BRITISH STANDARD
CONFIRMED
SEPTEMBER 1989
BS 6338:1982
ISO 4520:1981
Specification for
Chromate conversion
coatings on
electroplated zinc and
cadmium coatings
UDC 669.058:[669.585.7 + 669.738.57]:661.875.274.7/.8
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BS 6338:1982
This British Standard, having
been prepared under the
direction of the Surface Coatings
(other than Paints) Standards
Committee, was published
under the authority of the Board
of BSI and comes into effect on
31 December 1982
© BSI 07-1999
The following BSI references
relate to the work on this
standard:
Committee reference SRC/9
Draft for comment 77/50321 DC
ISBN 0 580 13108 4
Cooperating organizations
The Surface Coatings (other than Paints) Standards Committee, under whose
direction this British Standard was prepared, consists of representatives from
the following:
Aluminium Federation
Assay Offices Committee of Great Britain
Association of Metal Sprayers
British Constructional Steelwork Association
British Jewellers’ Association
British Jewellery and Giftware Federation Ltd.
British Metal Finishing Suppliers’ Association*
British Steel Industry
Chemical Industries Association
Electricity Supply Industry in England and Wales
Engineering Equipment Users’ Association
Galvanizers’ Association
Greater London Council
Institute of Metal Finishing*
Institute of Sheet Metal Engineering*
Institute of Vitreous Enamellers
Institution of Corrosion Science and Technology
Metal Finishing Association*
Ministry of Defence
Motor Cycle Association of Great Britain
Society of Motor Manufacturers and Traders Limited*
Vitreous Enamel Development Council
Zinc Development Association
The organizations marked with an asterisk in the above list, together with the
following, were directly represented on the Technical Committee entrusted
with the preparation of this British Standard:
Electronic Engineering Association
Magnesium Industry Council
Telecommunication Engineering and Manufacturing Association
Amendments issued since publication
Amd. No. Date of issue Comments
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BS 6338:1982
© BSI 07-1999 i
Contents
Page
Cooperating organizations Inside front cover
National foreword ii
0 Introduction 1
1 Scope and field of application 1
2 References 1
3 Method of application of chromate conversion coatings 1
4 Classification 1
5 Requirements 1
Table 1 — Classification of chromate conversion coatings 2
Table 2 — Requirements for corrosion resistance 2
Publications referred to Inside back cover
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BS 6338:1982
ii © BSI 07-1999
National foreword
This British Standard has been prepared under the direction of the Surface
Coatings (other than Paints) Standards Committee. It is identical with
ISO 4520 “Chromate conversion coatings on electroplated zinc and cadmium
coatings”, prepared with the active participation and approval of the
United Kingdom, by Sub-committee 3, Electrodeposited coatings and related
finishes, of Technical Committee 107, Metallic and other non-organic coatings,
and published in 1981 by the International Organization for Standardization
(ISO).
Terminology and conventions. The text of the international standard has
been approved as suitable for publication as a British Standard without
deviation. Some terminology and certain conventions are not identical with those
used in British Standards; attention is drawn especially to the following.
The comma has been used as a decimal marker. In British Standards it is current
practice to use a full point on the baseline as the decimal marker.
Wherever the words “International Standard” appear, referring to this standard,
they should be read as “British Standard”.
A related standard for ISO 2081:1973 and ISO 2082:1973, referred to in clause 1
for information only, is BS 1706:1960 “Electroplated coatings of cadmium and
zinc on iron and steel”.
A British Standard does not purport to include all the necessary provisions of a
contract. Users of British Standards are responsible for their correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.
Cross-references
International standard Corresponding British Standard
ISO 3613-1980 BS 5411 Methods of test for metallic and related coatings
Part 13:1982 Chromate conversion coatings on zinc and
cadmium
(Identical)
ISO 3768-1976 BS 5466 Methods for corrosion testing of metallic coatings
Part 1:1977 Neutral salt spray test (NSS test)
(Identical)
ISO 3892-1980 BS 5411 Methods of test for metallic and related coatings
Part 14:1982 Gravimetric method for determination of
coating mass per unit area of conversion coatings on
metallic materials
(Identical)
Summary of pages
This document comprises a front cover, an inside front cover, pages i and ii,
pages 1 and 2, an inside back cover and a back cover.
This standard has been updated (see copyright date) and may have had
amendments incorporated. This will be indicated in the amendment table on the
inside front cover.Li
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BS 6338:1982
© BSI 07-1999 1
0 Introduction
Coatings of zinc and cadmium are chromate treated
in order to retard the formation of corrosion
products on the surfaces of coatings exposed in
corrosive atmospheres. Chromate treatment is
particularly effective in retarding the formation of
white corrosion products which form on zinc and
cadmium coatings under certain conditions.
Thicknesses of electroplated coatings suitable for
chromate treatment are covered by the relevant
International Standards, i.e. ISO 2081 and
ISO 2082.
1 Scope and field of application
This International Standard specifies requirements
relating to chromate conversion coatings on zinc and
cadmium intended to give protection against
corrosion. Finishes for giving particular colours only
or specifically to improve paint adhesion are not
covered by this International Standard.
2 References
ISO 2081, Metallic coatings — Electroplated
coatings of zinc on iron and steel.
ISO 2082, Metallic coatings — Electroplated
coatings of cadmium on iron and steel.
ISO 3613, Chromate conversion coatings on zinc and
cadmium — Test methods.
ISO 3768, Metallic coatings — Neutral salt spray
test (NSS test).
ISO 3892, Conversion coatings on metallic
materials — Determination of coating mass per unit
area — Gravimetric methods..
3 Method of application of chromate
conversion coatings
Chromate conversion coatings are normally applied
by dipping. Chromating solutions are usually acidic
and contain hexavalent chromium salts together
with other salts which may be varied 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 is also affected by pH and
temperature, the duration of treatment and the
nature and surface condition of the coating being
treated. Bright, transparent films on zinc and
cadmium coatings can be obtained by dipping in
appropriate solutions. On zinc, they can also be
obtained by bleaching iridescent films in alkaline
solutions or in phosphoric acid.
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 order
to prevent the dissolution of the hexavalent
chromium. The drying of the article shall be carried
out at a temperature not exceeding 60 °C to prevent
cracking due to dehydration of the chromate
coating. (Any heat treatment for the relief of
hydrogen embrittlement shall be carried out before
a chromate conversion coating is applied.)
4 Classification
Finishes can be applied ranging from thick, dark,
olive-green coatings with good protective properties
to thin, transparent, sometimes bluish films of
attractive appearance but with limited protective
value.
Electroplaters 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 bleached 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 colourless
bleached coatings.
Finishes are divided into two classes, each of which
comprises two types; their most important
characteristics are listed in Table 1.
Finishes may be characterized by class alone or by
class and type designation. Typical examples are:
a) Fe/Cd 8 c 2
b) Fe/Zn 25 c 1A
where
Fe refers to the basis metal (iron or steel;
Cd and Zn refer to the electroplated coating
(cadmium or zinc);
8 and 25 are the thicknesses, in micrometres, of
the cadmium and zinc coatings;
c refers to the chromate conversion coating;
2 and 1 are the classes of the chromate
conversion coatings;
A designates the type of chromate conversion
coating.
5 Requirements
5.1 General
Chromate conversion coatings harden with age by
gradual dehydration. They should, therefore, be
handled carefully for the first 24 h after treatment,
and any tests (including corrosion tests) shall be
deferred until the expiry of that period.
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BS 6338:1982
2 © BSI 07-1999
5.2 Adhesion of coloured coatings
Coloured coatings shall be adherent and shall be
tested by one of the methods specified in ISO 3613.
5.3 Corrosion resistance
When subjected to the neutral salt spray test
specified in ISO 3768, the time for formation of
white corrosion products on chromated cadmium or
zinc coatings shall not be less than the values given
in Table 2.
NOTE These requirements are not applicable to the edges of
the test specimen.
Table 1 — Classification of chromate conversion coatings
Table 2 — Requirements for corrosion
resistance
Class Designationa Type Typical appearance
Coating mass per unit
area
g/m2
(see ISO 3892)
Corrosion protection
A Clear Transparent clear,
sometimes with a bluish
tinge
u 0,5 Slight, for example
against staining
during handling or
against high humidity
in mildly corrosive
conditions
1
B Bleached Transparent with slight
iridescence
u 1,0
2
C Iridescent Yellow iridescent 0,5 to 1,5 inclusive Considerable,
including protection
against certain
organic vapours
D Opaque Olive green, shading to
brown or bronze
>1,5
a In addition, black coatings can be produced by several methods. Such coatings may have different degrees of corrosion protection
and may also differ in coating mass per unit area.
Designation Possible classificationsa
Minimum time to
formation of white
corrosion products, h
A 1, 1A 6
B 1B 24
C 2, 2C 72
D 2D 96
a See clause 4, which describes how finishes may be
characterized by class alone or by class and type designation.
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BS 6338:1982
© BSI 07-1999
Publications referred to
See national foreword.
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BS 6338:1982
ISO 4520:1981
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