1、Designation: B 457 67 (Reapproved 2003)Standard Test Method forMeasurement of Impedance of Anodic Coatings onAluminum1This standard is issued under the fixed designation B 457; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method describes the conditions and equipmentfor measuring the impedance of anodic coatings on alumin
3、um.Such measurements have been used to evaluate the quality ofseal of an anodic coating. The test method does not prescribethe procedure for producing the anodic coating, nor thepostanodizing treatment usually described as “sealing.”1.2 This test method is applicable to the rapid, nondestruc-tive te
4、sting of anodic coatings. The interpretation of results andcorrelation of data with service experience and other tests arenot within the scope of this test method.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the
5、user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Nature of Test2.1 Impedance is an electrical characteristic described as thetotal opposition of all circuit elements to the flow of alternatingcurren
6、t. Inductive effects of anodic coatings are negligible andthe impedance is presumed to be made up of resistance andcapacitive reactance according to the following relationships:Z 5 =R21 Xc2(1)andXc5 1/2 p fC (2)where:Z = impedance,R = resistance,Xc= capacitive reactance,f = frequency, andC = capacit
7、ance.2.2 Both resistance and capacitance are associated with amaterial constant and a geometric factor:R 2 r l/A!(3)where:R = resistance,r = specific resistance,l = length of conductor, andA = area of cross section of conductor.C 5 eA/l!(4)where:C = capacitance,e = dielectric constant,A = area of op
8、posing plates of capacitor, andl = distance between plates.2.3 For measuring impedance of anodic coatings, the mate-rial constants are partly associated with the anodic coating andpartly with the electrolyte employed in the test cell. Forsimplification these constants are assumed not to vary. Thevar
9、iations in measured impedance then depend directly ongeometric factors.23. Apparatus3.1 Impedance BridgesCommercially available a-c im-pedance bridges with the following characteristics are satisfac-tory:33.1.1 Range1 F (1000 nF) to 0.0001 F (0.1 nF) fullscale with a dissipation factor of 0 to 2.3.1
10、.2 Frequency1000 Hz.3.1.3 Accuracy1 %.3.1.4 VoltageAdjustable so as to be not greater than 0.5 Vas applied to the test specimen.1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 onChromate Conver
11、sion Coatings.Current edition approved Feb. 10, 2003. Published May 2003. Originallyapproved in 1967. Last previous edition approved in 1998 as B 457 67 (1998).2TheAlcoa Impedance Test forAnodic Coatings (AZTAC) evaluates a 0.129-cm2 (0.02-in.2) test area and expresses impedance in kilohms. Instruct
12、ions are availablefrom Alcoa Process Development Laboratories, P.O. Box 2970, Pittsburgh, PA15230.3The Z-Scope, formerly manufactured by Twin City Testing Corp., Tonawanda,NY was designed to measure the impedance of anodic coatings on aluminum.AZTAC values (the impedance of a 0.129-cm2(0.02-in.2) te
13、st area) can be readdirectly in kilohms with this instrument.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2 Cell (Fig. 1) for containing the electrolyte during test. Itmay be constructed from the following components:43.2.1 Glas
14、s Tube, 9 mm (0.35 in.) in outside diameter by 9cm (3.54 in.).3.2.2 Rubber Gasket or Seal.53.2.3 Rubber StopperAny convenient size with a hole incenter to accommodate the glass tube.3.2.4 Platinum or Type 300 Stainless Steel Counter Elec-trode, 9 cm (3.54 in.) long by 0.75 to 1.0 mm (0.030 to 0.040i
15、n.) diameter wire.3.3 Wire Leads, two, insulated, approximately 1 m (40 in.)long complete with C-clamps or alligator clips shall be used formaking connections to the test specimen and cell electrode.3.4 Support Strand, complete with a utility clamp to holdthe electrolytic test cell.3.5 Glass or Stai
16、nless Steel Tank containing pure boilingwater is required for conditioning the test specimen.3.6 Container for Cell Standardization A 150 to 500-mLType 300 stainless steel beaker or standard 1-in.2test cell6maybe used.4. Electrolyte4.1 The electrolyte shall be a 312 % solution of sodiumchloride prep
17、ared by dissolving 36 g of the salt in 1 L ofdistilled or deionized water.5. Test Specimen5.1 The area to be tested should be sufficiently large toaccommodate the test cell, sufficiently flat to prevent leakageof the solution from the probe, and free from mechanicaldefects such as scratches. After a
18、n anodic coating has beensealed, moisture is lost from the coating thus producing anincrease in impedance. The rate at which moisture is lostincreases as the relative humidity decreases; therefore, therelative humidity of the testing environment should be con-trolled and never below 25 %. Specimens
19、should be testedwithin 1 h after the anodic coating has been sealed. If thiscannot be done or if the part has been subjected to an elevatedtemperature drying treatment, the test area should be condi-tioned as follows:5.1.1 Wipe the test area with acetone, methylethylketone,trichloroethylene, or simi
20、lar solvent.5.1.2 Immerse the test area in pure boiling water for 30 s.5.1.3 Rinse in cold water.5.1.4 Blot dry and test without further delay.6. Procedure6.1 Penetrate the anodic coating in a small area with a sharpinstrument and connect the lead from the low-impedanceterminal of the bridge to the
21、exposed aluminum substrate.6.2 Connect the other lead from the bridge to the counterelectrode of the test cell. Be certain the lead wires are nottwisted around each other. Turn on the bridge and allow it towarm up.6.3 Place the test cell on the area to be tested and applyenough pressure, 490 to 1230
22、 MPa (71 to 178 psi) to preventleakage of the electrolyte (the cell is spring-loaded to ensuresufficient and constant pressure). Make certain the bottom ofthe gasket on the test cell is level with the test surface.6.4 Fill the test cell at least half full with electrolyte andcommence a 2-min soak pe
23、riod. The counter electrode shouldnot contact the test specimen.6.5 Turn on and obtain a preliminary “balance” of the bridgein accordance with the manufacturers instructions (not neces-sary if a direct-reading instrument is used).6.6 After the electrolyte has been in contact with the testsurface for
24、 a period of 2 min, obtain a final balance of thebridge and record the dissipation factor (tan d). Next, recordthe capacitance (in nanofarads) as indicated by either thebalancing series component (CS) or the balancing parallelcomponent ( CP).6.7 Calculate the impedance, Z, in kilohms, as follows:7Z
25、5 159/Cscos d (5)Cs5 Cp1 1 tan d !2#6.8 Different locations should be measured until at least twovalues are within 10 %. These are then averaged and multipliedby the correlation factor (See Section 7).7. Cell Standardization7.1 Impedance is inversely proportional to the test area. Theapparatus descr
26、ibed herein tests an area of 0.129 cm2(0.02in.2). Use of this relatively small test area facilitates themeasurement of small parts and curved surfaces. It may be4Another test cell satisfactory for use with this method is the one used with theKocour Electronic Thickness Tester Model 995 available fro
27、m Kocour Co., 4800 S.St. Louis Ave., Chicago, IL.5Type A seal available from Kocour Co. or Twin City Testing Corp., 2440Franklin Ave., St. Paul, MN, has been found satisfactory.6Available from Twin City Testing Corp.7Direct-reading instruments, such as the Z-Scope, are available, and the use ofsuch
28、equipment obviates mathematical computations (see 3.1).FIG. 1 Diagram of Test Cell ConstructionB 457 67 (2003)2desirable to modify this apparatus so that larger areas can bemeasured. Because this area may vary owing to variations incell gasket size and wear during use, the cell should bestandardized
29、 daily.7.2 Cell standardization is accomplished by measuring theimpedance on a 6.45-cm2(1-in.2) surface and comparing thismeasurement with results obtained with the cell. The 6.45-cm2(1-in.2) test surface must be free from surface scratches. Therequired area may be obtained by appropriately masking
30、alarger anodized specimen using wax of lacquer. Several appli-cations of the masking material will ensure against uncoatedareas, especially at edges. Standard 1-in.2test cells are avail-able and may be used as an alternative way of arriving at a1-in.2area. If this type cell is used beaker immersion
31、is notrequired.7.3 Immerse the specimen with the 6.45-cm2(1-in.2) area ofexposed anodic coating in a stainless steel beaker containingthe 312 % sodium chloride electrolyte. Connect one lead fromthe impedance bridge to the beaker, the other to an area ofexposed aluminum on the test specimen.7.4 Measu
32、re the impedance as outlined in Section 6, andrecord as A.7.5 Using the test cell, make at least two impedancemeasurements and average on this same area of the specimenused for standardization. Record this as B. If the two measure-ments are not within 10 %, an additional measurement shouldbe made an
33、d the two values within 10 % averaged.7.6 Obtain the correction factor as follows:Correction factor 5 50A/B (6)8. Temperature Correction8.1 Impedance decreases with increasing temperature. Ifmeasurements are made at widely varying temperatures, theyshould be adjusted to a constant, that is, 25C (77F
34、).Example:Z77 F5 ZT/1 0.011 T 77!#(7)where:ZT= impedance value recorded at ambient,T = ambient temperature, F, andZ77 F= corrected reading.9. Interpretation of Results9.1 The relationship between impedance value and qualityof the anodic coating shall be as agreed by the manufacturerand the purchaser
35、. A minimum acceptable value can beestablished for any anodic coating thickness being used.Generally, higher values should be obtained with thickeranodic coatings. However, an increase in coating thicknessdoes not compensate for a poor seal. Low values will beobtained regardless of coating thickness
36、 when the quality ofseal is poor.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof
37、 infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this s
38、tandard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake you
39、r views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).FIG. 2 Diagram of Test Set-UpB 457 67 (2003)3
