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ASTM C743-87(2004)e1

(Test Method)

Standard Test Method for Continuity of Porcelain Enamel Coatings

Standard Test Method for Continuity of Porcelain Enamel Coatings

ABSTRACT
This test method provides for easy detection of discontinuities and areas of light coverage that are not readily seen by visual inspection on corrosion protective porcelain enamel coatings. These two issues limit the protection to the metal substrate. Apparatus includes a nondestructive coating thickness gage, either an ac or a dc high-voltage discharge test instrument with a continuously variable output voltage, and a mesh wire gauze test probe. This test uses porcelain enameled glass coating specimens of any size which should be removed from an edge or the back of a specimen and then properly conditioned as specified. If the probe is wider than the specimen, the specimen edges should be masked using electrical insulating tape.
SCOPE
1.1 Porcelain enamel coatings are applied to metals to provide protection from corrosion as well as to enhance their appearance. This test method permits the easy detection of discontinuities and areas of light coverage, not readily seen by visual inspection, which limit the protection to the substrate. Somewhat similar tests applicable to the thicker glass coatings used for chemical reaction vessels are found in Test Methods C 536 and C 537.
1.2 Values stated in SI units are to be regarded as the standard. Inch-pound units are provided for information only.
This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

Status
Historical
Publication Date
31-Jul-2004
ICS
25.220.50 - Enamels
Technical Committee
B08 - Metallic and Inorganic Coatings
Drafting Committee
B08.12 - Materials for Porcelain Enamel and Ceramic-Metal Systems
Current Stage
Ref Project

Relations

Replaces
ASTM C743-87(1998) - Standard Test Method for Continuity of Porcelain Enamel Coatings
Effective Date
01-Aug-2004
Replaced By
ASTM C743-87(2009) - Standard Test Method for Continuity of Porcelain Enamel Coatings
Effective Date
15-Apr-2009
Referred By
ASTM C286-22 - Standard Terminology Relating to Porcelain Enamel and Ceramic-Metal Systems
Effective Date
01-Aug-2004

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ASTM C743-87(2004)e1 - Standard Test Method for Continuity of Porcelain Enamel CoatingsASTM C743-87(2004)e1 - Standard Test Method for Continuity of Porcelain Enamel Coatings
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ASTM C743-87(2004)e1 - Standard Test Method for Continuity of Porcelain Enamel Coatings
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
e1
Designation:C743–87(Reapproved2004)
Standard Test Method for
Continuity of Porcelain Enamel Coatings
This standard is issued under the fixed designation C 743; 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.
e NOTE—Keywords were added editorially in September 2004.
1. Scope 3.3 RMS voltage—root mean square voltage, or “average”
voltage, shown on ac test meters.
1.1 Porcelain enamel coatings are applied to metals to
3.4 peak voltage—for ac meters, the voltage that will
provide protection from corrosion as well as to enhance their
discharge across the air gap or across a defect. The peak
appearance. This test method permits the easy detection of
voltage is the RMS voltage multiplied by 1.414.
discontinuities and areas of light coverage, not readily seen by
visual inspection, which limit the protection to the substrate.
4. Apparatus
Somewhat similar tests applicable to the thicker glass coatings
4.1 Anondestructive coating thickness gage with a range of
used for chemical reaction vessels are found in Test Methods
at least 0 to 0.5 mm in 0.03-mm increments (0 to 20 mils in
C 536 and C 537.
1.0-mil increments).
1.2 Values stated in SI units are to be regarded as the
4.2 Either an ac or a dc high-voltage discharge test instru-
standard. Inch-pound units are provided for information only.
ment with a continuously variable output voltage over the
1.3 This standard does not purport to address all of the
range of at least 0 to 4 kV as indicated on a built-in voltmeter.
safety problems, if any, associated with its use. It is the
Theinstrumentshallbecapableofbeingsetto0.1kVandshall
responsibility of the user of this standard to establish appro-
have an insulated cable and probe and a means for indicating
priate safety and health practices and determine the applica-
(light, meter, or bell) a voltage discharge. Before use in this
bility of regulatory limitations prior to use.
test, the high-voltage discharge test instrument shall be cali-
2. Referenced Documents brated in accordance with the procedure set forth inAnnexA1.
4.3 A test probe constructed of 100 mesh wire gauze as
2.1 ASTM Standards:
illustrated in Fig. 1.
C 536 Test Method for Continuity of Coatings in Glassed
Steel Equipment by Electrical Testing
5. Test Specimens
C 537 Test Method for Reliability of Glass Coatings on
5.1 Porcelain enameled specimens of any size may be used
Glassed Steel Reaction Equipment by High Voltage
in this test. The enamel shall be removed from an edge or the
3. Terminology backofthespecimentoprovideanelectricalgroundforthetest
instrument. If the probe is wider than the specimen, the edges
3.1 air-gap voltage—voltage needed to arc through a defect
of the specimen shall be masked with electrical insulating tape
that is open to both the surface of the porcelain enamel and the
to prevent discharges from occurring at the edges where
metal substrate. The length of the air gap is estimated by
porcelain coverage may be light.
measuring the enamel thickness.
3.2 overvoltage—difference between an applied test voltage
6. Conditioning
and the air-gap voltage for the porcelain enamel being tested.
6.1 Before testing, wash the specimens with a 1 weight %,
solution of trisodium phosphate on a soft cellulose sponge,
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
rinse with tap water, and then rinse with distilled water. Then
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.12 on
dry the specimens thoroughly in air.
Materials for Porcelain Enamel and Ceramic-Metal Systems.
Current edition approved Aug. 1, 2004. Published September 2004. Originally
7. Procedure
approved in 1973. Last previous edition approved in 1998 as C 743 – 87 (1998).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
7.1 Measuring the Enamel Thickness—Measuretheenamel
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
thickness at a minimum of three locations on the specimen.
Standards volume information, refer to the standard’s Document Summary page on
Make these measurements to the nearest 0.03 mm (1.0 mil) in
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
e1
C743–87 (2004)
FIG. 1 Test Probe
accordance with the operating instructions for the thickness in terms of d-c overvoltage or peak a-c overvoltage (these are
gage. Use the maximum value obtained as the enamel thick- identical). Determination of the test voltage depends on
ness. whether a d-c or an a-c test instrument is used.
7.2 Selecting the Test Voltage—The test voltage to be used 7.2.3.1 DC Test Instruments—Determine the test voltage in
depends upon the purpose for conducting the test. The test can accordance with 7.2.2, substituting the specified overvoltage
be used to determine (1) the presence of defects open to both for the selected overvoltage as in 7.2.2.2 and 7.2.2.3.
theenamelsurfaceandthebasemetal,( 2)thelocationofareas 7.2.3.2 AC Test Instruments—Divide the specified overvolt-
of light coverage in addition to defects open to both the enamel age by 1.414, then determine the test voltage as in 7.2.2,
surface and the base metal, or (3) compliance with a specifi- substituting the specified overvoltage divided by 1.414 for the
cation.According to the user’s purpose for conducting the test, selected overvoltage as in 7.2.2.2 and 7.2.2.3.
the following three instructions correspond respectively to (1), 7.2.3.3 Probe the specimen as shown in 7.3.
(2), or ( 3) above: 7.3 Probing the Specimens:
7.2.1 Test Method A—For Open Defects: 7.3.1 Connect the ground wire of test instrument to the base
7.2.1.1 Determine the air-gap voltage that corresponds to metal of the specimen.
the enamel thickness from the calibration curve described in 7.3.2 Turn on the test instrument.
the annex. This value is the test voltage. 7.3.3 Set the output voltage (as indicated on the built-in
7.2.1.2 Probe the specimen in accordance with 7.3. voltmeter) to the value selected in 7.2.
7.2.2 Test Method B—For Light Coverage and Open De- 7.3.4 Let the voltage stabilize for 15 s.
fec
...

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SCOPE
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1.1 These test methods cover accelerated determination of the resistance of porcelain enamel coatings on aluminum alloys to spalling from exposure to moisture or weathering. Test Method A,2 using a 5 % solution of ammonium chloride, requires 96-h immersion while Test Method B,3 using a 1 % solution of antimony trichloride, is completed after 20 h of immersion. The spalling tendency is evaluated by the same criteria in both methods. While either method is suitable for magnesium silicon alloys, such as 6061, Test Method B is preferred for simple alloys or commercially pure aluminum, such as 1100.  
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4.1 When a porcelain enamel is first subjected to abrasion of the type involved in these tests, the rate of wear or attrition is relatively low. As the enamel is subjected to continued abrasion, the rate of wear increases until it reaches a steady value. Thereafter, the rate of wear remains almost constant until the enamel is penetrated and the underlying ground coat or metal exposed. The abrasion that occurs during the period of increasing rate-of-weight loss is defined as surface abrasion and results in reduced gloss and cleanability with high-gloss enamels and a modification of color, appearance, or surface texture, or combination thereof with low-gloss enamels. The abrasion that occurs during the period of steady rate-of-weight loss is defined as subsurface abrasion and results in the destruction of the continuity of the coating. These two types of abrasion are not necessarily proportional, and since it is desirable to be able to determine the resistance of porcelain enamel to both types of abrasion, it is necessary to deal with each one separately.
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5.1 The results of this test indicate whether or not a sheet steel will cause reboiling under one given set of enameling conditions and enable the enamel processor to establish a possible source of imperfections in the finished coating.  
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