1、Designation: C 743 87 (Reapproved 2009)Standard 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 revi
2、sion. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 Porcelain enamel coatings are applied to metals toprovide protection from corrosion as well as to enhance theirappearance. This
3、 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 chemical reaction vessels are found in Test MethodsC 536 an
4、d 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 theresponsibility of the user of this standard to establish appr
5、o-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 TestingC 537 Test Method for Reliability of Glass Coatings onGl
6、assed 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 bymeasuring the enamel thickness.3.2 overvoltagedifference betw
7、een 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 across the air gap or across a defect. The peakvoltage is the R
8、MS 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-ment with a continuously variable output voltage over thera
9、nge 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 use in thistest, the high-voltage discharge test instrument s
10、hall 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 enamel shall be removed from an edge or theback of the spe
11、cimen 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 coverage may be light.6. Conditioning6.1 Before testing, was
12、h 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 Measure the enamelthickness at a minimum of three locations o
13、n the specimen.Make these measurements to the nearest 0.03 mm (1.0 mil) in1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.12 onMaterials for Porcelain Enamel and Ceramic-Metal Systems.Current edit
14、ion approved April 15, 2009. Published June 2009. Originallyapproved in 1973. Last previous edition approved in 2004 as C 743 87 (2004)1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume i
15、nformation, 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 the thicknessgage. Use the maximum value obtained as the ename
16、l 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) the location of areasof light coverage in addition to defects o
17、pen 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 Test Method AFor Open Defects:7.2.1.1 Determine the air-gap volta
18、ge 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 Determine the air-gap voltage that corresponds tothe enamel th
19、ickness 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 warepassing this test will have a long, corrosion-free service life isi
20、ncreased 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 aSpecificationSpecifications for continuity of coating will bein terms of d-c
21、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, substituting the specified overvoltagefor the selected overvoltage as i
22、n 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 7.2.2.3.7.2.3.3 Probe the specimen as shown in 7.3.7.3 Probin
23、g 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 Let the voltage stabilize for 15 s.7.3.5 Hold the probe with a l
24、ight 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 discontinuity to the ground provided by thebase metal. This spa
25、rk 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 ofdiscontinuities.7.4.2 When a discontinuity is located, a failure in thecoa
26、ting is present.FIG. 1 Test ProbeC 743 87 (2009)28. 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 (Test MethodA, B, orC),8.1.5 Overvoltage used,8.1.6 Name and model
27、 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 precision or biasof this test method since the result merely states whet
28、her 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. CALIBRATING THE TEST INSTRUMENTSA1.1 Even though each test instrum
29、ent 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 RequiredThree specimens of each of 15enamels are required to determine th
30、e 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 Procedure:A1.3.1 Puncturing the Enamel Coating:A1.3.1.1 Connect the gr
31、ound 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 with a light pressure at approxi-mately 30 to the surface of the sp
32、ecimen.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 thehigh-voltage probe in a manner similar to that outlined inA1.3.1
33、.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 the testspecimens. This procedure may be expedited by starting th
34、ereprobing 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 air-gapvoltage.A1.4 Measuring the Enamel ThicknessMeasure theename
35、l 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 for the thickness gage. Average the three measure-ments to give t
36、he 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 the form y=a+bx) throughthese averages as follows:a 5 (x2(y 2 (x(x
37、y!/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.NOTE A1.1The equations given here are for hand computation ordesk
38、calculator use only. If the least squares line is fitted by digitalcomputer, any accurate library program may be used.C 743 87 (2009)3ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standa
39、rd 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 responsible technical committee and must be reviewed every five years andif
40、 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 consideration at a meeting of theresponsible technical committee, whi
41、ch 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 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 (2009)4
