Metallography Znalloy

Prévia do material em texto

10. A.D. Romig, Jr. and W.R. Sorenson, Uranium Alloys: Sample Preparation for Transmission Electron 
Microscopy, J. Microsc., Vol 132, 1983, p 203 
11. Radiological Health Handbook, U.S. Department of Health, Education, and Welfare, Public Health 
Service, Food and Drug Administration, Bureau of Radiological Health, Rockville, MD, 1970 
12. “Occupational Health Guideline for Uranium and Insoluble Compounds,” U.S. Department of Health 
and Human Services, Washington, DC, 1978 
13. “Hygienic Guide Series—Uranium,” American Industrial Hygiene Association, Detroit, MI 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its 
Alloys 
 
Introduction 
ZINC AND ZINC ALLOY specimen preparation techniques are discussed in this article. Typical structures 
observed in these specimens are also covered, as is the effect of alloying elements and processes. The etchants 
used for many of the micrographs included in this article are listed in Table 1. 
Table 1 Etchants for zinc and zinc alloys 
Etchant Composition 
1(a) 200 g CrO3, 15 g Na2SO4, 1000 mL H2O 
2(b) 50 g CrO3, 4 g Na2SO4, 1000 mL H2O 
3 200 g CrO3 and 1000 mL H2O 
4 5 mL HNO3 and 100 mL H2O 
5 5 g FeCl3, 10 mL HCl, 240 mL alcohol 
6(c) 375 mL H3PO4, and 625 mL ethyl alcohol 
7 1 drop HNO3 and 10 mL amyl alcohol 
(a) For rolled zinc-copper alloys, the Na2SO4 content can be reduced to 7.5 g. If desired, a smoothly etched 
surface can be obtained by increasing the Na2SO4 to 30 g. 
(b) This etchant can be prepared by mixing one part (by volume) etchant 1 and three parts H2O. 
(c) Electrolytic; current density, 0.1 to 0.2 A/cm2 (0.6 to 1.3 A/in.2) for 3 to 8 s 
 
 
 
 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
Microstructures of Zinc and Zinc Alloys 
The natural impurities, contaminants, and alloying additions present in commercial zinc materials have 
extremely limited solid solubility. They readily produce alterations in cast or wrought microstructures and 
changes in one or more properties. High-purity zinc, UNS Z13002, for example, is 99.99% Zn with maximum 
limits of 0.003% each on lead, iron, and cadmium and is almost free of mircosegregation (Fig. 1, 2). Nominal 
compositions of the alloys depicted in this article are noted in the captions. 
 
Fig. 1 Special high-grade zinc, UNS Z13002 [99.99% Zn (min), 0.003% Pb (max), 0.003% Fe (max), 
0.003% Cd (max)], as-cast. Almost free of microsegregation. Etchant 1, Table 1. 100× 
 
Fig. 2 Same alloy as Fig. 1 under polarized light illumination to show the extent of grain growth from 
original etched grain boundaries within large grains. Etchant 1, Table 1. 100× 
The elements commonly found in zinc are lead, cadmium, iron, copper, aluminum, titanium, and tin. Lead, 
cadmium, tin, and iron are natural impurities in zinc and are also added to zinc to develop desired properties. 
Zinc casting alloys are primarily zinc-aluminum with small additions of other elements, such as copper and 
magnesium. Wrought zinc alloys for rolled products generally contain lead, iron, cadmium, copper, or titanium 
alone or in combination and usually in concentrations under 1%. The effects on microstructure produced by 
these elements are described as follows. 
Zinc has a familiar role as a protective coating for steel in galvanizing processes. Pure zinc and zinc-aluminum 
alloys are used in continuous hot dip processes. The galvanneal process uses zinc-iron alloys (Ref 1). Batch 
process hot dip galvanizing uses high-grade zinc (UNS Z15001, with impurities less than 0.10%; UNS Z13001, 
with impurities less than 0.010%; and prime western zinc, UNS Z19001) (Ref 2). The interaction between base 
materials and coatings results in interesting profiles of microstructures (Ref 1, 3, 4, 5). 
Lead. The solubility of lead in solid zinc is extremely limited. A monotectic is formed at 418 °C (784 °F) and a 
lead content of 0.9%, and zinc crystals and liquid exist in equilibrium down to the eutectic temperature of 318 
°C (604 °F). As a result, lead appears in cast zinc and zinc alloys at the dendrite boundaries in the form of 
small, spherical droplets or surface films (Fig. 3). Because of their softness, the droplets can be easily pulled out 
during polishing, leaving holes that appear black in the microstructure. Special care in polishing is required to 
retain the lead particles. 
 
Fig. 3 Prime western zinc, UNS Z19001 [98% Zn (min), 1.4% Pb (max), 0.05% Fe (max), 0.20% Cd 
(max)], as cast. The dark spots are lead particles at the grain boundaries. Etchant 1, Table 1. 100× 
When rolled, the particles of lead are elongated in the rolling direction and are not located preferentially at the 
recrystallized grain boundaries. In zinc-aluminum alloys, lead induces intergranular corrosion; concentrations 
must be maintained below 0.004%. Lead was added to UNS Z33520 to illustrate this effect in Fig. 4 and 5. 
 
Fig. 4 Fracture surface of the 10 mm (0.375 in.) diameter end of a tension test bar die cast from alloy 3 
(UNS Z33520) to which 0.018% Pb was added (0.005% Pb is allowed). Exposed 10 days to wet steam at 
95 °C (205 °F). Dark ring is intergranular corrosion. See also Fig. 5 Not polished, not etched. 6× 
 
Fig. 5 Micrograph of edge of fracture surface in Fig. 4 Subsurface intergranular corrosion (top) causes 
swelling and decreases mechanical properties. Deliberate addition of 0.018% Pb to the alloy 
approximates the contamination that might occur from the use of remelted scrap. As-polished. 100× 
The cadmium present in most commercial zinc products is in solid solution and produces no change in 
microstructure, except coring in the cast structure. In rolled zinc, the cadmium remains in solid solution, 
increasing strength, hardness, and creep resistance and raising the recrystallization temperature (Fig. 6, Fig. 7). 
In zinc-aluminum alloys, because cadmium lowers resistance to intergranular corrosion, concentrations must 
remain below 0.003%. 
 
Fig. 6 Hot-rolled special zinc [99% Zn (min), 0.6% Pb (max), 0.03% Fe (max), 0.50% Cd (max)], under 
polarized light; grains are clearly defined. Etchant 1, Table 1. 250× 
 
Fig. 7 Same alloy as Fig. 6 except cold rolled and photographed under polarized light. Note distortion of 
the grains caused by cold working. Etchant 1, Table 1. 250× 
Iron, when present in zinc in amounts exceeding approximately 0.001%, appears in the microstructure as an 
intermetallic compound containing approximately 6% Fe. The particle size is controlled by the amount of iron 
present and the thermal history of the part. Fine particles in a casting can be coalesced to a coarser form by 
prolonged heating at 370 °C (700 °F). 
Cast specimens with fast cooling (Fig. 8) and very slow cooling (Fig. 9) show the zeta-phase intermetallics. 
Four distinct phases are seen in the profiles of galvanized coating on steel (Ref 5, Fig. 1) and galvannealed 
coating (Ref 1, Fig. 7). 
 
Fig. 8 Zn-0.025Fe alloy, permanent mold, rapid cooling, annealed 40 h at 380 °C (716 °F). Zeta-phase 
intermetallic compounds appear as fine precipitate through annealing. Electrolytic polish. Etchant: 
similar to etchant 1, Table 1 (but only 10 g Na2SO4). 500× 
 
Fig. 9 Zn-0.025Fe alloy, hot graphite mold, slow cooling. Zeta-phase intermetallic compounds. 
Electrolytic polish. Electrolytic etch: etchant 6, Table 1 (short time). 200× 
The iron-zinc compound, like lead, precipitates at dendrite boundaries. When a zinc casting is rolled, the iron-
zinc particles are elongated in the rolling direction, along with any leadparticles present. The presence of iron 
particles in the proper concentration and distribution in rolled zinc assists in control of grain size. Iron in zinc-
aluminum alloys is present as FeAl3 particles, which can significantly lower ductility. Alloy ZA-27, UNS 
Z35841, with 0.05% Fe added (Fig. 10) and with 0.013% Fe added (Fig. 11) results in an FeAl3 intermetallic. 
 
Fig. 10 ZA-27 alloy, UNS Z35841, with 0.05% Fe (0.075% Fe max allowed), as sand cast. Structure 
consists of intermetallic FeAl3 particles (dark gray) in a matrix of particles (light) and particles (light 
gray). As-polished. 100× 
 
Fig. 11 ZA-27 alloy, UNS Z35841, with 0.13% Fe (excess, 0.075% Fe max allowed), as sand cast. 
Structure is intermetallic FeAl3 particles (dark gray) in a matrix of phase and phase. Structure is 
much coarser than in Fig. 10 As-polished. 100× 
Copper, when present in zinc in amounts to approximately 1%, is in solid solution and results in a cored 
structure. During hot rolling at approximately 205 °C (400 °F), the copper is retained in supersaturated solid 
solution. On cooling, some of the zinc-copper phase precipitates at the final recrystallized grain boundaries 
(Fig. 12). During long exposures near room temperature, phase will continue to precipitate at grain boundaries 
and finally in the interior of the grains, ultimately forming T phase (ternary eutectic). When cold rolled, phase 
precipitates rapidly and abundantly in the cold-worked structure (Fig. 13). In concentrations beyond 1% in zinc-
aluminum alloys, phase precipitates as an interdendritic phase. 
 
Fig. 12 Zinc containing 1% Cu, UNS Z44330, hot rolled. Polarized light illumination clearly defines the 
zinc-copper phase at grain boundaries. Etchant 1, Table 1. 250× 
 
Fig. 13 Cold-rolled Zn-1Cu alloy, UNS Z44330, photographed under polarized light. Note the severe 
distortion of grains caused by cold working (compare with Fig 12). Etchant 1, Table 1. 250× 
Die cast alloy UNS Z35531, alloy 5, containing 1% Cu is seen in Fig. 14. Aging for 10 days at 95 °C (205 °F) 
increases the amount of precipitation in the zinc solid solution (Fig. 15). An alloy with 0.9% Cu, UNS Z35841, 
ZA-27, shows primary cored aluminum-rich dendrites with peritectic + and white -phase particles (Fig. 
16). The same alloy in a sand cast specimen that was treated 3 h at 360 °C (680 °F) shows course -phase 
particles at old dendritic boundaries (Fig. 17). With longer lower-temperature exposure, the phase is 
converted to a fine T phase (ternary eutectic) (Fig. 18). Continuously cast, the same alloy has a finer 
microstucture. The presence of the -phase particles varies with the size of the bar being cast (Fig. 19, 20). 
 
Fig. 14 Alloy 5 (UNS Z35531, ASTM AC41A, Zn-4.1Al-0.055Mg-1.0Cu), as die cast. Alloy 5 has more 
copper and magnesium and higher strength and hardness than alloy 3. Etchant 2, Table 1. 1000× 
 
Fig. 15 Same as Fig. 14 except aged 10 days at 95 °C (205 °F). Aging had the same effect on the die cast 
structure as for alloy 3 (UNS Z33520) (Fig. 24b). Etchant 2, Table 1. 1000× 
 
Fig. 16 ZA-27 alloy (UNS Z23841, Zn-11Al-0.9Cu-0.02Mg), as sand cast. Primary, cored aluminum-rich 
dendrites surrounded by peritectic + . White particles are phase. Less eutectic is apparent than in 
Fig. 1, 2, 23, 24 Etchant 1, Table 1. (a) 100×. (b) 500× 
 
Fig. 17 Same alloy as Fig. 16 sand cast, homogenized 3 h at 360 °C (680 °F), and furnace cooled. 
Structure is fully stabilized phase decomposed into + lamellar eutectoid. Coarse -phase particles 
are present at old dendritic boundaries. Etchant 1, Table 1. 500× 
 
Fig. 18 Same alloy as Fig. 16 sand cast, heat treated 12 h at 250 °C (480 °F), and furnace cooled. 
Structure consists of coarse eutectoid + phase and eutectic. The metastable phase has been 
converted to fine T (ternary eutectic). Etchant 1, Table 1. 500× 
 
Fig. 19 Same alloy as Fig. 16 except continuous cast in a 25 mm (1 in.) diameter bar. There is a much 
finer microstructure, with more eutectic and no coarse -phase particles. Compare also with Fig. 20. 
Etchant 1, Table 1. (a) 100×. (b) 500× 
 
Fig. 20 Same alloy as Fig. 16 except continuously cast in a 150 mm (6 in.) diameter bar. Same 
constituents as Fig. 16, but a finer structure. Some coarse -phase particles (white) are evident. Compare 
with Fig. 19(a) and (b), which show the same alloy cast in a smaller section. Etchant 1, Table 1. (a) 100×. 
(b) 500× 
Titanium. The solid solubility of titanium in zinc is very limited. At approximately 0.12% Ti, a lamellar 
eutectic of zinc and TiZn15 (4.66% Ti) forms (Fig. 21). The eutectic forms at dendrite boundaries in a casting. 
The TiZn15 compound decreases the cast grain size of zinc and restricts grain growth in hot-rolled zinc. In 
rolled strip, particles of the compound are elongated in the rolling direction. Zinc grain growth is limited to the 
spacing between the stringers of compound. 
 
Fig. 21 Cast zinc with 0.6% Cu and 0.14% Ti. Eutectic (zinc and titanium-zinc phases) at grain 
boundaries. Both etched in etchant 1, Table 1. (a) 100×. (b) Showing the lamellar eutectic. Coarse needles 
of titanium-zinc phase are parallel to the polishing plane; fine needles, perpendicular. 250× 
Aluminum. A lamellar eutectic forms at 5% Al between aluminum ( ) and zinc ( ) solid solution at 382 °C 
(720 °F). The constituent of the eutectic is stable only above 275 °C (527 °F); at lower temperatures, it 
transforms eutectoidally into and phases (Fig. 22, 23). Although the solid solubility of aluminum in zinc at 
the eutectic temperature is approximately 1%, castings containing as little as 0.10% Al display the eutectic 
structure in interdendritic areas. At the normal aluminum concentration in standard zinc die-casting alloys 
(4.0% Al), the rate of attack by the melt on iron is sufficiently low to permit die casting in hot-chamber 
machines in which the operating mechanism is immersed continuously in the molten alloy. 
 
Fig. 22 ZA-8 alloy (UNS Z35636, Zn-8Al-1Cu-0.02Mg), as sand cast. Both etched in etchant 1, Table 1. 
(a) Dendrite structure at a lower magnification, 100×. (b) Coarse, zinc-rich dendrites in a matrix of + 
eutectic phase. 500× 
 
Fig. 23 Same alloy as Fig. 22 except as pressure die cast. Same constituents as Fig. 22, but pressure die 
casting has yielded a much finer microstructure. Etchant 2, Table 1. 500× 
During solidification of hypoeutectic zinc die-casting alloys containing approximately 4% Al, such as alloy 3 in 
Fig. 24(a), the first material to freeze appears as primary particles of zinc-rich solid solution ( phase). Later, 
the remaining liquid solidifies as eutectic composed of phase and the unstable high-temperature constituent . 
Aluminum acts as a grain refiner in cast zinc; together with the high solidification rates of the die casting 
process, this results in a fairly fine equiaxed grain structure, which is primarily responsible for the strength, 
ductility, and toughness of zinc die castings. 
 
Fig. 24 Alloy 3 (ASTM AG40A, Zn-4.1Al-0.035Mg). (a) As die cast. Structure is zinc solid solution 
surrounded by eutectic. (b) Same as (a) except aged 10 days at 95 °C (205 °F). Aging increased the 
amount of precipitation in the zinc solid solution. Both etched in etchant 2, Table 1. 1000× 
When die castings are aged at room temperature or a slightly elevated temperature, a precipitation reaction 
occurs in the zinc-rich phase. In a freshly made die casting, phase may contain approximately 0.35% Al in 
solution. During five weeks at room temperature, this will decrease to approximately 0.05%, the difference 
appearing as minute particles of within the -phase structure. Most of the aluminum can be precipitated in 
much less time by aging at a slightly elevated temperature (Fig. 24b). Similar aging effectsare observed in 
alloys containing copper, which is added to increase tensile strength and hardness. Copper additions to 1.5% 
yield die castings with reasonably stable properties and dimensions. Low-temperature stabilization annealing is 
occasionally used to achieve maximum stability. The zinc-aluminum alloys extend the amount of primary 
phase; in the ZA-8 (Fig. 22) and ZA-12 (Fig. 25) alloys, this is phase but is replaced by in the ZA-27 alloy 
(Fig. 11). 
 
Fig. 25 ZA-12 alloy (UNS Z35631, Zn-11Al-0.9Cu-0.02Mg), as sand cast. Coarse, zinc-rich dendrites in a 
matrix of eutectic + phase. This alloy contains less of the eutectic than ZA-8 (Fig. 22, Fig. 23). Etchant 
1, Table 1. (a) 200×. (b) 500× 
The casting method will influence the cooling rate and the microstructure that results. Figure 26 shows 
permanent mold, pressure die cast, and continuous cast specimens of the same alloy that was sand cast in Fig. 
25. 
 
Fig. 26 Same alloy as Fig. 25 (a) As-cast in a permanent mold. Same constituents as Fig. 25, but 
permanent mold casting results in a finer microstructure. Etchant 1, Table 1. (b) Same constituents, but 
pressure die casting results in a much finer microstructure. Etchant 4, Table 1. (c) Continuous cast in an 
80 mm (3.25 in.) diameter ingot results in finer structure than the sand casting. Etchant 1, Table 1. All: 
500× 
Wrought alloys based on the eutectoid composition of 78% Zn and 22% Al are of commercial interest because 
of their superplastic properties. Microstructures developed in this alloy depend on the heat treatment used (Fig. 
27). 
 
Fig. 27 Zn-22Al alloy (eutectoid composition). (a) Superplastic, fine-grained structure obtained by 
annealing at 350 °C (660 °F) and water quenching. (b) Same alloy held 1 h at 350 °C (660 °F) and air 
cooled. Structure consists of lamellar and granular and , both products of eutectoid transformation. 
Etchant 2, Table 1. 2500× 
Magnesium additions in concentrations of 0.01 to 0.03% to the ZA-8, ZA-12, and ZA-27 alloys increase 
strength and hardness but decrease ductility. 
Tin forms a low-melting eutectic with zinc at 91% Sn at 198 °C (388 °F). The solubility of tin in solid zinc is 
extremely restricted, and the zinc-tin eutectic appears in alloys containing as little as 0.001% Sn. Figure 28 
shows the dendritic distribution of tin. 
 
Fig. 28 Zn-1Sn permanent mold with rapid cooling. There is a dendritic distribution of tin. Mechanically 
polished. Etchant: similar to etchant 1, Table 1 (but only 10 g Na2SO4). 500× 
The only deliberate use of tin additions to zinc is in certain hot dip galvanizing operations. In some galvanizing 
operations, tin additions are widely used to regulate the formation of bright, smooth, large white spangles in the 
coating. Tin present in hot-rolled zinc can cause hot shortness, the tendency of an alloy to separate along grain 
boundaries under stress near its melting point. In zinc-aluminum alloys, tin induces intergranular corrosion; 
concentrations must be maintained below 0.002%. 
References cited in this section 
1. R.W. Leonard, Continuous Hot Dip Coatings, Corrosion: Fundamentals, Testing, and Protection, Vol 
13A, ASM Handbook, ASM International, 2003, p 786–793 
2. T.J. Langgill, Batch Process Hot Dip Galvanizings, Corrosion: Fundamentals, Testing, and Protection, 
Vol 13A, ASM Handbook, ASM International, 2003, p 794–802 
3. H.E. Townsend, Continuous Hot Dip Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 
International, 1994, p 339–348 
4. S.G. Fountoulakis, Continuous Electrodeposited Coatings for Steel Strip, Surface Engineering, Vol 5, 
ASM Handbook, ASM International, 1994, p 349–359 
5. D. Wetzel, Batch Hot Dip Galvanized Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 
International, 1994, p 360–371 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
Specimen Preparation 
Sectioning. The initial sample can be removed from a larger mass of material by sawing, breaking, or shearing 
(see the article “Metallographic Sectioning and Specimen Extraction” in this Volume). Because zinc alloys are 
comparatively soft, final sectioning of specimens is sometimes performed using special techniques. Either 
abrasive or diamond wheels are employed, because they produce less metal flow than sawing or shearing. 
Mounting. Most specimens are mounted using conventional cold resin materials (see the article “Mounting of 
Specimens” in this Volume). Hot compression is not used, because the zinc will deform and recrystallize. 
Specimens of rolled zinc and zinc alloys can be secured by clamping. Several specimens are mounted together 
in a screw clamp using thin spacers of soft zinc between specimens and heavy strips of zinc between the clamp 
plates and the outermost specimens. The assembly is tightly clamped to prevent seepage of etchants between 
specimens. The zinc spacers are of known structure and serve as convenient standards of comparison for 
determining if the specimens have been prepared correctly. 
Grinding and polishing of cast zinc can cause distortion to a depth 20 to 100 times as great as the deepest 
scratch. Therefore, in each stage of grinding and polishing, considerably more metal should be removed than 
the amount required for eliminating the scratches that remain from the previous stage. It is easier to prepare a 
distortion-free surface on specimens of fine-grained zinc than on specimens of coarse-grained, soft zinc. 
Wet grinding on a belt grinding machine using 60- and 180-grit silicon carbide abrasives is suitable for zinc and 
zinc alloys. Local heating from grinding must be minimized using water cooling, because heat can cause 
structural changes too deep to remove by polishing. 
Rough polishing is performed using 240-, 320-, 400-, and then 600-grit (65, 45, 35, and 20 m) silicon carbide 
papers. These papers are less susceptible to loading than emery papers. Some authorities recommend applying 
wax to the polishing papers so that they do not become embedded with zinc particles. Specimens should be 
rotated 90° relative to the direction of polishing after each polishing step. For soft (pure) zinc, polishing can be 
carried out by hand on papers supported on a flat surface. Zinc alloys are polished on a wheel using the same 
grades of paper. A low wheel speed (250 rpm maximum) during polishing will minimize overheating of the 
specimen, as will application of water to the silicon carbide papers and polishing in intervals of a few seconds, 
allowing the specimen to cool before polishing resumes. 
Fine polishing is performed using magnesium oxide (MgO) or alumina (Al2O3). A method for preparing 
specially graded wet-polishing abrasives is described in Ref 6. In fine polishing, the first two wheels are 
covered with smooth canvas, the third wheel with felt or billiard cloth. A soft-nap polishing cloth is used for 
fine polishing. Hands as well as the specimen must be washed between polishing steps to prevent carryover of 
coarser grit from previous steps. Overpolishing and its consequences can be avoided by etching between 
polishing steps. 
Zinc alloys that have intermetallic phases etch differently than unalloyed zinc. Because the intermetallic phases 
remain in relief, excessive polishing and etching should be avoided. Some of the steps listed previously can 
often be eliminated for alloys with intermetallic phases. 
Polishing through all four wheels is necessary only for specimens with microstructures requiring high 
magnifications for resolution. Most low-magnification examinations can proceed after polishing on the third 
wheel. An ethanol powder mixture instead of water should be used when polishing galvanized steel. The 
specimen is thencleaned ultrasonically with alcohol and blown dry with warm air. Vibratory polishing of zinc 
and zinc alloys will produce surfaces with improved response to polarized light. 
Macroetching. Use of concentrated hydrochloric acid (HCl) at room temperature, followed by rinsing and 
wiping off the resulting black deposit, produces satisfactory grain contrast on copper-free zinc and zinc alloys. 
Etchant 1 in Table 1 may be used for zinc containing 1% Cu or less, but with this etchant, grain contrast is not 
well defined. An etchant equal to HCl for producing grain contrast has not been found for the zinc alloys 
containing copper. 
Microetching. The most useful etchants for microscopic examination of zinc and zinc alloys are aqueous 
solutions of chromic acid (CrO3) to which sodium sulfate (Na2SO4) has been added. The grades of CrO3 used 
for chromium plating are adequate. The compositions of etchants commonly used are given in Table 1. Any use 
of chromic acid and the other etchants must done with strict adherance to the safety and regulatory 
requirements. Chromic acid is toxic, may cause cancer by inhalation, and must not enter ground water, bodies 
of water, or sewage systems. See the Material Safety Data Sheets available from the supplier for conditions of 
use. 
Etching should follow soon after final polishing. The specimen should be cleaned in alcohol, then running 
water, and etched while wet. To avoid staining, the use of etchant 1 or 2 in Table 1 should be followed 
immediately by a rinse in etchant 3. The specimen is then thoroughly washed in running water, dipped in 
alcohol, and dried with a stream of warm, clean air. 
Table 2 lists recommendations for etchants and etching times for zinc and zinc die-casting alloys. The etching 
time may be longer or shorter for specific etching conditions; a minor difference in solution temperature may 
affect etching time. In addition, as indicated in Table 2 for cast or rolled zinc, etching time is often decreased as 
the magnification to be used is increased. Etchant 4 in Table 1 may also be used for etching zinc pressure die 
cast and galvanized specimens. Etching should proceed for 4 to 5 s, followed by rinsing in water and drying in 
warm air. 
Table 2 Etchants and etching times for zinc and zinc die-casting alloys 
Time, s, for examination at Specimen metal(a) Etchant (from Table 1) 
250× 1000× 
Cast or rolled zinc 1 5 1 
Die cast alloy 3 (Z33520), 5 (Z35531), or 7 (Z33523) 2 1 1 
Zinc-coated steel 7 1–30 1–30 
(a) Selected die cast alloys given by common name (UNS) 
Electrolytic etching has been used to differentiate two intermediate phases of the zinc-copper system ( phase 
and phase). The electrolyte is a 17% aqueous solution of CrO3. The polished specimen is the anode, and a 
small coil of platinum wire in the bottom of the dish or beaker holding the electrolyte serves as the cathode. The 
specimen is connected to the current source before immersion in the electrolyte. At a current density of 0.2 
A/cm2 (1.5 A/in.2), and phases are approximately equally attacked. At higher current densities, phase is 
preferentially attacked; at lower current densities, phase is preferentially attacked. 
In a common procedure, the specimen is first etched at 0.78 A/cm2 (5 A/in.2). Gamma, if present, will be 
attacked; phase will not be attacked. The specimen is then repolished and etched at 0.15 A/cm2 (1 A/in.2), 
which will reveal any phase present. Further details on electrolytic etching of zinc alloys are available in Ref 
7. 
References cited in this section 
6. J.L. Rodda, Preparation of Graded Abrasives for Metallographic Polishing, Trans. AIME, Vol 99, 1932, 
p 149–158 
7. J.L. Rodda, Notes on Etching and Microscopical Identification of the Phases Present in the Copper-Zinc 
System, Trans. AIME, Vol 124, 1937, p 189–193 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
Preparation of Zinc Coating Specimens 
The preparation of specimens of zinc coatings on steel presents several challenges due to the difference in 
mechanical and chemical properties of the materials. The electrochemical properties that make zinc coatings 
desirable as a cathodic protection will not cease while the sample is prepared. 
Sectioning. Shearing sufficient to cut the base material may deform the coating. An abrasive cut-off wheel is 
recommended. 
Mounting. Gaps around the specimen caused by resin shrinkage are to be avoided, because water, alcohol, or 
polishing materials could be trapped in the gap. Degrease the specimen with acetone before mounting. The cold 
mounting resins mentioned previously, such as slow-curing epoxies, generally have little shrinkage. 
Grinding and Polishing. The previous general guide is applicable. Fine polishing with a diamond paste on a 
slow-speed wheel is recommended. Care must be taken to keep the surface of the entire specimen flat, because 
grinding will remove material of various constituents at different rates. Soft-nap cloth should not be used, 
because it may create more relief than is desired. Zinc reacts in water, causing stains, so alcohol rinses are 
recommended, followed by forced-air drying. 
Etching. Amyl alcohol/nital, such as etchant 7, Table 1, is commonly used. The concentration of acid can vary 
from 0.5 to 2%, and etching time will vary with the type of coating. Amyl alcohol is a toxic substance; the 
etchant must be prepared under a hood, and skin contact must be avoided. Profiles of galvanized, galvannealed, 
and electrogalvanized steel are given in Fig. 29, 30 and 31. 
 
Fig. 29 Hot dip galvanized 1006, UNS G10060, steel. The galvannealed process produced a coating with 
no free zinc. Coating weight: 275 g/m2 (0.9 oz/ft2). Etchant: amyl-nital. 550× 
 
Fig. 30 Hot dip galvanized 1006, UNS G10060, steel, without annealing. Zinc-iron compounds are 
present at the interface, while the remainder of the coating is free zinc. Coating weight: 320 g/m2 (1.05 
oz/ft2). Etchant: amyl-nital. 550× 
 
Fig. 31 Electrogalvanized 1006, UNS G10060, steel. Careful etching with a chromic acid/sodium sulfate 
etchant, with a concentration intermediate between etchant 1 and 2 in Table 1, reveals the interface 
between layers of zinc deposited in individual cells in a continuous multicell electrogalvanizing line. Note 
the absence of a zinc-iron alloy layer at the interface with the steel. Coating weight: 80 g/m2 (0.26 oz/ft2). 
Etchant: 8 to 10 g CrO3, 1 g Na2SO4, 100 mL H2O. 1000× 
Examination. A collection of three-dimensional images depicting the surface morphology and the profile of 
coated steel produced using a scanning electron microscope can be found in Ref 7. Examination of the 
thickness of various intermetallic layers provides information on the formability and corrosion resistance of the 
coated sheet. 
Reference cited in this section 
7. J.L. Rodda, Notes on Etching and Microscopical Identification of the Phases Present in the Copper-Zinc 
System, Trans. AIME, Vol 124, 1937, p 189–193 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
Microexamination 
Grain size is best determined under polarized light illumination, which displays each grain as a different shade, 
depending on orientation (Fig. 2, 6, 7, 32). Grain counts can be made with good accuracy. Grain boundaries are 
poorly defined under bright-field illumination. 
 
Fig. 32 Alloy 5, UNS Z35531, die cast with rapid freezing, unetched. (a) Polarized light illumination 
shows grain size but not alloy phases. Grains are larger than primary crystallites in (b), indicatingthat 
the zinc grains extend into the eutectic. (b) Fine primary crystallites of solid solution in fine lamellar 
eutectic. The fine structure imparts toughness and high strength to the alloy. Compare with Fig. 14 and 
15 As-polished. Etchant 2, Table 1. 250× 
Dendrite arm spacing can be measured on micrographs of selected areas using the linear intercept procedure in 
ASTM E 112 (Ref 8). Soundness values (porosity levels) can also be obtained in cast alloys by the use of 
quantitative analysis on a volume percent basis. Specimens to be examined for corrosion should be in the as-
polished condition. 
Reference cited in this section 
8. “Test Methods for Determining the Average Grain Size,” E 112, Annual Book of ASTM Standards, Vol 
03.01, ASTM 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
Acknowledgment 
This article is adapted from “Zinc and Zinc Alloys,” Metallography and Microstructures, ASM Handbook, 
Volume 9, 1985, that had been revised for that edition by L. Mongeon and R.J. Barnhurst. 
 
 
Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM 
Handbook, ASM International, 2004, p. 933–941 
Metallography and Microstructures of Zinc and Its Alloys 
 
References 
1. R.W. Leonard, Continuous Hot Dip Coatings, Corrosion: Fundamentals, Testing, and Protection, Vol 
13A, ASM Handbook, ASM International, 2003, p 786–793 
2. T.J. Langgill, Batch Process Hot Dip Galvanizings, Corrosion: Fundamentals, Testing, and Protection, 
Vol 13A, ASM Handbook, ASM International, 2003, p 794–802 
3. H.E. Townsend, Continuous Hot Dip Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 
International, 1994, p 339–348 
4. S.G. Fountoulakis, Continuous Electrodeposited Coatings for Steel Strip, Surface Engineering, Vol 5, 
ASM Handbook, ASM International, 1994, p 349–359 
5. D. Wetzel, Batch Hot Dip Galvanized Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM 
International, 1994, p 360–371 
6. J.L. Rodda, Preparation of Graded Abrasives for Metallographic Polishing, Trans. AIME, Vol 99, 1932, 
p 149–158 
7. J.L. Rodda, Notes on Etching and Microscopical Identification of the Phases Present in the Copper-Zinc 
System, Trans. AIME, Vol 124, 1937, p 189–193 
8. “Test Methods for Determining the Average Grain Size,” E 112, Annual Book of ASTM Standards, Vol 
03.01, ASTM