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IS : 11232 - 1985
Indian St1andard
SPECIFICATlON FOR
CHROMATE CONVEE~SION COATINGS
ON ALUMINIUM
Corrosion Protection Sectional Committee, SMDC 29
Chairman
S11n1C.P.DE
Representing
Naval
Chemical
&
Metallurgical Lauoratory,
Bombay
Members
DRS. N. PANDEY ( Alternate I to
Shri C. P. De)
Du M. B. DESIIMUIOI ( Alternate II to
Shri C. P. De)
Da M. BALAKltISHNAN
Pyrene-Rai Metal Treatments Ltd, Bombay
Smn A. D. lNAMDAR ( Alternate)
S11n1 B. GoswAMY
Tata Engiineering & Locomotive
Co
Ltd,
Jamshedpur
SHRI N. K. NAG ( Alternate )
SnnI V. K. JAIN
Oil & Natural Gas Commission, Dehra Dun
S11Rr K. S. BnATJA ( Alternate)
JOINT
DIRECTOR
STANDARDS
Research Designs & Standa1 ds Organization,
( CARRIAGE l )
Lucknow
DY DIRECTOR ( MET-4 ) ( Alternate )
SHRI R. L. KAPOOlt
Ministry of Shipping & Transport ( Roads\\' ing )
Dn V. M. Kr•~LKAR
Engineers India Ltd , New Delhi
Dn M. D. MAHESHWARI
The Indian Tube Co Ltd, Jamshedpur
Smu K. R. NATA RAJAN (Alternate)
S1mr K. P. MUKHERJEE
National Metallurgical Laboratory, Jamshedpur
DR lNDER S1NGH ( Alternate)
SHrn R. C. MUKHERJEE
Steel Authority of India ( Rourkela Steel Plant ),
Rourh:la.
SHRI NntANJ AN DASH ( Alternate)
Sinn R. P. NAGAR
Tata Consulting Engineers, Bombay
SHRI D. B. MEHTA ( Alternate)
SHRI M. M. NATH
Lloyds Tar Products Ltd, New Delhi
Smu L. PUGAZIIENTHY
Indian Lead Zinc Information Centre, New Delhi
DR K. S. RAJAGOPALAN
Central Electro-chemical
Research
Institute,
Karaiku<li
D1~ K. BALAKRISHN AN ( Alternate)
Snn1 S. RAMAJ AYAM
Indian Telephone Industries Ltd, Bangalore
SHRI N. S. NANJUNI>A RAo (Alternate)
( Continued on page 2 )
© Copyright 1985
INDIAN STANDARDS INSTITUTION
This publication i.-;i protected under the Indian Copyright A,t ( XIV of 1957 ) and
, reproduction in whole or in part by any meana except with written permission of the
publisher shal1 be deemed to be ,1.n infringement of copyright under the ~ai<l Act.
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IS : 11232 • 1985
( Continued from page I )
Representing
Ministry of Defence ( R & D)
Members
D.R N. P. RAO
Snn.1J. BANERJEE (Alternate)
DR SANDIP KUMAR RoY
SHRI A. K. SAHU
S1m1 D. K. Roy ( Alternate )
S11R1 S. R. SAHU
Indian Explosives Ltd, Rishra
National Test House, Calcutta
Steel Authority of India Ltd ( Bokaro Steel Plant),
Bokaro Steel City
Smn K. ANNAIAH ( Alternate )
Smn D. SEN
Ministry of Defence ( DGOF )
Smn D. SKN GUPTA
Oil India Ltd, Calcutta
S1rn,1 D. K. SINGH
Bharat Heavy Electricals Ltd
SnRI R. M. SINGHAL ( Alternate I )
Sttm R. N. MrsnRA ( Alternate II )
Smn K. S. SRINIVASAN
National Buildings Organization, New Delhi
SnRI SuAsm KANT (Alternate)
Smu G. K. T AKIAlt
Fibreglass Pilkington, Bombay
S run A. S Ii Aiu t<' ( Alternate )
DR K. M. VRKMA
Fertilizer ( P & D ) India Ltd, Sindri
S11R1 R. K. SINHA ( Alternate)
Snnr K. RA<mAVENDRAN,
Director General, ISI ( Ex-officio Member)
Director ( Struc & Met )
Secretary
Smu B. K. MuKHOPADHYAY
Assistant Dinector ( Metals), ISI
•
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IS: 11232 - 1985
Indian !Standard
SPECIFICA~TION FOR
CHROMATE CONVERSION COATINGS
ON ALUMINIUM
0.
FOREWORD
0.1 This Indian Standard was adopted by the In<lian Standards
Institution on 14 March 1985, after the draft finalized by the
Corrosion Protection Sectional Cornmittee had been approved by the
Structural and Metals Division Council.
0.2 Aluminium and aluminium alloys are chromate treated in order to
retard corrosion; as a base for organic fihns including paints, plastics and
adhesives; and as a protective coating having a ]ow electrical contact
impedance/ resistance.
0.3 For the purpose of deciding whether a particular require1nent of this
standard is complied with, the final value, observed or calculated,
expressing the result of a test or analysis, shall be rounded off in accordance with IS : 2-1960*. The number of significant places retained in the
rounded off value should be the same as that of the specified value in this
standard.
1. SCOPE
1.1 This standard specifies requirements relating to rinseu chromate
conversion coatjngs on aluminiurn alloys. It is intended to give
protection against corrosion and to act as a base for other coatings.
Coatings that require no rinsing after application are not covered by this
standard.
2. METHOD OF APPLICATION OF CHROMATE COATINGS
2.1 Chromate conversion coatings are applied by dipping, spraying and
brushing. Chroma ting solutions arc usually acidic and contain
hexavalent chromium salts together with other salts which may be varied
•Rules for rounding off numerical va]ues ( rel'ised ).
3
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IS : 11232 - 1985
to affect the appearance and hardness of the film. The colour of the
film, and, therefore, the type of conversion coating depends on the
composition of the chromating solution, but it is also affected by the pH
and temperature, the duration of the treatment, and the nature and
surface condition of the alloy being treated.
2.2 These coatings receive a final water rinse.
If hot water is used as
the final rinse after the chromating process, it is essential that the time
of rinsing should be kept as short as possible in oder to prevent the dissolution of t!i.e hexavalent chromium. The drying of the coating shall
be carried out at a temperature not exceeding 60°C to prevent cracking
due to dehydration of the chromate coating.
3. CLASSIFICATION
3.1 Ji'inishes can be applied ranging in colour from brown, thick coatings
providing maximum corrosion protection to yellow, intermediate
thickness coatings suitable as an organic film base or to colourless, thin
coatings suitable for low electrical contact resistance.
3.2 Finishers can seldom guarantee to supply exact shades of colour with
chromate conversion coatings. If it is necessary to have exact shades of
colour, it is possible to dye chromate coatings to obtain a wide range of
colours, but they can only be expected to give an order of added
corrosion resistance similar to that provided by the undyed coatings. It
should be noted that colour and collour uniformity wi11 be somewhat
between one alloy and other and from a polished surface to an etched
surface. Iridescence and variations: in colour density from one area of
the surface to another are normal and shall not be considered a sign of
poor quality.
The finishe~ are divided into three classes and their·most important
characteristics are as follows:
C!11ss
Appearance
Coating We£ght
Corrosion Protection
I.
Yellow to brown
More than
0·4 g/ml'l
Maximum corrosion resistance
generally ussd as final finish
2.
Colourless to
yellow
O· l to
·r g IC)
1nw
O.,~
Moderate corrosion resistance,
used as a paint base and for
bonding to rubber
3.
Colourless
Less than
O·l g/ m 2
Decorative, slight corrosion
resistance low electrical con..
tact resistance
4-
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IS : 11232
• 1985
4. REQ,UIREMENTS
4.1 General -
Chromate conversion coatings harden with age by
gradual dehydration. They should, therefore, be handled carefully for
the first 24 hours after treatment, and any tests ( including corrosion
tests ) shall be deferred until the expiry of that period.
4.1.1 The basis metal should be free from visible defects, such as
blisters, gouges, non-metallic inclusions, pits or porosity which will be
detrimental to the appearance and performance of the film.
4.2 Adhesion - The coatings shall be adherent and non-powdery.
When specified, the chromate conversion coating shall pass the organic
coating adhesion test in Appendix A.
4.3 Corrosion Resistance - When subjected to the neutral salt spray
test ( see IS : 9844-198 l • ) , two separate panels of the coating shall withstand exposure for the hours shown in Table l, without giving evidence,
to the unaided eye, of more than a total of eight isolated spots or pits,
not larger than I 1nn1 in diameter. Each individual panel shall not
have more than five isolated spots or pits, none larger than I mm in
diameter on their respective surfaces. Spots within 10 mm of the edges
of the panels are not counted.
TABLE 1 EXPOSURE TIME
COATING
NoN-JiEAT- TREATABLE
CrJAss
WROUGHT ALLOYS
HEAT-TREATABLE ALLOYS CAST ALLOYS WITH
A NOMINAL S1LIOON
AND GAST ALLOYS WITH A
CoNTENT OF MoRE
NOMINAL SILICON CONTHAN 1 PERCENT
TENT LESS THAN 1 PERCENT
Hours
Hours
Hovrs
1
500
336
48
2
250
168
24
3
168
120
12
No-rE - The exposure times are indicative of the relative corrosion resistance of
the various coating classes on the different alloys, but no direct relationship exists
between performance in the neutral salt spray test and performance in service.
4.4 Coating Weight - The weight of the coating per unit area of
coated surface shall conform to the values given in 3.2 when tested in
accordance with the method given in IS : 9838-1981 t.
•Methods of testing corrosion resistance of electroplated and anodized aluminium
coatings by neutral salt spray test.
t Method for determination of coating mass per unit area of conversion coatings on
metallic materials by gravimetric method.
5
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1S: 11232 - 1985
4.4.1 It should be recognized that heavier coating weights do not
always provide better performance, particularly when using the coating
for an organic film or adhesive base.
4.4.2 The presence of a Class 3 coating shall be confirmed by the spot
test ( see Appedix B ).
4.5 Test Specimens and Sampling
4.5.1 Test specimens should be of the same alloy and surface condition
as the articles represented. Panels should be 75 X 250 x 0·8 mm.
4.5.2 The sampling plan for testing the coatings shall be as agreed to
between the interested parties.
APPE1~DIX A
( Clause 4-2 )
ORGANIC COATING ADHESION TEST
A-1. SCOPE
A-1.1 The chromated specimen shall be coated with the organic coating
system specified by the purchaser.
1:25 micrometres in thickness.
The coating shalJ be between 25 and
A-2. APPARATUS
A-2.1 Cutting Tool - Sharp razor blade, scalpel, knife or other cutting
device having a cutting edge angle between J5 and 30 degrees that wiIJ
make either a single cut or several cuts at once. It is of particular
importance that the cutting edge be in good condition.
A-2.2 Cutting Guide -
If cuts aire made manually ( as opposed to a
mechanical apparatus ) a steel or other hard metal straightedge or
template to ensure straight cuts.
A-2.3 Rule - Rule graduated in 0·5 mm.
A-2.4 Tape - Semi-transparent pressure sensitive tape, 25 mm wide with
an adhesive strength of 40 ± 2·8 g/mm width. The tape shall be capable
of maintaining that strength ± 5 percent for I year. An adhesive
composed of resin and 100 percent crude rubber is reported to meet these
characteristics. The backing of the tape may consist of fibre•reinforced
cellulose acetate, unplasticized polyvinyl chloride, or polyester film.
A-2.5 Rubber Eraser -
On the end of a pencil.
6
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IS : 11232 • 1985
A .. 2.6 Illumination -
Normal reading light.
A-2. 7 Magnifying Glass -
2 x to 10 ><.
A-3. PROCEDURE
A-3.1 Place the coated specimen ( panel or part ) under the magnified
glass with suitable illumination. For organic coatings up to 50 µ.m thick.
Make eleven parallel cuts l mm apart. The cuts should penetrate
through to the base. The cuts shall be :20 mm long. Brush the area
with a soft brush oi- tissue to remove any flakes or loose particles.
Make an additional number of cuts at 90° to and centred on the
original cuts. Brush the area as before and inspect the incisions to make
certain that all have reached the base material.
Remove two complete laps of tape from the roll and discard.
Remove an additional length at a steady rate and cut a piece about
75 mm long. Place the centre of the tape over the grid of cuts and
smooth into place with a finger. To ensure good contact with the film
rub the tape firmly with the eraser on the end of a pencil. Observe for
uniform contact.
Within 90 ± 30 s of application, reinove the tape by grasping the
free end rapidly and smoothly pulling it off as close to an angle of 180°
as possible.
A-3.2 Inspect the grid area for removal of coating from the substrate
using the illuminated magnifier.
following scale:
Rate the adhesion according to the
5.
The edges of the cuts are completely srno::>th; none oft he squares
of the lattice are detached.
4.
Small flakes of the coating are detached at the intersections; less
than 5 percent of the area is affected.
Small flakes of the coating are detached along edges and at
intersections of cuts. The area affected is 5 to 10 percent of the
lattice.
The coating has flaked along the edges and on parts of the
squares. The area affected is 15 to 35 percent of the lattice.
The coating has flaked along the, edges of cuts in large ribbons
and whole squares have dNached. The area affected is 35 to
65 percent of the lattice.
Flaking and detachment worse than Grade l.
3.
2.
l.
0.
A-3.3 Repeat the test in two other locations on the panel or part.
A-4. REPORT
A-4.1 Report the number of tests, their naean and range.
7
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IS : 11232 - 1985
APPEN :D IX
(: Clause ~t4.2 )
B
SPOT TEST FOR CHROMATE CONVERSION COATING
B-1. OUTLINE OF THE TEST
B-1.1 A solution of ferric nitrate, hydrochloric acid and potassiurn
ferricyanide reacts with untreated alu1ninium to give blue green colour.
Chromated aluminium takes time to develop this colour, and this tjme
depends upon the thickness of chromate coating or film formed.
B-2. REAGENTS
B-2.1 Solution I -
Dissolve 2 g of ferric nitrate ( AR ) in 20 ml of
reagent grade hydrochloric acid and dilute with 473 ml of distilled
water.
B-2.2 Solution II - Dissolve 2 g of potassium ferricyanide in 473 m 1 of
distilled water.
B-2.3 The two solutions should be stored separately in brown bottles and
mixed together in equal volumes be:fore use.
mixed solution is limited to only 7 days.
The storage life of the
B-3. PROCEDURE
B-3.1 Take a piece of untreated alurr1inium plate, without any chromate
conversion coa6ng and remove the oxide layer by dipping it for about
half a minute in 2 N sodium hydroxide solution. This shall be used as
a reference plate for comparison purposes.
B-3.2 Place a drop of mixed solution ( see B-2.3 ) by means of a glass
rod on both, the untreated plate, and the treated aluminium plate ".vith
chromate conversion coating. The solution will turn blue green
immediately on the untreated plate, while the time to change the colour
to blue green on the treated plated will vary with the thickness of
conversion coating.
8