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BSI British Standards
Coatings on metal fasteners –
Part 1: Specification for general
requirements and selection guidelines
BS 7371-1:2009
BS 7371-1:2009 BRITISH STANDARD
Publishing and copyright information
The BSI copyright notice displayed in this document indicates when the
document was last issued.
© BSI 2009
ISBN 978 0 580 55382 0
ICS 21.060.01, 25.220.01
The following BSI references relate to the work on this standard:
Committee reference FME/9
Draft for comment 08/30133813 DC
Publication history
First published January 1992
Second (present) edition, August 2009
Amendments issued since publication
Date Text affected
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BRITISH STANDARD
© BSI 2009 • i
BS 7371-1:2009
Contents
Foreword iii
1 Scope 1
2 Normative references 1
3 Terms and definitions 1
4 Selection of coatings 1
5 Preparation before coating 2
6 Hydrogen embrittlement 2
7 Thickness and coating weight 3
8 Sampling for inspection 3
9 Delivery conditions 4
10 Information to be supplied to the coater and designation 4
Annexes
Annex A (informative) Selection guidelines 5
Annex B (informative) Avoidance of hydrogen embrittlement 9
Annex C (informative) Surface areas of bolts, screws and nuts 10
Annex D (informative) Delivery conditions 14
Bibliography 15
List of figures
Figure C.1 – Constituent surface areas of a bolt or screw 11
List of tables
Table A.1 – Protective value and service condition number 5
Table A.2 – Coating selection 6
Table A.3 – Coating recommendations for parallel threads 8
Table C.1 – Surface areas of screws and bolts 11
Table C.2 – Surface areas of nuts for electroplated coatings 12
Table C.3 – Surface areas of nuts for non-electroplated coatings 12
Summary of pages
This document comprises a front cover, an inside front cover,
pages i to iv, pages 1 to 16, an inside back cover and a back cover.
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BRITISH STANDARD
© BSI 2009 • iii
BS 7371-1:2009
Foreword
Publishing information
This part of BS 7371 is published by BSI and came into effect on
31 August 2009. It was prepared by Subcommittee FME/9/1, Mechanical
properties of fasteners, under the authority of Technical Committee
FME/9, Fasteners. A list of organizations represented on this committee
can be obtained on request to its secretary.
Supersession
This part of BS 7371 supersedes BS 7371-1:1991, which is withdrawn.
Relationship with other publications
Other published Parts of BS 7371 are as follows:
• BS 7371-2, Coatings on metal fasteners – Part 2: Specification for
torque/clamping force relationship;
• BS 7371-3, Coatings on metal fasteners – Part 3: Specifications for
electroplated zinc; 1)
• BS 7371-4, Coating on metal fasteners – Part 4: Specification
for electroplated nickel, nickel/chromium and copper/nickel/
chromium coatings;
• BS 7371-6, Coatings on metal fasteners – Part 6: Specification for
hot dipped galvanized coatings;
• BS 7371-7, Coatings on metal fasteners – Part 7: Specification for
mechanically applied zinc and zinc based coatings;
• BS 7371-8, Coatings on metal fasteners – Part 8: Specification for
sherardized coatings;
• BS 7371-9, Coatings on metal fasteners – Part 9: Specification for
phosphate or phosphate and oil coatings;
• BS 7371-10, Coatings on metal fastenings – Part 10: Specification
for organic coatings;
• BS 7371-11, Coatings on metal fasteners – Part 11: Specification
for zinc flake non-electrolytically applied cured coatings; 2)
• BS 7371-12, Coatings on metal fasteners – Part 12: Requirements
for imperial fasteners.
The Parts of BS 7371 listed describe coatings for:
a) corrosion protection;
b) appearance;
c) electrical conductivity;
d) controlled lubricity;
e) identification; and
f) avoidance of hydrogen embrittlement.
Attention is drawn to Clause 4 and Annex A which provides advice on
the suitability and limitations of the coatings for these purposes.
1) Being reviewed, replaced by BS EN ISO 4042 but remains current.
2) Obsolescent, replaced by BS EN ISO 10683 but remains current.
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BRITISH STANDARD
Information about this document
This is a full revision of the standard, and introduces the following
principal changes:
• Change to Figure C.1.
• Exclusion of cadmium.
• General revision.
Product certification/inspection/testing. Users of this British Standard
are advised to consider the desirability of third-party certification/
inspection/testing of product conformity with this British Standard.
Users seeking assistance in identifying appropriate conformity
assessment bodies or schemes may ask BSI to forward their enquiries
to the relevant association.
Presentational conventions
The provisions of this standard are presented in roman (i.e. upright)
type. Its requirements are expressed in sentences in which the principal
auxiliary verb is “shall”.
Commentary, explanation and general informative material is presented
in smaller italic type, and does not constitute a normative element.
Contractual and legal considerations
This publication does not purport to include all the necessary provisions
of a contract. Users are responsible for its correct application.
Compliance with a British Standard cannot confer immunity from
legal obligations.
The attention of the user of this Part of BS 7371 is drawn to
the restrictions placed on the use and supply of hexavalent
chromium-containing substances by ECC Directive 2000/53/EC on
End of Life Vehicles [1] and amendment of Annex II by Commission
Decision June 2002 [Document No. C(2002) 2238] [2], and Non-Use on
Motor Vehicles marketed after July 2007.
Attention is drawn to The Waste Electrical and Electronic
Equipment (WEEE) Directive (2002/96/EC) [3], which came into
force on 31 December 2006.
Attention is drawn to the restrictions placed on the marketing
and use of cadmium and its compounds by in the Environmental
Protection (Controls on Injurious Substances) (No. 4) Regulations
1992 [4]. Cadmium has thus been excluded from this Specification.
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BS 7371-1:2009
1 Scope
This Part of BS 7371 specifies general requirements that apply to
coatings covered by all other Parts of BS 7371, including:
a) preparations before coating;
b) hydrogen embrittlement avoidance;
c) sampling for inspection;
d) thickness testing;
e) area tables for metric parallel threaded fasteners (area tables for
imperial fasteners can be found in BS 7371-12); and
f) selection of coatings (including the opportunity to select
non-hexavalent chromium-containing coatings).
Specific requirements are given in each Part of BS 7371.
2 Normative references
The following references are indispensable for the application of this
document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document
(including any amendments) applies.
BS 6041:1981+A1:1981, Method of sampling of electrodeposited metallic
coatings and related finishes: procedures for inspection by attributes
3 Terms and definitions
3.1 significant surfaces
areas visible on a part when assembled
3.2 batch average thickness
average thickness of the coating on all components of a batch
3.3 production run
batches of parts processed continuously without any changes in
coating techniques or constituents within a single working day
4 Selection of coatings
The coating shall be selected with regard to:
a) corrosion resistance;
b) appearance;
c) shape and size;
d) embrittlement;
e) conductivity;
f) lubricity;
g) application limitations;
h) colour (if applicable); and
i) whether hexavalent chromium can be tolerated.
NOTE Annex A provides guidance on suitable coatings for specific
components.
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5 Preparation before coating
Prior to coating, fastener parts shall be free of oil, grease, scale, oxides
and other foreign matter.
NOTE 1 The cleaning process should not induce hydrogen embrittlement
(see Clause 6).
NOTE 2 It is essential that the cleaning process is not detrimental to the
substrate.
6 Hydrogen embrittlement
6.1 Cleaning processes
Fastener parts heat-treated or cold-worked to a surface hardness of
320 HV to 390 HV, and also property classes3) 9.8, 10.9, and 10.9, shall
be cleaned with an inhibited acid, alkaline or mechanical process.
NOTE 1 The time during which parts are immersed in inhibited acid
cleaning agents increases the risk of the introduction of potential hydrogen
embrittlement. Immersion time in the inhibited acid should therefore be of
minimum duration. Cathodic cleaning processes should be avoided.
Special cleaning processes using non-acidic methods such as dry
honing, abrasive blasting or alkali derusting shall be used to clean
parts which have been heat-treated or cold-worked to a surface
hardness greater than 390 HV or property class 12.9 and above.
NOTE 2 Hydrogen embrittlement might cause premature failure of
parts which are heat-treated or cold-worked to a surface hardness of
320 HV and above when subjected to stress. The property classes affected
include 9.8 and above. Although baking after coating will minimize the
risk of failure, the process can never be guaranteed to be completely
effective. Annex B includes a more detailed explanation.
6.2 Coating processes
Fastener parts shall be coated in accordance with the appropriate
Parts of BS 7371.
NOTE The risk of hydrogen-embrittlement failure of parts with a
surface hardness greater than 390 HV, and also property class 12.9 and
above, cannot be precluded unless coating processes are used which do
not involve electrolysis from aqueous solutions. Only high-efficiency,
non-cyanide type electroplating solutions or alternative non-embrittling
coatings should be used (see Annex B).
6.3 De-embrittlement processes
NOTE 1 Although property class 8.8 might marginally exceed 320 HV
surface hardness, it is not neccessary to take these precautions provided
that the surface hardness of this particular property class does not
exceed 350 HV.
Except for those parts described in Note 1, parts heat-treated or
cold-worked to a surface hardness greater than 320 HV (and also
property classes 9.8 and above which have been electroplated)
shall be de-embrittled by baking. Parts shall not be heated to a
temperature above that at which they were originally tempered.
3) Property classes are designated in BS EN ISO 898-1.
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The baking process specified shall apply to the parts within the scope
of this standard with the exception of:
a) parts tempered below 180 °C;
b) parts susceptible to hydrogen embrittlement;
c) parts with coatings adversely affected by a temperature of
180 °C to 210 °C; and
d) parts adversely affected by a temperature of 180 °C to 210 °C.
The heating process shall commence within 1 h after the completion
of electroplating.
Except for those parts listed in a) to d), parts shall be heated to
achieve a temperature of 180 °C to 210 °C within 4 h (maximum)
after the completion of electroplating and shall be maintained at this
temperature for the appropriate time as given in 1) to 6) below. These
times shall be treated as minimum times at temperature.
1) Bolts and screws of property classes 9.8, 10.9 and 10.9 4 h.
2) Bolts and screws of property class 12.9 (see 6.1) 6 h.
3) Spring steel parts hardness range 400 HV to 500 HV 8 h.
4) Spring steel parts hardness range 500 HV to 600 HV 12 h.
5) Case-hardened tapping screws 2 h.
6) Case-hardened thread rolling screws 6 h.
NOTE 2 It should not be assumed that the specified baking process
will completely remove hydrogen embrittlement in all cases. Other
conditions of duration and temperature can be used if they have been
shown to be effective for the parts listed in a) to d). Parts which have
been tempered at temperatures below 180 °C need baking at a lower
temperature. Generally, parts baked at lower temperatures require longer
times at that temperature. Some types of steel are more susceptible to
hydrogen embrittlement than others and the baking times given might
be inadequate in some circumstances. Certain coatings, e.g. tin, and
the physical properties of some parts might be adversely affected by a
temperature of 180 °C to 210 °C. In such cases, lower temperatures and
longer times will be required.
7 Thickness and coating weight
Thickness and coating weight shall be evaluated using the appropriate
methods specified in each Part of BS 7371, which include:
a) local thickness;
b) batch average thickness; and
c) mass per unit area.
NOTE To assist in calculations, areas of standard metric bolts, screws and
nuts are defined in Annex C.
8 Sampling for inspection
Sampling for thickness measurement shall be in accordance with
BS 6041:1981+A1.
Sampling for corrosion testing or torque/clamping force measurement
shall be in accordance with BS 6041:1981+A1, Table 3.
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9 Delivery conditions
All requirements of the coating specifications shall be conformed to
at the time of delivery of the fasteners or accessories to the location
specified by the purchaser (see Annex D).
NOTE The purchaser should state the location in the enquiry or order.
10 Information to be supplied to the coater
and designation
The following information shall be supplied to the coater.
a) The number of this British Standard, i.e. BS 7371-1:2009.
b) The Part of BS 7371 appropriate to the coating required.
c) The coating grade or thickness as applicable.
d) The requirement for lubrication, if applicable.
e) The requirement, if any, for stress relieving before coating, and
the stress relieving conditions.
f) Special requirements, e.g. additional protection for storage (see
Clause 9 and Annex D).
g) The requirements of subsequent operations such as the
application of adhesives.
The coating shall be specified using at least the following designation
(see footnote A) to Table A.2):
“Coating in accordance with BS 7371-X:20XX: Grade ”.
The lubricated condition shall be designated by the letter “L” after
the grade identification (see Annex A).
EXAMPLE
An electroplated zinc coating of grade Zn8C with added lubrication is
designated thus:
BS 7371-3:2009: Grade Zn8CL.
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Annex A (informative) Selection guidelines
A.1 Guidelines on the selection of coatings
A.1.1 Protective value and service condition number
The protective value and service life of a coating will vary according
to the type of coating, its thickness, the use of a chromate conversion
treatment (or other coating where appropriate), and so on.
For the purpose of this standard, degrees of severity are defined in
Table A.1 and equated to a service condition number.
A.1.2 Coating selection
Coatings should be selected using the data in Table A.2 and Table A.3
in accordance with the criteria defined in Clause 4 and in conjunction
with the appropriate Part of BS 7371.
NOTE Increased thicknesses may be specified to extend resistance to rust
provided that thread dimensions before plating are adjusted.
All coatings except phosphate and oil coatings will not substantially
deteriorate when stored in a good storage environment for a period
of 3 months.
Table A.1 Protective value and service condition number
Designation number and service condition Description and application examples
0
Very mild
Exposure to indoor atmospheres and not subject to
condensation, wear or abrasion
Example: a product with temporary protective coating for
storage
1
Mild
Exposure to indoor atmospheres with rare condensation
and subject to minimum wear or abrasion
Example: domestic furniture screws
2
Moderate
Exposure mostly to dry indoor atmospheres but subject to
occasional condensation, wear or abrasion
Examples: fasteners for motor vehicle interiors, industrial
machinery
3
Severe
Exposure to condensation, perspiration, infrequent
wetting by rain, and cleaners
Examples: fasteners for window fittings, washing
machines, bicycles
4
Very severe
Exposure to harsh conditions or subject to frequent
exposure to moisture, cleaners and saline solutions, plus
likely damage by denting, scratching or abrasive wear
Examples: plumbing, motor vehicle underbody fasteners
NOTE For exceptionally severe service conditions for outdoor use (for example, electrical transmission towers,
buildings and bridges, etc.) where very long service is required, consider hot dip galvanizing to BS 7371–6 or
mechanical galvanizing to BS 7371-7 or sheradizing to BS 7371-8. For the extra thread tolerances required, see
ISO 965-5:1998, thread tolerance 6AZ.
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Table A.2 Coating selection
Coating Grade A) Embrittlement B) Galvanic C) Conductive D) Lubricity E) Appearance
and
chromium
content F)
Service
condition
number
Electroplated
zinc G)
Zn5A ✗ ✗ ✗ — Silver6+ 1
Zn5B ✗ ✗ ✗ — Silver6+ 1
Zn5C ✗ ✗ ✗ — Yellow6+ 2
Zn5D ✗ ✗ ✗ — Olive drab6+ 3
Zn5V ✗ ✗ ✗ — Silver-blue3+ 1
Zn5W ✗ ✗ ✗ — Silver-irid.3+ 1
ZN8A ✗ ✗ ✗ — Silver6+ 1
Zn8B ✗ ✗ ✗ — Silver6+ 1
Zn8C ✗ ✗ ✗ — Yellow6+ 2
Zn8D ✗ ✗ ✗ — Olive drab6+ 3
Zn8V ✗ ✗ ✗ — Silver-blue3+ 1
Zn8W ✗ ✗ ✗ — Silver-irid.3+ 1
Zn12A ✗ ✗ ✗ — Silver6+ 1
Zn12B ✗ ✗ ✗ — Silver6+ 2
Zn12C ✗ ✗ ✗ — Yellow6+ 3
Zn12D ✗ ✗ ✗ — Olive drab6+ 4
Zn12V ✗ ✗ ✗ — Silver-blue3+ 1
Zn12W ✗ ✗ ✗ — Silver-irid.3+ 2
Mechanically
plated zinc G)
Zn5AM — ✗ ✗ ✗ Silver6+ 1
Zn5BM — ✗ ✗ ✗ Silver6+ 1
Zn5CM — ✗ ✗ ✗ Yellow6+ 2
Zn5DM — ✗ ✗ ✗ Olive drab6+ 3
Zn5VM — ✗ ✗ ✗ Silver-blue3+ 1
Zn5WM — ✗ ✗ ✗ Silver-irid.3+ 1
Zn8AM — ✗ ✗ ✗ Silver6+ 1
Zn8BM — ✗ ✗ ✗ Silver6+ 1
Zn8CM — ✗ ✗ ✗ Yellow6+ 2
Zn8DM — ✗ ✗ ✗ Olive drab6+ 3
Zn8VM — ✗ ✗ ✗ Silver-blue3+ 1
Zn8WM — ✗ ✗ ✗ Silver-irid.3+ 1
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Table A.2 Coating selection (continued)
Coating Grade A) Embrittlement B) Galvanic C) Conductive D) Lubricity E) Appearance
and
chromium
content F)
Service
condition
number
Zn12AM — ✗ ✗ ✗ Silver6+ 1
Zn12BM — ✗ ✗ ✗ Silver6+ 2
Zn12CM — ✗ ✗ ✗ Yellow6+ 3
Zn12DM — ✗ ✗ ✗ Olive drab6+ 4
Zn12VM — ✗ ✗ ✗ Silver-blue3+ 1
Zn12WM — ✗ ✗ ✗ Silver-irid.3+ 2
Organic OA — — — ✗ Coloured H) 1
OB — — — ✗ Coloured 2
OC — — — ✗ Coloured 3
OD — — — ✗ Coloured 4
Zinc flake A — ✗ ✗ ✗ Silver greyc 3
B — ✗ ✗ ✗ Silver greyc 4
Hot dip
galvanized
— — ✗ ✗ ✗ Silver/grey 4
NOTE 1 The use of cadmium is severely restricted, cadmium has been removed from this table.
NOTE 2 For exceptionally severe service conditions for outdoor use (for example, electrical transmission
towers, buildings and bridges, etc.) where very long service is required, consider hot dip galvanizing to
BS 7371-6 or mechanical galvanizing to BS 7371-7 or sheradizing to BS 7371-8. For the extra thread tolerances
required, see BS ISO 965-5:1998, thread tolerance 6AZ.
A) The designation can be extended to include requirements such as lubricity, colour, etc., in accordance with the
appropriate Part of BS 7371 applicable to the specific coating.
B) ✗ indicates the potential for embrittlement of parts during the coating process, see Clause 6.
C) ✗ indicates that the method of protection is galvanic.
D) ✗ indicates an ability to form part of an electrically conductive assembly at voltages of 12 V and greater with little
resistance.
E) ✗ indicates that lubricity is, or can be,