4.Summary of Test Method
4.1The test method determines the potential independent
critical pitting temperature (CPT)by way of a potentiostatic
technique using a temperature scan and a specimen holder that
is designed to eliminate the occurrence of crevice corrosion
(see Fig.1).The specimen is exposed,either entirely or in part,
depending on test cell configuration to a 1M NaCl solution,
initially at 0°C.After an initial temperature stabilization
period,the solution is heated at a rate of 1°C/min.About 60s
before the temperature scan is commenced,the specimen is
anodically polarized to a potential above the pitting potential
range.This potential is held constant during the whole tem-
perature scan.A potential of 700mV versus SCE (25°C)has
been found suitable for most stainless steels.The current is
monitored during the temperature scan,and the CPT is defined
as the temperature at which the current increases rapidly,which
for practical reasons is defined as the temperature at which the
current density exceeds 100µA/cm 2for 60s.Pitting on the
specimen is confirmed visually after the test.
5.Significance and Use
5.1This test method provides a prediction of the resistance
to stable propagating pitting corrosion of stainless steels and
related alloys in a standard medium (see Note 1).The CPT test
can be used for product acceptance,alloy development studies,
and manufacturing control.In the case of product acceptance,
the supplier and user must agree upon the preconditioning of
the specimen with regard to surface finish.The test is not
intended for design purposes since the test conditions acceler-
ate corrosion in a manner that does not simulate any actual
service environment.
5.2Another method to determine the potential independent
CPT with an electrochemical technique has been discussed in
the literature (1-4).This test method involves a potentiody-
namic (potential sweep)procedure performed on specimens at
different temperatures.A comparison (2)of the test method
described in this test method and the potentiodynamic tech-
nique has indicated no difference in the test result obtained.
6.Apparatus
6.1The apparatus necessary for determining the CPT con-
sists of instruments for measuring electronic signals,a tem-
perature controlling apparatus,a specimen holder,and a test cell.The instruments for measuring electro
nic signals may be integrated into one instrument package or may be individual components.Either form of instrumentation can provide ac-ceptable data.Typical test equipment consists of the following:(1)potentiostat (2)potential measuring instrument (3)current measuring instrument (4)temperature controller (5)tempera-ture measuring instrument (6)test cell (7)specimen holder,and (8)electrodes.6.2Potentiostat —The potentiostat shall be able to apply the constant potential to within 1mV at a current density of 10mA/cm 2.The applied potential is changed either automatically or manually by shifting the potential from the open circuit potential to another more noble potential.6.3Potential Measuring Instrument —Requirements shall be in accordance with the section on Potential Measuring Instruments in Test Method G 5.6.4Current Measuring Instruments —An instrument that is capable of measuring a current accurately to within 5%of the actual value.The typical current densities encountered during the CPT test are in the range of 1µA/cm 2to 10mA/cm 2.6.5Temperature Controller:6.5.1Thermostat equipment is required that can provide cooling and heating of the test solution in the temperature range from 0°C to approximately 100°C.Further,the tempera-ture controller is used to provide controlled heating,which gives the test solution temperature a temperature increase rate of 1°C/min in the range from 0°C to approximately 100°C.6.5.2Above 10°C,the average rate of temperature change of the test solution shall be 1.060.3°C/min,where the average is calculated over a temperature range of 10°C.6.6Temperature Measurement Instrumentation,
shall be capable of measuring the temperature of the test solution with an accuracy of 60.4°C.6.7Test Cell:6.7.1Option 1,G5Type —The test cell should be similar to the one described in Test Method G 5.Other similar polariza-tion cells may be equally suitable.The gas purger should distribute the gas in numerous small bubbles.6.7.2Option 2,Flushed-port Cell —This cell design is based on that published by R.Qvarfort (3)and includes the specimen holder in the design.The advantages of this cell design are that the specimen edges and back do not need to be machined,the specimen does not have to be mounted inside the cell,and crevice corrosion at the contact area of the cell port is completely eliminated,even at elevated test temperatures.See Appendix X2for a description of this cell.The gas purger should distribute the gas in numerous small bubbles.6.7.3The test cell shall be able to contain a test solution volume of minimum 100mL per square centimetre test area.A maximum dilution of 15%of the test solution during the test period is allowed in case a flushed port cell or similar arrangement is used.6.8Specimen Holder:6.8.1Any part of the specimen holder coming in contact with the test solution during testing shall be made of an inert material,and any seal shall not allow leakage of electrolyte.6.8.2The specimen holder shall have a design that
ensures
FIG.1Determination of
CPT
no occurrence of crevice corrosion at the contact area between specimen holder and specimen.
6.8.3Two examples of specimen holder designs in accor-dance with this standard are shown in Appendix X2and Appendix X3.The major difference between the specimen holder designs lies in the allowable specimen geometry and the number of surfaces on the specimen that are being tested simultaneously.
6.9Electrodes:
6.9.1Auxiliary(Counter)Electrode—Requirements shall be in accordance with the section Auxiliary Electrodes in Test Method G5with the exception that only one counter electrode is necessary for CPT testing.The electrode material shall be of
a type which can be considered inert under the test conditions.
6.9.2Reference Electrode—The reference electrode shall be kept at room temperature outside the actual test cell.The reference electrode shall be capable of ensuring a constant reference potential within65mV during the entire test procedure(see Note2).Electrical contact to the test solution shall be p
rovided by the use of a luggin capillary placed in the test solution.Requirements shall otherwise be in accordance with the section on Reference Electrode in Test Method G5. N OTE2—It may be difficult to ensure a fully constant reference potential due to the large variations in temperature of the test solution; therefore,the allowable is65mV.This does,however,not affect the measured potential independent CPT(1).
7.Test Specimens
7.1Finish—Any geometry and surfacefinish(see Note3) compatible with the chosen specimen holder as specified in6.8 may be used.
N OTE3—The state of the surface may be dependent on the time and location of storage between thefinal mechanical or chemical surface treatment and testing.The time and location of storage may,therefore,in some situations be considered an integral part of the surfacefinish. 7.2Sampling—When using this test method to meet product acceptance criteria,the means of sampling of a test specimen shall be decided by agreement between the parties involved.
7.3Test Area—A minimum test area of1cm2shall be used.
7.4Specimens removed from a work piece or component by shearing,cutting,burning,and so forth shall have the affected edges removed by grinding or machining,unless it is explicitly intended to study the effects of these edge factors.
8.Reagents and Materials
8.1Purity of Reagents—Reagent grade chemicals shall be used in all tests.Unless otherwise indicated,it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available.7Other grades may be used, provided it isfirst ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
8.2Purity of Water—Unless otherwise indicated,references to purified water shall be understood to mean reagent water as defined by Type IV of Specification D1193.
8.3Standard Test Solution—To prepare1L of1M sodium chloride(NaCl)solution,dissolve58.45g sodium chloride (NaCl)in purified water to a total solution volume of1L.The solution can be made up in bulk and stored for one month at room temperature.
8.4Purging Gas—Nitrogen gas of minimum99.99%purity should be used.
9.Applied Potential
9.1Standard Potential—An anodic potential of700mV versus SCE(25°C)is used.This has been found appropriate for most stainless steels(1).
9.2Alternative Potential:
9.2.1If uncertainty exists concerning whether the standard potential is sufficiently high to obtain the potential independent CPT,a test at800mV versus SCE(25°C)may be performed.
A significant deviation between the CPT obtained at700mV and800mV will indicate a need for a reevaluation and new choice of potential.
N OTE4—Using a lower potential than the standard potential of700mV versus SCE(25°C)is fully acceptable,provided the determined CPT still is potential independent.To change the measurement range provided by the standard test conditions,a new test solution composition will have to be chosen.Following the choice of test solution,a test potential that ensures the determination of a potential independent CPT will have to be determined.
9.2.2Evaluation of differences in obtained CPT at the two potentials should take into account the repea
tability of the test method.The homogeneity of the material used for the two different potentials shall also be considered before an alterna-tive potential is used.
10.Procedure
10.1Sample Mounting,Cleaning and Placement:
10.1.1The recommendations given in Practice G1are to be followed,where applicable,unless otherwise stated in this procedure.
10.1.2Clean the specimen just before immersion in the electrolyte by degreasing with a suitable detergent,rinsing in purified water,followed by ethanol or similar solvent,and air drying.After degreasing,handle the specimen with clean gloves,soft clean tongs,or equivalent preventive measures,to avoid surface contamination.
10.2Test Solution Preparation:
10.2.1Prepare the test solution according to Section8. 10.2.2Bring the solution to an initial temperature at or below3°C.
10.3Mount the specimen in the specimen holder.
10.4Place the specimen,counter electrode,salt bridge probe,and other components in the empty test cell.
editor evaluating revision10.5Fill the cell with cold(#3°C)solution.
10.6Ensure that the salt bridge isfilled with the test solution and is free of air bubbles,particularly in the restricted space at the tip.A lugging probe should contain a wick,or equivalent device,to ensure electric contact even when small gas bubbles are formed during the test.
7Reagent Chemicals,American Chemical Society Specifications,American Chemical Society,Washington,DC.For suggestions on the testing of reagents not listed by the American Chemical Society,see Analar Standards for Laboratory Chemicals,BDH Ltd.,Poole,Dorset,U.K.,and the United States Pharmacopeia and National Formulary,U.S.Pharmacopeial Convention,Inc.(USPC),Rockville,
MD.
10.7Commence bubbling nitrogen gas through the solution. N OTE5—The purpose of the purging gas is to enhance the stirring of the test solution.No reduction in the oxygen level of the test solution is intended.Presence or absence of oxygen has been shown not to affect the test result(1).
10.8Connect the electrodes to the potentiostat and data recording device and the connections for temperature measure-ment and control.Let the system cool and stabilize at061°C for a minimum of10min.
10.9Record the open circuit potential(OCP)of the test specimen shortly before the test is begun.
10.10Set the data acquisition for electrode current and solution temperature.The sampling rate shall correspond to a minimum of two measurements every minute to follow solu-tion temperature and the variation of the current on the working electrode.
10.11Apply the constant anodic potential to the working electrode.
10.12The potential shall be applied for6065s before the temperature increase.
10.13The temperature is then ramped at1°C/min.
10.14Continue measuring the temperature until the CPT has been determined or the maximum required temperature is reached.The CPT is determined when the current density reaches100µA/cm2and remains above this level for a minimum of60s.Terminate the test and data acquisition after either of these limits has been reached.
10.15Completion of Test—Dismount the specimen as soon as possible after test completion.Inspect the specimen to locate pits(see Note6).Rinse the specimen in water,clean with ethanol(95%is suggested)or detergent,rinse again with water,and then air dry.
N OTE6—Pits may be difficult to locate if the test is stopped shortly after pitting initiation.Pits may be located based on leaking rust or by using a needle to uncover pits hidden below a thin metal or oxidefilm.The lack of visible pitting may indicate that general corrosion has occurred,for example,transpassive corrosion.However,only a more thorough exami-nation of the tested specimen can give a possible explanation.
11.Visual Examination of Test Electrode
11.1Confirm the existence of pits and the absence of crevice corrosion using a microscope at20X magnification. Note the location of pits relative to the test geometry(in the center,on edges,at the botto
m center,at the bottom edge,and so forth).
11.2Any crevice corrosion observed on the specimen after testing means that the test results are invalid and must be discarded.
11.3If required,a more thorough examination of the elec-trode can involve measurement of pit density and pit depths on specimen electrodes,according to Guide G46,using a micro-scope at20X magnification.
12.Data Analysis
12.1Measured current as a function of time shall be converted to current density values.The data may be presented as in Fig.1,which shows an example of a current density versus temperature plot.
12.2Evaluation of the CPT:
12.2.1Standard Evaluation—The critical pitting tempera-ture is taken as the temperature at which the current increases above100µA/cm2(see Note7)and stays above this critical current density for a minimum of60s;see Fig.1.
N OTE7—The CPT is defined as the lowest temperature at which stable propagating pitting occurs.For practical reasons,this is being translated to the temperature at which the current density increases above a certain level(100µA/cm2has been chosen as a standard in this test method).A 60s delay is introduced in order to ensure that the observed current increase originates from stable propagating pitting and not short lived current peaks originating from metastable pitting.
12.2.2Alternative Evaluation:
12.2.2.1For materials that generally exhibit a very high passive current density(for example some,but not all,stainless steel welds)or low pitting propagation rate(some,but not all, nickel-base alloys)a different critical current density may be chosen,but generally this should be avoided.Any change in the evaluation criteria shall be noted specifically in the report. 12.2.2.2Comparison of CPTs obtained with different criti-cal current densities is very difficult and should generally not be attempted.
12.2.3The registered temperature in the solution will not be exactly the same as the temperature of the specimen,because the solution temperature is continuously changed during most of the test.A conversion of the increasing solution temperature to specimen temperature should be performed and the CPT should be defined relative to the temperature of the specimen.
12.2.4The conversion between solution and specimen tem-perature may be done either by direct measurement of the specimen surface temperature during the test or by using a suitable calibration formula based on an earlier parallel mea-surement of specimen temperature and solution temperature.A detailed guideline of how to obtain a suitable calibration formula is given in Annex A1.
13.Report
13.1Report the following mandatory information:
13.1.1Test identification number and date of test.
13.1.2Critical pitting temperature(CPT).CPTs below10°C shall only be reported as below10°C or<10°.
13.1.3Formula for conversion of test solution temperature to specimen temperature shall be reported.If no conversion of the solution temperature to specimen temperature has been done,this shall be stated specifically in the test report.
13.1.4Location of pits on the tested surface.
13.1.5Test area.
13.1.6Material identification data.
13.1.7Type of test cell and test solution volume.
13.1.8Surfacefinish and approximate geometry of the tested specimen including the approximate storage time be-tweenfinal surfacefinish preparation and testing.
13.1.9If no pits were observed despite an observed rapid current increase,this lack of visual pitting identification shall be noted explicitly.The evaluation of such deviations should lead to a more thorough examination of the specimen.
13.1.10If parameters deviating from the standard values in this test method have been used,then all deviations shall be reported.
13.2Optional Reporting—If required,a more
elaborate
report can,additional to the mandatory report,contain one or more of the following information:
13.2.1Test identification number;specimen number;mate-rial;heat number;product form;solution temperature at CPT; open circuit potential.
13.2.2Together with the basic test results,it is recom-mended that the data from the CPT test should be presented graphically,as shown in Fig.1.
13.2.3In addition the following data are useful to report,(a) the scatter or deviation in the CPT values based on multiple runs if available,(b)pit geometry,number of pits formed and their depth in accordance with Guide G46.
13.3The example data record sheet in Appendix X4,or equivalent,may be used for reporting.
14.Precision and Bias8
14.1Interlaboratory Test Program—An interlaboratory study was run in which the critical pitting temperatures were determined for four grades of stainless steels with laboratory ground surface.Ten laboratories participated in the study.Each laboratory tested three tofive test specimens of each of the four materials.Practice E691was followed for the design and analysis of the data.
14.2Precision:See Table1.The terms,repeatability limit and reproducibility limit,are used as specified in Practice E177.The repeatability and reproducibility limits were ob-tained by multiplying the respective standard deviations by2.8. N OTE8—The high reproducibility limit for material UNS S31254is believed to be caused by problems with temperature calibration,which is most critical for materials with high CPTs.The variations and reported procedures in the round robin results were,however,judged to be insufficient to exclude the values from two laboratories(out of ten laboratories),which otherwise would have resulted in that the repeatabil-ity limit would have been67.4°C and the reproducibility limit would have been612.5°C.
14.3Bias—This test method has no bias,because the electrochemically(potentiostatic)determined potential inde-pendent CPT is defined by this test method and no accepted reference standard exists.
15.Keywords
15.1critical pitting temperature;electrochemical test;pit-ting corrosion;stainless steel
ANNEX
(Mandatory Information)
A1.GUIDELINES FOR CALIBRATING THE SPECIMEN TEMPERATURE VERSUS THE TEST SOLUTION TEMPERATURE
A1.1The registered temperature in the solution will not be exactly the same as the temperature of the specimen,because the solution temperature is continuously changed during most of the testing time.
A1.2The temperature lag is further enhanced when using theflushed port cell or similar test cells and specimen holders, where the specimen is placed partly outside the test solution.In these cases there is an added cooling or heating of the specimen from the outside.
A1.3The temperature lag between solution and specimen can be minimized by providing adequate stirring of the solution.A combination of mechanical stirring and dispersed gas bubbling has been found beneficial(see Ref(1)).
A1.4When calibrating the specimen temperature versus the test solution temperature,the following guidelines for the calibration are recommended;
A1.4.1The calibration should be performed by comparing the specimen temperature and the test solution temperature at 10°C intervals or less in the temperature range in question.
A1.4.2The specimen temperature calibration formula should be calculated based on a linear order regression analysis.
A1.4.3The calibration shall be performed under identical conditions to a real CPT test except that no control of the electrochemical potential of the specimen is required.It is recommended to standardize the specimen geometry and size in order to avoid large variations in the specimens heat capacity,which may influence the accuracy of the calibration. A1.4.4The specimen temperature shall be measured by installing a thermistor or similar device inside the specimen. The thermistor shall be located as close to the surface in contact with the solution as possible and at the same time centrally located relative to the specimen geometry,that is,the thermistor should be located on the shortest line between the center of the specimen and the exposed surface,but still as close to the surface as possible.
A1.4.5Thefinal accuracy of the temperature measurement of the specimen during the calibration should be60.4°C.
8Supporting data are available from ASTM Headquarters.Request RR:G01-1017.
TABLE1Precision
Material,
UNS No.
Average
CPT,°C
Repeatability
Standard
Deviation,
s r
Reproducibility
Standard
Deviation,s R
95%
Repeatability
Limit,r
95%
Reproducibility
Limit,R
S3160019.5  1.9  2.8  5.47.9
S3180351.9  1.7  2.4  4.9  6.7
N0890454.1  3.7  3.711.311.3
S3125482.7  3.4  6.69.418.5A
A See Note
8.