ASTM E1558-2004 Standard Guide for Electrolytic Polishing of Metallographic Specimens

Designation:E1558?09(Reapproved2014)

Standard Guide for

Electrolytic Polishing of Metallographic Specimens1

This standard is issued under the?xed designation E1558;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(′)indicates an editorial change since the last revision or reapproval.

1.Scope

1.1This guide deals with electrolytic polishing as a means of preparation of specimens for metallographic purposes. Procedures are described for polishing a variety of metals.

N OTE1—References(1-133)2on electrolytic polishing will provide the reader with speci?c information beyond the scope of this guide.

1.2The values stated in SI units are to be regarded as standard.No other units of measurement are included in this standard.

1.3This standard does not purport to address all of the safety concerns,if any,associated with its use.It is the responsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.Speci?c safety precautions are described in Section5and6.3.1.

2.Referenced Documents

2.1ASTM Standards:3

E7Terminology Relating to Metallography

E407Practice for Microetching Metals and Alloys

3.Terminology

3.1De?nitions—All terms used in this guide are either de?ned in Terminology E7or are discussed in3.2.

3.2De?nitions of Terms Speci?c to This Standard:

3.2.1electrolytic polish(electropolish)—A method of pol-ishing metals and alloys in which material is removed from the surface by making the metal the anode in an electrolytic bath.

4.Signi?cance and Use

4.1Advantages of Electrolytic Polishing:

4.1.1For some metals,a high quality surface?nish can be produced that is equivalent to,or better than,that which can be obtained by mechanical methods.

4.1.2Once procedures have been established,satisfactory results can be obtained rapidly with reproducibility.

4.1.3There can be a marked saving of time if many specimens of the same material are polished sequentially. 4.1.4Electropolishing a selected area on the surface of a relatively large metal part can be accomplished nondestructively,that is,without the need for sectioning to remove a piece.

4.1.5Soft,single-phase metals,which may be difficult to polish by mechanical methods,may be successfully electrop-olished.

4.1.6The true microstructure of a specimen can be obtained because artifacts(such as disturbed metal,scratches,and mechanical twins)produced on the surface,even by careful grinding and mechanical polishing operations,can be removed. These features are important in low-load hardness testing, X-ray diffraction studies,and in electron microscopy,where higher resolution puts a premium on undistorted metal sur-faces.

4.1.7After electropolishing is completed,etching can often be accomplished by reducing the voltage(generally to about one-tenth that required for polishing)for a short time before it is turned off.

N OTE2—Not all electropolishing solutions produce good etching results.

4.2Disadvantages of Electrolytic Polishing:

4.2.1Many of the chemical mixtures used in electropolish-ing are poisonous or dangerous if not properly handled(see Section5).These hazards are similar to those involved in the mixing and handling of etchants,see Test Methods E407. 4.2.2In multi-phase alloys,the polishing rate of each phase may be different.The result may be a non-planar surface. 4.2.3Electropolished surfaces may be slightly undulated rather than perfectly planar and,therefore,may not be suitable for examination at all magni?cations.

4.2.4The rate of polishing in areas adjacent to various 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.

1This guide is under the jurisdiction of ASTM Committee E04on Metallography

and is the direct responsibility of Subcommittee E04.01on Specimen Preparation.

Current edition approved Oct.1,2014.Published December2014.Originally

approved https://www.wendangku.net/doc/0310371090.html,st previous edition approved in2009as E1558–09.DOI:

10.1520/E1558-09R14.

2The boldface numbers in parentheses refer to the references at the end of this

standard.

3For referenced ASTM standards,visit the ASTM website,https://www.wendangku.net/doc/0310371090.html,,or

contact ASTM Customer Service at service@https://www.wendangku.net/doc/0310371090.html,.For Annual Book of ASTM

Standards volume information,refer to the standard’s Document Summary page on

the ASTM website.

Copyright?ASTM International,100Barr Harbor Drive,PO Box C700,West Conshohocken,PA19428-2959.United States

4.2.5Dimples,pits,and waviness limit applications involv-ing surface phenomena,coatings,interfaces,and cracks.Edges tend to be attacked preferentially,resulting in edge rounding.

4.2.6Artifacts may be produced by electropolishing.

4.2.7Specimen mounting materials may react with the electrolyte.

4.2.8The electropolished surfaces of certain materials may be passive and difficult to etch.

4.2.9Metal removal rates by electropolishing are usually quite low,typically about1μm/min,and all of the prior induced damage from cutting and grinding may not be re-moved if preparation is stopped after a600-grit SiC grind and electropolishing times are short.

4.2.10A large number of electrolytes may be needed to polish the variety of metals encountered by a given laboratory. Considerable time may be required to develop a procedure for

a new alloy.

5.General Safety Precautions

5.1Before using or mixing any chemicals,all product labels and pertinent Material Safety Data Sheets(MSDS)should be read and understood concerning all of the hazards and safety precautions to be https://www.wendangku.net/doc/0310371090.html,ers should be aware of the type of hazards involved in the use of all chemicals used,including those hazards that are immediate,long-term,visible,invisible, and with or without odors.

5.1.1Consult the product labels and MSDS for recommen-dations concerning proper protective clothing.

5.1.2All chemicals are potentially dangerous.All persons using any electrolyte should be thoroughly familiar with all of the chemicals involved and the proper procedure for handling, mixing,and disposing of each chemical,as well as any combinations of those chemicals.

5.1.3When pouring,mixing,or etching,always use the proper protective equipment(glasses,gloves,apron,etc.)and it is strongly recommended to always work under a certi?ed and tested fume hood.This is imperative with etchants that give off noxious odors or toxic vapors.In particular,note that solutions containing perchloric acid must be mixed and used in an exclusive hood equipped with a wash down feature to avoid accumulation of explosive perchlorates.

5.1.4Table1includes speci?c safety precautions for the mixing or use of some electrolytes.The user should take care to observe each of these speci?c precautions.

5.2Some basic suggestions for the handling and disposal of electrolytes and their ingredients are as follows:

5.2.1As previously stated,it is good practice to always work under a certi?ed fume hood when mixing and utilizing any electrolyte and it is imperative with those electrolytes that give off noxious odors or toxic vapor.Additionally,the electrolytes in Groups I and II must be treated with extra caution because dried perchlorates can accumulate in hood ductwork and on work surfaces creating the potential for a powerful accidental explosion.Therefore,these electrolytes must only be used in an exclusive hood equipped with a wash down feature.To avoid the accumulation of explosive,dry perchlorates,the hood should undergo a wash down cycle following each use.

5.2.2When pouring,mixing,or using electrolytes,always use the proper protective equipment(eyewear,gloves,apron, and so forth).

5.2.3Use proper devices(glass or plastic)for weighing, measuring,mixing,containing,and storage of solutions.

5.2.4When mixing electrolytes,always add reagents to the solvent unless speci?c instructions indicate otherwise.

5.2.5When using an electrolyte,always avoid direct physi-cal contact with the electrolyte and the https://www.wendangku.net/doc/0310371090.html,e tongs or some other indirect method of handling specimens.

5.2.6Methanol is a cumulative poison hazard.Where etha-nol or methanol are listed as alternates,ethanol is the preferred solvent.Methanol should be used in a properly designed chemical fume hood.

5.2.7All spills should be cleaned up and disposed of properly,no matter how small the spill.

5.2.8Properly dispose of all solutions that are not identi?ed by composition and concentration.

5.2.9Store,handle,and dispose of chemicals according to the manufacturer’s recommendations.Observe printed cau-tions on reagent containers.

5.2.10Information pertaining to the toxicity hazards and working precautions of chemicals,solvents,acids,bases,and so forth,being used(such as MSDS)should be available for rapid consultation.

5.3Many of the electrolytes in the following listing can be exceedingly dangerous if carelessly handled.The pertinent safety precautions for each class of electrolyte should be read before any electrolyte is mixed or used.

5.4Electrolytes containing perchloric acid and acetic anhy-dride are very dangerous to mix and may be unpredictable in use.Many industrial?rms and research laboratories forbid the use of such mixtures.Certain cities also have ordinances prohibiting the use of such potentially explosive mixtures. These facts are considered sufficient reason for recommending against their use.

5.5Mixtures of oxidizable organic compounds and power-ful oxidizing agents are always potentially dangerous.After some use,any electrolyte will become heavily laden with ions of the metals polished.These ions may interfere with further polishing or catalyze the decomposition of the electrolyte.The electrolyte then must be discarded in accordance with appro-priate regulations.

5.6Most electrolytes(with few exceptions)should be mixed and stored in clean glass containers and never be in contact with foreign materials or organic compounds.The exceptions are those electrolytes containing?uorides and strong alkaline solutions that should be mixed and stored in polyethylene or other appropriate material containers.Electro-lytes must never be allowed to become concentrated by evaporation.All electrolytes should be discarded appropriately as soon as they have exceeded their immediate usefulness. 5.7Specimens mounted in bismuth or bismuth-containing metals must not be electropolished in perchloric acid solutions because this mounting medium may react explosively with the electrolyte.Likewise,bismuth or bismuth-containing alloys must not be electropolished in solutions containing

perchloric

TABLE1Electrolytes for Electropolishing

Class Use Formula Cell Voltage Time Remarks Group I(Electrolytes Composed of Perchloric Acid and Alcohol With or Without Organic Additions)Use in a washdown/perchloric rated fume hood. I-1Al and Al alloys with less than ethanol(95%)800mL30to8015to60s 2percent Si distilled water140mL

perchloric acid(60%)60mL

steels—carbon,alloy,stainless35to6515to60s

Pb,Pb-Sn,Pb-Sn-Cd,Pb-Sn-Sb12to3515to60s

Zn,Zn-Sn-Fe,Zn-Al-Cu20to60...

Mg and high Mg alloys......nickel cathode I-2stainless steel and aluminum ethanol(95%)800mL35to8015to60s

perchloric acid(60%)200mL

I-3stainless steel ethanol(95%)940mL30to4515to60s

perchloric acid(65%)60mL

I-4steel,cast iron,Al,Al alloys,Ni,

Sn,Ag,Be,Ti,Zr,U,

heat-resisting alloys ethanol(95%)

2-butoxy ethanol

perchloric acid(30%)

700mL

100mL

200mL

30to6515to60s one of the best formulas for

universal use

I-5steels—stainless,alloy,

high-speed;Fe,Al,Zr,Pb ethanol(95%)

glycerin

perchloric acid(30%)

700mL

100mL

200mL

15to5015to60s universal electrolyte comparable to

I-4

I-6Al,Al-Si alloys ethanol(95%)

diethyl ether

perchloric acid(30%)760mL

190mL

50mL

35to6015to60s particularly good with Al-Si alloys

I-7Mo,Ti,Zr,U-Zr alloy methanol(absolute)

2-butoxy ethanol

perchloric acid(60%)600mL

370mL

30mL

60to1505to30s

I-8Al-Si alloys methanol(absolute)

glycerin

perchloric acid(65%)840mL

125mL

35mL

50to1005to60s

I-9vanadium methanol(absolute)

2-butoxy ethanol

perchloric acid(65%)590mL

350mL

60mL

303s three-second cycles repeated at

least seven times to prevent heating

germanium titanium zirconium 25to35

58to66

70to75

30to60s

45s

15s

polish only

polish and etch simultaneously

I-10aluminum methanol(absolute)

nitric acid

perchloric acid(60%)950mL

15mL

50mL

30to6015to60s

I-11steels—carbon,alloy,stainless

Ti,high-temperature alloys,Pb,

Mo methanol(absolute)

butylcellosolve

perchloric acid

600mL

360mL

60mL

30–405–60s good all purpose electropolish

I-12Al and Al alloys ethanol(95%)

perchloric acid 1000mL

200mL

102min not good for Al-Cu and Al-Si alloys.

Black?lm forms.Peel off after1–1.5

min and polish1min more.

I-13steel,Al,Ni,Sn,Ti,Be

stainless steel

Al3Ni ethanol(95%)

butylcellosolve

water

perchloric acid

700mL

100mL

137mL

62mL

2020s Mix ethanol and water,add

perchloric acid carefully.Then,add

butylcellosolve before use.

I-14Ni,Ag,or Cu alloys

Cd ethanol(95%)

butylcellosolve

perchloric acid

700mL

100mL

200mL

70–8015s

I-15Mo and Mo alloys methanol(absolute)

water

butylcellosolve

perchloric acid 600mL

13mL

360mL

47mL

20s Mix methanol and water,add

perchloric acid carefully.Add

butylcellosolve before use.

Group II(Electrolytes Composed of Perchloric Acid and Glacial Acetic Acid in Varying Proportions)Use in a washdown/perchloric rated fume hood.

II-1Cr,Ti,Zr,U,

Fe,steel—carbon,alloy,stainless acetic acid(glacial)

perchloric acid(60%)

940mL

60mL

20to601to5min good general-purpose electrolyte

II-2Zr,Ti,U,steel—carbon and alloy acetic acid(glacial)

perchloric acid(60%)900mL

100mL

12to700.5to2min

II-3U,Zr,Ti,Al,steel—carbon and

alloy acetic acid(glacial)

perchloric acid(60%)

800mL

200mL

40to1001to15min

II-4Ni,Pb,Pb-Sb alloys acetic acid(glacial)

perchloric acid(60%)700mL

300mL

40to1001to5min

II-53percent Si-Fe acetic acid(glacial)

perchloric acid(60%)650mL

350mL

...5min0.06A/cm2

II-6Cr acetic acid(glacial)

perchloric acid 1000mL

5mL

30–502–3min can lower voltage to25V by adding

5–15%water.

II-7Hf,steel—carbon and alloy acetic acid(glacial)

perchloric acid 1000mL

50mL

https://www.wendangku.net/doc/0310371090.html,ed to polish Hf wires.

Group III(Electrolytes Composed of Phosphoric Acid in Water or Organic Solvent)

III-1cobalt phosphoric acid(85%)1000mL 1.23to5min

III-2pure copper distilled water

phosphoric acid(85%)175mL

825mL

1.0to1.610to40min copper cathode

III-3stainless,brass,Cu and Cu

alloys except Sn bronze water

phosphoric acid(85%)

300mL

700mL

1.5to1.85to15min copper

cathode

III-4alpha or alpha plus beta brass,Cu-Fe,Cu-Co,Co,Cd water phosphoric acid (85%)600mL 400mL 1to 21to 15min copper or stainless steel cathode III-5Cu,Cu-Zn water pyrophosphoric acid 1000mL 580g 1to 210min copper cathode III-6

steel

diethylene glycol monoethyl ether

phosphoric acid (85%)500mL 500mL 5to 205to 15min 49°C

III-7Al,Ag,Mg

water ethanol (95%)phosphoric acid (85%)200mL 380mL 400mL 25to 304to 6min aluminum cathode,38to 43°C

III-8uranium

ethanol (absolute)glycerin (cp)phosphoric acid (85%)300mL 300mL 300mL ......III-9Mn,Mn-Cu alloys

ethanol (95%)glycerin phosphoric acid (85%)500mL 250mL 250mL 18...III-10Cu and Cu-base alloys

distilled water ethanol (95%)phosphoric acid (85%)500mL 250mL 250mL ...1to 5min III-11stainless steel ethanol (absolute),to pyrophosphoric acid 1L 400g ...10min good for all austenitic heat resistant

alloys,38°C plus

III-12Mg-Zn ethanol (95%)phosphoric acid (85%)625mL 375mL 1.5to 2.53to 30min III-13

uranium

ethanol (95%)ethylene glycol phosphoric acid (85%)445mL 275mL 275mL 18to 205to 15min 0.03A/cm 2

III-14Al-Mg alloys

water ethanol (95%)phosphoric acid (85%)250mL 380mL 400mL 50–602min III-15Cu-Pb alloys ethanol (95%)phosphoric acid (85%)620mL 380mL good up to 30%Pb

III-16

Neptunium

ethanol (95%)glycerol phosphoric acid (85%)400mL 400mL 800mL

after P1200–grit SiC,use 6-μm diamond on nylon before electropolishing.

Group IV (Electrolytes Composed of Sulfuric Acid in Water or Organic Solvent)

IV-1stainless steel water sulfuric acid 250mL 750mL 1.5to 61to 2min IV-2stainless steel,Fe,Ni water sulfuric acid 400mL 600mL 1.5to 62to 6min IV-3

stainless steel,Fe,Ni,Mo

water sulfuric acid 750mL 250mL 1.5to 62to 10min Mo—

0.3to 1min

particularly good for sintered Mo—0to 27°C

IV-4molybdenum water sulfuric acid 900mL 100mL 1.5to 60.3to 2min particularly good for sintered Mo—0to 27°C

IV-5

stainless steel

water glycerin sulfuric acid 70mL 200mL 720mL 1.5to 60.5to 5min IV-6stainless steel,aluminum

water glycerin sulfuric acid 220mL 200mL 580mL 1.5to 121to 20min IV-7molybdenum methanol (absolute)sulfuric acid 875mL 125mL 6to 180.5to 1.5min 0to 27°C IV-8

Ni-base superalloys

methanol (absolute)sulfuric acid 800mL 200mL

3020s for alloy 625

Group V (Electrolytes Composed of Chromic Acid in Water)V-1stainless steel water chromic acid 830mL 620g 1.5to 92to 10min V-2

Zn,brass

water chromic acid 830mL 170g

1.5to 1210to 60s Group VI (Mixed Acids or Salts in Water or Organic Solvent)VI-1stainless steel phosphoric acid (85%)sulfuric acid 600mL 400mL ......VI-2

stainless steel

water phosphoric acid (85%)sulfuric acid 150mL 300mL 550mL ...2min 0.3A/cm 2

VI-3stainless and alloy steel

water phosphoric acid (85%)sulfuric acid 240mL 420mL 340mL ...2to 10min 0.1to 0.2A/cm 2

VI-4stainless steel

water phosphoric acid (85%)sulfuric acid 330mL 550mL 120mL ...1min 0.05A/cm 2

VI-5bronze (to 9%Sn)

water phosphoric acid (85%)sulfuric acid 450mL 390mL 160mL

...1to 5min 0.1A/cm 2

VI-6bronze(to6%Sn)water

phosphoric acid(85%)

sulfuric acid 330mL

580mL

90mL

...1to5min0.1A/cm2

VI-7steel water

glycerin

phosphoric acid(85%)

sulfuric acid 140mL

100mL

430mL

330mL

...1to5min1to5A/cm2,38°C plus

VI-8stainless steel water

glycerin

phosphoric acid(85%)

sulfuric acid 200mL

590mL

100mL

110mL

...5min1A/cm2,27to49°C

VI-9stainless steel water

chromic acid

phosphoric acid(85%)

sulfuric acid 260mL

175g

175mL

580mL

...30min0.6A/cm2,27to49°C

VI-10stainless steel water

chromic acid

phosphoric acid(85%)

sulfuric acid 175mL

105g

460mL

390mL

...60min0.5A/cm2,27to49°C

VI-11stainless and alloy steel water

chromic acid

phosphoric acid(85%)

sulfuric acid 240mL

80g

650mL

130mL

...5to60min0.5to A/cm2,38to54°C

VI-12tantalum hydro?uoric acid

sulfuric acid 100mL

900mL

...9min graphite cathode,0.1A/cm2,32to

38°C

VI-13stainless steel water

hydro?uoric acid

sulfuric acid 210mL

180mL

610mL

...5min0.5A/cm2,21to49°C

VI-14zinc water

chromic acid

sulfuric acid

sodium dichromate

acetic acid(glacial)800mL

100g

46mL

310g

96mL

......0.002A/cm2,21to49°C

VI-15stainless steel hydrogen peroxide(30%)

(Caution)

hydro?uoric acid

sulfuric acid 260mL

240mL

500mL

...5min0.5A/cm2(Caution)Dangerous

VI-16stainless steel water

hydro?uoric acid

sulfuric acid 520mL

80mL

400mL

...1?2to4min0.08to0.3A/cm2

VI-17stainless steel water

chromic acid

nitric acid

hydrochloric acid

sulfuric acid 600mL

180g

60mL

3mL

240mL

......

VI-18bismuth glycerin

acetic acid(glacial)

nitric acid 750mL

125mL

125mL

121to5min0.5±A/cm2(Caution)This mixture

will decompose vigorously after a

short time.Do not try to keep.

VI-19magnesium ethylene-glycol-monoethyl ether

hydrochloric acid 900mL

100mL

50to6010to30s Bath should be stirred.Cool with

cracked ice below2°C

VI-20molybdenum,sintered and cast methanol(absolute)

hydrochloric acid

sulfuric acid 685mL

225mL

90mL

19to3520to35s Mix slowly.Heat is developed.Avoid

contamination with https://www.wendangku.net/doc/0310371090.html,e below

2°C.

Group VI(Mixed Acids or Salts in Water or Organic Solvent)—Continued

VI-21titanium ethanol(95%)

n-butyl alcohol

aluminum chloride(anhydrous)

(add very slowly)(Caution)

zinc chloride(anhydrous)900mL

100mL

60g

250g

30to601to6min(Caution)Anhydrous aluminum

chloride is extremely dangerous to

handle.

VI-22uranium acetic acid(glacial)

distilled water

chromic acid 750mL

210mL

180g

805to30min The chromic acid is dissolved in the

water before adding to the acetic

https://www.wendangku.net/doc/0310371090.html,e below2°C.

VI-23pure zinc ethanol(95%)

aluminum chloride(anhydrous)

(Caution)

zinc chloride(anhydrous)

distilled water

n-butyl alcohol 720mL

50g

225g

160mL

80mL

25to400.5to3min(Caution)Anhydrous aluminum

chloride is extremely dangerous to

https://www.wendangku.net/doc/0310371090.html,e below16°C.

VI-24zirconium.Polish and etch

simultaneously glycerin(Caution)

hydro?uoric acid

nitric acid

870mL

43mL

87mL

9to121to10min(Caution)will decompose on

standing,dangerous if kept too long

water

hydrochloric acid20mL

electrolyte until brown?lm is

dissolved

acid.Specimens mounted in organic mounting compounds, such as Bakelite,must not be electropolished in electrolytes containing perchloric acid as they may also react explosively.

5.8Speci?c Safety Precautions for Each Group of Electro-lytes:

5.8.1The electrolytes recommended for use are classi?ed into eight groups.Their chemical components are listed in the order of mixing.This ordering has been done to prevent possibly dangerous reactions.Unless other instructions are speci?cally given,the electrolytes are intended to be used in the temperature range from about18to27°C.Cooling may be necessary to maintain this range during use.

5.8.2Group I—(Electrolytes Composed of Perchloric Acid and Alcohol(Methanol or Ethanol)With or Without Organic Additions):

5.8.2.1These electrolytes are believed to be safe to mix and use provided the following safety precautions are followed. Use these electrolytes in an exclusive hood equipped with a wash down feature.The hood should undergo a wash down cycle following each use to avoid accumulation of explosive,dry perchlorates.Only small quantities should be mixed and stored in glass-stoppered bottles?lled to capacity.Any evapo-rated solvents should be replaced to keep the bottle?lled. Spent or exhausted polishing baths are to be promptly dis-carded in a manner consistent with prevailing regulations.The electrolytes are always to be protected from heat or?re.

N OTE3—In this,and all the following formulations,the term95% ethanol refers to a speci?cally denatured alcohol which is composed of95 parts by volume absolute ethanol and5parts by volume absolute methanol.In case this formulation is not available,the use of100% absolute ethanol is advised.Alcohol formulations containing benzene, gasoline,or other denaturing substances are likely to cause difficulties and their use is not recommended.

5.8.3Group II—(Electrolytes Composed of Perchloric Acid and Glacial Acetic Acid):

5.8.3.1Use these electrolytes in an exclusive hood equipped with a wash down feature.The hood should undergo a wash down cycle following each use to avoid accumulation of explosive,dry perchlorates.Very little heat is developed when perchloric acid is mixed with glacial acetic acid.In mixing,the

VI-26Sb methanol(absolute)

sulfuric acid

hydrochloric acid 300mL

50mL

30mL

6–102–4min pure https://www.wendangku.net/doc/0310371090.html,e Pt cathode and anode

lead wires.Agitate bath.Do not

touch polished surface with cotton.

VI-27Sb ethanol(95%)

glycerol

phosphoric acid

sulfuric acid

30mL

30mL

100mL

30mL

good for polarized light work

VI-28Bi water

phosphoric acid

sulfuric acid 200mL

100mL

200mL

good for polarized light work

VI-29Cr water

phosphoric acid

sulfuric acid 210mL

640mL

150mL

18stir bath or specimen

VI-30Ge methanol(absolute)

hydrochloric acid 1000mL

10mL

VI-31Nb water

sulfuric acid

hydro?uoric acid 300mL

100mL

100mL

40polish toα-alumina before

electropolishing

VI-32Nb methanol(absolute)

sulfuric acid

hydro?uoric acid 940mL

50mL

15mL

50–6010s

VI-33Ni-base superalloy methanol(absolute)

hydrochloric acid 170mL

30mL

3020s for Waspaloy and IN-100mod.Etch

at5V for4s.

Group VII(Alkaline Electrolytes)

VII-1gold water to

potassium cyanide

potassium carbonate

gold chloride 1000mL

80g

40g

50g

7.52to4min graphite cathode

VII-2silver water to

sodium cyanide

potassium ferrocyanide 1000mL

100g

100g

2.5To1min graphite cathode

VII-3silver water to

potassium cyanide

silver cyanide

potassium dichromate 1000mL

400g

280g

280g

...To9min graphite cathode,0.003to0.009

A/cm2

VII-4tungsten water to

trisodium phosphate 1000mL

160g

...10min graphite cathode,0.09A/cm2,38to

49°C

VII-5tungsten,lead water to

sodium hydroxide 1000mL

100g

...8to10min graphite cathode,0.03to0.06A/cm2

VII-6zinc,tin water to

potassium hydroxide 1000mL

200g

2to615min copper cathode,0.1to0.2A/cm2

VII-7W water

sodium hydroxide 1000mL

20g

5min

Group VIII(Mixture of Methyl Alcohol and Nitric Acid)

VIII-1Ni,Cu,Zn,Monel,brass,

Ni-chrome,stainless steel methanol(absolute)

nitric acid

660mL

330mL

40to7010to60s very useful but

dangerous

perchloric acid should be added to the acetic with stirring. These mixtures are normally perfectly safe to mix and use but, nonetheless,great care should be exercised with them.Tem-peratures must never be allowed to exceed29°C.They are ?ammable and must be guarded against?re or the evaporation of the acetic acid.Plastic parts are likely to be quickly damaged by exposure to such mixtures.

5.8.4Group III—(Electrolytes Composed of Phosphoric Acid in Water or Organic Solvents):

5.8.4.1These mixtures are generally quite easy to prepare. In mixing,they are handled exactly as a mineral acid;namely, the acid must be slowly poured into the water or solvent with constant stirring to prevent the formation of a heavy layer of acid at the bottom of the vessel.Some solid phosphoric acids are quite energetic in their combination with water,requiring extra care in their mixing.

5.8.5Group IV—(Electrolytes Composed of Sulfuric Acid in Water or Organic Solvents):

5.8.5.1The addition of sulfuric acid to water produces an extremely exothermic reaction.The acid must always be poured into the water slowly and with constant stirring. Cooling is necessary.Great care should be taken to prevent spattering.Even dilute solutions of sulfuric acid strongly attack the skin or clothing.Such solutions are also very hygroscopic. They vigorously attack most plastics.The mixtures of sulfuric acid with other inorganic acids are generally more useful as electrolytes.

5.8.6Group V—(Electrolytes Composed of Chromium Tri-oxide in Water):

5.8.

6.1The addition of crystalline chromium trioxide (CrO3)to water is simple,since very little heat is developed. The resulting chromic acid is a powerful oxidant.Under certain conditions it will liberate considerable quantities of free oxygen.It is generally dangerous,and possibly incendiary,in the presence of oxidizable materials.It cannot be safely mixed with most organic liquids,such as alcohols or glycerol.It can be safely mixed with certain saturated organic acids,but should not be mixed with acetic acid.Chromic acid solutions cannot be used in contact with plastic parts without their eventual destruction.Care should be taken to prevent its contact with the skin since repeated exposure to even dilute solutions of chromic acid or the chromates will cause persistent and painful ulcers that are slow to heal.Chromium trioxide is a human poison and is a carcinogen.

5.8.7Group VI—(Mixed Acids or Salts in Water or Organic Solutions):

5.8.7.1These mixtures are safe to mix and use providing the mixing is done properly.It must be remembered that in all cases,the acid is added to the solvent slowly and with constant stirring.If sulfuric acid is in the formula,it is added last and with particular care.If hydro?uoric acid or?uorides are part of a formula,polyethylene or other similar hydro?uoric acid-resistant vessels should be used.Particular care should be taken to avoid skin contact with acid?uorides since exposure to them,which may pass unnoticed at the time,may result in serious burns later.In those electrolytes containing anhydrous aluminum chloride,extreme care must be exercised.The reaction between this compound and water is almost explosive.Chromates and dichromates cannot be safely mixed with most organic liquids but can be mixed with saturated organic acids. Care should be taken to prevent contact with the skin.

5.8.8Group VII—(Alkaline Electrolytes):

5.8.8.1These mixtures can be grouped into two general categories,those containing cyanide,and those not containing cyanide.

(1)The use of cyanide by anyone not properly trained and familiar with it is extremely dangerous.Cyanides are among the quickest acting and most potent poisons likely to be encountered in the laboratory.Cyanide is so quick-acting and deadly that the administration of an antidote is usually inef-fectual.Extreme care must be taken that no droplet of the solution or crystal of the salt is ever left around where it can be accidentally picked up and carried to the mouth.

(2)Solutions of the alkali hydroxides are very useful for the polishing of certain amphoteric metals.Their attack on the skin is drastic,so great care should be exercised in their use. The dissolution of alkali hydroxides,such as NaOH,in water produces substantial heat.Add the pellets to the water a little bit at a time with constant stirring until the required concen-tration is obtained.If the temperature becomes excessive,allow the solution to cool back to ambient before adding more hydroxide.

5.8.9Group VIII—(Mixtures of Methyl Alcohol and Nitric Acid):

5.8.9.1Nitric acid can be mixed with methanol with appar-ent safety(mixtures with up to about33%nitric acid can be safely stored).This is done by adding the acid to the alcohol with careful stirring.HNO3cannot be safely mixed and stored with any higher alcohol except in very dilute solutions(for example,do not store solutions of more than3%HNO3in ethanol).Under certain conditions,extremely unstable or explosive nitro compounds,azides or fulminates can be formed in alcoholic HNO3solutions.The spontaneous decomposition of the mixture can also be catalyzed by impurities or heat.It should always be discarded as soon as it has served its immediate purpose.Due to its dangerous nature,it should not be employed if its use can be avoided.

6.Apparatus

6.1For the electropolishing of metal specimens in an appropriate electrolyte,a suitable electrolysis cell and a con-trollable power supply are needed.Simple laboratory apparatus,such as shown in Fig.1,can be assembled to perform this function.Many such arrangements are described in the literature.There are also several commercially available models of electropolishing apparatus for either laboratory or ?eld use.

6.2Whenever an attempt is made to polish large surface areas,the problems of obtaining sufficient current density and cooling of the specimen and electrolyte become troublesome. An adequate volume of electrolyte should be used so that localized overheating does not occur.Supplementary cooling may be required.In general,electropolishing of areas larger than1cm2is not recommended for metallographic work because of the increased difficulties encountered,but it is possible to polish larger

areas.

6.3Many variations are possible in the design of a cell for electrolysis.Because the current density is critical,predeter-mine the approximate area to be polished so that the conditions for optimum results can be achieved.This variable can be controlled by the use of simple ?xed masks,such as are used in commercial units,or by the use of protecting masks of insulating paints or tapes.

6.3.1The material used for the mask may be important as reactions with the electrolyte may occur which may simply degrade the mask material,or may lead to explosions (see discussions of electrolyte types I,II,V ,and VIII in 5).6.4Suitable power sources may be batteries,recti?er power supplies,or direct-current generators.Because as much as 150V dc may be required to polish some metals,precautions must be taken to avoid electrical shock.The ideal power source would be a true direct current supply with a closely controlled voltage.When alternating-current converted to direct-current power is used,the voltage and current characteristics needed for electropolishing can be controlled only by the use of resistance.One of the best ways of obtaining the power for electropolishing is to rectify the output of a variable-voltage transformer.When a single-phase,full wave,bridge-recti?er circuit is used,the output current is not satisfactory for all electropolishing unless the ripple voltage is reduced to some

small value by ?ltering.This is because a particular ionization level must often be maintained within close limits in the electrolytic cell.Sufficient ?ltering for small amounts of power is obtained with small chokes and capacitors.When heavy currents are needed,a better source of power is a three-phase,full-wave,bridge reci?er.

6.5Certain electrolytes are strong etchants of the metal when the polishing current is off.Therefore,the provision for rapid insertion and removal of the specimen is necessary.6.6The electrolyte ?ow rate may be an important variable.Certain electrolytes polish best when a vigorous ?ow rate is employed,using special electropolishing cells,while other electrolytes may require gentle agitation or no agitation at all.When using pumped solutions with specially designed polish-ing cells,the manufacturer’s practice for adjustment of the ?ow rate should be followed.

6.7The cathode material should be relatively inert in the electrolyte and,for best results,should be more noble than the metal or alloy being electropolished.A stainless steel cathode is recommended for all of the electrolytes in Table 1where no speci?c recommendation is given.Other useful cathode mate-rials include copper,nickel,graphite and platinum.The surface area of the cathode should be much greater than that of the anode (surface to be polished).

6.8Additional design features that are desirable in appara-tus for the electropolishing of metallographic specimens are as follows:

6.8.1Separation of the power and control portions from the polishing cell to protect them from corrosive fumes and solutions;

6.8.2Meters for the continuous indication of voltage and current;

6.8.3Polishing cycle controlled by an automatic timer;6.8.4Appropriate fume hood to vent gases formed while polishing;

6.8.5Convenient changing of electrolytes and washing and drying of the apparatus;and

6.8.6Corrosion resistant parts which are in contact with electrolyte.

7.Procedure

7.1The metallographer may be required to electropolish some metal or alloy which has not previously been encoun-tered.There are several general principles which can be applied advantageously in these cases.The problem should be viewed in comparison with known procedures and information gained through previous experience.It is generally helpful to compare the relationship of the major component with elements of the same general group in the periodic arrangement of the elements,and to study the phase diagram,if available,to predict the number of phases and their characteristics.Single-phase alloys are generally easier to electropolish than multi-phase alloys.Minor alloying elements may affect the material’s response to polishing in a given electrolyte.

7.2The conditions required for optimum polishing of a metal in a given electrolyte can be ascertained by plotting current density versus voltage curves.The resulting curves

will

FIG.1Simple Arrangement for

Electropolishing

approximate one of the two forms shown in Fig.2.Curve I is typical of electrolytes that either polish over a very wide range or will not polish at all.Curve II is characteristic of electrolytes that form an ionic ?lm.The dotted portion of the curve is added in recognition of certain published data and the observation that the formation of a polishing ?lm requires ?nite time.Polishing will occur between B and C and is usually best slightly before C.

7.3After the polishing range is determined,other constants such as preparation,electrolyte ?ow and time can be deter-mined experimentally.In the majority of cases,a mechanically prepared surface,as produced by grinding up to a 600-grit silicon carbide ?nish,is sufficient;however,with some alloys,a mechanically polished surface is desirable.Metal removal rates by electropolishing are low,typically around 1μm per minute.To remove all prior damage from grinding,either some rough polishing should be performed after the P1200-grit grinding step,or the electropolishing time may be extended,which may not be desirable.The surface to be electropolished should be thoroughly cleaned to obtain uniform attack by the electrolyte.

7.4Small specimens may be mounted in compression mounting media or in castable mounting media for ease in

handling for mechanical preparation.Both types of mounting media are generally impervious to attack by the electrolyte (see 5.7).When mounted samples are to be electropolished,elec-trical contact can be made through a small hole drilled through the back of the mount into the metal.

7.5To aid in selecting or developing an electrolyte for a new metal or alloy,the following characteristics should be considered:

7.5.1The electrolyte should be somewhat viscous.

7.5.2The electrolyte must be a good solvent of the anode under electrolysis conditions.Insoluble products that form adherent deposits on the face of the specimen may prevent polishing.

7.5.3It is desirable that the electrolyte not attack the metal when the current is not ?owing.However,this condition is not always achieved.

7.5.4One or more ions of large radii [example:(PO 4)?3,(ClO 4)?1,or (SO 4)?2(and occasionally certain large organic molecules)]are usually required in electrolytes.

7.5.5The electrolyte should be simple to mix,stable,and safe to handle.

7.5.6It is desirable that the electrolyte function at room temperature and not be sensitive to temperature changes.If electropolishing is being done on a continuing basis,the electrolyte can be cooled by placing the cell in a water-cooled container having a water inlet and outlet.

7.6The cathode material and size,and the anode-to-cathode spacing and orientation,are important variables.The cathode material should be more noble than the anode material (that is,the surface to be polished)and the cathode surface area should be substantially greater than the anode surface area to be polished.Generally,the cathode and anode surfaces should be aligned parallel to each other with a spacing of about 10–20mm.

7.7The depth of the specimen below the electrolyte surface during polishing may also in?uence polishing results.There should be adequate electrolyte above the area to be polished for best results.

7.8Table 2shows a listing of some of the most common troubles likely to be encountered in electropolishing and suggestions for solving the problems.

7.9Table 1contains a listing of the formulas and conditions for use of electrolytes suggested for the polishing of various metals.Table 3is a cross listing of the various electrolytes for speci?c metals and generally for their alloys.8.Keywords

8.1electrolytic polishing;electropolishing ;polishing;specimen preparation

(metallographic)

FIG.2Typical Curves for Determining Conditions for Electropol-

ishing

TABLE 2Electropolishing Procedural Problems and Corrections

Trouble

Possible Cause

Suggested Correction

Center of specimen deeply etched

no polishing ?lm 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 ?lm

(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 ?lm (1)increase voltage

(2)use more viscous electrolyte Waviness or streaks on polished surface

(1)insufficient time (2)incorrect agitation

(3)inadequate preparation (4)too much time

(1)increase or decrease agitation (2)better preparation

(3)increase voltage and decrease 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 ?lm

(1)increase voltage (2)better preparation (3)decrease time Pitting

(1)too long polishing (2)too high voltage

(1)better preparation (2)decrease voltage (3)decrease time

(4)

try different electrolyte

TABLE 3Suggested Electrolytes for Metals and Generally Their Alloys

Metal or Alloy

Electrolyte No.

Aluminum

I-1,I-2,I-4,I-5,I-6,I-8,I-10,I-12,I-13,II-3,III-7,III-14,IV-6Aluminum-silicon alloys I-6,I-8

Antimony II-4,VI-26,VI-27Beryllium I-4,I-13

Bismuth VI-18,VI-25,VI-28Cadmium I-14,III-4Cast iron I-4,II-1

Chromium II-1,II-6,VI-29,VIII-1Cobalt I-5,II-3,III-1,III-4,VIII-1

Copper

III-2,III-3,III-4,III-5,III-10,III-15,VIII-1Copper-zinc alloys III-3,III-4,III-5,III-10,V-2,VIII-1Copper-tin alloys III-10,VI-5,VI-6,VIII-1Copper-nickel alloys III-3,III-10,VIII-1Germanium I-9,VI-30Gold VII-1

Hafnium II-7,VII-11

Iron,pure

I-5,II-1,IV-2,IV-3Iron-silicon alloys I-5,I-6,I-8,II-5Iron-copper alloys III-3,III-4

Iron-nickel alloys I-5,II-1,II-2,II-4,IV-3,VIII-1Lead

I-1,I-5,I-11,II-4,VII-5Magnesium I-1,III-7,III-12,VI-19Manganese III-9

Molybdenum I-7,I-11,I-15,IV-4,IV-7,VI-20Neptunium

III-16

Nickel-chromium II-4,VIII-1

Nickel

I-4,I-13,I-14,II-4,IV-2,VIII-1Nickel aluminide I-13

Niobium VI-31,VI-32Silicon VII-5

Silver

I-14,III-7,VII-1,VII-2,VII-3

Steel,austenitic,stainless,and super alloys I-1,I-2,I-3,I-4,I-5,I-11,I-13,II-1,II-2,II-3,III-3,III-6,III-11,IV-1,IV-2,IV-3,IV-5,IV-6,IV-8,V-1,VI-1,VI-2,VI-3,VI-4,VI-7,VI-8,VI-9,VI-10,VI-11,VI-13,VI-15,VI-16,VI-17,VI-33,VIII-1Steel,carbon and alloy I-1,I-2,I-4,I-5,I-11,I-13,II-1,II-2,II-3,II-5,II-7,III-6,VI-3,VI-11Tantalum VI-12

Tin

I-4,I-13,VI-5,VI-6,VII-6Titanium I-4,I-9,I-13,II-1,II-2,II-3Tungsten VII-4,VII-5,VII-7

Uranium I-4,I-7,II-1,II-2,II-3,III-8,III-13Vanadium I-9

Zinc

I-1,I-5,III-12,V-2,VI-14,VI-23,VII-6,VIII-1Zirconium

I-4,I-7,I-9,II-2,

VI-24

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Electropolishing Reviews–References (1-14)Electropolishing Theory–References (15-34)Electropolishing Equipment–References (35-51)Electropolishing Evaluations–References (52-56)Electropolishing Solutions–References (57-130)

Safety–References (130-133)

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