ASTM E1558-99
(Guide)Standard Guide for Electrolytic Polishing of Metallographic Specimens
Standard Guide for Electrolytic Polishing of Metallographic Specimens
SCOPE
1.1 This guide deals with electrolytic polishing as a means of preparation of specimens for metallographic purposes. Procedures are described for polishing a variety of metals. Note 1-References (1-133) on electrolytic polishing will provide the reader with specific information beyond the scope of this guide.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific safety precautions are described in Section 5 and 6.3.1.
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Designation: E 1558 – 99
Standard Guide for
Electrolytic Polishing of Metallographic Specimens
This standard is issued under the fixed designation E 1558; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.1.3 There can be a marked saving of time if many
specimens of the same material are polished sequentially.
1.1 This guide deals with electrolytic polishing as a means
4.1.4 Electropolishing a selected area on the surface of a
of preparation of specimens for metallographic purposes.
relatively large metal part can be accomplished nondestruc-
Procedures are described for polishing a variety of metals.
tively, that is, without the need for sectioning to remove a
NOTE 1—References (1-133) on electrolytic polishing will provide the
piece.
reader with specific information beyond the scope of this guide.
4.1.5 Soft, single-phase metals, which may be difficult to
1.2 This standard does not purport to address all of the
polish by mechanical methods, may be successfully electropol-
safety concerns, if any, associated with its use. It is the
ished.
responsibility of the user of this standard to establish appro-
4.1.6 The true microstructure of a specimen can be obtained
priate safety and health practices and determine the applica-
because artifacts (such as disturbed metal, scratches, and
bility of regulatory limitations prior to use. Specific safety
mechanical twins), produced on the surface even by careful
precautions are described in Section 5 and 6.3.1.
grinding and mechanical polishing operations, can be removed.
These features are important in low-load hardness testing,
2. Referenced Documents
X-ray diffraction studies, and in electron microscopy, where
2.1 ASTM Standards:
higher resolution puts a premium on undistorted metal sur-
E 3 Methods of Preparation of Metallographic Specimens
faces.
E 7 Terminology Relating to Metallography
4.1.7 After electropolishing is completed, etching can often
E 407 Test Methods for Microetching Metals and Alloys
be accomplished by reducing the voltage (generally to about
one-tenth that required for polishing) for a short time before it
3. Terminology
is turned off.
3.1 Definitions—All terms used in this guide are either
NOTE 2—Not all electropolishing solutions produce good etching
defined in Terminology E 7 or are discussed in 3.2.
results.
3.2 Definitions of Terms Specific to This Standard:
4.2 Disadvantages of Electrolytic Polishing:
3.2.1 electrolytic polish (electropolish)—A method of pol-
4.2.1 Many of the chemical mixtures used in electropolish-
ishing metals and alloys in which material is removed from the
ing are poisonous or dangerous if not properly handled (see
surface by making the metal the anode in an electrolytic bath.
Section 5). These hazards are similar to those involved in the
4. Significance and Use
mixing and handling of etchants, see Test Methods E 407.
4.2.2 In multi-phase alloys, the polishing rate of each phase
4.1 Advantages of Electrolytic Polishing:
may be different. The result may be a non-planar surface.
4.1.1 For some metals, a high quality surface finish can be
4.2.3 Electropolished surfaces may be slightly undulated
produced that is equivalent to, or better than, that which can be
rather than perfectly planar and, therefore, may not be suitable
obtained by mechanical methods.
for examination at all magnifications.
4.1.2 Once procedures have been established, satisfactory
4.2.4 The rate of polishing in areas adjacent to various
results can be obtained rapidly with reproducibility.
inhomogeneities, such as nonmetallic inclusions and voids, is
usually greater than that in the surrounding matrix and tends to
exaggerate the size of the inclusions and voids.
This guide is under the jurisdiction of ASTM Committee E-4 on Metallography
4.2.5 Dimples, pits, and waviness limit applications involv-
and is the direct responsibility of Subcommittee E04.01 on Sampling, Specimen
ing surface phenomena, coatings, interfaces, and cracks. Edges
Preparation, and Photography.
Current edition approved October 10, 1999. Published December 1999. Origi-
tend to be attacked preferentially, resulting in edge rounding.
nally published as E 1558 - 93. Last previous edition E 1558 - 93.
4.2.6 Artifacts may be produced by electropolishing.
The boldface numbers in parentheses refer to the references at the end of this
4.2.7 Specimen mounting materials may react with the
standard.
Annual Book of ASTM Standards, Vol 03.01. electrolyte.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1558
4.2.8 The electropolished surfaces of certain materials may read and understood concerning all of the hazards and safety
be passive and difficult to etch. precautions to be observed. Users should be aware of the type
4.2.9 Metal removal rates by electropolishing are usually of hazards involved in the use of all chemicals used, including
quite low, typically about 1 μm/min, and all of the prior those hazards that are immediate, long-term, visible, invisible,
induced damage from cutting and grinding may not be re- and with or without odors.
moved if preparation is stopped after a 600-grit SiC grind and 5.1.1 Consult the product labels and MSDS for recommen-
electropolishing times are short. dations concerning proper protective clothing.
4.2.10 A large number of electrolytes may be needed to 5.1.2 All chemicals are potentially dangerous. All persons
polish the variety of metals encountered by a given laboratory. using any electrolyte should be thoroughly familiar with all of
Considerable time may be required to develop a procedure for the chemicals involved and the proper procedure for handling,
a new alloy. mixing, and disposing of each chemical, as well as any
combinations of those chemicals.
5. General Safety Precautions
5.1.3 Table 2 includes specific safety precautions for the
5.1 Before using or mixing any chemicals, all product labels mixing or use of some electrolytes. The user should take care
and pertinent Material Safety Data Sheets (MSDS) should be to observe each of these specific precautions.
TABLE 2 Electrolytes for Electropolishing
Class Use Formula Cell Voltage Time Remarks
Group I (Electrolytes Composed of Perchloric Acid and Alcohol With or Without Organic Additions)
I-1 Al and Al alloys with less than ethanol (95 %) 800 mL 30 to 80 15 to 60 s
2 percent Si distilled water 140 mL
perchloric acid (60 %) 60 mL
steels—carbon, alloy, stainless 35 to 65 15 to 60 s
Pb, Pb-Sn, Pb-Sn-Cd, Pb-Sn-Sb 12 to 35 15 to 60 s
Zn, Zn-Sn-Fe, Zn-Al-Cu 20 to 60 .
Mg and high Mg alloys . . nickel cathode
I-2 stainless steel and aluminum ethanol (95 %) 800 mL 35 to 80 15 to 60 s
perchloric acid (60 %) 200 mL
I-3 stainless steel ethanol (95 %) 940 mL 30 to 45 15 to 60 s
perchloric acid (65 %) 60 mL
I-4 steel, cast iron, Al, Al alloys, Ni, ethanol (95 %) 700 mL 30 to 65 15 to 60 s one of the best formulas for
Sn, Ag, Be, Ti, Zr, U, 2-butoxy ethanol 100 mL universal use
heat-resisting alloys perchloric acid (30 %) 200 mL
I-5 steels—stainless, alloy, ethanol (95 %) 700 mL 15 to 50 15 to 60 s universal electrolyte comparable to
high-speed; Fe, Al, Zr, Pb glycerin 100 mL I-4
perchloric acid (30 %) 200 mL
TABLE 1 Electropolishing Procedural Problems and Corrections
Trouble Possible Cause Suggested Correction
Center of specimen deeply etched no polishing film at center of specimen (1) increase voltage
(2) decrease agitation
(3) use more viscous electrolyte
Pitting or etching at edges of specimen too viscous or thick film (1) decrease voltage
(2) increase agitation
(3) use less viscous electrolyte
Sludge settling on surface insoluble anode product (1) try new electrolyte
(2) increase temperature
(3) increase voltage
Roughness or matte surface insufficient or no polishing film (1) increase voltage
(2) use more viscous electrolyte
Waviness or streaks on polished surface (1) insufficient time (1) increase or decrease agitation
(2) incorrect agitation (2) better preparation
(3) inadequate preparation (3) increase voltage and decrease time
(4) too much time
Stains on polished surface attack after polishing current is off (1) remove specimen while current is still on
(2) try less corrosive electrolyte
Unpolished spots (bullseyes) gas bubbles (1) increase agitation
(2) decrease voltage
Phases in relief insufficient polishing film (1) increase voltage
(2) better preparation
(3) decrease time
Pitting (1) too long polishing (1) better preparation
(2) too high voltage (2) decrease voltage
(3) decrease time
(4) try different electrolyte
E 1558
TABLE 2 Continued
Class Use Formula Cell Voltage Time Remarks
I-6 Al, Al-Si alloys ethanol (95 %) 760 mL 35 to 60 15 to 60 s particularly good with Al-Si alloys
diethyl ether 190 mL
perchloric acid (30 %) 50 mL
I-7 Mo, Ti, Zr, U-Zr alloy methanol (absolute) 600 mL 60 to 150 5 to 30 s
2-butoxy ethanol 370 mL
perchloric acid (60 %) 30 mL
I-8 Al-Si alloys methanol (absolute) 840 mL 50 to 100 5 to 60 s
glycerin 125 mL
perchloric acid (65 %) 35 mL
I-9 vanadium methanol (absolute) 590 mL 30 3 s three-second cycles repeated at
2-butoxy ethanol 350 mL least seven times to prevent heating
perchloric acid (65 %) 60 mL
germanium 25 to 35 30 to 60 s
titanium 58 to 66 45 s polish only
zirconium 70 to 75 15 s polish and etch simultaneously
I-10 aluminum methanol (absolute) 950 mL 30 to 60 15 to 60 s
nitric acid 15 mL
perchloric acid (60 %) 50 mL
I-11 steels—carbon, alloy, stainless methanol (absolute) 600 mL 30–40 5–60 s good all purpose electropolish
Ti, high-temperature alloys, Pb, butylcellosolve 360 mL
Mo perchloric acid 60 mL
I-12 Al and Al alloys ethanol (95 %) 1000 mL 10 2 min not good for Al-Cu and Al-Si alloys.
perchloric acid 200 mL Black film forms. Peel off after 1–1.5
min and polish 1 min more.
I-13 steel, Al, Ni, Sn, Ti, Be ethanol (95 %) 700 mL 20 20 s Mix ethanol and water, add
stainless steel butylcellosolve 100 mL perchloric acid carefully. Then, add
Al Ni water 137 mL butylcellosolve before use.
perchloric acid 62 mL
I-14 Ni, Ag or Cu alloys ethanol (95 %) 700 mL 70–80 15 s
Cd butylcellosolve 100 mL
perchloric acid 200 mL
I-15 Mo and Mo alloys methanol (absolute) 600 mL 20 s Mix methanol and water, add
water 13 mL perchloric acid carefully. Add
butylcellosolve 360 mL butylcellosolve before use.
perchloric acid 47 mL
Group II (Electrolytes Composed of Perchloric Acid and Glacial Acetic Acid in Varying Proportions)
II-1 Cr, Ti, Zr, U acetic acid (glacial) 940 mL 20 to 60 1 to 5 min good general-purpose electrolyte
Fe, steel—carbon, alloy, stainless perchloric acid (60 %) 60 mL
II-2 Zr, Ti, U, steel—carbon and alloy acetic acid (glacial) 900 mL 12 to 70 0.5 to 2 min
perchloric acid (60 %) 100 mL
II-3 U, Zr, Ti, Al, steel—carbon and acetic acid (glacial) 800 mL 40 to 100 1 to 15 min
alloy perchloric acid (60 %) 200 mL
II-4 Ni, Pb, Pb-Sb alloys acetic acid (glacial) 700 mL 40 to 100 1 to 5 min
perchloric acid (60 %) 300 mL
II-5 3 percent Si-Fe acetic acid (glacial) 650 mL . 5 min 0.06 A/cm
perchloric acid (60 %) 350 mL
II-6 Cr acetic acid (glacial) 1000 mL 30–50 2–3 min can lower voltage to 25 V by adding
perchloric acid 5mL 5–15 % water.
II-7 Hf, steel—carbon and alloy acetic acid (glacial) 1000 mL . . Used to polish Hf wires.
perchloric acid 50 mL
E 1558
TABLE 2 Continued
Class Use Formula Cell Voltage Time Remarks
Group III (Electrolytes Composed of Phosphoric Acid in Water or Organic Solvent)
III-1 cobalt phosphoric acid (85 %) 1000 mL 1.2 3 to 5 min
III-2 pure copper distilled water 175 mL 1.0 to 1.6 10 to 40 min copper cathode
phosphoric acid (85 %) 825 mL
III-3 stainless, brass, Cu and Cu water 300 mL 1.5 to 1.8 5 to 15 min copper cathode
alloys except Sn bronze phosphoric acid (85 %) 700 mL
III-4 alpha or alpha plus beta brass, water 600 mL 1 to 2 1 to 15 min copper or stainless steel cathode
Cu-Fe, Cu-Co, Co, Cd phosphoric acid (85 %) 400 mL
III-5 Cu, Cu-Zn water 1000 mL 1 to 2 10 min copper cathode
pyrophosphoric acid 580 g
III-6 steel diethylene glycol monoethyl 500 mL 5 to 20 5 to 15 min 120°F
ether
phosphoric acid (85 %) 500 mL
III-7 Al, Ag, Mg water 200 mL 25 to 30 4 to 6 min aluminum cathode, 100 to 110°F
ethanol (95 %) 380 mL
phosphoric acid (85 %) 400 mL
III-8 uranium ethanol (absolute) 300 mL . .
glycerin (cp) 300 mL
phosphoric acid (85 %) 300 mL
III-9 Mn, Mn-Cu alloys ethanol (95 %) 500 mL 18 .
glycerin 250 mL
phosphoric acid (85 %) 250 mL
III-10 Cu and Cu-base alloys distilled water 500 mL . 1 to 5 min
ethanol (95 %) 250 mL
phosphoric acid (85 %) 250 mL
III-11 stainless steel ethanol (absolute), to 1L . 10 min good for all austenitic heat resistant
pyrophosphoric acid 400 g alloys, 100°F plus
III-12 Mg-Zn ethanol (95 %) 625 mL 1.5 to 2.5 3 to 30 min
phosphoric acid (85 %) 375 mL
III-13 uranium ethanol (95 %) 445 mL 18 to 20 5 to 15 min 0.03 A/cm
ethylene glycol 275 mL
phosphoric acid (85 %) 275 mL
III-14 Al-Mg alloys water 250 mL 50–60 2 min
ethanol (95 %) 380 mL
phosphoric acid (85 %) 400 mL
III-15 Cu-Pb alloys ethanol (95 %) 620 mL good up to 30 % Pb
phosphoric acid (85 %) 380 mL
III-16 Neptunium ethanol (95 %) 400 mL after 600-grit SiC, use 6-μm
glycerol 400 mL diamond on nylon before
phosphoric acid (85 %) 800 mL electropolishing.
Group IV (Electrolytes Composed of Sulfuric Acid in Water or Organic Solvent)
IV-1 stainless steel water 250 mL 1.5 to 6 1 to 2 min
sulfuric acid 750 mL
IV-2 stainless steel, Fe, Ni water 400 mL 1.5 to 6 2 to 6 min
sulfuric acid 600 mL
IV-3 stainless steel, Fe, Ni, Mo water 750 mL 1.5 to 6 2 to 10 min particularly good for sintered
sulfuric acid 250 mL Mo— Mo—32 to 80°F
0.3 to 1 min
IV-4 molybdenum water 900 mL 1.5 to 6 0.3 to 2 min particularly good for sintered
sulfuric acid 100 mL Mo—32 to 80°F
IV-5 stainless steel water 70 mL 1.5 to 6 0.5 to 5 min
glycerin 200 mL
sulfuric acid 720 mL
IV-6 stainless steel, aluminum water 220 mL 1.5 to 12 1 to 20 min
glycerin 200 mL
sulfuric acid 580 mL
IV-7 molybdenum methanol (absolute) 875 mL 6 to 18 0.5 to 1.5 min 32 to 80°F
sulfuric acid 125 mL
IV-8 Ni-base superalloys methanol (absolute) 800 mL 30 20 s for alloy 625
sulfuric acid 200 mL
Group V (Electrolytes Composed of Chromic Acid in Water)
V-1 stainless steel water 830 mL 1.5 to 9 2 to 10 min
chromic acid 620 g
V-2 Zn, brass water 830 mL 1.5 to 12 10 to 60 s
chromic acid 170 g
E 1558
TABLE 2 Con
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