, Rowland [2] made a significant contribution to the technique

107

of metallographic preparation and etching of hot-dip galvanized and galvannealed coat- ings. In that work he discussed a number of etchants to be used on coatings depending upon their immersion time, chemical com- position, 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 differ- ence between adjacent phase layers. Row- land specified the use of different concen- trations of picric acid, ethyl alcohol, and water to etch short- and long-time immer- sion hot-dip galvanized coatings, galvan- nealed coatings, as well as coatings con- taining aluminum. He also developed two alternative solutions for the etching of gal- vanized 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 ap- proximately 10~m in thickness. Metallo- graphic 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 coat- ings 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-

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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, hot- dip coatings. The etchant was found to work well for hot-dip galvanized coatings contain- ing 0.00-0.15 effective wt.% aluminum [5] that were deposited on a number of differ- ent steel substrates. The etchant also per- formed well for coatings that were annealed under a variety of temperature/time condi- tions. The preparation technique also uses, in part, the work of Drewein et al. [6]. Although Drewein's techniques were devel- oped for electrodeposited coatings, modihca- tions have been made for improved struc- tural 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 sec- tions. 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

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 an- other 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 flat- ness 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 mount- ing media.

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

Edges of I n ~ ~ 0 ~ Indicator

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Spacers

Sheet .. sample

' ~ Edge of Interest

FIG. 2. Individual sheet sample in the process of be- ing incorporated into the stack arrangement.

Preparation for Hot-Dip Coatings 109

been completely exposed. The thickness of the mount is then measured using a mi- crometer. The mount is rough ground to re- move 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 mi- crometer 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 dur- ing grinding to prevent edge rounding of the samples. Edge flatness is critical for ana- lyzing the zinc coating located at the outer- most 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 direc- tion of spin of the wheel [6]. The mount should then be rotated 90 and held in a sim- ilar 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 lead- ing edges in this manner again minimizes rounding the edges of interest. After grind- ing 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 uni- form 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

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 al- cohol 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 dur- ing ultrasonic cleaning. The mount is blown dry and inspected. Immediate cleaning of the mount in alcohol is crucial in maintain- ing 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 grind- ing), and then the mount is cleaned with al- cohol (as described earlier) after each paper.

POLISHING

Polishing can begin with a stationary nap- less 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 sub- sequent 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 dummy mount should be used to work the paste into the new polishing cloth. Using a dummy mount prior to the actual mount will prevent large scratches from be- ing 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 mi- croscope. The scratches should appear in all directions, with no parallel scratches remain- ing from the last grinding step. This proce- dure 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.

1 1 0

Polishing can then be continued on a sta- tionary Struers DP NAP cloth charged with l~m paste, or Struers DP spray, HQ. This polishing cloth need only be charged infre- quently 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 re- main, polishing should continue for an ad- ditional 20-30 s, followed by cleaning and examination. The finish polishing step is per- formed 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-pre- pared 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 etch- ing solution be prepared with amyl alcohol and not ethanol. Amyl alcohol-based etch- ants etch more slowly than ethanol-based mixtures, thus allowing for more control dur- ing 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 im- mersed into the crucible containing no hydrofluoric acid, and it is slightly agitated for approximately 20 s. The sample is re- moved from the crucible and immediately

C. E. Jordan et al.

placed (metal surface side up) into the beaker containing ethanol. The mount is then re- moved, 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 re- peated using the ratio of 2:1 for the etch- ing 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 gal- vanneal coatings, and Fig. 5(a) is a commer- cial galvanneal product. All three types of coat ings-hot-dip galvanized, simulated galvanneal, and commercial galvanneal- exhibited good relief of structure using the described preparation technique and etch- ant. Nomarski differential interference con- trast 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 20 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 effective wt.% 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 stabilized interstitial free steel that was annealed for (a) 1 s at 450C; (b) 5 s at 450C; (c) 10 s at 450C; (d) 20 s at 450C; and (e) 60 s at 450C.

(e)

consistent with those for the zinc-rich Fe-Zn eta phase. Therefore, the x-ray data indicate that the as-galvanized coating contained es- sentially all eta phase. Examination of the coating using wavelength dispersive spec- troscopy (WDS) analysis confirmed the pres- ence of blocky crystals of an i ron-aluminum- zinc intermetallic c o m p o u n d located at the coating/steel interface.

Figure 4(a-e) shows examples of the sim- ulated galvanneal coatings generated during thermal processing of an as-galvanized coat-

ing (like that shown in Fig. 3) deposi ted onto an interstitial free (IF) steel. Because the gal- vanized coating has undergone a diffusional transformation upon heating, these coatings have developed a more complex structure of iron-zinc phases. The x-ray spec t rum presented in Fig. 7 was obtained from the coating shown 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 cor respond 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 gal- vannealed (alloyed) coating by conventional x-ray analysis.

For the shorter hold time annealed coat- ings [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 com- mercial galvanneal in Fig. 5(a) is very simi- lar 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 inten- sity (counts) versus 20 values of the as- galvanized coating in Fig. 3.

Preparation for Hot-Dip Coatings

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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 per- pendicular 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 gen- erated during polishing. It has been sug- gested 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 elec- tron image of the surface of the coating after heat treatment but prior to sample prepara- tion. 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 sim- ulation 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 over- polish the samples.

The most essential part of the metallo- graphic 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-

114 C. E. Jordan et al.

rosion of the coating. Precautions should also be taken to maintain edge flatness. The coatings are approximately 8-10~m in thick- ness 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 tex- ture of the coatings in cross section.

SUMMARY

A new etchant for hot-dip galvanized coat- ings has been developed. It has proven successful for hot-dip galvanized, laboratory- simulated galvanneal, and commercial gal- vanneal coatings. The maintenance of coat- ing sample flatness and cleanliness was found to be critical in the metallography of hot-dip galvanized and galvannealed coatings.

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 gal- vanized coatings, Trans. ASM 40:983 (1948).

3. J. R. Kilpatrick, A new etching technique for gal- vanneal and hot-dipped galvanized coatings, Prac- tical 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 electro- deposited iron-zinc alloy coatings on steel, Materi- als 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).

The authors thank National Steel, Armco Steel, LTV Steel, Dofasco, Rouge Steel, Cockerill Sambre, and Noranda for their sponsorship of this work. Received November 1992; accepted May 1993.