ASTM B457-67(2008)e1
(Test Method)Standard Test Method for Measurement of Impedance of Anodic Coatings on Aluminum
Standard Test Method for Measurement of Impedance of Anodic Coatings on Aluminum
ABSTRACT
This test method describes the conditions and equipment for measuring the impedance of anodic coatings on aluminum. Such measurements have been used to evaluate the quality of seal of an anodic coating. The test method does not prescribe the procedure for producing the anodic coating, nor the postanodizing treatment usually described as "sealing." This test method is applicable to the rapid, nondestructive testing of anodic coatings. The interpretation of results and correlation of data with service experience and other tests are not within the scope of this test method. The impedance of anodic coatings is defined in terms of resistance and capacitive reactance. The apparatus to be used shall have the characteristics specified for the following: (1) impedance bridge range, frequency, accuracy, and voltage, (2) cell components such as glass tube, rubber gasket or seal, rubber stopper, and platinum or Type 300 stainless steel counter electrode, (3) wire leads, (4) support strand, (5) glass or stainless steel tank, and (6) container for cell standardization. The conditions for the test specimen and electrolyte, and the procedure of measurement, cell standardization, and temperature correction are detailed. The test cell construction and test set-up are illustrated.
SCOPE
1.1 This test method describes the conditions and equipment for measuring the impedance of anodic coatings on aluminum. Such measurements have been used to evaluate the quality of seal of an anodic coating. The test method does not prescribe the procedure for producing the anodic coating, nor the postanodizing treatment usually described as “sealing.”
1.2 This test method is applicable to the rapid, nondestructive testing of anodic coatings. The interpretation of results and correlation of data with service experience and other tests are not within the scope of this test method.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, 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.
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Designation: B457 − 67 (Reapproved 2008)
StandardTest Method for
Measurement of Impedance of Anodic Coatings on
Aluminum
This standard is issued under the fixed designation B457; 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 (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—The units statement in subsection 1.3 was corrected editorially in April 2008.
1. Scope
R = resistance,
X = capacitive reactance,
1.1 Thistestmethoddescribestheconditionsandequipment c
f = frequency, and
for measuring the impedance of anodic coatings on aluminum.
C = capacitance.
Such measurements have been used to evaluate the quality of
seal of an anodic coating. The test method does not prescribe 2.2 Both resistance and capacitance are associated with a
the procedure for producing the anodic coating, nor the material constant and a geometric factor:
postanodizing treatment usually described as “sealing.”
R 2 r ~l/A! (3)
1.2 This test method is applicable to the rapid, nondestruc-
where:
tive testing of anodic coatings.The interpretation of results and
R = resistance,
correlation of data with service experience and other tests are
r = specific resistance,
not within the scope of this test method.
l = length of conductor, and
1.3 The values stated in SI units are to be regarded as the
A = area of cross section of conductor.
standard. The values given in parentheses are for information
C 5 e~A/l! (4)
only.
where:
1.4 This standard does not purport to address all of the
C = capacitance,
safety concerns, if any, associated with its use. It is the
e = dielectric constant,
responsibility of the user of this standard to establish appro-
A = area of opposing plates of capacitor, and
priate safety and health practices and determine the applica-
l = distance between plates.
bility of regulatory limitations prior to use.
2.3 For measuring impedance of anodic coatings, the mate-
2. Nature of Test
rial constants are partly associated with the anodic coating and
partly with the electrolyte employed in the test cell. For
2.1 Impedanceisanelectricalcharacteristicdescribedasthe
simplification these constants are assumed not to vary. The
total opposition of all circuit elements to the flow of alternating
variations in measured impedance then depend directly on
current. Inductive effects of anodic coatings are negligible and
geometric factors.
the impedance is presumed to be made up of resistance and
capacitive reactance according to the following relationships:
3. Apparatus
2 2
Z 5 =R 1X (1)
c
3.1 Impedance Bridges—Commercially available a-c im-
pedance bridges with the following characteristics are satisfac-
and
tory:
X 51/2πfC (2)
c
3.1.1 Range—1 µF (1000 nF) to 0.0001 µ F (0.1 nF) full
where: scale with a dissipation factor of 0 to 2.
Z = impedance,
TheAlcoaImpedanceTestforAnodicCoatings(AZTAC)evaluatesa0.129-cm
2 (0.02-in. ) test area and expresses impedance in kilohms. Instructions are available
This test method is under the jurisdiction ofASTM Committee B08 on Metallic from Alcoa Process Development Laboratories, P.O. Box 2970, Pittsburgh, PA
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.07 on 15230.
Conversion Coatings. The Z-Scope, formerly manufactured by Twin City Testing Corp., Tonawanda,
Current edition approved April 1, 2008. Published April 2008. Originally NY was designed to measure the impedance of anodic coatings on aluminum.
2 2
approved in 1967. Last previous edition approved in 2003 as B457 – 67 (2003). AZTAC values (the impedance of a 0.129-cm (0.02-in. ) test area) can be read
DOI: 10.1520/B0457-67R08E01. directly in kilohms with this instrument.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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B457 − 67 (Reapproved 2008)
3.4 Support Strand, complete with a utility clamp to hold
the electrolytic test cell.
3.5 Glass or Stainless Steel Tank containing pure boiling
water is required for conditioning the test specimen.
3.6 Container for Cell Standardization—A150 to 500-mL
2 6
Type 300 stainless steel beaker or standard 1-in. test cell may
be used.
4. Electrolyte
4.1 The electrolyte shall be a 3 ⁄2 % solution of sodium
chloride prepared by dissolving 36 g of the salt in 1 L of
distilled or deionized water.
5. Test Specimen
5.1 The area to be tested should be sufficiently large to
accommodate the test cell, sufficiently flat to prevent leakage
of the solution from the probe, and free from mechanical
defects such as scratches. After an anodic coating has been
sealed, moisture is lost from the coating thus producing an
increase in impedance. The rate at which moisture is lost
FIG. 1 Diagram of Test Cell Construction
increases as the relative humidity decreases; therefore, the
relative humidity of the testing environment should be con-
trolled and never below 25 %. Specimens should be tested
within 1 h after the anodic coating has been sealed. If this
cannot be done or if the part has been subjected to an elevated
temperature drying treatment, the test area should be condi-
tioned as follows:
5.1.1 Wipe the test area with acetone, methylethylketone,
trichloroethylene, or similar solvent.
5.1.2 Immerse the test area in pure boiling water for 30 s.
5.1.3 Rinse in cold water.
FIG. 2 Diagram of Test Set-Up
5.1.4 Blot dry and test without further delay.
6. Procedure
6.1 Penetrate the anodic coating in a small area with a sharp
3.1.2 Frequency—1000 Hz.
instrument and connect the lead from the low-impedance
3.1.3 Accuracy—1%.
terminal of the bridge to the exposed aluminum substrate.
3.1.4 Voltage—Adjustable so as to be not greater than 0.5 V
as applied to the test specimen.
6.2 Connect the other lead from the bridge to the counter
electrode of the test cell. Be certain the lead wires are not
3.2 Cell (Fig. 1) for containing the electrolyte during test. It
twisted around each other. Turn on the bridge and allow it to
may be constructed from the following components:
warm up.
3.2.1 Glass Tube, 9 mm (0.35 in.)
...
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