ASTM D2674-72(1998)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D 2674 – 72 (Reapproved 1998)
Standard Methods of
Analysis of Sulfochromate Etch Solution Used in Surface
Preparation of Aluminum
This standard is issued under the fixed designation D 2674; 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.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 These methods offer a means for controlling the effec-
bility of regulatory limitations prior to use.
tiveness of the etchant which is normally used for preparing the
surface of aluminum alloys for subsequent adhesive bonding.
2. Purity of Reagents and Water
As the etchant reacts with the aluminum, hexavalent chromium
2.1 Purity of Reagents—Reagent grade chemicals shall be
is converted to trivalent chromium; a measure of the two and
used in all tests. Unless otherwise indicated, it is intended that
the difference can be used to determine the amount of dichro-
all reagents shall conform to the specifications of the Commit-
mate used.
tee on Analytical Reagents of the American Chemical Society,
1.2 The sulfochromate solution can be replenished by re-
where such specifications are available. Other grades may be
storing the sodium dichromate and the sulfuric acid to the
used, provided it is first ascertained that the reagent is of
original formulation levels. The lower limit of usefulness will
sufficiently high purity to permit its use without lessening the
vary depending upon solution storage, adhesives used, critical
accuracy of the determination.
nature of bond capability, variety of metals processed, etc. and
2.2 Purity of Water—Unless otherwise indicated, references
should be determined. Replenishment will be limited to the
to water shall be understood to mean reagent water conforming
number of times the chemical ingredients can be restored and
to ASTM Specification D 1193, for Reagent Water.
maintained to the required levels and should be determined by
the user. Sludge collecting in the bottom of a tank should be
METHOD A
minimized by periodic removal of sludge. For some applica-
tions, the hexavalent chromium should not fall below the
3. Apparatus
trivalent chromium content.
3.1 pH Meter.
1.3 A typical aqueous sulfochromate acid etch solution
formulation is as follows: 4. Reagents
Sodium dichromate (Na Cr O ) 4.5 oz/gal (33.7 g/litre)
2 2 7
4.1 Methyl Orange Indicator Solution—Prepare in accor-
Sulfuric acid (sp gr 1.84) 45 oz/gal (337.1 g/litre)
dance with ASTM Practices E 50, for Apparatus, Reagents, and
1.4 Maintenance of the aqueous concentrations is suggested
Safety Precautions for Chemical Analysis of Metals.
as follows by appropriate additions of sodium dichromate or
4.2 Sodium Hydroxide, Standard Solution (1.0 N)—
sulfuric acids.
Dissolve 40 g of sodium hydroxide (NaOH) in 1 liter of water
Sodium dichromate 3.0 to 6.0 oz/gal (22.5 to 44.9 g/litre) and standardize.
Sulfuric acid 40 to 50 oz/gal (299.6 to 374.5 g/litre)
5. Procedure
1.5 Method A is intended for measuring the sulfuric acid
5.1 Pipet a 5-mL sample of sulfochromate etch solution into
content of a sulfochromate solution. Method B is intended for
a 250-mL beaker. Add 100 mL of water. Titrate the sample with
measuring the hexavalent and trivalent Chromium content of a
the NaOH solution to a pH of 3.5, using a pH meter.
sulfochromate solution. Method C is intended as an alternative
5.2 An alternative method may be used whereby, instead of
method for measuring the hexavalent and trivalent Chromium
the pH meter, 1 drop of methyl orange indicator solution is
content of a sulfochromate solution.
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
“Reagent Chemicals, American Chemical Society Specifications,” Am. Chemi-
cal Soc., Washington, D.C. For suggestions on the testing of reagents not listed by
These methods are under the jurisdiction of ASTM Committee D-14 on the American Chemical Society, see “Reagent Chemicals and Standards,” by Joseph
Adhesives and are the direct responsibility of Subcommittee D14.80 on Metal Rosin, D. Van Nostrand Co., Inc., New York, N. Y., and the “United States
Bonding Adhesives. Pharmacopeia.”
Current edition approved April 3, 1972. Published May 1972. Originally Annual Book of ASTM Standards, Vol 11.01.
published as D 2674–67 T. Last previous edition November 1967. Annual Book of ASTM Standards, Vol 03.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 2674
added and, the titration continued until the color of the solution with the standard ferrous ammonium sulfate solution (using 1
loses all trace of orange. drop of the ferrous complex indicator) (Note 1) until the
indicator turns pink and then add about 5 ml of the standard
6. Calculation
ferrous ammonium sulfate solution in excess. Record the total
6.1 Calculate the sulfuric acid content of the sulfochromate
volume of the standard ferrous solution added. Back-titrate
solution, A in oz/gal, B in g/litre, as follows:
with the potassium permanganate solution until the color of the
solution becomes very slightly green.
A 5 1.3 VN (1)
B 5 9.81 VN (2)
NOTE 1—Barium diphenylamine sulfonate indicator may be used if
preferred. This indicator gives a purple to green end point. If performed
where:
with care, back titration is not necessary.
V 5 milliliters of sodium hydroxide solution, and
NOTE 2—Upon reaching the end point, the solution will be colorless,
N 5 normality of sodium hydroxide solution. then it will carry over by itself to a slightly green color. Any violet color
will indicate the back titration has gone over the end point.
6.2 Replenish the sulfuric acid content of the solution as
follows:
8.2 Record the amount of potassium permanganate used to
reach this end point. Use an average of three determinations in
W 5 ~A 2 A!V (3)
1 1 1
the calculation.
W 5 ~B 2 B!V (4)
2 1 2
9. Calculation
where:
W 5 weight of sulfuric acid (H SO , sp gr 1.84) to be
1 2 4 9.1 Calculate the hexavalent chromium as sodium dichro-
added to solution, oz,
mate (Na Cr O ·2H O)
...