ISO/TR 16689:2012

Anodizing of aluminium and its alloys — Experimental research on possible alternative sealing quality test methods to replace the phosphoric acid/chromic acid immersion test — Evaluation of correlations

ISO/TR 16689:2012 contains data from an evaluation of candidates to replace the chromic/phosphoric acid solution (CPA) test for the quality of sealing of anodic oxidation coatings on aluminium.

Anodisation de l'aluminium et ses alliages — Recherche expérimentale sur les méthodes alternatives possibles d'essai de qualité d'étanchéité pour remplacer l'essai d'immersion dans l'acide phosphochromique — Évaluation des corrélations

General Information

• Status: Published
• Publication Date: 18-Apr-2012
• ICS: 25.220.20 - Surface treatment
• Technical Committee: ISO/TC 79/SC 2 - Organic and anodic oxidation coatings on aluminium
• Drafting Committee: ISO/TC 79/SC 2 - Organic and anodic oxidation coatings on aluminium
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 05-Sep-2018

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 16689
First edition
2012-05-01

Anodizing of aluminium and its alloys —
Experimental research on possible
alternative sealing quality test methods to
replace the phosphoric acid/chromic acid
immersion test — Evaluation of
correlations
Anodisation de l'aluminium et ses alliages — Recherche expérimentale
sur les méthodes alternatives possibles d'essai de qualité de colmatage
pour remplacer l'essai d'immersion dans l'acide phosphochromique —
Évaluation des corrélations




Reference number
ISO/TR 16689:2012(E)
©
ISO 2012

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ISO/TR 16689:2012(E)

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ii © ISO 2012 – All rights reserved

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ISO/TR 16689:2012(E)
Contents Page
Foreword . iv
Introduction . v
1  Scope . 1
2  Literature research . 2
2.1  General . 2
2.2  Acidified sulfite test (Kape test) . 2
2.3  Acetic acid/sodium acetate solution test . 2
2.4  Chromic/phosphoric acid solution test . 3
2.5  Sulfuric acid solution test. 4
3  Materials and experimental . 4
3.1  Anodizing . 4
3.2  Sealing . 4
3.3  Measurements of sealing quality . 5
4  Results . 7
4.1  Masking of cut surfaces . 7
4.2  Bare aluminium and dissolution in the dissolution tests . 7
4.3  Hot sealing . 7
4.4  Cold sealing (two step) . 13
4.5  Nickel-based medium temperature sealing . 17
4.6  Nickel-free medium temperature sealing . 21
5  Discussion . 24
6  Conclusion . 26
Annex A (Informative) Qualanod working group report . 27
Bibliography . 30

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ISO/TR 16689:2012(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 16689 was prepared by Technical Committee ISO/TC 79, Light metals and their alloys, Subcommittee
SC 2, Organic and anodic oxidation coatings on aluminium.
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ISO/TR 16689:2012(E)
Introduction
The chromic/phosphoric acid solution (CPA) test is the main test used internationally to assess the quality of
[1] [2]
sealing of anodic oxidation coatings on aluminium. The method is described in ISO 3210 , ASTM B680 ,
[3] [4] [5] [6]
EN 12373-6 and EN 12373-7 . ISO 7599 and EN 12373-1 designate it to be the referee test, as do the
[7] [8]
voluntary standards of Qualanod and the AAMA (American Architectural Manufacturers’ Association) .
The CPA test was originally proposed by two workers at Alcoa, J. H. Manhart and W. C. Cochran, in the early
[9]
1970s . They compared it for hot-water sealing with various simple laboratory tests including other acid
[10]
dissolution tests, some of which were in regular use at that time and were described in ISO 2932 . Since the
adoption of the CPA test, practical experience has revealed that low-coating mass loss is an indication of
good sealing quality and of the ability of the coating to resist staining and blooming in many types of service.
There is mounting concern in Europe over the use of this test because the test solution contains hexavalent
chromium [Cr(VI)] which is a human carcinogen via inhalation. Chromic acid was included, 2010-12-15, in The
European Chemicals Agency candidate list of substances of very high concern for authorization. Special
authorization will have to be obtained for the use of such substances in every application.
In 2007 Qualanod initiated a study to identify potential alternative tests. It was decided to restrict this to acid
dissolution tests because it was expected that they would behave in a manner most similar to the CPA test. A
list of criteria was drawn up for alternative tests to be assessed against. These criteria included ones that
would favour easy-to-use immersion tests. The technical literature was reviewed and a shortlist of tests
produced.
The next stage was to carry out experimental work to determine whether the alternative tests were
comparable to the CPA test for a range of sealing methods. Sapa Technology offered to undertake this project.
Sapa found that neither of the acid immersion tests evaluated were suitable alternatives to the CPA test. This
was because they responded very differently depending on the sealing method. It is believed that the
response of any immersion test is dependent on the solution composition. Sapa also found that the
admittance test was good at distinguishing sealing quality for all the sealing methods. However, admittance is
a property of the whole of the anodized coating whereas the CPA test is surface-specific, providing a
prediction of the likelihood of surface degradation during service.
[11]
This Technical Report contains an edited version of Sapa Technology technical report D09-0179 .
It is believed that future investigations should focus on finding a test method that will enable the prediction of
superficial, cosmetic degradation during exposure to the weather. This would not include the ability of an
anodized coating to protect the aluminium from pitting corrosion, which can already be assessed using a salt
spray test. Rather, it would assess the susceptibility to weathering effects such as staining, blooming, chalking,
resmutting and iridescence.

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TECHNICAL REPORT ISO/TR 16689:2012(E)

Anodizing of aluminium and its alloys — Experimental research
on possible alternative sealing quality test methods to replace
the phosphoric acid/chromic acid immersion test — Evaluation
of correlations
1 Scope
This Technical Report contains data from an evaluation of candidates to replace the chromic/phosphoric acid
solution (CPA) test for the quality of sealing of anodic oxidation coatings on aluminium.
Following a review by Qualanod (see Working Group report in Annex A), it was agreed with Sapa Technology
that the candidate tests for evaluation would be as follows:
[10]
 acetic acid/sodium acetate solution (AASA) test as described in ISO 2932 , a method used in the
1970s;
[9]
 sulfuric acid solution (SA) test as described by Manhart and Cochran .
[3]
The evaluation consists of a comparison of the candidates with the CPA (EN 12373-6 ), dye absorption
[12] [13]
(EN 12373-4 ) and admittance tests (EN 12373-5 ) using four different sealing methods:
 hot-water sealing;
 cold sealing;
 medium-temperature (midtemp) sealing using a nickel-containing solution;
 midtemp sealing using a nickel-free solution.
An immersion test based on the CPA test, but without the inclusion of chromic acid, was excluded due to the
similarity with the SA test. The scope of the work to develop a new phosphoric acid method was considered
too comprehensive for this project.
In general, the sealed coating (pores filled by hydration) loses mass and thickness linearly with dissolution
time. Different sealing methods (or sealing conditions of time, temperature, pH, composition of sealing
solution) result in different pore-filling material with differences in resistance to acid dissolution. When
considering replacing the CPA test with an alternative acid dissolution test, there are some criteria for a new
test. If possible, the response to the test should be similar for different sealing methods, i.e. it should be
possible to use the same standard even if the sealing method is different. There should be a significant
difference in the mass loss for a good and a bad sealing.
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ISO/TR 16689:2012(E)
2 Literature research
2.1 General
A comprehensive survey of the methods of testing the sealing quality of anodic coatings was given by
[9] [14]
Manhart and Cochran and by Kape in the 1970s. A more recent survey was made in 1987 by Wernick
[15]
and al. where the main acid dissolution tests are:
 acidified sulfite test (Kape test);
 AASA test;
 CPA test.
These tests are explained below, see 2.2 to 2.4.
In Figure 1 is shown the correlation of several acid dissolution tests with sealing time for sulfuric acid coatings
[9]
published by Manhart and Cochran . Note that the curves generally exhibit the same shape with a difference
in the absolute value of the mass loss. The thickness of the anodic oxide is about 25 μm (estimated from
given anodizing conditions).
2.2 Acidified sulfite test (Kape test)
The test solution is a mixture of sodium sulfite, acetic acid and sulfuric acid at 90 °C to 92 °C and pH 2,5 such
that sulfur dioxide is evolved but mainly retained in solution. Test solution: 1 000 ml deionized water to which
have been added glacial acetic acid (20 ml/l to 40 ml/l) to give a pH of 3,6 to 3,8 followed by 5 N sulfuric acid
(10 ml/l to 15vml/l) to give a pH of 2,5 at room temperature. A predip is made 10 min in 50 % by volume nitric
acid at room temperature.
The mass of the sample is assessed before predip, after predip and after immersion in test solution.
Immersion of the sample for 20 min. Note that care should be taken such that the solution temperature does
not at any time exceed 92 °C or the sulfur dioxide dissolved in the solution will be boiled off.
For a coating of good quality the loss of mass between the first and second weighing is negligible (a
significant difference indicates an excessively porous coating). Assessment of total mass loss is made using
2 [15]
the mass loss between the second and third weighings. A maximum mass loss 20 mg/dm is permitted (not
[10]
specified in the standard). The test is described in the standard ISO 2932 which was withdrawn in 1991.
2.3 Acetic acid/sodium acetate solution test
[10]
This sealing quality test was made according to standard ISO 2932 . The method was used in the 1970s but
the standard was withdrawn in 1991 being replaced with the CPA test.
The test solution is a mixture of 100 ml/l acetic (glacial) acid, 0,5 g/l sodium acetate in deionized water at pH
2
2,3 to 2,5. Renewed solution after each test is recommended. Not more that 3 dm surface area of immersed
sample per litre of solution. Non-anodized areas are not taken into account when calculating the surface area
2
since the solution only slightly attacks bare metal (not more than 0,05 mg/cm ), unless the bare areas
exceeds 5 % of the total surface area of the sample. During immersion, 15 min, the solution is maintained at
2 [15]
boiling point. A maximum mass loss of 20 mg/dm is permitted . Furneaux and Wood pointed out that this
test might be less suitable for other sealing methods than conventional hot sealing (e.g. nickel-based cold
[16]
sealing) .

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ISO/TR 16689:2012(E)

Key
2
y   mass loss, expressed in mg/dm (log scale)
x   sealing time, expressed in minutes (log scale)
1   15 % H SO
2 4
2   2 % CrO –5 % H PO
3 3 4
3   acetic acid
4   6 % citric acid
5   acidified Na SO
2 3
6   20 % HNO
3
NOTE   This figure is reproduced with permission from the National Association for Surface Finishing,
1155 15th St., NW, Suite 500, Washington, DC 20005 USA.
[9]
Figure 1 — Correlation of several acid dissolution tests with sealing time for sulfuric acid coatings
2.4 Chromic/phosphoric acid solution test
[9]
This test was originally proposed by Manhart and Cochran in 1971 and was then adopted as the general
[1]
referee mass loss test previously described by ISOv3210 . The sealing quality is evaluated with a mass loss
[3]
test today according to EN 12373-6 . The mass loss test is destructive and frequently used as a complement
[12]
to the dye spot test (EN 12373-4 ). The better the sealing, the lower the mass loss in this test. The
specifications on the mass loss vary depending on the application, even though for normal applications a
2 [7]
mass loss of less then 30 mg/dm is needed for approval according to Qualanod .
The test solution is a mixture of 2 % by mass chromic acid and 5 % by mass phosphoric acid, operated at
37,8 °C for 15 min [the same solution is used at higher temperature for determination of oxide density
[17]
(EN 12373-2 )].
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ISO/TR 16689:2012(E)
Note the drying procedures associated with the weighing. Prior to weighing the sample is:
 degreased for 30 s in a suitable organic solvent (e.g. ethanol);
 left to dry 5 min in ambient atmosphere;
 placed in a drying oven pre-heated to 60 °C for 15 min;
 left to cool for 30 min over silica gel in a closed desiccator.
When this test is performed in a production line however the drying procedures are probably always simplified
(i.e. no drying in oven and no cooling down in desiccator). This sealing test is sometimes combined with a
[4]
10 min predip in an aqueous solution containing (470 ± 15) g/l nitric acid (EN 12373-7 ), specified according
[7]
to Qualanod .
2
The test solution should not be used for more than 10 dm surface area of immersed sample per litre of
solution. The result is similar as with Kape and AASA tests but with greater mass losses (sulfuric acid
[14] [9]
anodized coatings) . Some of the mentioned advantages with the CPA test are the stability, convenient
operating temperature, no attack of uncoated metal, a convenient test period and no unpleasant odour.
Thickness loss and mass loss occur at the same rate.
2.5 Sulfuric acid solution test
[9]
This method is described by Manhart and Cochran . The test solution contains sulfuric acid in deionized
water at 48,9 °C. The immersion time is 20 min. It is written that bare metal surfaces should be protected
since the test solution also dissolves the aluminium and that the test might need a nitric acid predip.
3 Materials and experimental
3.1 Anodizing
Anodizing trials were made in an in-house anodizing pilot plant at Sapa Technology in Finspång, Sweden. The
process sequence was: degreasing, alkaline etching, desmutter, anodizing, sealing. Profile samples for
2
anodizing were of alloy EN AW 6063 and temper T6. The anodized area was 1 dm (100 mm x 50 mm x
2
3 mm). An electrolyte with 185 g/l sulfuric acid at 20 °C was used anodizing at a current density of 1,5 A/dm
and, if nothing else is stated, with a target thickness of (20 ± 1) μm which requires 42 min anodizing.
3.2 Sealing
Details about the sealing additives used and conditions used during tests are shown in Table 1. Cold sealing
was always made in combination with a hot sealing (i.e. dual step sealing) being 10 min. Note that the test
conditions on purpose go outside the recommended working conditions.
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ISO/TR 16689:2012(E)
Table 1 — List of tested sealing additives with recommended working conditions and test
conditions
Type of Product Manufacturer Chemical Working conditions Test conditions
sealing name
°C pH min/µm °C pH min/µm
Hot Almeco Henkel Anti-smut 97 5,8 3 90/97 5,2/5,8 1/2/3
Seal SLX
Midtemp Houghto Houghton Nickel acetate 74– 5,5– 0,55 80 5.8 0,25/0,4/
seal A620 Chemicals 85 6,1 0,55/1/
1,5/2
Alfiseal Alufinish Mono- and 86– 5,8– 3 88 6,0 1/2/3
969 dihexadecyl 90 6,1
disulfonic
diphenyloxide,
disodium salt
Cold PS41 Metachem Nickel fluoride 28– 5,8– 0,8–1,2 20/25/ 5,5/5,8/ 0,5/0,75/
32 6,4 30 6,0 1
(6,3)
NOTE The cold sealing step was followed by 10 min hot sealing at 96 °C.

3.3 Measurements of sealing quality
3.3.1 Acid dissolution tests
These are mass loss tests that assess the resistance to dissolution by acid solutions.
3.3.1.1 CPA test
[3]
The sealing quality was evaluated with a mass loss test according to EN 12373-6:1998 . A mass loss of less
2 [7]
than 30 mg/dm is needed for approval according to Qualanod .
The test solution is a mixture of 2 % by mass chromic acid and 5 % by mass phosphoric acid, operated at
[17]
37,8 °C [the same solution is used at higher temperature for determination of oxide density (EN 12373-2 )].
Note the drying procedures associated with the weighing. Prior to weighing the sample is:
 degreased for 30 s in a suitable organic solvent (e.g. ethanol);
 left to dry 5 min ambient atmosphere;
 placed in a drying oven pre-heated to 60 °C for 15 min;
 left to cool for 30 min over silica gel in a closed desiccator.
When this test is performed in a production line however the drying procedures are probably always simplified
(i.e. no drying in oven and no cooling down in desiccator).
This sealing test is sometimes combined with a 10 min predip in an aqueous solution containing (470 ± 15) g/l
[4] [7]
nitric acid (EN 12373-7 ), specified according to Qualanod .
2
The test solution should not be used for more than 10 dm surface area of immersed sample per litre of
solution. The mass loss of a bare aluminium substrate under test conditions was evaluated in 4.2.
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ISO/TR 16689:2012(E)
3.3.1.2 AASA test
The test solution is a mixture of 100 ml acetic (glacial) acid, 0,5 g sodium acetate in deionized water (total
volume 1 000 ml) pH 2,3 to pH 2,5. During immersion, 15 min, the solution is maintained at boiling point. After
2
the test the sample is rinsed in deionized water, dried and reweighed. A maximum mass loss of 20 mg/dm is
[15]
permitted .
2
Renewed solution after each test is recommended. Not more that 3 dm surface area of immersed sample per
litre of solution. Not anodized areas are not taken into account when calculating the surface area since the
2 2
solution only slightly attacks bare metal (not more than 0,05 mg/cm (5 mg/dm ) according to the standard)
unless the bare areas exceed 5 % of the total surface area of the sample. The mass loss of a bare aluminium
substrate under test conditions was evaluated in 4.2.
3.3.1.3 SA test
[1]
The test solution described contains 15 % sulfuric in deionized water at 48,9 °C . For simplicity the
temperature of the test solution in the trials made in this project was kept at 50 °C. The immersion time is
[9]
20 min .
It is written that bare metal surfaces should be protected since the test solution also dissolves the aluminium
[9]
and that the test might need a nitric acid predip . The mass loss of a bare aluminium substrate under test
conditions in the sulfuric acid test was evaluated in 4.2.
3.3.2 Admittance test
[13]
The sealing quality was also evaluated using the admittance test according to EN 12373-5 . The instrument
used for the admittance measurements was an Anotest YD from Fischer (ring diameter 13 mm).
Measurements were performed (if nothing else is mentioned) approximately 24 h after sealing and (according
to the standard) the measuring probe was left in the electrolyte 2 min before reading the result. Approved
value for an oxide thickness of 20 μm is approximately 20 μS. Values above this value are not approved.
Unlike the dye spot test and the mass loss test the admittance measurement takes into account the total oxide
film thickness such that it is sensitive to the sealing of the pores (pore filling) in the bulk.
Note that, depending on sealing additive used, different results might be achieved (i.e. the sealing additive
might influence the results obtained). The use of admittance measurements where cold (nickel fluoride)
[7]
sealings have been used is not recommended according to Qualanod . The presence of heavy metals (like
nickel) in the oxide might increase the conductivity and therefore the admittance of the oxide. Nevertheless, in
the datasheets for Alfiseal 985 (nickel fluoride cold sealing from Alufinish) impedance is however mentioned
as one method to control the sealing quality (earliest 15 h after sealing).
The test is simple, fast (2 min) and in principle non-destructive (contact to the base metal is made with a
screw preferably in one end of the profile).
[18]
Some admittance measurements were taken from a previous work reported in D07-0223 .
3.3.3 Dye spot test
The samples were evaluated with a dye spot test according to EN 12373-4. Rating 0 to 2 is accepted and 3 to
[7]
5 not accepted according to Qualanod (rating 0 is good quality; rating 5 is poor sealing quality) .
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ISO/TR 16689:2012(E)
4 Results
4.1 Masking of cut surfaces
In Annex A is shown the report of the Working Group (Qualanod) on the replacement for the CPA test for
sealing quality. In the conclusions of this report are given three possible candidates to replace the CPA test:
the AASA test; the SA test; a phosphoric acid test (similar to CPA test but without the chromic acid).
The SA and phosphoric acid tests suffer the disadvantage that those acids attack bare aluminium. Because of
this a means of protecting the cut edges of test coupons would have to be developed.
Initial trials were made where masking of the cut surfaces were made using nail polish. The nail polish was
applied on cut surfaces after the initial weighing and removed using acetone (masked surfaces) followed by
ethanol (full sample) before the second weighing. The preliminary results show that this could be a possible
method to use. It is important however that the cut surfaces are smooth enough to facilitate the removal of the
nail polish after performed test. More tests are needed however. For reasons described in 4.2 below the work
with developing a masking method was not completed.
4.2 Bare aluminium and dissolution in the dissolution tests
When investigating the mass loss of a bare aluminium substrate under test conditions in the different acid
dissolution tests the mass loss turned out to be very low, see Table 2, within the region of the accuracy of the
measurement (the result from the CPA test is even negative). The total mass loss on a sample where the cut
surface (bare aluminium surface) corresponds to 2 %, 5 % and 10 % of the total sample surface respectively
2
was estimated to be less then 0,3 mg/dm . This is very low and we concluded that a masking method was not
needed in cases where the bare aluminium surface is less than 5 % (the same criteria as for the AASA test).
Table 2 — Measured mass loss on a bare aluminium substrate
Dissolution test Sample area Mass loss Calculated mass losses for different percentages of
2 2 2
(dm ) (mg/dm ) bare aluminium (mg/dm )
2% 5% 10%
CPA 1,1 -2,3 - - -
AASA 1,1 2,5 0,05 0,12 0,25
SA 1,1 1,3 0,03 0,06 0,13

4.3 Hot sealing
4.3.1 Mass loss
In Table 3 the recommended working conditions in production and the sealing solution conditions during test
are shown. Note that the test conditions on purpose go outside the recommended working conditions.
Figures 2 to 6 shows the mass loss on samples sealed with hot sealing (at pH 5,8 and at 97 °C if nothing else
is stated). Figure 3 shows the mass loss response in the CPA test when compared with the mass loss in the
sulfuric acid test; note different y-axes. Note the difference in response to the lowered sealing temperature
(90 °C, 2 min/μm). Figure 4 shows the mass loss response in the CPA test when compared with the mass
loss in the AASA test; note different y-axes. The response is similar even though the absolute value of the
mass loss is lower for the AASA test. However, the AASA test generates mass losses with a larger variation
for similar sealed samples (for example, compare the two sealed for 1 min/μm). In Figures 5 to 6 are shown
[9]
also the data presented by Manhart and Cochran as well as measured mass losses from samples from a
production plant.
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ISO/TR 16689:2012(E)
The mass loss in the AASA test is very flat in the relevant sealing time interval, such that it would be difficult to
separate a good sealing from a bad, see Figures 5 to 6. The test results are easily within the maximum
2
permitted mass loss 20 mg/dm in spite of the poor sealing conditions. This is valid also for the CPA test, i.e.
2
the test results are easily within the maximum permitted mass loss 30 mg/dm . The most significant difference
in the mass loss with sealing time is seen for the SA test. Also included is the average mass loss of two
samples from a production plant, which were measured about one week after production.
Table 3 — Recommended working and test conditions
Type of Product Manufacturer Concentration Working conditions Test conditions
sealing name (g/l)
°C pH min/µm °C pH min/µm
Hot Almeco Henkel 2 97 5,8 3 90/97 5,2/5,8 1/2/3
Seal
SLX

100
SA test
80
CPA test
AASA test
60
40
20
0
Sealing conditions

...