1、Designation: C 743 87 (Reapproved 2004)e1Standard Test Method forContinuity of Porcelain Enamel Coatings1This standard is issued under the fixed designation C 743; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last re
2、vision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTEKeywords were added editorially in September 2004.1. Scope1.1 Porcelain enamel coatings are applied to metals toprovide protection
3、 from corrosion as well as to enhance theirappearance. This test method permits the easy detection ofdiscontinuities and areas of light coverage, not readily seen byvisual inspection, which limit the protection to the substrate.Somewhat similar tests applicable to the thicker glass coatingsused for
4、chemical reaction vessels are found in Test MethodsC 536 and C 537.1.2 Values stated in SI units are to be regarded as thestandard. Inch-pound units are provided for information only.1.3 This standard does not purport to address all of thesafety problems, if any, associated with its use. It is there
5、sponsibility 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 Documents2.1 ASTM Standards:2C 536 Test Method for Continuity of Coatings in GlassedSteel Equipment by Electrical Test
6、ingC 537 Test Method for Reliability of Glass Coatings onGlassed Steel Reaction Equipment by High Voltage3. Terminology3.1 air-gap voltagevoltage needed to arc through a defectthat is open to both the surface of the porcelain enamel and themetal substrate. The length of the air gap is estimated byme
7、asuring the enamel thickness.3.2 overvoltagedifference between an applied test voltageand the air-gap voltage for the porcelain enamel being tested.3.3 RMS voltageroot mean square voltage, or “average”voltage, shown on ac test meters.3.4 peak voltagefor ac meters, the voltage that willdischarge acro
8、ss the air gap or across a defect. The peakvoltage is the RMS voltage multiplied by 1.414.4. Apparatus4.1 A nondestructive coating thickness gage with a range ofat least 0 to 0.5 mm in 0.03-mm increments (0 to 20 mils in1.0-mil increments).4.2 Either an ac or a dc high-voltage discharge test instru-
9、ment with a continuously variable output voltage over therange of at least 0 to 4 kV as indicated on a built-in voltmeter.The instrument shall be capable of being set to 0.1 kV and shallhave an insulated cable and probe and a means for indicating(light, meter, or bell) a voltage discharge. Before us
10、e in thistest, the high-voltage discharge test instrument shall be cali-brated in accordance with the procedure set forth in Annex A1.4.3 A test probe constructed of 100 mesh wire gauze asillustrated in Fig. 1.5. Test Specimens5.1 Porcelain enameled specimens of any size may be usedin this test. The
11、 enamel shall be removed from an edge or theback of the specimen to provide an electrical ground for the testinstrument. If the probe is wider than the specimen, the edgesof the specimen shall be masked with electrical insulating tapeto prevent discharges from occurring at the edges whereporcelain c
12、overage may be light.6. Conditioning6.1 Before testing, wash the specimens with a 1 weight %,solution of trisodium phosphate on a soft cellulose sponge,rinse with tap water, and then rinse with distilled water. Thendry the specimens thoroughly in air.7. Procedure7.1 Measuring the Enamel Thickness Me
13、asure the enamelthickness at a minimum of three locations on the specimen.Make these measurements to the nearest 0.03 mm (1.0 mil) in1This test method is under the jurisdiction of ASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.12 onMaterials
14、 for Porcelain Enamel and Ceramic-Metal Systems.Current edition approved Aug. 1, 2004. Published September 2004. Originallyapproved in 1973. Last previous edition approved in 1998 as C 743 87 (1998).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service
15、 at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.accordance with the operating instructions for
16、the thicknessgage. Use the maximum value obtained as the enamel thick-ness.7.2 Selecting the Test VoltageThe test voltage to be useddepends upon the purpose for conducting the test. The test canbe used to determine (1) the presence of defects open to boththe enamel surface and the base metal, ( 2) t
17、he location of areasof light coverage in addition to defects open to both the enamelsurface and the base metal, or (3) compliance with a specifi-cation. According to the users purpose for conducting the test,the following three instructions correspond respectively to (1),(2), or ( 3) above:7.2.1 Tes
18、t Method AFor Open Defects:7.2.1.1 Determine the air-gap voltage that corresponds tothe enamel thickness from the calibration curve described inthe annex. This value is the test voltage.7.2.1.2 Probe the specimen in accordance with 7.3.7.2.2 Test Method BFor Light Coverage and Open De-fects:7.2.2.1
19、Determine the air-gap voltage that corresponds tothe enamel thickness from the calibration curve described inthe annex.7.2.2.2 Arbitrarily select the overvoltage to be used. (In-creasingly smaller defects are located with this test as theovervoltage is increased. Therefore, confidence that warepassi
20、ng this test will have a long, corrosion-free service life isincreased as the overvoltage is increased.)7.2.2.3 Add the selected overvoltage to the air-gap voltage.This value is the test voltage.7.2.2.4 Probe the specimen in accordance with 7.3.7.2.3 Test Method CFor Compliance with aSpecificationSp
21、ecifications for continuity of coating will bein terms of d-c overvoltage or peak a-c overvoltage (these areidentical). Determination of the test voltage depends onwhether a d-c or an a-c test instrument is used.7.2.3.1 DC Test InstrumentsDetermine the test voltage inaccordance with 7.2.2, substitut
22、ing the specified overvoltagefor the selected overvoltage as in 7.2.2.2 and 7.2.2.3.7.2.3.2 AC Test InstrumentsDivide the specified overvolt-age by 1.414, then determine the test voltage as in 7.2.2,substituting the specified overvoltage divided by 1.414 for theselected overvoltage as in 7.2.2.2 and
23、 7.2.2.3.7.2.3.3 Probe the specimen as shown in 7.3.7.3 Probing the Specimens:7.3.1 Connect the ground wire of test instrument to the basemetal of the specimen.7.3.2 Turn on the test instrument.7.3.3 Set the output voltage (as indicated on the built-involtmeter) to the value selected in 7.2.7.3.4 Le
24、t the voltage stabilize for 15 s.7.3.5 Hold the probe with a light pressure at approximately30 to the surface of the specimen.7.3.6 Probe the specimen at a rate of 3 to 4 in./s (75 to 100mm/s).7.4 Locating Discontinuities:7.4.1 When the probe passes over a discontinuity, a sparkwill arc across the d
25、iscontinuity to the ground provided by thebase metal. This spark can be seen and heard. In addition, mostinstruments are equipped with an auxiliary light that flasheswhen a discontinuity is located. Some instruments also have anaudible signal or digital counter to aid in the detection ofdiscontinuit
26、ies.7.4.2 When a discontinuity is located, a failure in thecoating is present.FIG. 1 Test ProbeC 743 87 (2004)e128. Report8.1 The report shall include the following:8.1.1 Title of test, ASTM designation, and issue.8.1.2 Enamel thickness,8.1.3 Air-gap voltage,8.1.4 Method of selecting test voltage (T
27、est Method A, B, orC),8.1.5 Overvoltage used,8.1.6 Name and model of test equipment,8.1.7 Whether or not discontinuities were present,8.1.8 Type of specimen tested,8.1.9 Number of specimens tested, and8.1.10 Type of enamel tested.9. Precision and Bias9.1 No statement is made about either the precisi
28、on or biasof this test method since the result merely states whether thereis conformance to the criteria for success specified in theprocedure.10. Keywords10.1 continuity; glass coating; high voltage test; light cov-erage; porcelain enamel; porcelain enamel thicknessANNEX(Mandatory Information)A1. C
29、ALIBRATING THE TEST INSTRUMENTSA1.1 Even though each test instrument is equipped with abuilt-in voltmeter, the discharge voltage across a given air gapwill vary from instrument to instrument. Therefore, eachinstrument must be calibrated by determining its air-gapvoltage curve.A1.2 Specimens Required
30、Three specimens of each of 15enamels are required to determine the air-gap voltage for thetest instrument. These 15 enamels shall spread over the rangeof thicknesses from 0.1 to 0.5 mm (4 through 18 mils) asuniformly as possible. These specimens shall be prepared asoutlined in 4.1 and 5.1.A1.3 Proce
31、dure:A1.3.1 Puncturing the Enamel Coating:A1.3.1.1 Connect the ground wire of the test instrument tothe base metal of the specimen.A1.3.1.2 Turn on the test instrument.A1.3.1.3 Raise the output voltage of the test instrument toits maximum and allow it to stabilize for 15 s.A1.3.1.4 Hold the probe wi
32、th a light pressure at approxi-mately 30 to the surface of the specimen.A1.3.1.5 Probe the specimen at a rate of 75 to 100 cm/s (3to 4 in./s).A1.3.1.6 Discard all specimens that are not punctured bythis treatment.A1.3.2 Reprobing the Specimens:A1.3.2.1 Reprobe the specimens that were punctured by th
33、ehigh-voltage probe in a manner similar to that outlined inA1.3.1.1 through A1.3.1.4, except that the test voltage shall be100 V initially and shall be increased in increments of 100 Vuntil a discharge occurs. A15-s stabilization period shall beobserved for each increase in voltage before reprobing
34、the testspecimens. This procedure may be expedited by starting thereprobing of duplicate specimens at 300 Volts lower than thebreakdown voltage noted for the first reprobed specimen, thenincreasing the voltage by increments of 100 V. The dischargevoltage determined in this manner is defined as the a
35、ir-gapvoltage.A1.4 Measuring the Enamel ThicknessMeasure theenamel thickness at three locations on the surface of thespecimen. Make these measurements close to the area wherethe discharge occurred. Make these measurements to thenearest 0.025 mm (1.0 mil) in accordance with the operatinginstructions
36、for the thickness gage. Average the three measure-ments to give the average enamel thickness which is assumedequivalent to the length of the air gap.A1.5 CalculationsCalculate the average enamel thick-ness and air-gap voltage for each specimen. Plot these averagesand fit the least squares line (of t
37、he form y=a+bx) throughthese averages as follows:a 5 (x2(y 2 (x(xy!/N(x22 (x!2# (A1.1)b 5 N(xy 2 (x(y!/N(x22 (x!2#where:a = intercept,b = slope,N = number of specimens,x = average thickness of an enamel, mils, and average airgap voltage of an enamel, kV.The resulting curve is the calibration curve.N
38、OTE A1.1The equations given here are for hand computation ordesk calculator use only. If the least squares line is fitted by digitalcomputer, any accurate library program may be used.C 743 87 (2004)e13ASTM International takes no position respecting the validity of any patent rights asserted in conne
39、ction 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 infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsi
40、ble 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 standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful co
41、nsideration 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 your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100
42、 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).C 743 87 (2004)e14
