Designation:G48–03
Standard Test Methods for
Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride
Solution1
This standard is issued under thefixed designation G48;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
1.1These test methods cover procedures for the determina-tion of the resistance of stainless steels and related alloys to pitting and crevice corrosion(see Terminology G15)when exposed to oxidizing chloride environments.Six procedures are described and identified as Methods A,B,C,D,E,and F.
1.1.1Method A—Ferric chloride pitting test.
1.1.2Method B—Ferric chloride crevice test.
1.1.3Method C—Critical pitting temperature test for nickel-base and chromium-bearing alloys.
1.1.4Method D—Critical crevice temperature test for nickel-base and chromium-bearing alloys.
editor evaluating revision1.1.5Method E—Critical pitting temperature test for stain-less steels.
1.1.6Method F—Critical crevice temperature test for stain-less steels.
1.2Method A is designed to determine the relative pitting resistance of stainless steels and nickel-base,chromium-bearing alloys,whereas Method B can be used for determining both the pitting and cre
vice corrosion resistance of these alloys. Methods C,D,E and F allow for a ranking of alloys by minimum(critical)temperature to cause initiation of pitting corrosion and crevice corrosion,respectively,of stainless steels,nickel-base and chromium-bearing alloys in a standard ferric chloride solution.
1.3These tests may be used to determine the effects of alloying additives,heat treatment,and surfacefinishes on pitting and crevice corrosion resistance.
1.4The values stated in SI units are to be regarded as the standard.Other units are given in parentheses for information only.
1.5This 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.
2.Referenced Documents
2.1ASTM Standards:
A262Practices for Detecting Susceptibility to Intergranu-lar Attack in Austenitic Stainless Steels2
D1193Specification for Reagent Water3
E691Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method4
E1338Guide for the Identification of Metals and Alloys in Computerized Material Property Databases5
G1Practice for Preparing,Cleaning,and Evaluating Cor-rosion Test Specimens6
G15Terminology Relating to Corrosion and Corrosion Testing6
G46Guide for Examination and Evaluation of Pitting Corrosion6
G107Guide for Formats for Collection and Compilation of Corrosion Data for Metals for Computerized Database Input6
3.Terminology
3.1Definition of Terms Specific to This Standard:
3.1.1critical crevice temperature,n—the minimum tem-perature(°C)to produce crevice attack at least0.025-mm (0.001-in.)deep on the bold surface of the specimen beneath the crevice washer,edge
attack ignored.
3.1.2critical pitting temperature,n—the minimum tem-perature(°C)to produce pitting attack at least0.025-mm (0.001-in.)deep on the bold surface of the specimen,edge attack ignored.
3.2The terminology used herein,if not specifically defined otherwise,shall be in accordance with Terminology G15.
1These test methods are under the jurisdiction of ASTM Committee G01on Corrosion of Metals,and are the direct responsibility of Subcommittee G01.05on Laboratory Corrosion Tests.
Current edition approved May10,2003.Published July2003.Originally approved in1976.Last previous edition approved in2000as G48–00.
2Annual Book of ASTM Standards,V ol01.03.
3Annual Book of ASTM Standards,V ol11.01.
4Annual Book of ASTM Standards,V ol14.02.
5Annual Book of ASTM Standards,V ol02.05.
6Annual Book of ASTM Standards,V ol03.02. 1
Copyright©ASTM International,100Barr Harbor Drive,PO Box C700,West Conshohocken,PA19428-2959,United States.
Definitions provided herein and not given in Terminology G 15are limited only to this standard.
4.Significance and Use
4.1These test methods describe laboratory tests for com-paring the resistance of stainless steels and related alloys to the initiation of pitting and crevice corrosion.The results may be used for ranking alloys in order of increasing resistance to pitting and crevice corrosion initiation under the specific conditions of these methods.Methods A and B are designed to cause the breakdown of Type 304at room temperature.
4.2The use of ferric chloride solutions is justified because it is related to,but not the same as,that within a pit or crevice site on a ferrous alloy in chloride bearing environments (1,2).7The presence of an inert crevice former of consistent dimension on a surface is regarded as sufficient specification of crevice geometry to assess relative crevice corrosion susceptibility.4.3The relative performance of alloys in ferric chloride solution tests has been correlated to performance in certain real environments,such as natural seawater at ambient temperature (3)and strongly oxidizing,low pH,chloride containing envi-ronments (4),but several exceptions have been reported (4-7).4.4Methods A,B,C,D,E,and F can be used to rank the relative resistance of stainless steels and nickel base alloys to pitting and crevice corrosion in chloride-containing environ-ments.No statement can be made about resistance of alloys in environments that do not contain chlorides.
4.4.1Methods A,B,C,D,E,and F were designed to accelerate the time to initiate localized corrosion relative to most natural environments.Consequently,the degree of corro-sion damage that occurs during testing will generally be greater than that in natural environments in any similar time period.4.4.2No statement regarding localized corrosion propaga-tion can be made based on the results of Methods A,B,C,D,E or F.
4.4.3Surface preparation can significantly influence results.Therefore,grinding and pickling of the speci
men will mean that the results may not be representative of the conditions of the actual piece from which the sample was taken.
N OTE 1—Grinding or pickling on stainless steel surfaces may destroy the passive layer.A 24-h air passivation after grinding or pickling is sufficient to minimize these differences (8).
4.4.4The procedures in Methods C,D,E and F for measuring critical pitting corrosion temperature and critical crevice corrosion temperature have no bias because the values are defined only in terms of these test methods.
5.Apparatus
5.1Glassware —Methods A,B,C,D,E,and F provide an option to use either wide mouth flasks or suitable sized test tubes.Condensers are required for elevated temperature testing when solution evaporation may occur.Glass cradles or hooks also may be required.
5.1.1Flask Requirements ,1000-mL wide mouth.Tall form or Erlenmeyer flasks can be used.The mouth of the flask shall
have a diameter of about 40mm (1.6in.)to allow passage of the test specimen and the support.
5.1.2Test Tube Requirements ,the diameter of the test tube shall also be about 40mm (1.6in.)in diameter.If testing requires use of a condenser (described below),the test tube length shall be about 300mm (about 12in.);otherwise,the length can be about 150to 200mm ( 8in.).5.1.3Condensers,Vents and Covers :
5.1.3.1A variety of condensers may be used in conjunction with the flasks described in 5.1.1.These include the cold finger-type (see,for example,Practices A 262,Practice C)or Allihn type condensers having straight tube ends or tapered ground joints.Straight end condensers can be inserted through a bored rubber stopper.Likewise,a simple U tube condenser can be fashioned.
N OTE 2—The use of ground joint condensers requires that the mouth of the flask have a corresponding joint.
5.1.3.2U Tube Condensers ,fitted through holes in an appropriate size rubber stopper can be used in conjunction with the 300-mm test tube described in 5.1.2.
5.1.3.3When evaporation is not a significant problem,flasks can be covered with a watch glass.Also,flasks as well as test tubes can be covered with loosely fitted stoppers or plastic or paraffin type wraps.
N OTE 3—Venting must always be considered due to the possible build up of gas pressure that may result from the corrosion process.
5.1.4Specimen Supports :
5.1.4.1One advantage of using test tubes is that specimen supports are not required.However,placement of the specimen does create the possible opportunity for crevice corrosion to occur along the edge.
N OTE 4—See 14.2concerning edge attack.
5.1.4.2When using flasks,specimens can be supported on cradles or hooks.Cradles,such as those shown in Fig.1,eliminate the necessity for drilling a support hole in the test specimen.While the use of hooks requires that a specimen support hole be provided,the hooks,as contrasted to the cradle,are easier to fashion.Moreover,they create only one potential crevice site whereas multiple sites are possible with the cradle.
N OTE 5—A TFE-fluorocarbon cradle may be substituted for glass.
5.1.4.3The use of supports for Methods B,D,and F crevice corrosion specimens is optional.
5.2Water or Oil Bath ,constant temperature.
5.2.1For Methods A and B,the recommended test tempera-tures are 2262°C or 5062°C,or both.
5.2.2For Methods C,D,E,and F,the bath shall have the capability of providing constant temperature between 0°C and 85°C 61°C.
5.3Crevice Formers—Method B :
5.3.1Cylindrical TFE-fluorocarbon Blocks ,two for each test specimen.Each block shall be 12.7-mm (0.5in.)in diameter and 12.7-mm high,with perpendicular grooves 1.6-mm (0.063in.)wide and 1.6-mm deep cut in the top of each cylinder for retention of the O-ring or rubber bands.Blocks can be machined from bar or rod stock.
7
The boldface numbers in parentheses refer to the list of references at the end of this
standard.
5.3.2Fluorinated Elastomers O-rings,or Rubber Bands ,(low sulfur (0.02%max)),two for each test specimen.
N OTE 6—It is good practice to use all O-rings or all rubber bands in a given test program.
5.3.2.1O-rings shall be 1.75mm (0.070in.)in cross section;one ring with an inside diameter of about 20mm (0.8in.)and one with an inside diameter of about 30mm (1.1in.).Rubber bands shall be one No.12(38-mm (1.5-in.)long)and one No.14(51-mm (2-in.)long).
N OTE 7—Rubber bands or O-rings can be boiled in water prior to use to ensure the removal of water-soluble ingredients that might affect corrosion.
5.4Crevice Formers—Methods D and F :
5.4.1A Multiple Crevice Assembly (MCA),consisting of two TFE-fluorocarbon segmented washers,each having a number of grooves and plateaus,shall be used.The crevice design shown in Fig.2is one of a number of variations of the
multiple crevice assembly that is in use and commercially available.8
5.4.2Reuse of Multiple Crevice Assemblies ,when as-sembled to the specified torque,the TFE-fluorocarbon seg-mented washers should not deform during testing.Before reuse,each washer should be inspected for evidence of distortion and other damage.If so affected,they should be discarded.In some cases,the crevice formers may become stained with corrosion products from the tested alloy.Gener-ally,this staining can be removed by immersion in dilute HCl (for example,5-10%by volume)at room temperature,fol-lowed by brushing with mild detergent and through rinsing with water.
5.4.3Fasteners ,one alloy UNS N10276(or similarly resis-tant alloy)fastener is required for each assembly.Each assembly comprises a threaded bolt and nut plus two washers.The bolt length shall be sized to allow passage through the mouth of the glassware described in 5.1.5.5Tools and Instruments :
5.5.1A
6.35-mm (1⁄4-in.)torque limiting nut driver is required for assembly of the Methods D and F crevice test specimen.
5.5.2Low Power Microscope ,(for example,203magnifi-cation)for pit detection.
5.5.3Needle Point Dial Depth Indicator or Focusing Mi-croscope ,to determine the depth of pitting or crevice corro-sion,or both.
5.5.4Electronic Balance (optional),to determine specimen mass to the nearest 0.0001g.
5.5.5Camera (optional),to photographically record the mode and extent of any localized corrosion.
8
The sole source of supply of the apparatus known to the committee at this time is Metal Samples Co.,Inc.,P.O.Box 8,Route 1Box 152,Munford,AL 36268.If you are aware of alternative suppliers,please provide this information to ASTM Headquarters.Your comments will receive careful consideration at a meeting of the responsible technical committee,1which you may
attend.
FIG.1Examples of Glass Cradles that Can Be Used to Support the
Specimen
FIG.2TFE-fluorocarbon Crevice
Washers
6.Ferric Chloride Test Solution
6.1For Methods A and B,dissolve100g of reagent grade ferric chloride,FeCl3·6H2O,in900mL of Type IV reagent water(Specification D1193)(about6%FeCl3by mass).Filter through glass wool orfilter paper to remove insoluble particles if present.
6.2For Methods C,D,E,and F,dissolve68.72g of reagent grade ferric chloride,FeCl3·6H2O in600mL of reagent water and add16mL of reagent grade concentrated(36.5–38.0%) hydrochloric acid(HCl).This will produce a solution contain-ing about6%FeCl by mass and1%HCl resulting in a pH controlled environment over the test temperatures(9).
7.Test Specimens
7.1A test specimen25by50mm(1by2in.)is recom-mended as a standard size,although various shapes and sizes can be tested by this method.All specimens in a test series should have the same dimensions when comparisons are to be made.Unless end-grain pitting is an integral part of the evaluation,the proportion of end-grain surface to specimen surface should be kept as small as possible given the limita-tions of specimen sizes because of the susceptibility of end-grain surfaces to pitting.
N OTE8—The thickness of the specimen in Method B can influence the tightness of the crevice and the test results.
N OTE9—End-grain attack in Methods C,D,E,and F may not be as prevalent in a test in which low test temperatures are anticipated.
7.2When specimens are cut by shearing,the deformed material should be removed by machining or grinding prior to testing unless the corrosion resistance of the sheared edges is being evaluated.It is good practice to remove deformed edges to the thickness of the material.
7.3For Methods D and F,a sufficient hole should be drilled and chamfered in the center of the specimen to accommodate the bolt and insulating sleeve used to attach the crevice device.
7.4All surfaces of the specimen should be polished to a uniformfinish.A120-grit abrasive paper has been found to provide a satisfactory standardfinish.Wet polishing is pre-ferred,but if dry polishing is used,it should be done slowly to avoid overheating.The sharp edges of a specimen should be rounded to avoid cutting rubber bands or O-rings.
N OTE10—While a polished surface is preferred for uniformity,the test may be varied at the discretion of the investigator to evaluate other surface finishes,such as a millfinish.
7.5Measure the dimensions of the specimen,and calculate the total exposed area of interest.
7.6Clean specimen surfaces with magnesium oxide paste or equivalent,rinse well with water,dip in acetone or methanol, and air-dry.
7.7Weigh each specimen to the nearest0.001g or better and store in a desiccator until ready for use(see Practice G1).
8.Method A—Ferric Chloride Pitting Test
8.1Procedure:
8.1.1Pour600mL of the ferric chloride test solution into the1000-mL test beaker.If specimens larger than the standard are used,provide a solution volume of at least5mL/cm2(30 mL/in.2)of specimen surface area.Transfer the test beaker to a constant temperature bath and allow the test solution to come to the equilibrium temperature of interest.Recommended temperatures for evaluation are2262°C and5062°C. 8.1.2Place the specimen in a glass cradle and immerse in the test solution after it has reached the desired temperature. Maintain test solution temperature throughout the test.
8.1.3Cover the test vessel with a watch glass.A reasonable test period is72h,although variations may be used at the discretion of the investigator and depend on the materials being evaluated.
8.1.4Remove the specimens,rinse with water and scrub with a nylon bristle brush under running water to remove corrosion products,dip in acetone or methanol,and air-dry. Ultrasonic cleaning may be used as a substitute method in cases in which it is difficult to remove corrosion products from deep pits.
8.1.5Weigh each specimen to0.001g or better and reserve for examination(see Practice G1).
9.Method B—Ferric Chloride Crevice Corrosion Test
(10)
9.1Procedure:
9.1.1Add150mL of ferric chloride solution to each test tube,insert a rubber stopper,and place the tube in a thermo-stated bath until it comes to the equilibrium temperature of interest.Recommended temperatures for evaluation are226 2°C and5062°C.
9.1.2Fasten two TFE-fluorocarbon blocks to the test speci-men with O-rings or a double loop of each of two rubber bands as shown in Fig.3.Use plastic gloves to avoid hand contact with metal surfaces during this operation.Use the small O-ring or the No.12rubber band for the25-mm(1-in.)dimension and the large O-ring or the No.14rubber band for the50-mm (2-in.)dimension.
9.1.3After the test solution has reached the desired tem-perature,tilt the tube at a45°angle and slide the test specimen to the bottom of the tube,replace the stopper,and return the tube to the bath.
9.1.4A reasonable test period is72h,although variations may be used at the discretion of the investigator and depend on the materials being evaluated.Specimens may be inspected at intervals by decanting the test solution into a clean beaker
and (a)Specimen After Test with Attack at Four Crevices Under Rubber Bands and Under One Block
(b)Assembled Crevice Test Specimen
FIG.3Ferric Chloride Crevice Test
Specimen
sliding the specimen from the test tube.Rinse the specimen in water and examine under the four points of contact for the O-rings or rubber bands and under both TFE-fluorocarbon blocks.If further exposure i
s required,the specimen and solution should be returned to the test tube before the specimen drys and the test tube should be immediately returned to the bath.
N OTE 11—The removal of specimens for inspection may affect the rate of corrosion,and caution should be observed when comparing these results with those obtained from constant immersion tests.
9.1.5After the test has been completed,remove the test specimens,rinse with water,and scrub with a nylon bristle brush under running water to remove corrosion products,dip in acetone or methanol,and air-dry.Ultrasonic cleaning may be used in those cases in which it is difficult to remove corrosion products from deep pits or crevices.
9.1.6Weigh each specimen to the nearest 0.001g or better and reserve for examination.
10.Method C—Critical Pitting Temperature Test for
Nickel-Base and Chromium-Bearing Alloys:
N OTE 12—See Table 1.
10.1Procedure :
10.1.1Pour 600mL of acidified ferric chloride test solution into the test container.If specimens larger than the standard are used,provide a solution volume of at least 5mL/cm 2(30mL/in.2)of surface area.Cover the container with a watch glass,transfer to a constant temperature bath,and allow to come to the equilibrium temperature of interest.Verify the temperature before starting the test.
10.1.2If test tubes are used instead of a 1000-mL test container,150mL of test solution shall be added.
10.1.3Place the specimen in a glass cradle and immerse in the test solution after it has reached the desired temperature.The starting temperature may be estimated by the following equation (11,12).
CPT ~°C !5~2.53%Cr !1~7.63%Mo !1~31.93%N !241.0
(1)
Testing shall begin at the nearest increment of 5°C,esti-mated by the above equation.The minimum temperature of test is 0°C and the maximum temperature of test is 85°C.No more than one specimen shall be placed in a test container.Replace the watch glass on the container.Maintain the temperature (61°C (61.8°F))throughout the test period.The standard test period is 72h.This test period was evaluated by interlaboratory
testing,16.1.2,and is required to produce attack on some highly corrosion resistant nickel-based alloys.The test period must be reported as indicated in 15.1.
N OTE 13—Critical pitting or crevice testing is likely to require three to five replicate samples to determine the critical temperature for each alloy/condition.
N OTE 14—While testing may be done at a higher temperature,85°C was the maximum temperature of the test in the round robin.
10.1.4At the end of the test period,remove the specimen,rinse with water,scrub with a nylon bristle brush under running water to remove corrosion products,dip in acetone or metha-nol,and dry.
10.1.5Inspection is done in accordance with Section 14.Pitting corrosion is considered to be present if the local attack is 0.025mm (0.001in.)or greater in depth.
10.1.5.1If minimum or greater pitting attack is observed,lower the bath temperature 5°C,and using a new specimen and fresh solution,repeat steps 10.1.1to 10.1.5,or
10.1.5.2If less than minimum pitting attack is observed,raise the bath temperature 10°C,and using a new specimen and fresh solution,repeat steps 10.1.1-10.1.5.
11.Method D—Critical Crevice Temperature Test for
Nickel-Based and Chromium-Bearing Alloys (13):
N OTE 15—See Table 1.
11.1Procedure :
11.1.1Apply the two crevice washers to the specimen.A TFE-fluorocarbon tubular sleeve (for electrical isolation),and a bolt,a nut,and flat washers of UNS N10276may be used to fasten the crevice washers to the specimen,as illustrated in Fig.4.Insulating sleeves shall be used around the bolt and the specimen shall be checked for electrical contact with the bolt.The torque on the bolt influences the tightness of the crevice and the test results.A torque of 0.28Nm (40in.-oz)shall be applied using a 6.35-mm (1⁄4-in.)drive torque limiting nut-driver.The torque must be reported as indicated in 15.1.
N OTE 16—The torque of 0.28Nm (40in.-oz)was evaluated by inter-laboratory testing,16.1.2.
N OTE 17—(a )Titanium bolts,nuts,and flat washers may also be used to attach the crevice washers to the specimen.(b )Different crevice devices may give different results for the same torque.
11.1.2Pour 600mL of acidified ferric chloride test solution into the 1000-mL test container.Cover the container with a watch glass,transfer to a constant temperature bath,and allow to come to equilibrium temperature of interest.Verify the temperature before starting the test.
TABLE 1Results of First Interlaboratory Test Program
N OTE 1—Minimum temperature (°C)to produce attack at least 0.025-mm (0.001-in.)deep on bold surface of specimen.Edge attack ignored.
Alloy/Laboratory
Method C—CPT Critical Pitting Corrosion Temperature (C)Method D—CCT Critical Crevice Corrosion Temperature (C)UNS S31603UNS N08367UNS S44735UNS N06022UNS S31603UNS N08367UNS S44735UNS N06022120/20/2075/A /A 85/85/85>85/>85/>85<0/<0<030/30/3042/35/4250/A /50220/20/2070/70/7080/80/80>85/>85/>85<0/<0<025/25/2535/35/A 50/55/55320/20/2085/85/8575/85/85>85/>85/>85<0/<0/<025/30/3035/40/4055/60/60419/1975/8081/81>85/>85<0/<034/3440/4067/67520/20/2075/75/7570/70/75>85/>85/>85<0/<0/<020/20/2045/45/456
20/20
75/80
75/85
>85/>85
<0/<0
30/30
40/40
55/55
A
Test run but no attack
observed.
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